Mini Project- Implementation & Evaluation of Wireless LANs

Mini Project –

Implementation and Evaluation of Wireless LANs

BSc (Hons) Computer & Network technologies

Author: University of HertfordshireDate created:Date revised: 2009

AbstractThe following resources come from the 2009/10 BSc in Computer and Network Technologies (course number 2ELE0072) from the University of Hertfordshire. All the mini projects are designed as level two modules of the undergraduate programmes. The objectives of this module are to Demonstrate within a private network environment:• The implementation of a wireless local are networks (WLANs) topology with diverse physical parameters• The real-time performance evaluation of the individual WLAN transmission characteristics in the presence of standard

transport protocols.

This mini-project involves the implementation of an “infrastructure” wireless network, the generation and transmission of packets and the measurement of network performance for TCP transport protocols by means of the “Wireshark” benchmarking tool. Parameters most likely to affect network performance such as the transmission medium’s signal-to-noise ratio, the propagating signal’s latency and jitter and the packet loss rate will be determined.

© University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License.

Contents MiniProject Objectives Impact of wireless environment on networks Wireless networks The Wireless Spectrum Physical Impairments: Noise Physical Impairments: Interference Physical impairments: Fading Contention for the Medium Security WLANs: IEEE 802.11 Family IEEE 802.11 Standard WLAN characteristics TCP Flow Control Flow Control and throughput TCP Congestion Avoidance Address Resolution between IP and Underlying Networks Address Resolution protocol (ARP) Routing to another LAN RARP, BOOTP, DHCP protocols Data Link and Physical layers Mac Layer Functions Credits

In addition to the resources found below there are supporting documents which should be used in combination with this resource. Please see:

Mini Projects - Introductory presentation. Mini Projects - E-Log.Mini Projects - Staff & Student Guide.Mini Projects - Standard Grading Criteria.Mini Projects - Reflection.

You will also need the ‘Mini Project Implementation and Evaluation of Wireless LANs’ text document.Wireless LANs MiniProject 2

Wireless LANs MiniProject 3

MiniProject Objectives

Impact of the wireless environment on networks

Overview of current mobile wireless technologies

Introduce the basic operation of IEEE 802.11

Impact of wireless environment on networks

The wireless spectrumPhysical impairmentsContention for the shared mediumSecurity


Wireless networks

IEEE 802.11CharacteristicsModes of operationAssociation, authentication and

privacy


The Wireless Spectrum

30 MHz 30 GHz3 GHz300 MHz

Broadcast TV• VHF: 54 to 88 MHz, 174 to 216 MHz• UHF: 470 to 806 MHz

FM Radio• 88 to 108 MHz

Digital TV• 54 to 88 MHz, 174 to 216 MHz, 470 to 806 MHz


The Wireless Spectrum (cont…)


3G Broadband Wireless• 746-794 MHz, 1.7-1.85 GHz, 2.5-2.7 GHz

Cellular Phone• 800-900 MHz

Personal Communication Service (PCS)• 1.85-1.99 GHz


The Wireless Spectrum (cont..)


Wireless LAN (IEEE 802.11b/g)• 2.4 GHz

Local Multipoint Distribution Services (LMDS) • 27.5-31.3 GHz

Bluetooth• 2.45 GHz

Wireless LAN (IEEE 802.11a)• 5 GHz


Physical Impairments: Noise

Unwanted signals added to the message signal May be due to signals generated by natural

phenomena such as lightning or man-made sources, including transmitting and receiving equipment as well as spark plugs in passing cars, wiring in thermostats, etc.

Sometimes modeled in the aggregate as a random signal in which power is distributed uniformly across all frequencies (white noise)

Signal-to-noise ratio (SNR) often used as a metric in the assessment of channel quality


Physical Impairments: Interference

Signals generated by communications devices operating at roughly the same frequencies may interfere with one another Example: IEEE 802.11b and Bluetooth devices,

microwave ovens, some cordless phones CDMA systems (many of today’s mobile wireless

systems) are typically interference-constrained Signal to interference and noise ratio (SINR) is

another metric used in assessment of channel quality


Physical impairments: Fading

Strength of the signal decreases with distance between transmitter and receiver: path loss

Slow fading (shadowing) is caused by large obstructions between transmitter and receiver

Fast fading is caused by scatterers in the vicinity of the transmitter


Contention for the Medium

If A and B simultaneously transmit to C over the same channel, C will not be able to correctly decode received information: a collision will occur

Need for medium access control mechanisms to establish what to do in this case (also, to maximize aggregate utilization of available capacity)

A

packets

B

C


Security

Safeguards for physical security must be even greater in wireless communications

Encryption: intercepted communications must not be easily interpreted

Authentication: is the node who it claims to be?


WLANs: IEEE 802.11 Family

802.11 working group Specify an open-air interface between a wireless client

and an access point or among wireless clients IEEE 802.11a

Up to 54 Mbps in the 5 GHz band Uses orthogonal frequency division multiplexing (OFDM)

IEEE 802.11b (Wi-Fi) 11 Mbps (with fallback to 5.5, 2 and 1 Mbps) in the 2.4

GHz band IEEE 802.11g

20+ Mbps in the 2.4 GHz band


IEEE 802.11 Standard

Final draft approved in 1997 Operates in the 2.4 GHz industrial, scientific and medical

(ISM) band Standard defines the physical (PHY) and medium access

control (MAC) layers Note that the 802.11 MAC layer also performs functions

that we usually associated with higher layers (e.g., fragmentation, error recovery, mobility management)

Initially defined for operation at 1 and 2 Mbps Extensions (IEEE 802.11b, IEEE 802.11a, etc.) allow

for operation at higher data rates and (in the case of 802.11a) different frequency bands


WLAN characteristics


Wireless PANs, LANs and WANs

WLAN Basic Infrastructure

3ELE0049 Optical Communication Systems17


IEEE 802.11 Based Architecture

TCP Flow Control

TCP inherently supports flow control to prevent buffer overflow at the receiver Useful for fast sender transmitting to slower

receiver Receiver advertises a window (wnd) in

acknowledgements returned to the sender Sender cannot send more than wnd

unacknowledged bytes to the receiver

Src Dest

Limits amount ofdata that destinationmust buffer

© Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen

TCP Flow Control Example

Sender Receiverwnd = 1200

500 bytes

500 bytes

wnd = 200

200 bytes

wnd = 500

500 bytes



Flow Control and throughput Let rtt be the round-trip time, i.e., the time from sending a segment until

an acknowledgement (ACK) is received Let t = wnd/b be the time to transmit a full “window” of data, where b is

link bandwidth For a link with a high delay-bandwidth product (rttb), the flow control

window can limit throughput for the connection In this case, t rtt Throughput is wnd/rtt

Sender Receiver

t

rttwndbytes



TCP Congestion Avoidance Congestion avoidance (control) was added to

TCP in an attempt to reduce congestion inside the network

A much harder problem … Requires the cooperation of multiple senders Must rely on indirect measures of congestion

Implemented at sender

Src Dest

Attempts to reducebuffer overflow insidethe network Wireless LANs MiniProject 22


Address Resolution between IP and Underlying Networks Most hosts attached to a LAN by an interface board that only

understands LAN addresses. E.g. every Ethernet board is equipped with a 48-bit Ethernet address.

The boards send and receive frames based on 48-bit Ethernet (MAC) addresses. They know nothing about the 32-bit IP addresses.

Address Resolution Protocol (ARP) maps the IP addresses onto data link layer addresses (e.g., MAC address being the hardware address that is sent back to the host.

Every hardware devices’ MAC address can be found on the network interface card (NIC), located inside the host.

The MAC address is hard coded, which means that it cannot (usually) be altered by software

University of Illinois, produced by M. T. Harandi, J. Hou, I. Gupta, N. Vaidya and K Nahrstedt


Address Resolution protocol (ARP) The ARP protocol operates between the network layer and the

data link layer in the Open System Interconnection (OSI) model.

The phrase “address resolution” refers to the process of finding a MAC address of a host (computer) on a network.

The address is resolved using a protocol in which a short frame (data link layer “packet”) is broadcast on the local network by the host attempting to transmit data (client).

The server on the receiving end processes the frame. The address resolution procedure is completed when the client receives from the server, a response containing the server’s address.

University of Illinois, produced by M. T. Harandi, J. Hou, I. Gupta, N. Vaidya and K Nahrstedt


RARP, BOOTP, DHCP protocols

ARP: Given an IP address, return a hardware addressRARP: Given a hardware address, give me the IP addressDHCP, BOOTP: Similar to RARPHosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol: dynamically

get address: “plug-and-play” host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg


Data Link and Physical layers

Medium Access Control (MAC) sublayer

Physical Layer convergence procedure

(PLCP) sublayer

Physical mediumDependent (PMD)

sublayer

MAC sublayermanagement

PHY sublayermanagement

stationmanagement

Data LinkLayer

PhysicalLayer


Mac Layer Functions 802.11 MAC Layer Functions The following summarizes primary 802.11 MAC functions,

especially as they relate to infrastructure wireless LANs: Scanning: The 802.11 standard defines passive and active

scanning methods by which station scans individual channels to find searches for access points.

In passive scanning the station scans individual channels to find the best access point signal.

The access points periodically broadcasts a beacon, and the station receives these beacons while scanning and takes note of the corresponding signal strengths.

The beacons contain information about the access point, including service set identifier (SSID), supported data rates, etc.

The station can use this information along with the signal strength to compare access points and decide upon which one to use. Passive scanning is mandatory.


Mac Layer Functions (cont..)

In active scanning the station initiates the process by broadcasting a probe frame and all access points within range respond with a probe response.

Active scanning enables a station to receive immediate response from access points, without waiting for a beacon transmission.

The issue, however, is that active scanning imposes additional overhead on the network because of the transmission of probe and corresponding response frames.


Mac Layer Functions (cont..)

Authentication: Authentication is the process of proving identity, and the 802.11 standard specifies two forms:

Open system authentication and shared key authentication. Open system authentication is mandatory, and it's a two step process.

A station first initiates the process by sending an authentication request frame to the access point.

The access point replies with an authentication response frame containing approval or disapproval of authentication indicated in the Status Code field in the frame body.


This resource was created by the University of Hertfordshire and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

Slides 19 to 22 are reproduced with permission.© Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen

Slides 23 & 24 are reproduced with permission. University of Illinois, produced by M. T. Harandi, J. Hou, I. Gupta, N. Vaidya and K Nahrstedt

The remaining material:

© University of Hertfordshire 2009

This work is licensed under a Creative Commons Attribution 2.0 License.

The name of the University of Hertfordshire, UH and the UH logo are the name and registered marks of the University of Hertfordshire. To the fullest extent permitted by law the University of Hertfordshire reserves all its rights in its name and marks which may not be used except with its written permission.

The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence.

The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher.


Mini Project- Implementation & Evaluation of Wireless LANs

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