Alex Stancu_WiFi Mesh Optimization for Bucharest City Center

8/12/2019 Alex Stancu_WiFi Mesh Optimization for Bucharest City Center

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University Politehnica of Bucharest

Faculty of Electronics, Telecommunications and Information Technology

Design and Optimization of a Wi -F i/mesh access network case study

on Orange Romania network

Diploma Thesis submitted in partial fulfillment of the requirements for the Degree of

Engineer in the domain Electronics and Telecommunications study

program Networks and Software for Telecommunications

Thesis Advisors Student

Prof.Dr.Ing Eugen BORCOCI Liviu-Alexandru STANCU

Ing. Cristian PATACHIA-SULTANOIU

2012


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Statement of Academic Honesty

I hereby declare that the thesis Design and Optimization of a Wi-Fi/mesh

access network

case study on Orange Romania network

, submitted to the Facultyof Electronics, Telecommunications and Information Technology in partialfulfillment of the requirements for the degree of Engineer/Master of Science in thedomain Electronics and Telecommunications , study program Networks and Software

for Telecommunications , is written by myself and was never before submitted to anyother faculty or higher learning institution in Romania or any other country.

I declare that all information sources sources I used, including the ones I foundon the Internet, are properly cited in the thesis as bibliographical references. Text

fragments cited as is or translated from other languages are written between quotesand are referenced to the source. Reformulation using different words of a certain textis also properly referenced. I understand plagiarism constitutes an offence punishable

by law.

I declare that all the results I present as coming from simulations andmeasurements I performed, together with the procedures used to obtain them, are realand indeed come from the respective simulations and measurements. I understandthat data faking is an offence punishable according to the University regulations.

Bucharest, July 2012

Liviu-Alexandru STANCU

_________________________


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Table of Contents

List of Figures ............................................................................................................................................. 5

List of tables ................................................................................................................................................ 7

Acronyms .................................................................................................................................................... 8Introduction ................................................................................................................................................. 9

Chapter I Overview of Wi-Fi technology (IEEE 802.11) ....... ........ ....... ....... ....... ........ ....... ....... ....... ........11

1. Wi-Fi Brief history .......................................................................................................................11

2. Wireless LANs General description ..............................................................................................11

3. Architecture and components of 802.11 ....... ....... ........ ....... ....... ........ ....... ....... ....... ........ ....... ....... ....12

4. Network Services ............................................................................................................................13

4.1 Distribution system services ..........................................................................................................13

4.2 Station services .............................................................................................................................14

5. IEEE 802.11 architecture .................................................................................................................15

5.1 The hidden node problem ..............................................................................................................15

5.2 The exposed node problem ............................................................................................................16

6. IEEE 802.11 a/b/g/n spectrum and channels used ....... ....... ....... ........ ....... ....... ....... ........ ....... ....... ....16

7. WLAN topologies ...........................................................................................................................19

8. Issues in 802.11 WLANs .................................................................................................................20

8.1 Interference ...................................................................................................................................20

8.2 Interference mitigation ..................................................................................................................21

8.3 Attenuation ...................................................................................................................................228.4 Attenuation mitigation ...................................................................................................................23

9. Wireless Mesh Networking .............................................................................................................24

10. Power over Ethernet (PoE) ..........................................................................................................24

11. Cisco Unified Wireless Network Solution (CUWN) ....... ....... ........ ....... ....... ....... ........ ....... ....... ....24

12. Mobility ......................................................................................................................................27

Chapter II Introduction in designing and optimizing a wireless network ....... ....... ........ ....... ....... ....... ........ .29

1. Performing a site survey ..................................................................................................................29

2. Designing ........................................................................................................................................30

3. Optimization ...................................................................................................................................30

Chapter III Description of hardware solutions and software tools used ....... ....... ........ ....... ....... ........ .......31

1. Hardware solution ...........................................................................................................................31

1.1 Cisco Aironet 1552E Outdoor Access Point ...................................................................................31

1.2 Cisco AIR-ANT2547V-N Antenna ................................................................................................31

1.3 Cisco Wireless LAN Controller 4404 ............................................................................................32

2. AirMagnet Survey software tool ......................................................................................................32

3. AirMagnet Planner software tool ....... ........ ....... ....... ....... ........ ....... ........ ....... ....... ....... ........ ....... ......36

Chapter IV Practical design of the Wi-Fi/mesh network ....... ........ ....... ....... ....... ........ ....... ....... ....... ........411. Site Survey ......................................................................................................................................41


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2. Survey results analysis ....................................................................................................................50

3. Designing the Wireless Mesh Network ....... ....... ....... ........ ....... ....... ........ ....... ....... ....... ........ ....... .....51

Conclusions ................................................................................................................................................57

References ..................................................................................................................................................58

Annex 1 further survey results..................................................................................................................59


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List of Figures

Figure I-1 Basic service set .........................................................................................................................12Figure I-2 Independent and Infrastructure BSSs [9] .....................................................................................13Figure I-3 IEEE 802.11 architecture ............................................................................................................15Figure I-4 The hidden node problem: A wants to transmit to B; C is the hidden station ....... ........ ....... ........ ..15Figure I-5 The exposed node problem: C is the exposed station ....... ........ ....... ....... ....... ........ ....... ....... ........ .16Figure I-6 Comparison between 802.11 a/g/n transmission speeds [12] .......................................................17Figure I-7 Channelization Scheme [11] .......................................................................................................18Figure I-8 Channelization scheme for 5 GHz band [5] .................................................................................18Figure I-9 Star topology [17].......................................................................................................................19Figure I-10 Tree topology [17] ....................................................................................................................19Figure I-11Mesh topology [17] ...................................................................................................................20Figure I-12Example of harmonics interference [18].....................................................................................21Figure I-13Free space loss for different frequencies [24] .............................................................................22Figure I-14 An example of multipath propagation and its result [25] ...........................................................23Figure I-15 Comparison between a classic WLAN and a mesh WLAN [26] ........... ........ ....... ....... ........ .......24Figure I-16 Example of a Wireless Mesh Network [30] ...............................................................................26

Figure III-1 Cisco Aironet 1550 Series Outdoor AP [37] .............................................................................31Figure III-2 Cisco AIR-ANT2547V-N Antenna Radiation Patterns [38] ......................................................32Figure III-3 AirMagnet Survey project type selection ..................................................................................33Figure III-4 AirMagnet Survey GPS map importing ....................................................................................34Figure III-5 AirMagnet Survey Environment Information ...........................................................................34Figure III-6 AirMagnet Survey GPS Port configuration ....... ........ ....... ....... ....... ........ ....... ....... ....... ........ ......35Figure III-7 AirMagnet Survey type of survey selecting ..............................................................................36Figure III-8 AirMagnet Planner map recalibration .......................................................................................37Figure III-9 AirMagnet Planner - attenuation areas ....... ....... ........ ....... ....... ....... ........ ....... ....... ........ ....... ......38Figure III-10 AirMagnet Planner - AP exclusion areas ................................................................................38Figure III-11 AirMagnet Planner - desired coverage areas (blue) .................................................................38Figure IV-1Old Town Center with coverage areas ........ ....... ........ ....... ....... ....... ........ ....... ....... ........ ....... ......41

Figure IV-2 Non-GPS survey path example ................................................................................................42Figure IV-3 a) GPS survey with "on-site" recalibrated map; b) GPS survey with map imported from theInternet .......................................................................................................................................................42Figure IV-4a Selari Street APs found .......................................................................................................43Figure IV-5 Stavropoleos Street - APs found...............................................................................................43Figure IV-6 Smardan Street - APs found .....................................................................................................43Figure IV-7 Covaci Street - APs found ........ ....... ........ ....... ....... ....... ........ ....... ....... ........ ....... ....... ........ ........43Figure IV-8 Franceza Street - APs found ....... ........ ....... ....... ....... ........ ....... ........ ....... ....... ....... ........ ....... ......44Figure IV-9 Lipscani Street - APs found .....................................................................................................44Figure IV-10 Survey results for GPS-aided survey ......................................................................................44Figure IV-11 Selari Street - Orange WiFi coverage .....................................................................................44Figure IV-12 Stavropoleos Street - Orange WiFi coverage ..........................................................................44Figure IV-13 Smardan Street - Orange WiFi coverage ................................................................................45Figure IV-14 Franceza Street - Orange WiFi coverage ................................................................................45Figure IV-15 Covaci Street - Orange WiFi coverage ...................................................................................45Figure IV-16 Lipscani Street - Orange WiFi coverage .................................................................................45Figure IV-17 Old Town Center - Orange WiFi coverage .............................................................................45Figure IV-18 APs and their maximum received signal ....... ....... ....... ........ ....... ....... ........ ....... ....... ....... ........ .46Figure IV-19 SNR map for Franceza Street .................................................................................................47Figure IV-20 SNR map for Selari Street ......................................................................................................47Figure IV-21 SNR map for Covaci Street ....................................................................................................47Figure IV-22 SNR map for Smardan Street .................................................................................................47Figure IV-23 SNR map for Lipscani Street..................................................................................................47Figure IV-24 SNR map for Stavropoleos Street ...........................................................................................47Figure IV-25 SNR map for Old Town Center ..............................................................................................48Figure IV-26 Franceza Street - Predictive PHY ...........................................................................................48Figure IV-27 Covaci Street - Predictive PHY ..............................................................................................48
http://localhost/var/www/apps/conversion/tmp/scratch_10/lucrare_diploma_alex_stancu_v10.docx#_Toc328933546http://localhost/var/www/apps/conversion/tmp/scratch_10/lucrare_diploma_alex_stancu_v10.docx#_Toc328933547http://localhost/var/www/apps/conversion/tmp/scratch_10/lucrare_diploma_alex_stancu_v10.docx#_Toc328933549http://localhost/var/www/apps/conversion/tmp/scratch_10/lucrare_diploma_alex_stancu_v10.docx#_Toc328933549http://localhost/var/www/apps/conversion/tmp/scratch_10/lucrare_diploma_alex_stancu_v10.docx#_Toc328933547http://localhost/var/www/apps/conversion/tmp/scratch_10/lucrare_diploma_alex_stancu_v10.docx#_Toc328933546


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Figure IV-28 Smardan Street - Predictive PHY ...........................................................................................48Figure IV-29 Lipscani Street - Predictive PHY ....... ....... ....... ........ ....... ....... ........ ....... ....... ....... ........ ....... .....48Figure IV-30 Selari Street - Predictive PHY ................................................................................................49Figure IV-31 Stavropoleos Street - Predictive PHY .....................................................................................49Figure IV-32 Predictive Physical Data Rates for Old Town Center ....... ........ ....... ....... ....... ........ ....... ....... ....49Figure IV-33 Noise level map for Old Town Center, integrated in Google Earth .........................................49Figure IV-34 Interference map for Old Town Center Orange WiFi ...........................................................50Figure IV-35 Interference level for Old Town Center (survey path in red) ...................................................50Figure IV-36 AirMagnet Planner map with the already deployed network ...................................................52Figure IV-37 AirMagnet Planner Advisor suggestion ..................................................................................53Figure IV-38 Solution for extending the wireless mesh network ..................................................................53Figure IV-39 Further improvements on the design.......................................................................................54Figure IV-40 Further possible optimization .................................................................................................55


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AcronymsAP Access Point

ACK Acknowledge

AWPP Adaptive Wireless Path Protocol

BSS Basic Service Set

CSMA Carrier Sense Multiple Access

CSMA/CA Carrier Sense Multiple Access with Collision Avoidance

CSMA/CD Carrier Sense Multiple Access with Collision DetectionCUWN Cisco Unified Wireless Network

CTS Clear To Send

CCI Co-Channel Interference

CAPWAP Control And Provisioning for Wireless Access Points

DOCSIS Data Over Cable Service Interface Specification

DSSS Direct Sequence Spread SpectrumDS Distribution System

EIRP Effective Isotropic Radiated Power

ETSI European Telecommunications Standard Institute

FHSS Frequency Hopping Spread Spectrum

HR-DSSS High Rate Direct Sequence Spread Spectrum

ISM Industrial, Scientific and Medical

IEEE Institute of Electrical and Electronics Engineers

ISO International Organization for Standardization

LWAPP Lightweight Access Point Protocol

LLC Logical Link Control

MAC Medium Access ControlMAP Mesh Access Point

MIMO Multiple Input Multiple Output

NMEA National Marine Electronics AssociationOSI Open System Interconnection

OFDM Orthogonal Frequency Division Multiplexing

MPLS Multi Protocol Label Switching

PHY Physical layer

PLCP Physical Layer Convergence Protocol

PMD Physical Medium Dependent

RFSM Radio Frequency Spectrum ManagementRRM Radio Resource Management

RTS Request To Send

RAP Root Access Point

SSID Service Set Identifier

SINR Signal to Interference Noise Ratio

SFP Small Form-Factor Pluggable

SPA Signal Propagation AssessmentSTA Station in a WLAN

WCS Wireless Control System

WLC Wireless LAN ControllerWLAN Wireless Local Area Network


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Introduction

Wireless mesh networking comes as a response for the increased need in networkingnowadays. One of the main goals of wireless networks is providing Internet access to devices suchas laptops, tablets or smartphones. Wired networks are no longer a viable option for these devices,

because it would cancel the idea of mobility, their main advantage. Wireless Internet access can be

provided also by 3G networks, but Wi-Fi networks come with a series of advantages. Wi-Finetworks operate in a free spectrum, 2.4 or 5 GHz, so from this point of view, they are cheaper.They can provide increased speeds that reach, in theory, up to 600 Mbps with the new 802.11nstandard. A downside would be coverage, but with the new mesh networking implementationscoverage is being improved over classical Wi-Fi networks.

The purpose of this document is to present the steps necessary for designing and optimizinga wireless mesh network. The starting point was a Wi-Fi mesh network implemented in the OldTown Center in Bucharest by Orange Romania and the objectives were to extend the coverage ofthis network and to optimize the already implemented network for better coverage and data rates, if

possible. I achieved these objectives first by conducting a survey, then by analyzing survey results,and then by designing and optimizing the network, considering the previous steps. This was donewith the help of two specialized tools, AirMagnet Survey and AirMagnet Planner.


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Chapter I Overview of Wi-Fi technology (IEEE 802.11)

1. Wi-Fi Brief historyWi-Fi technology has its origins in 1985, when the ISM band for unlicensed use was

released. Later, the popularization of computers at home created the need to interconnect severaldevices, and WLANs became attractive as it was cheaper, no wires were required and it gavemobility to laptops. Communication protocols available at that time were designed for voicesignals, not for wireless broadband data, so IEEE (Institute of Electrical and Electronics Engineers)in the USA and ETSI (European Telecommunications Standards Institute) in Europe started theefforts to specify a new protocol. These specifications were oriented towards data packets, using IP(Internet Protocol). The idea behind was to emulate the successful Ethernet links wirelessly.

In 1996, ETSI released the first specification for a wireless LAN (HiperLAN/1). It wascapable of reaching an air data rate of 10 Mbps, in a range of 50m, by using a single FSK/GMSK(Frequency Shift Keying with Gaussian Minimum-Shift Keying) carrier, with a special layer forChannel Access and Control (CAC).

IEEE released its first specification, named 802.11, in 1997. The theoretical air rate was 1 or2 Mbps and it was specified for the 2.4GHz band. It used a carrier with CCK (Complementary CodeKeying) and DSSS (Direct Sequence Spread Spectrum) or Frequency Hopping and CSMA/CA(Carrier Sense Multiple Access/Collision Avoidance) as a method for medium access control.

There are several specifications that have been released by IEEE since 1997, each solving aspecific issue related to WLAN implementation. Those specifications are identified with 802.11followed by a letter (e.g. a, b, g, n etc.). The standard evolved, and in 1999 was released a newspecification (802.11b), which had a theoretical data rate of 11 Mbps. Since they thought that higherthroughputs would not be possible using single carrier solutions, due to the symbol duration

becoming shorter that the multipath spread, the same amendment included a new technology, foruse in the 5 GHz band: 802.11a, to increase air data rate to 54 Mbps. In 2000, ETSI released a new

specification, HiperLAN/2, very similar to 802.11a. In 2003, the OFDM technology used in802.11a (5 GHz) was extended to the 2.4 GHz band and became known as 802.11g. The latestspecification released by IEEE is 802.11n, entitled Enhancement for higher throughput and itestimates a maximum theoretical data rate of 600 Mbps.

Due to the complicated implementation of HiperLAN (more layers of the OSI protocol stackwere affected and needed to be specific) and to the fact that IEEE 802.11 devices had alreadyreached a price that was too low to compete with at the time ETSIs release was ready, the formernever received much commercial implementation and 802.11 is now implemented worldwide.

2. Wireless LANs General description IEEE 802.11 has specifications over layer 1 (PHY) and layer 2 (Data Link) of the ISO/OSI

model. There are fundamental characteristics that differentiate significantly wireless networks overtraditional wired LANs. First of all, an address in wired LANs represents a physical, fixed location,while in WLANs an address refers a station that is mobile, within a specific range. Thus, IEEE802.11 physical layers are known to have the following features:

use a medium that has no absolute boundaries outside of which transceivers wouldnot be capable to transmit;

are unprotected from other signals that share the medium; the medium is less reliable than wired physical layers; have dynamic topologies; lack of full connectivity (not every station can hear every other station); propagation properties are asymmetric and time-varying.


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Wireless LANs can handle, as IEEE 802.11 Standard specifies, mobile stations (that accessLANs while moving), as well as portable stations (stations that can be moved from one place toanother, but use the network only from a fixed location). Because the medium used forcommunicating is air, due to its propagation effects one cannot distinguish between portable andmobile stations. Even stationary stations can seem mobile because of these effects. Another majordifference between wired and wireless networks is that the stations from the latter may often have

the receiver powered off, because of the power management (being mobile the stations often are battery powered), so they are not always ready to receive. For higher OSI layers , WLANs mustappear as any other network, therefore the data link layer consists of LLC (Logical Link Control)and MAC (Medium Access Control), with the observation that MAC has a specific functionalitydue to the medium.

3. Architecture and components of 802.11 The IEEE 802.11 architecture has four major components that cooperate in order to provide

WLANs features: Stations (STA)

The entities between which the data is transferred in a network are called stations.Typically, stations are computing devices with wireless network interfaces (e.g. laptop,smartphone etc.).

Access points (AP)Access points are devices that offer, as the name suggests, an entry point for the

stations in the network. A special type of AP, also named a portal, provides a veryimportant function: the wireless-to-wired bridging function. In other words, frames on an802.11 network must be converted to another type of frame for delivery to the othernetworks, and this job is completed by a portal.

Wireless mediumThe frames in the network must be transferred from station to station by using a

wireless medium. The standard defines several different physical layers, such as radiofrequency (RF) physical layers, or infrared physical layers, though the RF layers have proven far more popular.

Distribution system (DS)The distribution system is used when several access points are used to cover a large

area, to handle the communication between them. This communication is necessary fortracking the movements of mobile stations, in order to provide mobility. The distributionsystem is, for the matter, the logical component of 802.11 used to forward frames to theirdestination. It may also be called the backbone network.

802.11 networks are flexible by design. Three possible implementations of a WLAN are

foreseen in the standard: InfraBSS (Infrastructure Basic Service Set), IBSS (Independent BasicService Set) and ESS (Extended Service Set).

A basic service set is a logical group of 802.11 stations communicating with one another.Because of the physical layer limitations (radio coverage), WLANs that have to cover a largegeographical area may consist of several basic blocks, namely basic service sets (BSSs).

Figure I-1 Basic service set


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Figure I-1 shows two basic service sets, each having two stations, the oval shaperepresenting a BSS coverage area within which each member stations of the BSS may remain incommunication. The area is called the Basic Service Area (BSA), and if a station leaves its BSA itcan no longer communicate with other stations of the BSA. A BSS requires a specialized station,known as an access point (AP), being the central point of communications for all stations in a BSS.In other words, the client stations do not communicate directly to one another. Rather, they

communicate with the AP and the AP forwards the frames to the destination. The AP might beequipped with an uplink port that connects the BSS to a wired network. Because of this feature, thistype of BSS is known also as InfraBSS (Infrastructure Basic Service Set). In order to obtainnetwork services in an infrastructure network, stations must associate with an access point. Access

points may choose to grant or deny the access, based on the contents in the association request. Thestandard does not compel a number of mobile stations that an access point can serve, but a mobilestation can only be associated with a single access point at a time.

The second type of possible implementation is the Independent BSS (IBSS). Stations in anIBSS communicate directly with each other and must be within direct communication range. Hence,an AP is no longer needed. Generally speaking, an IBSS is composed of a small number of stationsand has a specific purpose, for a short period of time. These are the reasons why IBSSs aresometime referred to as ad hoc networks.

The third type of implementation is ESS (Extended Service Set). Multiple infrastructureBSS can be connected via their uplink interfaces. The collection of BSSs interconnected via the DSis known as the ESS. The standard does not impose a specific backbone technology, but it requiresthe DS to provide a specified set of services.

An important concept is that an ESS behaves, from the LLC point of view, as an IBSS. Thestations within an EES can communicate, and can move between BSSs (in the same ESS)transparently to the LLC. 802.11 standard does not mention anything about the relative physical

positions of the BSSs (they can partially overlap, they can be fully disjointed; distances betweenBSSs are not limited).

4. Network Services802.11 standard provides network services necessary for a successful implementation, but

not the implementation itself of those services. The architectural service set is divided into twocategories: station services and distribution system services.

4.1 Distribution system services Distribution the service through which the messages are distributed by the DS.

The standard does not stipulate how the messages are distributed, but it states howthe DS needs to obtain the information needed to determine the target AP for thedestination. This information is provided by the administrative services.

Integration the service that allows the DS to transmit the messages towards or

from a portal (the interface between a DS and a non-IEEE 802.11 network)

Figure I-2 Independent and Infrastructure BSSs [9]


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These first two services form the message distribution services. The followingservices form the second distribution system services category, namely the managementservices:

Association in order to deliver a message within a DS, one has to know the access point towards the destination. This information is provided to the DS through theassociation service. Before a station can transmit a message through an AP, it needs

to become an associate of that AP and the DS will use this correspondence for itsdistribution service. If security protocols are used in the network, the association ismade just before the authentication. An AP that will drop all network protocol trafficfrom a station before authentication is made.

Reassociation this service is required by a station for changing the associationfrom an AP to another AP, when the station moves from a BSS to another, within thesame ESS. A station may request reassociation with the same AP if it wants tochange the association attributes.

Disassociation a service requested by a station or by an AP in order to terminate anexisting association. This is a notification and it cannot be refused. A station mustrequest disassociation whenever it leaves the network. An AP may requestdisassociation of the stations, so it can be removed from the network (formaintenance or other reasons).

4.2 Station servicesThese services are present in every station (including APs), are used by the MAC layer and

consist of: authentication, deauthentication, preauthentication, privacy. Two of these services,authentication and privacy, are designed to create a network closer to the features of wirednetworks, induced by the controlled and closed nature of the transmission medium.

Authentication wired networks have a physical security that preventsunauthorized access. Wireless networks, however, because the transmission medium

has no precise boundaries, provide an authentication service used by all stations to present their identity to the stations they will communicate with. Authentication ismade for taking the wireless link to the level of a physical, wired link. Twoauthentication schemes exist, one using an open system authentication (any stationcan become authenticated with any other), the other one using a shared keyauthentication.

Preauthentication authentication is a service requested before establishing anassociation and may require a considerable amount of time, depending on theauthentication protocol being used. Given that fact, the authentication service can beinvoked independently of the association service, thus improving BSS transitionmobility performance.

Deauthentication this service is required when an existing authentication needs to be terminated. Deauthentication determines disassociation of a station; it may berequested by any authenticated part and cannot be refused, because it is anotification.

Data confidentiality (privacy) this service provides the ability to protect thecontent of the messages through cryptographic algorithms. The default state for astation is to send the messages unprotected. If this is unacceptable for the sender, heshould not send the data. On the other hand, if this is unacceptable for the receiver,he should discard the data received.


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5. IEEE 802.11 architectureThe following figure presents 802.11 network logical architecture.

Figure I-3 IEEE 802.11 architecture

MAC sublayer utilizes CSMA (Carrier Sense Multiple Access) procedure, as Ethernet does.The main difference between them is that in Ethernet a station can detect collisions and CSMA/CD(CSMA Collision Detection) is used. In order to detect collisions in WLAN, a station would have totransmit and receive on the same frequency, thus leading to oscillation. Because this is an unwantedsituation, stations must try to avoid collisions and CSMA/CA (CSMA Collision Avoidance) is used.Due to the noise in the wireless transmission medium, effects of interference and multipath fading,

a mechanism with confirmation of the transmitted frames is used. If a confirmation frame (ACK) isnot received in a specified amount of time, the unconfirmed frame will be retransmitted.Using radio waves as a physical layer involves a more complex physical layer; thereby it

consists of two sublayers: PLCP (Physical Layer Convergence Protocol), used to map the MACframes onto the medium and PMD (Physical Medium Dependent) system to transmit the frames.

5.1 The hidden node problem

If two stations, A and C are not accessible to one another, and if another station B is between the first two and accessible to each of them, the transmission from A to B may beinterrupted by the transmission from C to B. Neither A or C is aware of this collision. The situation

is presented in the figure I-4.

Figure I-4 The hidden node problem: A wants to transmit to B; C is the hidden station


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5.2 The exposed node problem

Assuming that C is trying to transmit towards station D in the same time station B transmitstowards A. In this case, for station C, the transmission medium looks like being occupied and itshould delay the transmission, despite the fact that D does not hear the transmission from station B.The situation is presented in figure I-5.

Figure I-5 The exposed node problem: C is the exposed station

Both problems can be solved using a simple mechanism, which is optionally implemented inthe network. It is basically an algorithm, called multiple access with collision avoidance and it usescontrol frames, RTS and CTS (RTS Request To Send; CTS Clear To Send). The station that hassomething to transmit sends a RTS. The destination station then transmits a CTS. The stations thathear RTS and do not hear CTS are free to emit, because they do not perturb the destination station.If a station receives a RTS as well as a CTS it has to delay its transmission until it hears theconfirmation frame. If a station does not hear the RTS but it hears the CTS needs to delay thetransmission until it hears the confirmation frame. If two stations detect a clear link and try totransmit a RTS frame at the same time, the frames will collide and the stations cannot detect thecollision. Instead, if they do not receive a CTS frame within a given time, they each wait for arandom amount of time before trying again. The amount of time is defined by an exponential

backoff algorithm, the same used in Ethernet.

6. IEEE 802.11 a/b/g/n spectrum and channels usedThere are numerous amendments to the original 802.11 standard released in 1997. Some are

completed and even superseded by recent ones and some are still in development (they are active, but considered nevertheless a draft). The amendments were supposed mainly to facilitate higherdata rates and ranges for the WLANs. In the following paragraphs IEEE 802.11 a, b, g, and namendments will be considered and compared, preceded by the original standard.

The legacy standard appeared in 1997 and was designed for operating in the 2.4 GHz band.It used DSSS (Direct Sequence Spread Spectrum) or FHSS (Frequency Hopping Spread Spectrum)modulation, and a carrier with a bandwidth of 20 MHz, thus leading to a theoretical data rate of 1and 2 Mbps. It had an approximate indoor range of 20 meters and an outdoor one of 100 meters.

FHSS was largely deployed at that time because the electronics needed for FH modulation werecheap and did not require high power consumption. Another advantage was that a greater number ofnetworks could coexist due to frequency-hopping technique. The idea behind FH was changing thefrequency used for communication with a predetermined hopping pattern. On the other hand, DSSSwas using a spreading sequence to replace each bit of information with a wideband spreading code,thus leading to better signal to interference ratio and the ability to work with lower transmitter

power. Nevertheless, the legacy standard could not resist superseding by newer standards designed

for higher data rates. Therefore, in 1999 two new specifications appeared: 802.11a and 802.11b.The latter operated in the same 2.4 GHz band as the legacy standard, but had data rates up to 5.5Mbps and 11 Mbps. These rates were achieved by using a new modulation technique, called HR-DSSS (High Rate Direct Sequence Spread Spectrum). The channelization scheme used is the sameas with DSSS, meaning a 22 MHz bandwidth and 11 channels, 3 being non-overlapping. The rangeincreases too, to an approximately 38 meters indoors and 140 meters outdoors. At the same time


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802.11b introduced HR-DSSS physical layer, IEEE 802.11a presented the OFDM (OrthogonalFrequency Division Multiplexing) physical layer, but this time for 5 GHz band instead of 2.4 GHz.They decided for the 5 GHz band because they wanted higher data rates and the 2.4 GHz band wasalready being crowded, often with non-802.11 traffic. High data rates were, indeed, achieved, with amaximum theoretical rate of 54 Mbps. The standard provides the following data rates: 6, 9, 12, 24,48 and 54 Mbps, with a bandwidth of the carrier of 20 MHz.

In June 2003 a new specification was released, namely 802.11g. It was designed for backwards compatibility with 802.11b, so it had to operate in the 2.4 GHz band, and for data ratescomparable to 802.11a. As an advantage, because they are working at approximately half thefrequency, 802.11g devices have better range than 802.11a devices. The obvious drawback is themore crowded 2.4 GHz band. As a modulation scheme it uses OFDM technique and the same 11channels of 22 MHz bandwidth, and the data rates achieved are: 5.5, 11, 6, 9, 12, 24, 36, 48 and 54Mbps. Because backwards compatibility with 802.11b is required, in an environment with suchdevices the transmissions take place at the highest data rates available in 802.11b (11 Mbps). On theother hand, if only 802.11g devices are present, the transmissions take place at 54 Mbps.

The latest amendment of 802.11 standard, 802.11n, was released in 2009 and it wasdesigned for increasing speed comparable to wired networks performance. The high data rates are

provided by 802.11n using a new concept, MIMO-OFDM (Multiple Input Multiple Output OFDM).The goal of 802.11n standard is to provide speeds up to an incredible 600 Mbps, comparable towire-like communications. These speeds are achieved using new encoding algorithms, widerchannels and also a new technology, MIMO. MIMO utilizes multiple antennas and paralleltransmission schemes for ensuring a higher performance. 802.11n can use 20 or 40 MHz channelsand can either operate in 2.4 GHz or 5 GHz band. Up to 4 spatial streams are available for MIMOtechnology. Some important advantages of this standard are that multipath issues no longer appear,due to the new technology, and wireless range increases.

Figure I-6 Comparison between 802.11 a/g/n transmission speeds [12] IEEE 802.11 specifications include several requirements for transmission characteristics in

radio frequency spectrum. For example, the standard indicates the channelization scheme or thespectrum radiation of the signal. The 2.4 GHz band is divided into 13 channels in Europe (ETSI)and only 11 channels in the USA. The channels have a bandwidth of 22 MHz each, and have acenter frequency separated by only 5 MHz. These types of channels are used in 802.11b, 802.11gand 802.11n transmissions. In this configuration, a single group of three channels that do notoverlap exists: 1, 6 and 11. The following list presents the 13 channels and their center frequencies:1 2412 [MHz], 2 2417, 3 2422, 4 2427, 5 2432, 6 2437, 7 2442, 8 2447, 9 2452, 10

2457, 11 2462, 12 2467 and 13 2472.


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the regulator body above mentioned is called ANCOM. It stipulates that the maximum output power for a radio LAN must be 100 mW EIRP (Effective Isotropic Radiated Power) [15].

7. WLAN topologies Ad-hoc (point to point) topology

This type of connections, also known as peer-to-peer connections, enables wirelesscapable devices to communicate with each other directly, without connecting to anAccess Point. The main downside for this topology is that it is not scalable;

Star topology

The central node for this type of connection is a central controller, known as anAccess Point. The users do not communicate with each other directly, theycommunicate through the AP. This is the reason why the topology is also named a

point-to-multipoint star. It has the advantage of being scalable;

Figure I-9 Star topology [17] Tree topology

A tree topology combines multiple star topologies into a bus topology, having theadvantage of supporting future expandability of the network.

Figure I-10 Tree topology [17] Mesh topology

Mesh networks are networks in which nodes are mobile and have the ability tocommunicate directly to the adjacent nodes, without needing a central controllingdevice. The topology of a mesh can be constantly changing, as users enter or leavethe network. The main advantage of mesh networks is that they are very reliable and

self-healing, should the number of nodes be sufficiently high, due to the distributedcontrol and continuous reconfiguration.


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Figure I-11Mesh topology [17]

8. Issues in 802.11 WLANs

8.1 Interference

Radio frequency interference represents unwanted energy that overlaps with the efficientenergy used for transmitting data through the wireless medium, thus leading to problems with thereception of the signals. Interference can be caused by other 802.11 devices that transmit overapproximately the same frequency, or non-802.11 devices that work in the ISM band, mainly in the2.4 GHz band, such as Bluetooth devices, microwave ovens, cordless phones, security cameras,ZigBee devices etc.

The frequency that an Access Point operates on represents the center frequency used forcommunication, because both the transmitter and the receiver operate within a band of frequencies,usually several MHz wide. In other words, in the case of the transmitter, it is the frequency wheremost of the emitted power is concentrated, with a decrease in signal strength as we move from thecenter frequency. In the same way, a wireless Access Point receiver will be most sensitive forsignals near the center frequency, decreasing its sensitivity as we move from the center frequency.The center frequency of the receiver can be slightly different than the same frequency of thetransmitter, and the communication will still work, but the range of the communication willdecrease, because power decreases with the moving from the center frequency. A major factor indetermining the effects of radio interference is the band of frequencies around the center frequency.Considering a second radio signal that a receiver encounters, that is too close to its centerfrequency, depending on the bandwidth of the two signals, they can end overlapping too much,causing them to interfere. Furthermore, less power is needed to cause interference as the interferingsignals are closer.

A metric is proposed for measuring the interference between signals, namely Signal to Noiseand Interference Ratio (SNIR). According to this metric, two nodes are connected if their minimumSNIR is greater than a given threshold.

Interference occurs, most of the time, because of radio transmitters and other electronicequipment that operate within close range of each other. There are three main causes for

interference: Radio transmitting equipment that is incorrectly installed; A high-power radio signal from a nearby transmitter; Unwanted signals generated by the transmitter that are captured by the receiver

because of an improper shielding or filtering.

Unwanted signals, often called spurious radiation, are signals that the receiver does not needfor the communication to take place, but still it receives them. This type of signals often includesharmonic radiation, in the form of travelling or standing waves. The latter are a form of spuriousradiation and occur when two or more waves, having the same frequency, coexist and do not travelaway from their source. A typical cause for this type of waves is a damaged or incorrectlyterminated antenna cabling that transmits unwanted signals to the antenna.


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Harmonics are another source of interference. Considering a transmitter that produces asignal on a specific frequency, it also produces harmonics, meaning versions of the original signalthat appear, having less power, on multiples of the devices frequency (e.g. two, three, four timesthe frequency etc.). If one of these versions of the original signal falls on another locally usedfrequency, interference might appear [18].

Figure I-12Example of harmonics interference [18]

Another type of interference present in WLANs is Co-Channel Interference (CCI). Itusually appears in high-density WLANs, meaning an environment where client devices will be

positioned in densities greater than coverage expectations of a normal deployment. CCI affects thecapacity of the cell by reducing the available bandwidth. It appears where transmissions from one802.11 device enter the receiving range of other 802.11 devices on the same channel, thus reducingthe available spectrum and resulting performance. It can also cause delays in channel access orcollisions in transmissions that corrupt frames in transit. Adjacent-channel interference (ACI) mightalso appear in WLANs, because of two access points that operate in close proximity on overlappingchannels. Since WLANs implement a listen before talk protocol, based on CSMA/CA, anyinterference between WLAN networks tends to be resolved somehow, with the networks oftensharing the channel capacity. In contrast, interference from non-802.11 sources more often result inthe severe degradation of WLAN transmissions, because the protocols used differ from those of802.11.

8.2 Interference mitigation

Bandwidth and interference can be considered orthogonal. On one hand, larger bandwidthmeans larger throughput and higher data rates, but on the other hand, larger bandwidth meansincreased chances of interference between transmissions, affecting the data throughput.

No radio signal is absolutely immune to interference, regardless of frequency, thus leadingto two key components to effective interference management: constantly searching for interferencethat threatens the integrity of a WLAN and mitigating any interference discovered.

One method to dealing with the interference is migrating to another channel or band, namelythe 5 GHz spectrum used by 802.11a and some 802.11n devices. This spectrum is less likely tosuffer from interference, because not many devices operate in that band. Furthermore, in the 5 GHz

band there are 23 non-overlapping channels (20 MHz), as compared to just 3 in the 2.4 GHz band.The devices operating in 5 GHz band have less range than 2.4 GHz ones, but in terms of throughputthey are comparable.

Another method to deal with the problem is using RF Spectrum Management (RFSM orRadio Resource Management) tools, which are present in most of the WLAN systems nowadays.

The most important functions of the RRM are the automated setting of channel assignments andtransmit power levels and the reconfiguration of these parameters as radio and network conditionschange over time.


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Power control is a very sensitive issue in interference mitigation. Variable transmit powersresult in asymmetric links in the network and can lead to throughput starvation of some nodes (anode always overhears another nodes transmission, which is done with higher transmission power,and backs off from transmitting, thus leading to not being able to transmit). An adequate controlalgorithm must be used for controlling the power in order to improve the data throughput in thenetwork and obtain a starvation-free network. Intuitively, it works based on the following phrase:

if you want to shout, you need to listen more carefully so as not to disturb t hose who arewhispering [22].

8.3 Attenuation

Attenuation is the decrease in wireless signal strength as the signal propagates away fromthe antenna. An electromagnetic signal propagating through the air contains three sets of waves:direct or Line of Sight waves , sky waves and ground waves. Direct waves represent the group ofelectromagnetic waves propagating directly from the transmitter to the receiver, when a line of sightexists between the two. Sky waves are the group of waves that propagate from the transmittingantenna towards the sky and then being reflected by the ionosphere to the ground and so on until itreaches the receiver. In contrast, ground-wave propagation represents the group of waves thatfollow the contour of the Earth to reach their destination. Radiated energy from the Earth, trees,

buildings and other structures from the ground will affect the latter set of waves, whereas dust andwater particles in the atmosphere will affect mostly the sky-wave propagation model.

As the signal disperses in space, it is obvious that the received signal power will be less thanthe transmitted power, this effect being noticeable as the distance between the transmitter and thereceiver increases. In an ideal circumstance, signal power attenuation is proportional to the squareof that distance. This is called free space loss and is the most important factor concerning signal

power attenuation. Its numerical value is given by the following equation, and it has a constanttheoretical slope of 20 dB per decade [5]:

LdB = 32.444 + 20log 10 f MHz + 20log 10 d km

Figure I-13Free space loss for different frequencies [24]

In addition to free space loss, several other factors may influence the power attenuation ofthe signal: weather conditions, atmospheric attenuation and space rays [10]. Moreover, fading mayaffect signals because of the reflections, refractions and diffractions in the environment.


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8.3.1 Weather co ndit ions: rain, snow and fog If we refer to 802.11 devices, because of their operating frequencies of 2.4 GHz and 5 GHz,

they are not affected much from rain, snow and fog as devices operating at higher frequencieswould be. This happens due to the wavelength of the signal. At lower frequencies, the wavelength ismuch larger than obstacles such as rain drops or snow flakes, thus it is not affected.

8.3.2 Atmosp heric absorp t ionAtmospheric attenuation is not very relevant in the III-34 GHz frequency range, also due tothe wavelength of the signal. An approximate attenuation in the atmosphere is 0.00625 dB/km (or0.01 dB/mile).

8.3.3 Multi path fadin gMultipath appears when the signal travels to the receiver on more than the direct path,

because of the reflections, refractions and diffractions suffered on the way, each of the paths havinga different attenuation and distortion level. In the receiving antenna these signals sum up.Depending on the result, multipath can be constructive or destructive. In the first case, the signalsreceived from other paths than the direct one were in phase with the latter, thus when the antenna

added those signals up, it resulted an increase in the signal strength. On the other hand, if the signalsreceived are out of phase, the result of the summing can be close to zero, or even zero, meaning nosignal at all.

Figure I-14 An example of multipath propagation and its result [25] Multipath delay can cause Inter-Symbol Interference (ISI), meaning that symbols used in

802.11 signals to represent data overlap, making the receiver unable to distinguish between thesymbols transmitted. This leads to bit errors in the packets sent. These errors are detected at thereceiver through the cyclic redundancy check. When an error is detected a negative confirmation orno confirmation are sent. After the transmitter receives the negative confirmation or noconfirmation arrives and the time for that packet expires, it will resend the packet, when it regainsaccess to the medium. Because of the retransmissions, lower data throughput is experienced byusers when multipath propagation is significant [25].

8.4 Attenuation mitigation

When it comes to reducing the attenuation from weather conditions, atmosphericabsorptions and free space loss it is nothing one can do, and in WLANs they are not a concern. Onthe other hand, multipath propagation is somehow significant and it can be reduced by some means.

A solution to multipath propagation is antenna diversity. This means that the receiver willhave two antennas separated by a distance of one or two wavelengths. This will increase the odds ofreceiving a better signal on either of them. The principle is simple: antenna diversity works like aswitch that selects one antenna at a time, namely the antenna with better received signal. Anecessary condition is that both antennas have the same radiation characteristics. The goal ofantenna diversity is to increase the throughput by reducing the number of packets that are missed orretried.


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Another way to reduce multipath distortion is using a directional antenna that will only pickup signals coming from the direction of the transmitter, ignoring the reflected signals that travel tothe receiver on other paths and arriving at its sides or back.

9. Wireless Mesh Networking

The idea of mesh networking appeared as a response to the following paradox: in wirelessLANs, Access Points are wired to an infrastructure. Therefore a new idea was released, namelymaking a wireless backhaul between APs (radio connections) and only a few of them, depending onthe size of the WLAN, should be wired to an infrastructure. This would permit, for example, areashard-to-wire to be connected to the Internet more rapidly and easily and with lower-cost backhaul.Other advantages would be the ability of self-healing of the network, resilience and extensibility. Arecent amendment to IEEE 802.11 standard was released concerning this matter: IEEE 802.11s.

Figure I-15 Comparison between a classic WLAN and a mesh WLAN [26]

10. Power over Ethernet (PoE)

Power over Ethernet is a very important concept in designing a WLAN, because, whendesigning the network it makes sense to put the access points in locations suitable for radio

propagation purposes, rather than electrical power purposes. This can be achieved by using, on thesame LAN wired interface, the so called Power over Ethernet technology. The electrical powerneeded by a device, for example a wireless Access Point, could be transmitted through the sameEthernet cable used for transmitting data, utilizing the unused twisted pairs. PoE brings someadvantages, such as: simpler installation of the appliance, because only one set of wires is needed;in WLANs it simplifies the RF survey task, as an AP can be easily be moved and wired in [27].

11. Cisco Unified Wireless Network Solution (CUWN)

Unified Wireless Network is a solution provided by Cisco in order to deploy, control andmanage a secure, cost-effective and scalable Wireless LAN Network, in either an indoor or anoutdoor location. It is intended to be easy to setup and manage, and to deliver high performancewith a low total cost. In the following paragraphs I will focus only on a component of CUWN,namely Cisco Wireless Mesh Networking Solution, because the design and a possible futureimplementation of the WLAN rely on this solution.

Cisco Wireless Mesh Solution introduces a new network component, a Wireless LANController, which is intended to assume a central role in the network, thus making configuration andmanagement of the WLAN easier. Cisco Wireless Mesh is based on three key components:

Cisco Aironet 1500 Series Outdoor Mesh Access Point; Cisco Wireless LAN Controller;

Cisco Wireless Control System.


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Cisc o Aironet 1500 is a series of outdoor mesh access points designed for Ciscos WirelessMesh Solution. A 1500 Access Point is equipped for operating in two RF bands: 2.4 GHz for IEEE802.11 b/g/n client access and 5 GHz for communicating with other APs and data backhaul.Depending on their operating mode, two types of access points exist in this mesh architecture: RootAccess Points (RAPs) or Mesh Access Points (MAPs). The difference between Root APs(sometimes called Roof APs) and MAPs are that RAPs are c onnected to a WLC through a wired

connection, while a MAP must have a wireless connection to a RAP in his network, thus a wirelessconnection to a WLC.

Functions like authentication or association that were, in a classical WLAN, assumed byAPs, are now transferred to the Wireless LAN Controller. As a general rule, timing-dependentoperations, such as encryption or decryption of 802.11 frames or provisioning of real-time signalquality information with every received frame are handled by the AP. Less timing-dependentoperations are transferred to the WLC, in order to achieve simpler deploying and management ofthe WLAN. Two types of implementations are possible regarding WLCs: distributed or centralized.In a distributed deployment, WLCs are located in given areas of the location and manage the APs inthat area. On the other hand, in a centralized deployment, the WLCs are situated somewhere at acentralized location in the network. In Cisco 4400 Wireless LAN Controller Series, a WLC canmanage up to 100 APs.

The Wireless Control System (WCS) is an optional part of the mesh solution, but it isimportant because it provides a graphical platform for planning, configuration and management ofthe WLAN.

Several outdoor mesh APs can be deployed in a WLAN, forming parent, child and neighborrelationships. At least one of the APs must be connected through a wired connection to a WLC,assuming the role of a RAP. All other MAPs connect through a wireless link to a RAP in order toestablish a connection with the WLC. A parent represents the next hop from the MAP forcommunicating with the RAP. RAPs can only have parent roles in the wireless network. On theother hand, a child note represents the opposite of a parent node. A neighbor MAP is a node locatedat RF distance from the current MAP, but it is not a parent or a child. These node types are selecteddynamically, based on some values named ease values, through a Cisco proprietary routingalgorithm, AWPP (Adaptive Wireless Path Protocol).

AWPP is designed for providing some key characteristics needed in wireless routing:minimum resource consumption, fast convergence and ease of deployment [30]. Two types ofconstraints are considered in this algorithm: an additive constraint, number of hops, and a concaveconstraint, Signal to Noise Ratio (SNR) of the wireless link. These two constraints are used forcalculating a single value used as a metric, named ease value . A multiplier is also applied, based onsome SNR thresholds, in order to reflect the quality of the link. When selecting a parent, a MAPcalculates an adjusted eased value for making a decision. This is done with the following formula:adjusted ease = min (ease at each hop) / Hop count . A MAP chooses a neighbor with the highestadjusted ease value as a parent. Because the RF environment changes all the time, a problem canappear: the SNR of the links vary fast in time and thus a new parent should be selected all the time,making the mesh network not very stable. For solving this problem, an assumption is being made: a

parent offers a significantly better route to the RAP than any other potential parent. This isimplemented by giving the parent a 20% bonus ease on top of the parents calculated ease toreduce flapping between parents [30]. Loops might appear in the routes throughout the network.To ensure that this unwanted situation does not arise, MAPs discard the routes that contain theirown MAC address (routing information contains the MAC address of each hop to the RAP).

In order to maintain an accurate list of neighbors, a MAP continuously runs a backgroundscan of the available channels in 5 GHz. This is an advantage, because when a wireless link with a

parent drops, the MAP has already information regarding potential parents, making the switch to the

new parent faster.


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Figure I-16 Example of a Wireless Mesh Network [30]

A Wireless LAN Controller utilizes a protocol for managing APs in the network: CAPWAP(Control And Provisioning of Wireless Access Points). It is derived from a previous managing protocol, LWAPP (Lightweight Access Point Protocol). CAPWAP is used for creating wirelesslinks between a MAP and a WLC. These links are using a control channel and a data channel. Thecontrol channel is used for configuration, management and control of APs, and it is basically anencapsulated UDP packet, using port 5246. The data channel transports client traffic using UDP

port 5247. CAPWAP is also used for encapsulating AWPP information exchanged between meshAPs.

APs communicate with each other through 802.11 a/n links, creating a wireless backhaul.Data rates for these links are very important because they affect the overall data rate of the client.Optimum data rates are required for wireless backhaul. If the data rates are too low, the available

wireless channel bandwidth is not used. On the other hand, too high rates might lead to errors in packet transmission and communication failure. Wireless backhaul data rates also impact coverageand the number of APs needed in the wireless network, because a greater distance between MAPsmight be allowed by a lower bit rate. The following table shows the required SNR of backhaulwireless links for a number of desired data rates.

802.11a Data Rate(Mbps)

Minimum Required Link SNR(dB)

54 31

48 29

36 26

24 22


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Chapter II Introduction in designing and optimizing a wirelessnetwork

1. Performing a site survey

The first step in designing a wireless network, before conducting a site survey, is network planning. This is a very important phase in the design because its output information is used as aninput to site surveying.

Several data should be weighed when planning a network. Throughput considerations areneeded. The types of wireless devices to be chosen for implementation very much depend on thisrequirement. In most cases 802.11a or 802.11g devices will be chosen, due to their maximumtheoretical rate of 54 Mbps. Another important thing to have in mind is the desired coverage area.This leads to three types of implementation: coverage-based, capacity-based or a hybridimplementation. User density should be weighed as well, because people may gather more closelyin specific spots (e.g.: conference rooms, when talking about indoor locations). Mobility is also

important because recent applications require a continuous connectivity throughout the coveragearea. It is important to consider also the predicted number of users that will use the network andsome physical restrictions, such as the location of the wired backbone infrastructure or the

positioning of power outlets. In some cases aesthetic considerations may be required. Finally, siteenvironmental considerations and security requirements are needed [9]. Analyzing this informationwill provide input data for the next step, site surveying.

Radio frequency site surveying is the most important part in designing a wireless network. Ithelps the engineer that designs the wireless network to discover and understand the RF propagation,interference, RF coverage areas and it determines the appropriate placement of wireless devices, aswell as possible logistics problems.

When performing a site survey it is important to consider the goal of the network design:

capacity, coverage or a hybrid implementation that combines both aspects. On the other hand, inoutdoor deployments, other factors must be taken into consideration, such as: weather extremes,lightning, physical security and local regulations [33]. Site surveys offer some valuable informationin the design: the coverage an AP can provide, the number of AP needed for covering a given area,details about channels and power level settings required and types or gain of antennas to be used.Aided by this information, a user-to-AP ratio and then a throughput can be estimated. In contrast, ifa capacity-oriented deployment is considered, by estimating a desired throughput, the user densityand then user-to-AP ratio can be determined. In addition, the limitations of the backhaul wiredinfrastructure must be weighed or, in case of a wireless mesh architecture, the wireless backhaul, inorder to have a realistic design.

Before conducting a physical site survey some prerequisites are needed. First of all, theactual location of the survey must be visited and inspected, to gather basic information about the

possible extreme weather conditions, about how the wireless propagation environment mightchange over time, about where the wired backhaul is situated, power outlets etc. After that, userareas need to be identified in the location, as well as possible problem areas. Next, AP locations andantenna types have to be determined. The survey ends with proper documenting all the findings.

Two types of surveys exist: passive and active. A passive survey is done only to learn aboutthe traffic in 2.4 GHz or 5 GHz bands, by listening to the traffic the adapter hears, thus gatheringinformation about the 802.11 traffic that might interfere with the future network. Active surveyingmight be interpreted in two ways. On one hand it can represent a survey where the adaptor sendsand receives traffic, in order to identify, besides the existing Wi-Fi networks, the throughput inthose networks, or the success rate of the communication. On the other hand, active surveying mayrefer to the following situation: the operator goes to the location of the survey with one of the APshe has for the implementation and temporary installs it in different locations in order to measure its


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coverage in those areas, for obtaining valuable information about RF propagation. It is notrecommended to perform an active survey with other type of AP besides the one intended toimplement, because the results will not be accurate, and important inconsistencies might appear.

Documenting the site survey, step by step, is very important. The results of the physical sitesurvey establish an input to the next phase in network design, the actual design, by determining the

best configuration or several ones with their advantages and disadvantages, depending on the

criteria considered in the design.

2. Designing

In this phase of the project, the engineer actually designs the network, usually with the helpof a specialized tool. Having the input information from the previous phase, site surveying, namelythe number and types of AP to be used, the plan of the location where the network needs to beimplemented, the desired coverage for that location, the desired user throughput and possiblemobility requirements etc, the engineer must plan the network.

Designing consists in placing the APs on the plan of the location, guided by the informationreceived in the previous phases of the project. Depending on the software used for this job, the

planning can be done in two ways: manual or automatic. The latter is done by the tool after theinput data is introduced: the plan of the location with all the obstacles and their attenuationintroduced, the types of APs to be used for implementation, together with their antenna types andgain, their operating power and frequencies and most important the desired coverage area in thelocation. Having this information, the software then calculates the best model of deploying the APsand it simulates their coverage in the entire area. Using the manual method and having the sameinput information the engineer can place the APs in the location wherever he thinks it is best andthen the tool simulates the coverage area, as well as the channels recommended for the APs.

The output information in this phase of the project is the number and types of APs to beused, their position in the location of the future network and their basic configuration (e.g.: the

channels used for communication). This data serves as an input for the next phase of the process,the implementation of the network.

3. Optimization

After the implementation is complete, testing is done for evaluating the capabilities of thenetwork. If the results are not satisfactory, the next step in the process will be the optimization ofthe network. The results of the testing represent the input data for this next phase.

There are several problems that cause the theoretical and simulated network design to beinconsistent to the practical implementation. For example, considering an outdoor location, even if athorough site survey was done prior to implementation, RF propagation problems might appear due

to possible changes in the environment. This can lead to coverage problems or data rates problemsin some areas. If the network is designed to support an application such as VoIP, user mobilitymight not be guaranteed, its call lost, thus user experience could be disappointing. Also, if thenecessary data rate for a specific application is not ensured due to such problems, the same resultmight arise.

These problems that are identified after testing the newly implemented network can besolved before giving access to normal users. There are several methods to deal with the problems,which include: reconfiguring the APs to use only 20 MHz and non-overlapping channels, namely 1,6 and 11 (if we consider 2.4 GHz band); deploying additional APs, if possible, in the affected areas;implementing a solution for managing radio resources (e.g. Cisco Radio Resource Management);another possible solution is reducing the coverage areas for the APs, should co-channel interferenceemerge.

This is the final step in deploying a WLAN and it is important in order to receive a positivefeedback from its users.


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Chapter III Description of hardware solutions and software tools used

1. Hardware solution

The hardware solution used for implementing the wireless network is based on CiscoUnified Wireless Network Architecture. It consists of the following parts: Cisco Aironet 1552EOutdoor Access Points with Cisco Aironet Dual-Band Omnidirectional Antenna (AIR-ANT2547V-

N) and Cisco Wireless LAN Controller 4404.

1.1 Cisco Aironet 1552E Outdoor Access Point

Figure III-1 Cisco Aironet 1550 Series Outdoor AP [37]

1552E is an outdoor AP from Cisco Aironet 1550 Series [37], used for implementingoutdoor wireless mesh networks. It can work in both Wi-Fi frequency bands, 2.4 GHz and 5 GHz,supporting IEEE 802.11 b/g/n or 802.11 a/n technologies. 1552E has an improved 802.11n rangeand performance with 2x3 MIMO technology. It has a NEMA Type 4x certified enclosure, meaningthat it can be deployed in outdoor environments with no concerns about being damaged from theweather. The maximum data rate supported is 300 Mbps per radio. It integrates Cisco CleanAirtechnology, which consists in integrated spectrum intelligence for detecting, classifying andmitigating RF interference and it includes wireless, switching, routing and security capabilities. As

backhaul options it has one Gigabit Ethernet port, one Small Form-Factor Pluggable (SFP) Fiber port and, optionally, one DOCSIS 3.0 (8x4) Cable modem interface. Its operating temperature is between - 40 and 55C. As for the powering, 1552E has three options: 90 -480 VAC, 50-60 Hz, or 12VDC or with Power over Ethernet (PoE). It has three external antenna connections for three dual-

band antennas. The type of antenna used for implementation is described below.

1.2 Cisco AIR-ANT2547V-N Antenna

This is a dual-band (2.4 and 5 GHz), omnidirectional, stick antenna. It is designed foroperating in outdoor locations, together with Cisco Aironet Outdoor APs. Its operating frequencyvaries between 2400 and 2483 MHz, or 5150 and 5875 MHz, with a gain of 4 dBi or 7 dBiresp ectively. This type of antenna has a nominal input impedance of 50 and can operate attemperatures between - 40 and 85C. Figure III-2 shows its radiation patterns.


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Figure III-2 Cisco AIR-ANT2547V-N Antenna Radiation Patterns [38]

1.3 Cisco Wireless LAN Controller 4404

Cisco Wireless LAN Controller 4404 is part of Cisco 4400 Series Wireless LANControllers. A WLC is a device that coordinates the activity in a Cisco Unified Wireless Networkand helps with the automation of WLAN configuration and management. It assumes roles likeauthentication or association of wireless clients, which in a classical WLAN deployment were theroles of APs. In Ciscos solution, APs associate themselves with a WLC and they tun nel data andmanagement traffic to the WLC. Cisco WLC 4404 supports up to 100 APs and it has fourGigabit Ethernet ports. Real-time communication between APs and other WLCs are possible, withthe goal of having centralized security policies, QoS, RF management and mobility. As wirelesscapabilities it supports IEEE 802.11 a/b/g/n, while its wired capabilities are IEEE 802.3 10BASE -T, IEEE 802.3u 100BASE-TX specification, IEEE 802.1Q VLAN tagging and 802.1D SpanningTree Protocol [40]. Its operating temperature varies between 0 and 40C and its powering optionsare simple: 100 to 240 VAC, 50/60 Hz, 50 W.

Cisco WLCs support two types of transfer modes: Layer 2 tunneling, within Ethernetframes, and Layer 3 tunneling, within UDP packets. Cisco recommends the latter [30].

2. AirMagnet Survey software tool

AirMagnet Survey is a software tool from Fluke Networks used for conducting RF surveys

(collecting and analyzing radio frequency data). It is an easy-to-use tool that has an utmostimportance in designing a WLAN, because it helps an engineer understand the RF environment inwhich the WLAN will be deployed. AirMagnet Survey has a friendly and intuitive user interface. It


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can provide basic data that characterize the RF environment, such as signal strength, noise level orsignal-to-noise ratio or more complex data, such as data rates, channel allocation or user density.

For being able to gather this type of data, AirMagnet Survey utilizes a Wi-Fi adapter. Forexample, I used an USB Wireless adapter, namely Proxim ORiNOCO 8494 802.11a/b/g/n USBClient Adapter , for surveying the Old Center area in Bucharest.

Two types of projects can be created with AirMagnet Survey, meaning two types of surveys

can be conducted: indoor surveys or outdoor surveys. When creating a new project, only one type of project can be chosen. Since my objective was to design a WLAN for an outdoor location, namelythe Old Center area in Bucharest, I will further describe AirMagnet Survey from an outdoor survey

point of view.There are two ways of conducting an outdoor survey with AirMagnet: with or without the

aid of a GPS.

Figure III-3 AirMagnet Survey project type selection

In a non-GPS outdoor survey, the user must click on his location on the map for the tool torecord the data it measures at that time and associate it with that point. This can be done also in twoways: the first mode is named Sampling on click only, meaning that the tool saves the datameasured by the wireless adapter at that point exactly when the user clicks on his position on themap; the second way is named Auto -sample through path and it works in the following way:

between two clicks on the map, the tool automatically gathers and records data every one second

(this interval can be changed by the user), for a more accurate gathering of the RF data. On theother hand, if we choose to conduct a GPS aided survey, after importing the map of the locationtogether with its GPS coordinates, AirMagnet Survey records both the RF data and the GPScoordinates and integrates them automatically with the map, thus eliminating the need of clickingon the map and making surveying easier.

After defining the project type as a GPS Project, the next step would be importing the mapof the location to be surveyed. This can be done in multiple ways, as can be seen in fig. III-4. Twotypes of online maps can be loaded: MapPoint and Bing Maps . The advantage of this method isthat, together with the map, its GPS coordinates are included in the project. If the user wantsanother type of map, he can insert the desired map as a picture and then insert the coordinates fortwo diagonal corners of the map (the left top corner and the right bottom corner; the other points ofthe map will be interpolated by the tool). If the user does not know this data, he can leave thesefields blank and then recalibrate the map at the time of the survey, ideally by going to two diagonal

points of the map and recording the data that the GPS offers at those points.


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Figure III-4 AirMagnet Survey GPS map importing

After the map has been imported, some additional information must be introduced: SignalPropagation Assessment and AP Default Power. Signal Propagation Assessment (SPA) representsthe distance (in meters) used by the application to interpolate between captured data points. Foraccurate results, SPA should be set to a low value, e.g. one meter. This value leads to very accurateresults, but it is not practical, especially when performing an outdoor survey, because it would meanto cover by foot every single meter of the map. On the other hand, a too large value would lead tovery inaccurate results. The application has some default values for this parameter, depending on

the type of environment to be surveyed (the default distance of 45 meters was used for surveyingthe Old Center area, because it is a downtown location with severe RF obstruction). The second parameter, AP Default Power represents the default transmitting power in mWatts for the APs. Thisvalue is used when conducting data simulation. I used a value of 100 mW, this being the defaulttransmitting power for the APs considered for implementation.

Figure III-5 AirMagnet Survey Environment Information


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In order to conduct a GPS aided survey, an USB GPS is needed. The GPS must beconfigured to forward its data received from the satellites to a virtual COM port, and thenAirMagnet Survey needs to be configured to read the data from that port. This is done byconfiguring the GPS port from the tool with the same parameters the virtual COM port has. FigureIII-6 shows the configuration used in the Old Town Center area survey.

Figure III-6 AirMagnet Survey GPS Port configuration

After covering these steps, the application is ready for conducting a survey, but not beforeselecting the type of survey. Two types of surveys can be conducted: active survey or passive

survey. A passive survey is simple and it records the following parameters [35]: Wi-Fi channel the channel each AP is using as being captured by the wireless adapter; MAC Address the MAC address of each AP; Signal Strength the signal strength of all received packets from the AP; Noise Level the level of background energy in the 2.4 GHz or 5 GHz band; SNR (Signal/Noise Ratio) the signal strength compared to the noise level; SSID the name of the network to which an AP belongs; Current AP count the total number of APs detected.

The second type of survey is the active survey. It is more complex than a passive survey, because it gathers more information, such as data rates. In order to perform an active survey, theapplication must associate to a specific AP. It then transmits and receives data frames with a userdefined size with a delay between transmissions set by the user also, and it records the speed of thecommunication. Only one type of survey can be done at a time with a single wireless adapter.


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Figure III-7 AirMagnet Survey type of survey selecting

Selecting the type of survey is the last step in preparing for the survey. It is very importantfor the map to be calibrated; otherwise the results will be inaccurate and the foundation fordesigning the WLAN will be rickety, therefore the whole design will fail.

AirMagnet Survey calculates RF parameters in the following way [35]: Signal Strength is the power in dBm of the signal received by the antenna in the wireless

adapter. Its values vary between 0 dBm and -100 dBm, with -100 dBm and below this value being the weakest. A level of -76 dBm is recommended for data users and for voice users

signal strength should be around -67 dBm; Noise Level it represents the power level of any energy in the Wi-Fi spectrum that is notrecognized as 802.11 traffic. It is also measured in dBm and it varies between 0 and -100dBm. As a recommendation, if a network supports voice it should have a noise level below -90 dBm; otherwise it should have a noise level of -85 dBm;

Signal to Noise Ratio it is calculated as the difference between Signal Strength and NoiseLevel. It is measured in dB and it is recommended to have a value of around 40 dB;

Co-Channel Interference it appears between two or more APs that are operating on thesame channel. This formula is used for converting an APs signal into a quantifiable value(percentage): [(90 + S) * (100/60)], where S represents the APs signal strength. Theobt ained value represents the percentage of interference an AP introduce to other APsoperating on the same channel.

3. AirMagnet Planner software tool

AirMagnet Planner is an application used for wireless network planning. By considering a

Alex Stancu_WiFi Mesh Optimization for Bucharest City Center

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