WLAN

What is 802.11? A family of wireless LAN (WLAN) specifications developed by a working group at the Institute of Electrical and Electronic Engineers (IEEE) Versions: 802.11a, 802.11b, 802.11g, 802.11e, 802.11f, 802.11i

WLANs – 802.11 Protocol Architecture Physical Layer (PHY) Distributed Coordination Function (DCF) Point Coordination Function (PCF) Normal Data Traffic (Asynchronous) Real Time Traffic MAC

IEEE 802.11 - Physical Medium Specification Three Physical Media: INFRARED Narrowband Microwave Spread Spectrum

Infrared Infrared signals used to transmit data (similar to TV remotes!) Higher data rates possible (than spread spectrum) Line of sight point-to-point configuration required (or reflection surface will reflects signals) Too sensitive to obstacles. 10 m maximum range with no sunlight or heat interfere

Narrowband Microwave Typically used to link two WLANs together (for example, to link WLANs in two buildings) Microwave dishes required at both ends of link Unlike spread spectrum which operates in the unlicensed ISM band, narrowband microwave requires FCC licensing Exclusive license typically effective within a 17.5 mile radius

Spread Spectrum Distributed signals over multiple frequencies (to avoid eavesdropping or jamming) Frequency Hopping Spread Spectrum (FHSS) Sender transmits over a seemingly random series of frequencies Intended receiver aware of sequence of frequencies and hops accordingly Allows the coexistence of multiple networks in the same area by using different hopping sequences Direct Sequence Spread Spectrum (DSSS) DS systems use a carrier that remains fixed to a specific frequency band. The data signal is spread onto a much larger range of frequencies (at a much lower power level) using a specific encoding scheme.

Wireless LAN Classification Infrared (IR) LANs An individual cell of an IR LAN is limited to a single room, since infrared light does not penetrate opaque walls. Spread Spectrum LANs In most cases these LANs operate in the ISM (industrial, scientific, and medical) bands, so no FCC licensing is required for their use in the United States. Narrowband Microwave LANs These LANs operate at microwave frequencies but do no use spread spectrum. Some of these products operate at frequencies that require FCC licensing; others use one of the unlicensed ISM bands.

IEEE 802.11 Medium Access Control MAC layer covers three functional areas: Reliable data delivery Access control Security

Reliable Data Delivery Loss of frames due to noise, interference, and propagation effects Frame exchange protocol Source station transmits data Destination responds with acknowledgment (ACK) If source doesn‘t receive ACK, it retransmits frame

Wireless LAN MAC CSMA as Wireless MAC? Hidden and Exposed Terminal Problems make the use of CSMA an inefficient technique

Hidden Terminal Problem Collision A B C A talks to B C senses the channel C does not hear A’s transmission (out of range) C talks to B Signals from A and B collide

Exposed Terminal Problem Not possible A B C D B talks to A C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideally transmitted)

Hidden and Exposed Terminal Problems Hidden Terminal More collisions Wastage of resources Exposed Terminal Underutilization of channel Lower effective throughput

CSMA - Collision Avoidance CSMA-CA uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

Hidden Terminal Revisited … RTS A B CTS C CTS DATA A sends RTS B sends CTS C overheads CTS C inhibits its own transmitter A successfully sends DATA to B

Hidden Terminal Revisited How does C know how long to wait before it can attempt a transmission? A includes length of DATA that it wants to send in the RTS packet B includes this information in the CTS packet C, when it overhears the CTS packet, retrieves the length information and uses it to set the inhibition time

Exposed Terminal Revisited RTS B RTS C D Tx not inhibited CTS Cannot hear CTS B sends RTS to A (overheard by C) A sends CTS to B C cannot hear A’s CTS C assumes A is either down or out of range C does not inhibit its transmissions to D

Collisions Still possible – RTS packets can collide! Binary exponential backoff performed by stations that experience RTS collisions RTS collisions not as bad as data collisions in CSMA (since RTS packets are typically much smaller than DATA packets)

Access Control To deal with these two problems 802.11 supports two modes of access control DCF (Distributed Coordination Function) and PCF (Point Coordination Function). All implementations must support DCF, but PCF is optional.