Ch-17 Wireless Local Area Networks

Instructor: Dr. Mustafa Shakir

Overview of Wireless LANsuse wireless transmission mediumissues of high prices, low data rates,

occupational safety concerns, & licensing requirements now addressed

key application areas:LAN extension: Buildings with insufficient twisted

pair and new wire laying prohibitedcross-building interconnect: Connect LANs in nearby

buildingsnomadic access: Provides a wireless link between

LAN hub and mobile data terminal such as laptop computer

ad hoc networking: Peer collection of stations may configure themselves into a temporary network.

Single Cell LAN Extension

Access point coverage & comparisonAs a client moves away from the AP, the

transmission signals between the client and AP weaken.

Rather than decreasing reliability, the AP shifts to a slower data rate, which gives more accurate data transfer. This is called data rate or multi-rate shifting.

This happens without losing the connection, and without any interaction from the user.

Rate Shifting

Multi-rate implementation

The distance from an access point effects the available bandwidth. Multi-rate technology allows a step down in bandwidth to gain greater coverage distances.

If 11Mbps is required everywhere, the access points would need to be relocated, so that only the 11-Mbps circles are touching each other, with some overlap.

BandwidthThe maximum aggregate date rate can only

be achieved in a cell, if all remote units are operating at the highest rate. The number of users that can be supported by a single AP is dependent upon the bandwidth and the application needs.

Typical throughput will be lower than maximum data rate for all devices.

Multi Cell LAN Extension

Cross-Building Interconnectconnect LANs in nearby buildingspoint-to-point wireless linkconnect bridges or routers

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Bridging Defined

Bridges are used to connect two or more wired LAN’s, usually located within separate buildings, to create one large LAN.

Point-To-Point Wireless Bridging

Point-to-point wireless bridges, two LANs can be located up to 25 miles apart.

Antennas must have line-of-site. Obstacles cause communication problems.

With Cisco IOS, it is possible to use Fast Etherchannel or multi-link trunking, to aggregate up to three bridges together, yielding 33 Mbps.

Nomadic AccessWireless link between LAN hub & mobile data

terminal laptop or notepad computerenable employee to transfer data from portable

computer to serveralso useful in extended environment such as

campus or cluster of buildingsusers move around with portable computersmay wish access to servers on wired LAN

Infrastructure Wireless LAN

Ad Hoc Networking

temporary peer-to-peer network

The difference is that wireless LAN has stationary infrastruct. Having one or more cells while in adhoc there is no infrastructure.

Peer collection of stations within reach of each other.

Ad Hoc Topology

Peer-to-Peer (Ad Hoc) Topology (IBSS)

Can consist of 2 or more PCs with wireless network adapters.Sometimes called an Independent BSS.Limited range.

Wireless LAN Requirementsthroughput - efficient use wireless medium to maximize

capacityno of nodes - hundreds of nodes across multiple cellsconnection to backbone LAN - using control modulesservice area - 100 to 300 mlow power consumption - for long battery life on mobilestransmission robustness and securitycollocated network operationlicense-free operationhandoff/roamingdynamic configuration - addition, deletion, and

relocation of end systems without disruption to users

Technologyinfrared (IR) LANs

individual cell of IR LAN limited to single roomIR light does not penetrate opaque walls

spread spectrum LANsmostly operate in ISM (industrial, scientific, and

medical) bandsno Federal Communications Commission (FCC)

licensing is required in USAnarrowband microwave

microwave frequencies but not use spread spectrum

some require FCC licensing

Spread Spectrum LANConfiguration

usually use multiple-cell arrangementadjacent cells use different center

frequenciesconfigurations:

hub connected to backbone wired LANconnect to stations on wired LAN and in other cellsmay do automatic handoff

peer-to-peerno hubMAC algorithm such as CSMA used to control accessfor ad hoc LANs

Spread Spectrum LANsTransmission Issues

licensing regulations differ between countriesUSA FCC allows in ISM band:

spread spectrum (1W), very low power (0.5W)902 - 928 MHz (915-MHz band)2.4 - 2.4835 GHz (2.4-GHz band)5.725 - 5.825 GHz (5.8-GHz band)

2.4 GHz also in Europe and Japaninterference

many devices around 900 MHz: cordless telephones, wireless microphones, and amateur radio

fewer devices operating at 2.4 GHz; microwave ovenlittle competition at 5.8 GHz

IEEE 802.11 - BSSbasic service set (BSS) building blockmay be isolated may connect to backbone distribution system

(DS) through access point (AP)BSS generally corresponds to cell DS can be switch, wired network, or wireless

networkhave independent BSS (IBSS) with no AP IBSS

is adhoc network.

Basic Infrastructure Topology (BSS)

Building block of an 802.11 LAN that covers a single cell

When a device moves out of its BSS, it can no longer communicate with other members of the BSS.

Uses infrastructure mode, requires an access point (AP).

•All stations communicate through the AP, not directly with peers.

•A BSS has one service set ID (SSID).

Extended Service Set (ESS)possible configurations:

simplest is each station belongs to single BSScan have two BSSs overlapa station can participate in more than one BSSassociation between station and BSS dynamic

ESS is two or more BSS interconnected by DS

Extended Infrastructure Topology (ESS)

2 or more BSSs that are connected by a common distribution system

Allows the creation of a wireless network of arbitrary size and complexity.

All packets in an ESS must go through one of the APs.

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Wireless repeater

Channel 1

Access Point

Wireless Clients

Channel 1

Access Point

Wireless Repeater “Cell”

LAN Backbone

Medium Access ControlMAC layer covers three functional areas

reliable data deliveryaccess controlsecurity

Four Frame Exchangecan use four-frame exchange for better reliability

source issues a Request to Send (RTS) frame to destinationdestination responds with Clear to Send (CTS)after receiving CTS, source transmits data destination responds with ACK

RTS alerts all stations within range of source that exchange is under way

CTS alerts all stations within range of destination other stations don’t transmit to avoid collision and

the destination responds with an acknowledgement.

After receiving CTS the source transmits data frame and destination responds with acknowledgement

RTS/CTS exchange is required function of MAC but may be disabled

Distributed Coordination Function

DCF sublayer uses CSMA if station has frame to send it listens to mediumif medium idle, station may transmitelse waits until current transmission complete

no collision detection since on wireless networkDCF includes delays that act as a priority

scheme

MAC (DCF) Algortihm

Using an Inter Frame space IFS, the rules for CSMA access are as follows

1. A station with a frame to transmit senses the medium. If the medium is idle, it waits to see if the medium remains idle for a time equal to IFS.

2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over.

3. Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off a random amount of time and again senses the medium. If the medium is still idle, the station may transmit.

4.If the transmission is unsuccessful, which is determined by the absence of an acknowledgement, then it is assumed that a collision has occurred.