WiFi Networks: ieee 802. 11b Wireless lans Carey Williamson

MAC-Layer Retransmission

• If no ACK received “right away”, then the sender retransmits the frame again at the MAC layer
• indicates frame (or ACK) was lost/corrupted
• very short timeout (e.g., 1 msec)
• exponential backoff (doubling) if repeated loss
• Typically recovers before TCP would notice
• Max retransmission limit (e.g., 8)
• May do MAC-layer rate adaptation or frame fragmentation if channel error rate is high

Other MAC Protocols Supported

• Point Coordination Function (PCF)
• AP polls stations in turn to see if frames to send
• useful for real-time traffic
• Request-To-Send/Clear-To-Send (RTS/CTS)
• reservation-based approach (ask permission)
• useful for very large frames
• useful for solving the “hidden node” problem
• request asks for clearance (permission) to send
• request also indicates time required for transmit

Frame Formats

• Two frame formats available:
• long preamble
• short preamble
• Configuration option for NIC and AP
• Variable-size frames (max 2312 data bytes)
• 16-bit Cyclic Redundancy Code (CRC) for error checking of frames

Long Preamble = 144 bits

• Interoperable with older 802.11 devices

• Entire Preamble and 48 bit PLCP Header sent at 1 Mbps

128 bit Preamble
(Long)
16 bit
Start
Frame
Delimiter
Signal Speed
1,2,5.5,11
Mbps
Service
(unused)
Length
of
Payload
16 bit
CRC
Payload
0-2312 bytes
Transmitted at 1 Mbps
Transmitted at X Mbps
Frame Format (Long Preamble)
Short Preamble = 72 bits

• Preamble transmitted at 1 Mbps

• PLCP Header transmitted at 2 Mbps

• more efficient than long preamble

56 bit
Preamble
Payload
0-2312 bytes
Transmitted
at
1 Mbps
16 bit
Start
Frame
Delimiter
Signal Speed
1,2,5.5,11
Mbps
Service
(unused)
Length
of
Payload
16 bit
CRC
Transmitted
at
2 Mbps
Transmitted
at
X Mbps
Frame Format (Short Preamble)