Wireless & Personal Communication Systems – CSE5807 Lecture: 06 1 Wireless Personal Communications Systems – CSE5807 Lecture: 06 Stephen Giles and Satha

1Wireless & Personal Communication Systems – CSE5807Lecture: 06

Wireless Personal Communications Systems – CSE5807

Lecture: 06

Stephen Giles and Satha K. Sathananthan

School of Computer Science and Software Engineering

Monash University

AustraliaModified by Peter Granville August 2006

These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .


Wireless LAN Application Areas• LAN Extension:

– In number of environments, there is a role for the WLAN:

• Buildings with large open spaces – manufacturing plants, warehouses

• Historical buildings with insufficient twisted pair and where drilling holes for new wiring is prohibited

• Small offices where installation and maintenance of wired LANs is not economical

– In most cases an organization also has a Wired LAN. Typically the WLAN is linked into a wired LAN on same premises – LAN Extension

– Fig 13.1, 13.2 Stallings 2E, 1E


Wireless LAN Application Areas

• Cross-building interconnect:– To connect LANs in nearby buildings. Connecting wired or

wireless LANs by point-to-point wireless link.

– Devices connected are typically bridges or routers.

• Nomadic Access: – Wireless link between LAN hub and mobile data terminal

equipped with antenna

– Useful in an extended environment such as a campus or a business operating out of a cluster of buildings


Wireless LAN Application Areas

• Ad hoc networking:– Temporary peer-to-peer (no centralized server) network set

up to meet an immediate need

– A group of employees convene a business meeting and link theirs PCs in a temporary network just for the duration of the meeting.

– No infrastructure for an ad hoc LAN

– Fig 13.3 Stallings 2E, 1E


Wireless LAN

AP

WAN

AP AP

Backbone LANRouter


Wireless LAN

Ad hoc networks Infrastructure networks

Access Point

LAN WAN


Wireless LAN Requirements• Throughput

– The Media access control protocol should make efficient use of the wireless medium to maximize capacity

• Number of nodes– May need to support hundreds of nodes across multiple cells

• Connection to backbone LAN– In most cases, interconnection with stations on a wired backbone LAN is

required

• Service area– Typical cover area diameter 100 to 300m


Wireless LAN Requirements• Battery power consumption

– Mobile workers use battery powered workstations that need a long battery life

– A MAC protocol that requires mobile nodes to monitor access points constantly or engage in frequent handshakes with a base station would be inappropriate

– Sleep mode to reduce power consumption

• Transmission robustness and security– Unless properly designed, WLAN may be interference prone and easily

eavesdropped

– Must permit reliable transmission in a noisy environment and should provide some level of security from eavesdropping


Wireless LAN Requirements• License-free operation

– Users would prefer to buy and operate WLAN products without having to secure a license for the frequency band used by the LAN

• Handoff/roaming

– MAC protocol used should enable stations to move from one cell to another

• Dynamic configuration

– The MAC addressing and network management aspects of WLAN should permit dynamic and automated addition deletion and relocation of end systems without disruption to other users.


Wireless LAN Requirements• Collocated network operation

– As WLANs become more popular, it is likely for two or more to operate in the same area. Interference between the WLANs is possible and may thwart the normal operation of MAC protocol and may allow unauthorized access to a WLAN

• Kiviat graphs fig 13.4 Stallings 2E, 1E


Wireless LAN Technologies

Infrared (IR) Microwave Radio

Directed Omini-directional Diffused

Spread spectrum

Narrowband

Figs 13.5, 13.6, 13.7, Table13.1 Stallings 2E, 1E


Infrared Data Transmission Techniques• Directed Beam Infrared:

– Used to create point-to-point links.• Range depends on emitted power and degree of focusing.• Focused infrared data link can have range of kilometers.• Cross-building interconnect between bridges or routers.

• Ominidirectional:– Single base station (typically mounted on ceiling) within line of sight of

all other stations on LAN.• Base station broadcasts signal that can be received by infrared

transceivers.• Infrared transceivers transmit with directional beam aimed at base

station.

• Diffused:– All infrared transmitters focused and aimed at a point on diffusely

reflecting ceiling• Infrared radiation strikes ceiling and reradiated omnidirectionally. • Picked up by all receivers.


Wireless LAN Technologies• Infrared - Strengths

– Large spectrum available – potentially high data rates

– Unregulated worldwide

– Diffusely reflected, thus possible to use ceiling (light coloured) reflection to achieve coverage of an entire room

– Does not penetrate walls or opaque objects:

• More easily secured against eaves dropping

• Separate installation can be operated in every room in a building without interference, enabling construction of very large WLANs

– Equipment inexpensive and simple

– Uses intensity modulation, so receivers only need to detect the amplitude of the optical signals


Wireless LAN Technologies• Infrared – Weaknesses

– Many indoor environments experience intense infrared background radiation, this appears as noise at the receiver:

• Require use of higher power transmitters– Concern for eye safety

– Excessive power consumption

• Limits range


Spread Spectrum LAN Configuration• Multiple-cell arrangement

– Fig 13.2 Stallings 2E, 1E

– Within a cell, either peer-to-peer or hub.

• Peer-to-peer topology:

– No hub

– Access controlled with MAC algorithm => CSMA

– Appropriate for ad hoc LANs.

• Hub topology:

– Mounted on the ceiling and connected to backbone.

– May control access and act as multiport repeater.

– Automatic handoff of mobile stations.

– Stations in cell either:

• Transmit to / receive from hub only.


Narrowband Microwave LAN• Use of a microwave radio frequency band for signal

transmission.

• Relatively narrow bandwidth.

• Licensed by FCC in USA:– Licensed within specific geographic areas (radius 28km) to avoid

potential interference.

– Motorola - 600 licenses in 18-GHz range

– Encrypted transmissions prevent eavesdropping.

– License holder has a legal right to an interference free data communications channel

• Unlicensed:– RadioLAN introduced narrowband wireless LAN in 1995.– Uses unlicensed ISM spectrum => Operates at 10 Mbps in the 5.8-GHz band

• Used at low power (0.5 watts or less).

• Range = 50 m to 100 m.


License-Free Bands• No permission required for bandwidth usage.

– No licensing cost.– Limit on power of transmission exists.

• Risk of interference disrupting the communications, no legal remedy

• Industrial Scientific Medical (ISM) bands:– 900 MHz ISM Band

• 902 MHz – 928 MHz => Used in wireless home phones and wireless camera systems.

– 2.4 GHz ISM Band• 2.4 GHz – 2.5 GHz => Used by IEEE802.11, IEEE802.11b and IEEE802.11g devices.

– 5.8 GHz ISM Band• 5.725 GHz– 5.875 GHz

• Unlicensed national Information Infrastructure (UNNI) Bands:– Lower Band

• 5.15 GHz – 5.25 GHz

– Middle Band• 5.25 GHz – 5.35 GHz

– Upper Band• 5.725 GHz – 5.825 GHz


WLAN Organizations• Institute of Electrical and Electronic Engineers (IEEE).

• European Telecommunications Standards Institute (ETSI)

• The Wi-Fi Alliance

• Wireless LAN Association (WLANA)

• Infrared Data Association (IrDA)


WLAN Standards• IEEE802.11 family of standards.

– IEEE802.11

– IEEE802.11a

– IEEE802.11b

– IEEE802.11g

• HiperLAN– HiperLAN 1

– HiperLAN 2

• HomeRF


Other IEEE802.11 Standards• IEEE802.11c

– Define MAC procedure for the bridge operation.

• IEEE802.11e– Enhance current 802.11 MAC to expand support for

applications with QoS requirements.

• IEEE802.11f– Define procedure for Inter Access Point Protocol (IAPP).

• IEEE802.11i– Enhance the 802.11 MAC to enhance security and

authentication mechanisms.


IEEE 802 Protocol Layers


Protocol Architecture• Functions of physical layer:

– Encoding/decoding of signals.

– Preamble generation/removal (for synchronization).

– Bit transmission/reception.

– Includes specification of the transmission medium and topology.


Protocol Architecture• Functions of medium access control (MAC) layer:

– On transmission, assemble data into a frame with address and error detection fields.

– On reception, disassemble frame and perform address recognition and error detection.

– Govern access to the LAN transmission medium.

• Functions of logical link control (LLC) Layer:– Provide an interface to higher layers and perform flow and error control.


Separation of LLC and MAC• The logic required to manage access to a shared-access

medium not found in traditional layer 2 data link control.

• For the same LLC, several MAC options may be provided.


IEEE 802.11 Services


MAC Frame Format• MAC control

– Contains MAC protocol information.

• Destination MAC address

• Source MAC address

• Data

• Cyclic Redundancy Check (CRC)


Logical Link Control• Properties not shared by other control protocols:

– Must support multi-access, shared-medium nature of the link.

– Relieved of some details of link access by MAC layer.

• LLC Services:– Unacknowledged connectionless service

• No flow- and error-control mechanisms

• Data delivery not guaranteed

– Connection-mode service

• Logical connection set up between two users

• Flow- and error-control provided

– Acknowledged connectionless service

• Cross between previous two

• Datagrams acknowledged

• No prior logical setup


IEEE 802.11 Architecture• Access point (AP):

– MAC protocol by a central coordination function.

• Basic service set (BSS) :– Stations competing for access to shared wireless medium.

– Isolated or connected to backbone distribution system (DS) through AP.

• Distribution system (DS):– Can be a switch, a wired network or a wireless network.

• Extended service set (ESS): – Two or more basic service sets interconnected by DS.


IEEE 802.11 Architecture

AP

WAN

AP AP

Backbone LANRouter

BSS BSS BSS

ESS


IEEE 802.11 ServicesService Provider Used to support

Association Distribution System MSDU delivery

Authentication Station/AP LAN access and security

Deauthentication Station/AP LAN access and security

Disassociation Distribution System MSDU delivery

Distribution Distribution System MSDU delivery

Integration Distribution System MSDU delivery

MSDU delivery Station/AP MSDU delivery

Privacy Station/AP LAN access and security

Reassociation Distribution System MSDU delivery

MSDU – MAC Service Data Unit


Distribution of Messages Within a DS

• Distribution service:– Used to exchange MAC frames from station in one BSS to

station in another BSS.

• Integration service:– Transfer of data between station on IEEE 802.11 LAN and

station on integrated IEEE 802.x LAN.


Association-Related Services• Before DS can deliver data to or accept data from a

station, that station must be associated.

• Three transition types:– No transition

• Stationary or moves only within BSS.

– BSS transition• Station moving from one BSS to another BSS in same ESS.

– ESS transition• Station moving from BSS in one ESS to BSS within another

ESS.


Association-Related Services• Association

– Establishes initial association between station and AP.

• Reassociation

– Enables transfer of association from one AP to another, allowing station to move from one BSS to another.

• Disassociation

– Association termination notice from station or AP.


Access and Privacy Services• Authentication

– Establishes identity of stations to each other.

• Deauthentication– Invoked when existing authentication is terminated.

• Privacy– Prevents message contents from being read by unintended

recipient.


IEEE 802.11 Medium Access Control• MAC layer covers three functional areas:

– Reliable data delivery

– Access control

– Security


Reliable Data Delivery• More efficient to deal with errors at the MAC level than higher

layer (such as TCP).

• Frame exchange protocol:– Source station transmits data.

– Destination responds with acknowledgment (ACK).

– If source doesn’t receive ACK, it retransmits frame.

• Four frame exchange:– Source issues request to send (RTS).

– Destination responds with clear to send (CTS).

– Source transmits data.

– Destination responds with ACK.


Access Control• Distributed Foundation Wireless MAC (DFWMAC).

• Distributed Access:– Attractive for ad hoc network and bursty traffic.

– Handled by lower sublayer of the MAC layer, Distributed Coordination Function (DCF).

– Use CSMA.

– Ordinary asynchronous traffic directly uses DCF.

– DCF includes a set of delays to provide priority scheme.

• Optional Centralized Access:– Point Coordination Function (PCF) provides contention free

service.

– PCF is built on top of DCF.


Access Control


Medium Access Control Logic


Interframe Space (IFS) Values• Short IFS (SIFS):

– Shortest IFS

– Used for immediate response actions.

• Acknowledgment (ACK)

• Clear to send (CTS)

• Poll response

• Point coordination function IFS (PIFS):– Midlength IFS.

– Used by centralized controller in PCF scheme when using polls.

– Takes precedence over normal contention traffic.

• Distributed coordination function IFS (DIFS):– Longest IFS

– Used as minimum delay of asynchronous frames contending for access

– Used for all ordinary asynchronous traffic.


Communication Process

Beacon Contention-Free Period

(PCF Mode)

Contention Period

(DCF Mode)

Superframe

AP seizes control of medium here.

PIFSDIFS Contention Period

time

Stations in DCF mode contend for access here.


MAC Frame Format


Control Frames• Power save – poll (PS-Poll)

• Request to send (RTS)

• Clear to send (CTS)

• Acknowledgment

• Contention-free (CF)-end

• CF-end + CF-ack

Data Frames

• Data-carrying frames

– Data

– Data + CF-Ack

– Data + CF-Poll

– Data + CF-Ack + CF-Poll


Management Frame • Beacon

• Probe request

• Probe response

• Authentication

• Deauthentication

• Association request

• Association response

• Reassociation request

• Reassociation response

• Dissociation

• Announcement traffic indication message


IEEE 802.11 MAC Protocol• A protocol is needed to define rules for all stations to access

the common channel without conflict.

• Many proposals were submitted to IEEE 802.11 working group, CSMA/CA was chosen (1997).

• CSMA/CA is similar to CSMA/CD (used in Ethernet). A station must make sure the common channel is clear before any transmission attempt.

• Unlike CSMA/CD (Ethernet), a station cannot detect a collision. The receiver must reply with an acknowledgement immediately after receiving a frame.

• A station must choose a random future time for all its transmission (new or collided frames).


CSMA/CA

• To transmit a frame, a station first picks a random integer, r (or counter) from range [0,W-1].

• The value r is decreased by one when the common channel is detected idle for a short period of time known as slot time (e.g. 50sec).

• The station transmits the frame when r reaches zero.• If the channel is detected busy before r reaches zero, the station

stops decreasing r. The countdown will be reactivated when a long period of idle channel is detected (this period is known as DIFS – Distributed Interframe Space).

• After the completion of the transmission, the station must wait for a very short period (known as SIFS – Short IFS) for the acknowledgement (ack).

• The return of the ack confirms the transmission, otherwise, the station must repeat the first step.


Ack not received

transmission completed

Channel turns busy

CSMA/CA

The station transmits the frame

To transmit a frame, a station first picks a random integer, r (or counter) from range [0,W-1]

The r value is frozen. The station will continue to monitor the channel.The countdown will be reactivated when a long period of idle channel is detected (known as DIFS)

the station must wait for a very short period (known as SIFS) for an acknowledgement (ack).

For every slot time (eg. 50sec) where the common channel is sensed idle, r =r-1

when r =0channelturns idle for a DIFS

START

DONEack received

REPEAT START


CSMA/CA

• Initially, W is set to a small value (W=8 according to the IEEE 802.11 standard, this is known as the “minimum contention window”).

• As a station experiences collision, W is doubled.

• When W reaches a large value, it stays at that value (W=256 according to the IEEE 802.11 standard, this is known as the “maximum contention window”).


CSMA/CA Operation: Example-1

B

D

C

Consider this ad hoc WLAN:

Scenario:B is attempting to transmit a frame to D. The transmission is successful.

time

B was ready, B picked r = 3r = 0

B transmitted its frameACK from D

Slot time SIFS



B

D

C


Scenario:B, C are attempting transmissions to D. Both transmissions are successful.

time

B transmitted its frame. C detected a busy channel, rc was frozen.

ACK from D

Slot time

B, C are ready, B picked rB = 1 and C picked rc = 3

DIFS

rc was reactivated

C transmitted its frame here when rc = 0.



Scenario:B, C are attempting transmissions to D. A collision occurs before both transmissions are successful.

time

Transmissions of B and C collided here.

Slot time

B, C are ready, B picked rB = 1 and C picked rc = 1

No ACK from D

DIFS

B, C repeated the operation. B picked rB = 1 and C picked rc = 13

B transmitted its frame here when rB =0. rc was frozen here.

ACK from D

...

B

D

C


Hidden Station Problem

When A is transmitting a frame to B, since D is not in A’s coverage, D is a hidden station that D doesn’t sense a busy channel, thus D may starts a transmission that collides with A’s transmission

Hidden Station Problem in ad hoc Wireless LANs:

A BC D

A’s coverage B’s coverage


Exposed Station problem

While A is transmitting a frame to C, B senses a busy channel and concludes that it may not transmit any frame to D which is incorrect

Exposed Station Problem in ad hoc Wireless LANs:

A BC D

A’s coverage B’s coverage


Solution to Hidden/Exposed Station Problem

Four-way Handshaking

time

sender receiver

data

ACKBasic

operation

Additional

operation

RTS

CTS

RTS: Ready to send CTS: Clear to send


RTS/CTS Operation

B

D

C


Scenario:B is attempting to transmit a frame to D. The transmission is successful.

time

B was ready, B picked r = 3r = 0

B transmitted its frame after receiving CTS

ACK from D

Slot time

B transmitted RTSD replied with CTS


Handshaking

• The handshaking access method is an optional operation in IEEE 802.11.

• It is also used to improve performance (by reducing the bandwidth wastage due to a collision).

• Short frames are transmitted using Basic access method, and long frames are recommended to transmit using the handshaking access method.


IEEE 802.11: DCF & PCF

SIFS

PIFS

DIFSSIFS = Short Interframe spacePIFS = Point IFSDIFS = Distributed IFS

PCF (optional) DCF

SIFS

P Data P CD

IFS

PCF Operation:

PIFS

timeBackoffDataBusy

SIFS

PIFS

time


Required Reading

• W. Stallings, “Wireless Communications and Networks” Prentice-Hall, 2E and 1E

>> Chapter 13 & 14

Reference

• K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.

Documents

Wireless & Personal Communication Systems – CSE5807 Lecture: 06 1 Wireless Personal Communications Systems – CSE5807 Lecture: 06 Stephen Giles and Satha