Wireless LANs• Ethernet and all its enhancements is the major wired LAN

architecture today• Beyond Ethernet, the fastest growing LAN architecture is wireless• This architecture is being implemented within enterprises

Universities Medical institutions Businesses Libraries

• Also Implemented in hot-spots for customer conveniences Coffee shops Airline terminals Soon on actual airline flights

• Very useful for home networks• Being deployed as city-wide installations

Philadelphia San Francisco

Wireless Standards

• Most wireless deployments today are based on IEEE 802.11 standards

• This committee first established in 1990• 802.11 committee developed a number of

standards where each Has defined radio frequencies

• Most use frequencies in public domain• Called Industrial, Scientific, Medical (ISM) bands

Defined techniques for encoding bits over these frequencies

ISM Bands

802.11 LAN Overview

• Wireless Access Point (AP) Could be standalone – wireless island To provide connectivity, usually physically connected

to a wired LAN (switch) Installed at locations to provide optimal coverage Must have electrical power

• Client wireless adapter Usually built-in to PCs Each adapter has its own MAC address Can buy wireless adapter card for desktop PCs

Basic Service Set (BSS)

Extended Service Set

Wireless Installation

802.11 StandardsEncoding Schemes

• Frequency Hopping Spread Spectrum (FHSS)• Sends on one frequency for short period• Shifts to a second frequency for a period of time• After N hops, cycle repeated• If signal has bandwidth B, then uses N x B worth of

bandwidth

802.11 StandardsEncoding Schemes

• Direct Sequence Spread Spectrum (DSSS)

• Each data bit is replaced by a sequence of bits called a chip code

• If the chip code is N bits, then we need a bandwidth of N x Data Rate of original bit stream

802.11 StandardsEncoding Schemes

• Orthogonal Frequency-Division Multiplexing (OFDM)

• A form of Frequency Division• Transmit data simultaneously over a number of

equally spaced sub frequencies• Uses PSK and QAM modulation techniques in the

sub frequencies

802.11 Standards• Different networks can exist in the same

frequency Identified by a Service Set Identifier (SSID)

• Besides a Source and Destination address, frames include a third address, the SSID

802.11 StandardsAccess Mechanism

• Uses an contention access mechanism similar to Ethernet

• Cannot use Collision Detection Due to hidden node problem Two nodes might be able to communicate

with the same Access Point but may not ‘see’ each other

• Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) It includes a form of acknowledgement

contrary to Ethernet

802.11 StandardsAccess Mechanism

• Source station sends a short message Listens first (carrier sense) Waits a period of time (Distributed InterFrame Space)

then sends message This is a Request to Send message and sets timer

• Includes amount of time it needs channel

When/If destination receives message, it send a short Clear to Send message

• If CTS correctly received, Source sends data Sets timer Waits for an ACK message from destination

802.11 StandardsAccess Mechanism

• Source station sends a short message Listens first (carrier sense) Sends Request to Send message and sets

timer• Includes amount of time it needs channel

When/If destination receives message, it sends a short Clear to Send message

• If CTS correctly received, Source sends data Sets timer Waits for an ACK message from destination

802.11 StandardsAccess Mechanism

• If CTS not received before timeout, it is assume a collision occurred.

• When RTS is sent, it is seen by other stations

• Other stations set a timer to not attempt transmission for the time included in the RTS message

• This time is called a Network Allocation Vector (NAV)

CSMA/CA

802.11 Standards

• Original 802.11 Standard finalized in 1997 Used FHSS 1 and 2 Mbps

• 802.11b Standard ratified in 1999 Uses DSSS 11 Mbps, fallback to 5.5Mbps, 2 Mbps, 1Mbps Uses frequency range 2.4 Ghz – 2.485 Ghz This is divided into 11 channels (US)

802.11b Frequencies

Channel 1 2400 – 2424Channel 2 2405 – 2429Channel 3 2410 – 2434Channel 4 2415 – 2439Channel 5 2420 – 2444Channel 6 2425 – 2449…………..Channel 11 2450 – 2474

Channels 1, 2 ,3, 4, 5 OverlapThere are three non overlapping channels 1, 6, 11

Wireless LANs

• Care must be taken in wireless LAN designs

Note: Channels 1, 2, and 3 here refer to the three non-overlapping channels 1, 6, and 11

802.11 Standards

• 802.11g Standard ratified in 2003 Uses OFDM 54 Mbps, fallback to lower speeds Backward compatibility with 802.11b Most implementation are referred to as b/g Uses same frequency range 2.4 Ghz – 2.485 Ghz When 802.11b and 802.11g are mixed there is a

performance penalty

802.11 Standards

• 802.11a Standard ratified in 2001 Uses OFDM 54 Mbps, fallback to lower speeds Uses frequency range 5.47 Ghz – 5.725 Ghz Up to 24 non overlapping channels

Wireless Performance

• Never achieve rated speed. Closer to half advertised speed

• When 802.11b and 802.11g are mixed there is a performance penalty

• Performance substantially degraded when different Access Point using the same channels overlap

• Other devices operating in the same frequencies will degrade performance

• Performance degrades as distance from Access Point increases

• Obstructions affect performance

Wireless Access

• Can control who can access

• Can limit by MAC address

• Can require authentication

• On UMS networks, must register MAC address

Wireless security• The data travels over the air• Any station can ‘see/hear’ all data• Data can be encrypted

By the application By the wireless network Most are not encrypted.

• Most wireless access points provide for encryption Wired Equivalent Privacy (WEP) – original Wi-Fi Protected Access (WPA or WPA2) – latest

• Provides for different levels of encryption• Must configure Access Point and Client• Specified by IEEE 802.11i subcommittee

Access Points

• Like switches, there are different levels of Access Points Enterprise Home or small office environments

• Each has different levels of manageability and administration

• May have widely different levels of upgradeability

• Like switches, total cost is not the solely the purchase price of the Access Point

• For enterprise environments, choose products from reputable vendors that have such enterprise capabilities

Other 802.11 Standards

802.11e Quality of Service (QoS) – 2005

802.11f Inter Access Point Protocol - 2003

80211h Spectrum Management for

802.11a - 2004

802.11i Enhanced security - 2004

802.11n Higher speeds – 100Mbps + (9/2008)

802.11r Fast Roaming – (April 2008)

802.11v Wireless Management (?)

Bluetooth

• Another wireless technology• Designed for communication between

computer and devices Mouse Printers Cameras

• Also operates in the 2.4 GHz range Low power Distances in range of 10 meters

• Standard specified by IEEE802.15