LectA..ppt - 04/06/05

CDA 6505 Network Architecture

and Client/Server Computing

Wireless LANs

byZornitza Genova Prodanoff

Wireless LAN Applications

• LAN Extension• Cross-building interconnect• Nomadic Access • Ad hoc networking

02

LAN Extension

• Wireless LAN linked into a wired LAN on same premises– Wired LAN

• Backbone• Support servers and stationary workstations

– Wireless LAN• Stations in large open areas• Manufacturing plants, stock exchange trading floors,

and warehouses

Multiple-cell Wireless LAN

05

Cross-Building Interconnect

• Connect LANs in nearby buildings– Wired or wireless LANs

• Point-to-point wireless link is used• Devices connected are typically bridges or

routers

06

Nomadic Access

• Wireless link between LAN hub and mobile data terminal equipped with antenna– Laptop computer or notepad computer

• Uses:– Transfer data from portable computer to office

server– Extended environment such as campus

07

Ad Hoc Networking

• Temporary peer-to-peer network set up to meet immediate need

• Example:– Group of employees with laptops convene for a

meeting; employees link computers in a temporary network for duration of meeting

08

Wireless LAN Requirements

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

09

Wireless LAN Categories

• Infrared (IR) LANs• Spread spectrum LANs• Narrowband microwave

010

Strengths of Infrared Over Microwave Radio

• Spectrum for infrared virtually unlimited– Possibility of 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

011

Drawbacks of Infrared Medium

• Indoor environments experience infrared background radiation– Sunlight and indoor lighting– Ambient radiation appears as noise in an

infrared receiver– Transmitters of higher power required

• Limited by concerns of eye safety and excessive power consumption

– Limits range

012

IR Data Transmission Techniques

• Directed Beam Infrared• Ominidirectional• Diffused

013

Directed Beam Infrared

• Used to create point-to-point links• Range depends on emitted power and degree of

focusing• Focused IR data link can have range of kilometers

– Cross-building interconnect between bridges or routers

014

Ominidirectional

• Single base station within line of sight of all other stations on LAN

• Station typically mounted on ceiling• Base station acts as a multiport repeater

– Ceiling transmitter broadcasts signal received by IR transceivers

– IR transceivers transmit with directional beam aimed at ceiling base unit

015

Diffused

• All IR transmitters focused and aimed at a point on diffusely reflecting ceiling

• IR radiation strikes ceiling – Reradiated omnidirectionally – Picked up by all receivers

016

Spread Spectrum LAN Configuration

• Multiple-cell arrangement (Figure 13.2)• 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

017

Spread Spectrum LAN Configuration

• Hub topology– Mounted on the ceiling and connected to

backbone– May control access– May act as multiport repeater– Automatic handoff of mobile stations– Stations in cell either:

• Transmit to / receive from hub only• Broadcast using omnidirectional antenna

018

Narrowband Microwave LANs

• Use of a microwave radio frequency band for signal transmission

• Relatively narrow bandwidth• Licensed• Unlicensed

019

Licensed Narrowband RF

• Licensed within specific geographic areas to avoid potential interference

• Motorola - 600 licenses in 18-GHz range – Covers all metropolitan areas– Can assure that independent LANs in nearby

locations don’t interfere– Encrypted transmissions prevent

eavesdropping

020

Unlicensed Narrowband RF

• RadioLAN introduced narrowband wireless LAN in 1995– Uses unlicensed ISM spectrum– Used at low power (0.5 watts or less)– Operates at 10 Mbps in the 5.8-GHz band– Range = 50 m to 100 m

IEEE 802 Protocol Layers

022

IEEE 802.11 Wireless LANProtocol Architecture

• Functions of physical layer:– Encoding/decoding of signals– Preamble generation/removal (for

synchronization)– Bit transmission/reception– Includes specification of the transmission

medium

023

IEEE 802.11 Wireless LANProtocol 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

024

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

025

MAC Frame Format

• MAC control– Contains Mac protocol information

• Destination MAC address– Destination physical attachment point

• Source MAC address– Source physical attachment point

• CRC– Cyclic redundancy check

026

Logical Link Control

• Characteristics of LLC not shared by other control protocols:– Must support multiaccess, shared-medium

nature of the link– Relieved of some details of link access by MAC

layer

027

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

028

Differences between LLC and HDLC

• LLC uses asynchronous balanced mode of operation of HDLC (type 2 operation)

• LLC supports unacknowledged connectionless service (type 1 operation)

• LLC supports acknowledged connectionless service (type 3 operation)

• LLC permits multiplexing by the use of LLC service access points (LSAPs)

029

IEEE 802.11 Architecture

• Distribution system (DS)• Access point (AP)• Basic service set (BSS)

– Stations competing for access to shared wireless medium

– Isolated or connected to backbone DS through AP

• Extended service set (ESS) – Two or more basic service sets interconnected

by DS

IEEE 802.11 Services

031

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

032

Transition Types Based On Mobility

• 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

033

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

034

Access and Privacy Services

• Authentication– Establishes identity of stations to each other

• Deathentication– Invoked when existing authentication is

terminated• Privacy

– Prevents message contents from being read by unintended recipient

035

IEEE 802.11 Medium Access Control

• MAC layer covers three functional areas:– Reliable data delivery– Access control– Security

036

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

Medium Access Control Logic

039

Interframe Space (IFS) Values

• Short IFS (SIFS)– Shortest IFS– Used for immediate response actions

• Point coordination function IFS (PIFS)– Midlength IFS– Used by centralized controller in PCF scheme

when using polls• Distributed coordination function IFS (DIFS)

– Longest IFS– Used as minimum delay of asynchronous

frames contending for access

040

IFS Usage

• SIFS– Acknowledgment (ACK)– Clear to send (CTS)– Poll response

• PIFS– Used by centralized controller in issuing polls– Takes precedence over normal contention

traffic• DIFS

– Used for all ordinary asynchronous traffic

MAC Frame Format

042

MAC Frame Fields

• Frame Control – frame type, control information• Duration/connection ID – channel allocation time• Addresses – context dependant, types include

source and destination• Sequence control – numbering and reassembly• Frame body – MSDU or fragment of MSDU• Frame check sequence – 32-bit CRC

043

Frame Control Fields

• Protocol version – 802.11 version• Type – control, management, or data• Subtype – identifies function of frame• To DS – 1 if destined for DS• From DS – 1 if leaving DS• More fragments – 1 if fragments follow• Retry – 1 if retransmission of previous frame

044

Frame Control Fields

• Power management – 1 if transmitting station is in sleep mode

• More data – Indicates that station has more data to send

• WEP – 1 if wired equivalent protocol is implemented

• Order – 1 if any data frame is sent using the Strictly Ordered service

045

Control Frame Subtypes

• Power save – poll (PS-Poll)• Request to send (RTS)• Clear to send (CTS)• Acknowledgment• Contention-free (CF)-end• CF-end + CF-ack

046

Data Frame Subtypes

• Data-carrying frames– Data– Data + CF-Ack– Data + CF-Poll– Data + CF-Ack + CF-Poll

• Other subtypes (don’t carry user data)– Null Function– CF-Ack– CF-Poll– CF-Ack + CF-Poll

047

Management Frame Subtypes

• Association request• Association response• Reassociation request• Reassociation response• Probe request• Probe response• Beacon

048

Management Frame Subtypes

• Announcement traffic indication message• Dissociation• Authentication• Deauthentication

Wired Equivalent Privacy

050

Authentication

• Open system authentication– Exchange of identities, no security benefits

• Shared Key authentication– Shared Key assures authentication

051

Physical Media Defined by Original 802.11 Standard• Direct-sequence spread spectrum

– Operating in 2.4 GHz ISM band– Data rates of 1 and 2 Mbps

• Frequency-hopping spread spectrum– Operating in 2.4 GHz ISM band– Data rates of 1 and 2 Mbps

• Infrared– 1 and 2 Mbps– Wavelength between 850 and 950 nm

052

IEEE 802.11a and IEEE 802.11b

• IEEE 802.11a– Makes use of 5-GHz band– Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54

Mbps– Uses orthogonal frequency division

multiplexing (OFDM)– Subcarrier modulated using BPSK, QPSK, 16-

QAM or 64-QAM• IEEE 802.11b

– Provides data rates of 5.5 and 11 Mbps– Complementary code keying (CCK) modulation

scheme