IEEE 802.11 Wireless LAN DataLink Layer: Architecture, MAC Protocol, and Security, Slides of Computer Networks

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5: DataLink Layer 5a-

Wireless

5: DataLink Layer 5a-

IEEE 802.11 Wireless LAN

 wireless LANs: untethered/mobile networking  IEEE 802.11 standard:  MAC protocol  unlicensed frequency spectrum governed by pre-defined rules vs. restricted allocation  Basic Service Set (BSS) (a.k.a. “cell”) contains:  wireless hosts/stations (STA)  access point (AP): base station  BSS’s combined to form distribution system (DS) and extended service set (ESS)

• 5: DataLink LayerDocsity.com 5a-

 Interesting aspects to 802.11 protocols

relative to what we’ve seen already

 Both a polling (contention-free) mode and

CSMA/CA mode for dealing with

contention

 CSMA/CA – collision avoidance rather than

CD collision detection

 Some reliable data transfer aspects

including ACKS (unlike Ethernet)

5: DataLink LayerDocsity.com 5a-

5: DataLink Layer 5a-

Distributed Foundation

Wireless MAC

 Compromise was Distributed Foundation

Wireless MAC (DFWMAC)

 Distributed Access control mechanism with

an optional centralized control layer on top

of that

 Distributed Coordination Function (DCF) on top of physical layer  On top of that is optional Point Coordination Function (PCF) that provides contention free service

Access Control

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Superframes

CPs and CFPs alternate in a superframe

A superframe is an interval between two

beacon frame transmissions.

A beacon frame is broadcasted by APs in

BSSs or random stations in IBSSs.

It carries management information to the stations.

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IEEE 802.11 MAC Timing

PCF Superframe Construction

Medium Access Control Logic

IFS = interframe space

Each time fail increase time to wait before send

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Interframe Space (IFS) Values

 Actually three different 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

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IFS Usage

 SIFS

 Acknowledgment (ACK)  Clear to send (CTS)  Poll response( for PCF)

 PIFS

 Used by centralized controller in issuing polls (for PCF)  Takes precedence over normal contention traffic

 DIFS

 Used for all ordinary asynchronous traffic

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Polling

 Since PIFS smaller than DIFS, coordinator

can seize coordinator and lock all traffic (

at least traffic that obeys the rules) while

it polls and receives responses

 When polling coordinator sends a poll to a

station, it can respond using SIFS ( beating

the next PIFS and any DIFS)

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Reliable Data Delivery

 More efficient to deal with errors at the MAC level than higher layer (such as TCP)  Transport layer timeouts can take seconds

 Two 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

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Clear To Send (CTS)

 Station can make it more likely its frame

will get though by first sending a small

Request to Sent (RTS frame)

 The recipient will then reply CTS

 Avoids the hidden terminal problem