Topic 3 - Fundamental Concepts in Wireless Networks

Sensor networks are another form of infrastructureless network, with many similarities to ad-hock

Fundamental concepts in wireless networks Sharing Resources

Cellular concepts (reuse resources) WLAN (shared space) Adhock (shared resources) Sensor (shared resources, large space)

What is a Cell?

Cell is the Basic Union in The System defined as the area where radio coverage is given by one

base station. A cell has one or several frequencies, depending on

traffic load. Fundamental idea: Frequencies are reused, but not in

neighboring cells due to interference.

Cell characteristics

Implements space division multiplex: base station covers a certain transmission area (cell)

Mobile stations communicate only via the base station Advantages of cell structures:

higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

Different Types of Cells

Capacity & Spectrum Utilization Network capacity at required QoS

with conventional frequency plan

Subscriber growth

Time

Out of Capacity!!!

The need: Optimum spectrum

usage More capacity High quality of

service Low cost

I wish I could increase capacitywithout adding NEW BTS!

What can I do?

Cell Planning (1/3) The K factor and Frequency Re-Use Distance

K = i2 + ij + j2

K = 22 + 2*1 + 12

K = 4 + 2 + 1

K = 7i

j

1

2

34

5

6

7

Frequency re-use distance is based on the cluster size K

The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster

D = 3K * R

D = 4.58R

1

2

35

6

7

D

R

Cell Planning (2/3)

A3A1

A2G3

G1

G2C3

C2

B3B1

B2

F3F1

F2

D3D1

D2E3

E1

E2

G3G1

G2

F3F1

F2

C3C1

C2

A3A1

A2B3

B1

B2

E3E1

E2

D3D1

D2

7-cell reusepattern

Frequencyreuse

C1

Cell Planning (3/3) Cell sectoring

Directional antennas subdivide cell into 3 or 6 sectors

Might also increase cell capacity by factor of 3 or 6

Cell splitting Decrease transmission

power in base and mobile

Results in more and smaller cells

Reuse frequencies in non-contiguous cell groups

Example: ½ cell radius leads 4 fold capacity increase

Hierarchical Cell Structures (HCS) (1/2) HCS allows traffic to be directed to a preferred cell Each cell is defined in a particular layer The lower the layer, the higher the priority

Mobiles will select a cell on the lowest layer as long as it has “sufficient” signal strength, even if higher layer cell are stronger

WLAN: Definition

A fast-growing market introducing the flexibility of wireless access into office, home, or production environments.

Typically restricted in their diameter to buildings, a campus, single rooms etc.

The global goal of WLANs is to replace office replace office cabling and, additionally, to introduce a cabling and, additionally, to introduce a higher flexibility for ad hoc communication in, higher flexibility for ad hoc communication in, e.g., group meetingse.g., group meetings..

WLAN: Characteristics

Advantages: very flexible within radio coverage ad-hoc networks without previous planning possible wireless networks allow for the design of small, independent devices more robust against disasters (e.g., earthquakes, fire)

Disadvantages: typically very low bandwidth compared to wired networks (~11 – 54 Mbit/s)

due to limitations in radio transmission, higher error rates due to interference, and higher delay/delay variation due to extensive error correction and error detection mechanisms offer lower QoS

many proprietary solutions offered by companies, especially for higher bit-rates, standards take their time (e.g., IEEE 802.11) – slow standardization procedures standardized functionality plus many enhanced features these additional features only work in a homogeneous environment (i.e., when

adapters from the same vendors are used for all wireless nodes) products have to follow many national restrictions if working wireless, it

takes a very long time to establish global solutions

WLAN: Design goals

global, seamless operation of WLAN products low power for battery use (special power saving modes and

power management functions) no special permissions or licenses needed (license-free band) robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management protection of investment in wired networks (support the same

data types and services) security – no one should be able to read other’s data, privacy –

no one should be able to collect user profiles, safety – low radiation

transparency concerning applications and higher layer protocols, but also location awareness if necessary

WLAN: Technology Overview

Core technologies (IEEE 802.1x family) IEEE 802.11 (Wireless LAN)

IEEE 802.15 (Wireless PAN – Bluetooth)

IEEE 802.16 (Wireless M(etropolitan) AN) – Under development

Facilitating technologies RF-Id

IrDA

Home-RF PAN

LAN

MAN

WLAN: Technology

Can be categorized according to the transmission

technique being used Infrared (IR) LANs: Very limited coverage area (IR can’t

penetrate walls!)

Spread Spectrum LANs: Operate in Industrial, Scientific,

and Medical (ISM) bands

Narrowband Microwave LANS: Operate at microwave

frequencies but not using spread spectrum (in licensing or

ISM bands)

WLAN: infrared vs. radio transmission Infrared

uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.)

Advantages simple, cheap, available in many

mobile devices no licenses needed simple shielding possible

Disadvantages interference by sunlight, heat

sources etc. many things shield or absorb IR

light low bandwidth

Example IrDA (Infrared Data Association)

interface available everywhere

Radio typically using the license free

ISM band at 2.4 GHz Advantages

experience from wireless WAN and mobile phones can be used

coverage of larger areas possible (radio can penetrate walls, furniture etc.)

Disadvantages very limited license free frequency

bands shielding more difficult,

interference with other electrical devices

Example: WaveLAN, HIPERLAN, Bluetooth

WLAN: Spread Spectrum

Most popular category!

Spread Spectrum Communications Developed initially for military and intelligence

requirements

Essential idea: Spread the information signal over a

wider bandwidth to make jamming and interception

more difficult Frequency hopping

Direct sequence spread spectrum

WLAN: infrastructure vs. ad-hoc networksinfrastructure

network

ad-hoc network

APAP

AP

wired network

AP: Access Point

WLAN: Infrastructure-based networks Infrastructure networks provide access to other networks. Communication typically takes place only between the wireless nodes and the

access point, but not directly between the wireless nodes. The access point does not just control medium access, but also acts as a

bridge to other wireless or wired networks. Several wireless networks may form one logical wireless network:

The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio coverage.

Network functionality lies within the access point (controls network flow), whereas the wireless clients can remain quite simple.

Use different access schemes with or without collision. Collisions may occur if medium access of the wireless nodes and the access point

is not coordinated. If only the access point controls medium access, no collisions are possible.

Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes) The access point may poll the single wireless nodes to ensure the data rate.

Infrastructure-based wireless networks lose some of the flexibility wireless networks can offer in general: They cannot be used for disaster relief in cases where no infrastructure is left.

WLAN: ad-hoc networks No need of any infrastructure to work

greatest possible flexibility Each node communicate with other nodes, so no access point

controlling medium access is necessary. The complexity of each node is higher

implement medium access mechanisms, forwarding data Nodes within an ad-hoc network can only communicate if they can

reach each other physically if they are within each other’s radio range if other nodes can forward the message

WLAN: StandardsWireless

LAN

2.4 GHz 5 GHz

802.11(2 Mbps)

802.11b(11 Mbps)

802.11g(22-54 Mbps)

HiSWANa(54 Mbps)

802.11a(54 Mbps)

HiperLAN2(54 Mbps)

HomeRF 2.0(10 Mbps)

Bluetooth(1 Mbps)

HomeRF 1.0(2 Mbps)

802.11e(QoS)

802.11i(Security)

802.11f(IAPP)

802.11h(TPC-DFS)

WLAN: Standards (ii) IEEE 802.11 and HiperLAN2 are typically infrastructure-based networks,

which additionally support ad-hoc networking Bluetooth is a typical wireless ad-hoc network

IEEE 802.11b offering 11 Mbit/s at 2.4 GHz The same radio spectrum is used by Bluetooth

A short-range technology to set-up wireless personal area networks with gross data rates less than 1 Mbit/s

IEEE released a new WLAN standard, 802.11a, operating at 5 GHz and offering gross data rates of 54 Mbit/s Shading is much more severe compared to 2.4 GHz Depending on the SNR, propagation conditions and the distance between sender and

receiver, data rates may drop fast uses the same physical layer as HiperLAN2 does

HiperLAN2 tries to give QoS guarantees IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz.

Benefits from the better propagation characteristics at 2.4 GHz compared to 5 GHz Backward compatible to 802.11b

IEEE 802.11e: MAC enhancements for providing some QoS

Ad Hoc Networks: Definition

A network made up exclusively of wireless nodes without any access points operating in peer-to-peer configuration, grouped together in a temporary manner.

Ad Hoc Networks: Some Features Lack of a centralized entity All the communication is carried over the

wireless medium Rapid mobile host movements Limited wireless bandwidth Limited battery power Multi-hop routing

Ad Hoc Networks: Operation

Assumption Unidirectional link Adjustable power level Directional antenna GPS

Operation Broadcasting Routing Multicasting

Ad Hoc Networks: Challenges (i) Hidden terminal problem

A transmits to B C wants transmits to B C does not hear A’s transmission Collision

Exposed terminal problem B transmits to A C wants to transmit to D C hear B’s transmission Unnecessarily deferred

A B C

A B C D

Ad Hoc Networks: Challenges (ii) Challenges

Mobility Scalability Power

Minimizing power consumption during the idle time Minimizing power consumption during communication

QoS End to End delay Bandwidth management Probability of packet loss

Ad Hoc Networks: Broadcast (i) Objective:

paging a particular host sending an alarm signal finding a route to a particular host

Two types: Be notified -> topology change Be shortest -> finding route

A simple mechanism: Flooding Suffer from broadcast storm

Ad Hoc Networks: Broadcast (ii)

source

Be notified Be shortest

5 forwarding nodes4 hop time

source

6 forwarding nodes3 hop time

Ad Hoc Networks: Routing

Table Driven vs. On Demand DSDV, TORA, DSR, AODV

Hierarchical and Hybrid ZONE

Specific assumption Unidirectional link, Directional antenna, GPS

QoS-aware Power, Delay, Bandwidth

Ad Hoc Networks: Multicast

Parameter: The delay to send a packet to each destination The number of nodes that is concerned in

multicast The number of forwarding nodes

s

D

D

D

s

D

D

D

s

D

D

D

Ad Hoc Networks: Recommended Introductory Reading M. Frodigh, et al, "Wireless Ad Hoc

Networking: The Art of Networking without a Network," Ericsson Review, No. 4, 2000.

F. Baker, "An outsider's view of MANET," Internet Engineering Task Force document, 17 March 2002.

IEEE tutorial

Sensor Networks: Definition

A sensor network is a collection of collaborating sensor nodes (ad hoc tiny nodes with sensor capabilities) forming a temporary network without the aid of any central administration or support services. Sensor nodes can collect, process, analyze and

disseminate data in order to provide access to information anytime and anywhere.

Sensor Networks: Some Features Large number of sensors Low energy use Efficient use of the small memory Data aggregation Network self-organization Collaborative signal processing Querying ability

Sensor Networks: Operation

Sensors work in clusters Each cluster assigns a cluster head to manage its

sensors Three layers

Services layer Data layer Physical Layer

To compensate for hardware limitations (e.g. memory, battery, computational power): Applications deploy a large number of sensor nodes in the

targeted region.

Sensor Networks: Challenges (i) Hardware design Communication protocols Applications design Extending the lifetime of a sensor network Building an intelligent data collecting system Topology changes very frequently Sensors are very limited in power Sensors are very prone to failures

Sensor Networks: Challenges (ii) Sensors use a broadcast paradigm

Most networks are based on point to point communication

Sensors may not have a global identification (ID) Very large overhead

Dynamic environmental conditions require the system to adapt over time to changing connectivity and system stimuli

Sensor Networks: Aggregation Some sensor nodes are designed to

aggregate data received from their neighbors. Aggregator nodes cache, process and filter

data to more meaningful information. Aggregation is useful because:

Increased circle of knowledge Increased accuracy level Data redundancy

To compensate for sensor nodes’ failing

Sensor Networks: Dissemination Two ways for data dissemination:

Query driven: sink broadcasts one query and sensor nodes send back a report in response

Continuous update: sink node broadcasts one query and receives continuous updates in response (more energy consuming but more accurate)

Problems: Intermediate nodes failing to forward a message Finding the shortest path (a routing protocol) Redundancy: a sensor may receive the same data packet

more than once.

Sensor Networks: Advantages Coverage of a very large area through the scattering

of thousands of sensors. Failure of individual sensors has no major impact on

the overall network. Minimize human intervention and management. Work in hostile and unattended environments. Dynamically react to changing network conditions.

E.g. Maintain connectivity in case of unexpected movement of the sensor nodes.

Sensor Networks: Recommended Introductory Reading I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, E.

Cyirci, “ A survey on Sensor Networks”, Computer Networks, 38(4):393-422, March 2002.

Chee-Yong Chong, S. P. Kumar, “Sensor networks: evolution, opportunities, and challenges”, Proceedings of IEEE, pp 1247-1256, August 2003.

IEEE tutorial