Wireless Personal Communications Systems – CSE5807

1Wireless & Personal Communication Systems – CSE5807Lecture: 06

Wireless Personal Communications Systems – CSE5807

Lecture: 06

Stephen Giles and Satha K. SathananthanSchool of Computer Science and Software Engineering

Monash University

Australia

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


Wireless LAN Applications• LAN Extension:

– Wireless LAN linked into a wired LAN on same premises.• Cross-building interconnect:

– 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.

• Ad hoc networking:– Temporary peer-to-peer network set up to meet immediate

need.


Wireless LAN

AP

WAN

AP AP

Backbone LAN Router


Wireless LAN

Ad hoc networks Infrastructure networks

Access Point

LAN WAN


Wireless LAN Requirements• Throughput• Number of nodes• Connection to backbone LAN• Service area• Battery power consumption• Transmission robustness and security• License-free operation• Handoff/roaming• Dynamic configuration


Wireless LAN Categories

Infrared (IR) Microwave Radio

Directed Omini-directional Diffused

Spread spectrum

Narrowband


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 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.


Infrared Transmission• Advantages:

– Spectrum for infrared virtually unlimited => High data rates.– Infrared spectrum unregulated.– Equipment inexpensive and simple.– Reflected by light-colored objects.

• Ceiling reflection for entire room coverage.– Doesn’t penetrate walls.

• More easily secured against eavesdropping.• Less interference between different rooms.

• Disadvantages:– Indoor environments experience infrared background radiation.

• Transmitters of higher power required.• Limited by concerns of eye safety and excessive power consumption• Limits range.


Spread Spectrum LAN Configuration• Multiple-cell arrangement.

– 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:

– Licensed within specific geographic areas to avoid potential interference.– Motorola - 600 licenses in 18-GHz range – Encrypted transmissions prevent eavesdropping.

• 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.

• Potential interference is high.• 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 LAN Router

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


Required Reading • W. Stallings, “Wireless Communications and Networks”

Prentice-Hall, 2000.

>> Chapter 13 & 14

Reference• K. Pahlavan and K. Krishnamurthy “Principles of

Wireless Networks”, Prentice-Hall, 2002.

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Wireless Personal Communications Systems – CSE5807