EPL476 Mobile Networks

Wireless Technology Fundamentals

Instructor: Dr. Vasos Vassiliou

Slides adapted from Prof. Dr.-Ing. Jochen H. Schiller and W. Stallings

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)� Adhoc (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

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

K = i 2 + ij + j 2

K = 2 2 + 2*1 + 1 2

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

12

35

6

7

D

R

Cell Planning (2/3)

A3 A1A2

G3 G1G2 C3

C2

B3 B1B2

F3 F1F2

D3 D1D2

E3 E1E2

G3 G1G2

F3 F1F2

C3 C1C2

A3 A1A2

B3 B1B2

E3 E1E2

D3 D1D2

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 replace office cabling and, additionally, to office cabling and, additionally, to introduce a higher flexibility for ad hoc introduce a higher flexibility for ad hoc communication in, e.g., group meetingscommunication in, e.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 canᾼt 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 networks

infrastructure 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: StandardsWireles

sLAN

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 Aᾼs transmission� Collision

� Exposed terminal problem� B transmits to A� C wants to transmit to D� C hear Bᾼs 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

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.