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 standards This committee first established in 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

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

Standards Encoding 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

Standards Encoding 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

Standards Encoding 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

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

Standards Access 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

Standards Access 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

Standards Access 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

Standards Access 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

Standards Original  Standard finalized in 1997  Used FHSS  1 and 2 Mbps b  Standard ratified in 1999  Uses DSSS  11 Mbps, fallback to 5.5Mbps, 2 Mbps, 1Mbps  Uses frequency range 2.4 Ghz – Ghz  This is divided into 11 channels (US)

802.11b Frequencies Channel – 2424 Channel – 2429 Channel – 2434 Channel – 2439 Channel – 2444 Channel – 2449 ………….. Channel – 2474 Channels 1, 2,3, 4, 5 Overlap There 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

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

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

Wireless Performance Never achieve rated speed. Closer to half advertised speed When b and g 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 i 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 Standards eQuality of Service (QoS) – fInter Access Point Protocol hSpectrum Management for a iEnhanced security nHigher speeds – 100Mbps + (9/2008) rFast Roaming – (April 2008) vWireless 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