Wireless Communication Protocols and Technologies

Page 1Tatiana K. Madsen

Hans Peter SchwefelWCPT Spring 2004

Wireless Communication Protocols and Technologies

by Tatiana Madsen & Hans Peter Schwefel

• Mm1 Introduction. Wireless LANs (TKM)

• Mm2 Wireless Personal Area Networks and Bluetooth (TKM)

• Mm3 IP Mobility Support (HPS)

• Mm4 Ad hoc Networks (TKM)

• Mm5 Overview of GSM, GPRS, UMTS (HPS)



Course material

• Download slides www.kom.auc.dk/~tatiana• Books

– Jochen Schiller, ”Mobile Communications”, Addison-Wesley, 1st edition 2000, 2nd edition



Mobile vs Wireless

• Mobile vs Stationary• Wireless vs Wired

MobileWireless

user mobility: users communicate (wireless) “anytime, anywhere, with anyone”

device portability: devices can be connected anytime, anywhere to the network



Mobile vs Wireless

Wireless vs. mobile Examples stationary computer notebook in a hotel wireless LANs in historic buildings Personal Digital Assistant (PDA)



Outline

• Introduction - Historical perspectives• Properties of wireless medium• Basic MAC protocols for wireless communication• WLAN

– IEEE 802.11 standard

• Additional information• http://grouper.ieee.org/groups/802/11/• B. Crow et al, “IEEE 802.11 Wireless Local Area Networks”, IEEE

Comm. Magazine, September 1997



Chronological list of events of wireless systemsChronological list of events of wireless systems

Source: B. Furht, Handbook of Internet and Multimedia Systems and Applications. IEEE Press, 1999

1860s J.C. Maxwell postulates electromagnetic waves

1880s H.R. Hertz provides proof of electromagnetic waves

1895 G. Marconi demonstrates wireless communication and applies for patent

1913 Establishment of marine radio telegraphy

1921 Detroit police conducts field trials with mobile radio

1946 Bell Lab. deploys first commercial mobile radio telephone system

1950 Microwave links are developed

1980s Wide deployment of analog cellular systems

1992 Introduction of 2nd generation digital cellular systems

1993 Introduction of multiservices capabilities in the 2nd generation systems

2000 Third generation cellular systems with multimedia capabilities are introduced

2003 Start of commercial deployment of 3rd generation systems



Key problems

• Key problem - Path loss– Advent of the electron tube amplifier (de Forest, 1915)

• Key problem – Thermal noise– Claude Shannon, ”A mathematical theory of communication”, 1949– Advent of the Large Scale Integrated (LSI) circuits and Digital Signal

Processing (DSP)• Key problem – The limited spectrum

– Only one ”ether, unwanted interference between different users”– International Telecommunication Union (ITU) deals with these

problems



Source: “Mobile data feels pressure from the need for speed”, Network News, 2 June 1998 (CAP Gemini, September 1999)

Mobile Communications Networks

First Generation

• Analogue• Basic voice telephony• Low capacity• Limited local and

regional coverage• E.g. NMT, AMPS,

TACS, C-net

• Digital:– Circuit switched

• Voice plus basic data applications:

– Fax– SMS (small message

services)– Circuit-switched data

• Low data speed• Regional coverage,

with trans-national roaming

• E.g. GSM, D-AMPS, PDC, IS 95 CDMA

• Digital:– Packet and circuit

switched

• Advanced data — i.e. multimedia applications

• Fast data access• Global coverage• E.g. UMTS (WCDMA,

TD/CDMA), IMT-2000

Second Generation Third Generation

Wireless data already be introduced in second generation mobile

Development of mobile communications



20 155

Indoor

Pedestrian

High SpeedVehicular

Rural

Mobility & Range

Personal Area

VehicularUrban

IEEE 802.11a/b(WLAN),

Hiperlan2,MMAC

0.5 2

UMTS

GS

M

DECT

Possible UMTS extension for high speed data access

with roaming capability

Fixed urban

Total data rate per cell

10

BRAN

B-PAN PANBluetooth

1000 Mb/s



Mobile phones per 100 people 1999

0 10 20 30 40 50 60

Finland

Sweden

Norway

Denmark

Italy

Luxemburg

Portugal

Austria

Ireland

Switzerland

Great Britain

Netherlands

France

Belgium

Spain

Greece

Germany

2002

: 50

-70%

pen

etra

tio

n in

Wes

tern

Eu

rop

e



Worldwide wireless subscribers

0

100

200

300

400

500

600

700

1996 1997 1998 1999 2000 2001

Americas

Europe

Japan

others

total

http://www.3g.co.uk



Simple reference model

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio



Signal propagation ranges

distance

sender

transmission

detection

interference

• Transmission range

– communication possible

– low error rate

• Detection range

– detection of the signal possible

– no communication possible

• Interference range

– signal may not be detected

– signal adds to the background noise

Broadcast nature of channel



Signal propagation• Propagation in free space always like light (straight line)

• Receiving power proportional to 1/d² (d = distance between sender and receiver)

• Receiving power additionally influenced by

• fading (frequency dependent)

• shadowing

• reflection at large obstacles

• refraction depending on the density of a medium

• scattering at small obstacles

• diffraction at edges

reflection scattering diffractionshadowing refraction



• Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

• Time dispersion: signal is dispersed over time

interference with “neighbor” symbols, Inter Symbol Interference (ISI)• The signal reaches a receiver directly and phase shifted

distorted signal depending on the phases of the different parts

Multipath propagation

signal at sendersignal at receiver

LOS pulsesmultipathpulses



Wireless medium

• Time varying channel– Radio signals propagate according to reflection, diffraction and scattering

– The received signal power attenuates as for free space– Multipath propagation– Fading

• Burst channel errors• Broadcast nature of channel

• Half-duplex operation

1, , 2const

r



Wireless networks in comparison to fixed networks• Higher loss-rates due to interference

– emissions of, e.g., engines, lightning• Restrictive regulations of frequencies

– frequencies have to be coordinated, useful frequencies are almost all occupied

• Low transmission rates• Higher delays, higher jitter• Lower security, simpler active attacking

– radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones

• Always shared medium– secure access mechanisms important



• Hidden terminals– A sends to B, C cannot receive A – C wants to send to B, C senses a “free” medium – collision at B, A cannot receive the collision – A is “hidden” for C

• Exposed terminals– B sends to A, C wants to send to another terminal (not A or B)– C has to wait, CS signals a medium in use– but A is outside the radio range of C, therefore waiting is not

necessary– C is “exposed” to B

Hidden and exposed terminals

BA C



• Terminals A and B send, C receives– signal strength decreases proportional to the square of the distance– the signal of terminal B therefore drowns out A’s signal– C cannot receive A

• If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer

• Also severe problem for CDMA-networks - precise power control needed!

Near and far terminals

A B C



Classification of Wireless MAC protocols

Fixed assignment

Random assignment

Demandassignment

TDMA

FDMA

CDMA

MAC Protocols

ALOHA

CSMA

Token

Polling

GAMAs-ALOHA

FAMA



Access methods SDMA/FDMA/TDMA

• SDMA (Space Division Multiple Access)– segment space into sectors, use directed antennas – cell structure

• FDMA (Frequency Division Multiple Access)– assign a certain frequency to a transmission channel between a

sender and a receiver– permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast

hopping (FHSS, Frequency Hopping Spread Spectrum)• TDMA (Time Division Multiple Access)

– assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time



Random Access - Aloha

• Unslotted Aloha

• Slotted Aloha

packet 1 packet 2packet 2

rescheduled

packet 1packet 1

rescheduledUser 1

User 2

Basestation

successfultransmission

collisionsuccessful

transmissionsuccessful

transmission

timet0 t0+tp1 t1 t1+tp2

exp( 2 )S G G

exp( )S G G



Throughput curves of Aloha



Carrier Sense Multiple Access (CSMA)

• Aloha schemes “impolite” behavior• CSMA “listen before talk”

– Process of listening to the channel is not demanding– Carrier sensing does not relieve us from collisions– Variations of CSMA are due to behavior of users when the channel is

busy

• Non-persistent• 1-persistent• p-persistent



Non-persistent CSMA

• If the channel is busy, a terminal refrains from transmitting a packet and behaves exactly as if the packet collided.

• a - vulnerable period

a a

T

a



1-persistent CSMA

• Non-persistent CSMA: there are situations when the channel is idle, although one or more users have packets to transmit.

• 1-persistent: if the channel is idle, the user waits and transmits as soon as the channel becomes idle.



Slotted systems

• The wireless channel is said to be slotted if transmission attempts can take place at discrete instance in time.

• A slotted system requires network-wide time synchronization– in centralized network BS is used as a reference– in distributed networks it is more difficult

• slotted non-persistent and 1-persistent CSMA



Throughput curves



CSMA with Collision Detection

• Whenever the transmission of two or more packets overlap in time, all packets are lost and must be retransmitted

• In some local area networks (such as Ethernet) users can detect interference among several transmission (including their own) while transmission is in progress

• If a collision is detected during transmission, the transmission is aborted.

• Consensus reenforcement procedure

• Transmission period in the case of collision: 2 cd cr



Comparison of Throughput-Load curves



Collision detection in radio systems

• Wire: transmitted and received signals are of the same order of magnitude

• Wireless: the received signal is considerably weak compared with the transmitted

in radio systems CD is usually not implemented

ACK is required



Characteristics of wireless LANs

• Advantages

– very flexible within the reception area

– Ad-hoc networks without previous planning possible

– (almost) no wiring difficulties (e.g. historic buildings, firewalls)

– more robust against disasters like, e.g., earthquakes, fire

• Disadvantages - users expect the same services and capabilities

– typically very low bandwidth compared to wired networks (1-10 Mbit/s)

– many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11)

– products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like



Design goals for wireless LANs

– global, seamless operation– low power for battery use – no special permissions or licenses needed to use the LAN – robust transmission technology– simplified spontaneous cooperation at meetings – easy to use for everyone, simple management – protection of investment in wired networks – security (no one should be able to read my data), privacy (no one

should be able to collect user profiles), safety (low radiation)– existing applications should work



IEEE 802.11 standard• 802.3 Ethernet

• 802.5 Token ring

• 802.11 WLAN

• 802.15 WPAN

• Standards specify PHY and MAC, but offers the same interface to higher layers to maintain interoperability

access point

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLCLLC LLC

IEEE=Institute of Electrical and Electronics Engineers



802.11 - Layers and functions• PLCP Physical Layer Convergence Protocol

–clear channel assessment signal (carrier sense)

• PMD Physical Medium Dependent sublayer

–modulation, coding of signals• PHY Management

–channel selection• Station Management

–coordination of all management functions

PMD

PLCP

MAC

LLC

MAC Management

PHY Management

• MAC–access mechanisms, fragmentation, encryption

• MAC Management–association, re-association of a station to an AP, roaming–authentication, encryption–synchronization, power management

PH

YD

LC

Sta

tion

Man

agem

ent



802.11 - Architecture of an infrastructure network

•Station (STA)– terminal with access mechanisms

to the wireless medium and radio contact to the access point

•Basic Service Set (BSS)– group of stations using the same

radio frequency•Access Point

– station integrated into the wireless LAN and the distribution system

•Portal– bridge to other (wired) networks

•Distribution System– interconnection network to form one

logical network (EES: Extended Service Set) based on several BSS

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS

System architecture



802.11 - Architecture of an ad-hoc network

• Direct communication within a limited range

–Station (STA):terminal with access mechanisms to the wireless medium

–Independent Basic Service Set (IBSS):group of stations using the same radio frequency

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3



802.11 - Physical layer• 3 versions: 2 radio (typ. 2.4 GHz), 1 IR

– data rates 1 or 2 Mbit/s• FHSS (Frequency Hopping Spread Spectrum)

– separate different networks by using different hopping sequences– 79 hopping channels; 3 different sets with 26 hopping sequences per set

• DSSS (Direct Sequence Spread Spectrum)– method using separation by code– preamble and header of a frame is always transmitted with 1 Mbit/s, rest of

transmission 1 or 2 Mbit/s– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)– max. radiated power 1 W (USA), 100 mW (EU), min. 1mW

• Infrared– 850-950 nm, diffuse light, typ. 10 m range, indoor– carrier detection, synchronization



IEEE 802.11 MAC

• 802.11 supports 2 different fundamental MAC schemes:• The Distributed Coordination Function (DCF): all users have to contend

for accessing the channel. This is an implementation of ad hoc networks.• The Point Coordination Function (PCF): is based on polling and is

performed by an AP inside the BSS. In the IEEE 802.11 implementation of PCF is optionally.

• The PCF is required to coexist with the DCF: when the PCF is available in a network, there still is a portion of the time allocated to the DCF.



Access methods

– DFWMAC-DCF CSMA/CA (mandatory) – basic access method• collision avoidance via randomized „back-off“ mechanism• minimum distance between consecutive packets• ACK packet for acknowledgements (not for broadcasts)

– DFWMAC-DCF w/ RTS/CTS (optional) – handshaking access method

• avoids hidden terminal problem– DFWMAC- PCF (optional)

• access point polls terminals according to a list



Traffic services

– Asynchronous Data Service (mandatory)• exchange of data packets based on “best-effort”• support of broadcast and multicast

– Time-Bounded Service (optional)• implemented using PCF (Point Coordination Function)



Carrier Sensing

• Carrier sensing is performed at both the air interface, reffered to as physical carrier sensing, and at the MAC sublayer, reffered to as virtual carrier sensing.

• Physical c.s. detects activity in the channel via relative signal strength from other sources

• Virtual c.s. - from header information of frames. The duration field indicates the amount of time (in microseconds) after the end of the present frame the channel will be utilized. This time is used to adjust network allocation vector (NAV).

• The channel is marked busy if one of the c.s. indicate the channel is busy.



Priorities• Priorities

– priority access to the channel is controlled through the use of interframe space - mandatory periods of idle time.

– SIFS (Short Inter Frame Spacing)

• highest priority, for ACK, CTS, polling response– PIFS (PCF IFS)

• medium priority, for time-bounded service using PCF– DIFS (DCF, Distributed Coordination Function IFS)

• lowest priority, for asynchronous data service

t

medium busySIFS

PIFS

DIFSDIFS

next framecontention

direct access if medium is free DIFS



Random backoff time mechanism

• After DIFS period, a station computes a random backoff time• time is slotted to Slot_Time - to define IFS and backoff time• the r.b. Is an integer value that corresponds to a number of time slots• initially it is 0-7

– if the timer reached zero and medium is idle --> transmit– if the medium becomes busy --> freeze the timer– if collision --> new backoff time 0-15

• the idle period after DIFS is called contention window• this method promotes fairness



t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

802.11 - CSMA/CA basic access method

– station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

– if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

– if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)

– if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

slot timedirect access if medium is free DIFS



DFWMAC

• Network Allocation Vector (NAV) is time field that indicates the duration of the current transmission

• Backoff procedure is used to randomized access to the channel

t

SIFS

DIFS

ACK

defer access

otherstations

receiver

senderdata

DIFS

Medium busy RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

back

off



Trade-offs with RTS/CTS+ Collisons are avoided+ Hidden station problem is solved– Bandwidth reduction– Not with multicast and broadcast

Usage

With large frames

When collisions are likely



Fragmentation

t

SIFS

DIFS

data

ACK1

otherstations

receiver

senderfrag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

• Large frames handed down from the LLC to the MAC may require fragmentation to increase transmission reliability

• Fragmentation_threshold • the channel is not released until the whole frame is transmitted successfully or the source fails to receive

ACK for a fragment.



DFWMAC-PCF

PIFS

stations‘NAV

wirelessstations

point coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

• The beginning of a super frame is indicated by a beacon transmitted by AP. (synchronization)

• the minimum duration of PCF period is time required to send 2 frames + overhead + PCF-end-frame

• the maximum duration - time must be allotted for at least one frame to be transmitted during DCF period



DFWMAC-PCF

tstations‘NAV

wirelessstations

point coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contentionperiod

contention free period

t2 t3 t4



802.11 - MAC management

• Synchronization– try to find a LAN, try to stay within a LAN– timer etc.

• Power management– sleep-mode without missing a message– periodic sleep, frame buffering, traffic measurements

• Association/Reassociation– integration into a LAN– roaming, i.e. change networks by changing access points – scanning, i.e. active search for a network



Synchronization using a Beacon (infrastructure)

beacon interval

tmedium

accesspoint

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame



Synchronization using a Beacon (ad-hoc)

tmedium

station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

station2

B2 B2

random delay



Power management• Idea: switch the transceiver off if not needed• States of a station: sleep and awake• Timing Synchronization Function (TSF)

– stations wake up at the same time• Infrastructure

– Traffic Indication Map (TIM)• list of unicast receivers transmitted by AP

– Delivery Traffic Indication Map (DTIM)• list of broadcast/multicast receivers transmitted by AP

• Ad-hoc– Ad-hoc Traffic Indication Map (ATIM)

• announcement of receivers by stations buffering frames• more complicated - no central AP• collision of ATIMs possible (scalability?)



Power saving with wake-up patterns (infrastructure)

TIM interval

t

medium

accesspoint

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

p PS poll

p

d

d

d data transmissionto/from the station



Power saving with wake-up patterns (ad-hoc)

awake

A transmit ATIM D transmit data

t

station1

B1 B1

B beacon frame

station2

B2 B2

random delay

A

a

D

d

ATIMwindow beacon interval

a acknowledge ATIM d acknowledge data



802.11 - Roaming

• No or bad connection? Then perform:• Scanning

– scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer

• Reassociation Request– station sends a request to one or several AP(s)

• Reassociation Response– success: AP has answered, station can now participate– failure: continue scanning

• AP accepts Reassociation Request– signal the new station to the distribution system– the distribution system updates its data base (i.e., location information)– typically, the distribution system now informs the old AP so it can release

resources



IEEE 802.11 AuthenticationOpen System Authentication

Authentication request (OpenSystem Authentication)

Authentication response



IEEE 802.11 AuthenticationShared Key Authentication

Authentication request(Shared Key Authentication)

“challenge” text string

“challenge” text string,encrypted with shared key

WEP encryptionof challenge text

Positive or Negative responsebased on decryption result

WEP decryptionof encrypted text



IEEE 802.11 a and b IEEE 802.11b IEEE 802.11a

Time Table Standard in 1997 Standard in 2001

Frequency Band and bandwidth 2.4 GHz 5 GHz

Speed 11 Mbps 54 Mbps

Modulation Techniques Spread Spectrum OFDM (Orthogonal Frequency Division Multiplexing

Distance Coverage Up to 100 meters 20 meters - speed goes down with increased distance

Interoperability Current problems expected to be resolved in future

Problems now but expect resolution soon

Cost Cheaper - $300 for access point and $75 for adapter

More expensive - $500 in 01/2002 -

Interference with other devices Band is more polluted - significant interference here

Less interference because of few devices in this band



WLAN: IEEE 802.11 – future developments

• 802.11d: Regulatory Domain Update – completed• 802.11e: MAC Enhancements – QoS – ongoing

– Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol.

• 802.11f: Inter-Access Point Protocol – ongoing – Establish an Inter-Access Point Protocol for data exchange via the distribution

system.• 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM – ongoing • 802.11h: Spectrum Managed 802.11a (DCS, TPC) – ongoing • 802.11i: Enhanced Security Mechanisms – ongoing

– Enhance the current 802.11 MAC to provide improvements in security. • Study Groups

– 5 GHz (harmonization ETSI/IEEE) – closed – Radio Resource Measurements – started– High Throughput – started

Documents

Wireless Communication Protocols and Technologies