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IEEE 802.11 Wireless LAN: Physical Properties and MAC Protocol - Prof. Tracy Maples, Study notes of Computer Science

California State University (CSU) - Long BeachComputer Science
Prof. Tracy Maples

An overview of the ieee 802.11 wireless lan standard, focusing on its physical properties and the mac protocol used for collision avoidance. Three different physical media: frequency hopping, direct sequence, and infrared signals. It also explains the hidden node and exposed node problems and how the mac protocol solves them through the use of request to send (rts) and clear to send (cts) frames.

Typology: Study notes

Pre 2010

Uploaded on 08/18/2009

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CECS 474 Computer Network Interoperability
Wireless (802.11) Supplement
Wireless (802.11)
The most popular wireless LAN technology is the IEEE 802.11
standard.
Physical Properties
802.11 was designed to run over three different types of physical
media:
1. Spread spectrum using frequency hopping
2. Spread spectrum using direct sequence
3. Infrared signals
Spread spectrum spreads the signal over a wider frequency band
than normal so that interference from other devices will not have
as great an impact.
1. Frequency Hopping
The signal is transmitted over an (apparently) random sequence of
frequencies…first one, then a second, then a third, etc.
The sequence of frequencies is computed algorithmically using a
pseudorandom number generator. The sender and the receiver use
the same seed value to calculate synchronized frequencies.
802.11 defines a physical layer standard that uses frequency
hoping (over 79 1-Mhz-wide frequency bandwidths). It runs in the
2.4 Ghz frequency band of the elecromagnetic spectrum.
Dr. Tracy Bradley Maples (Spring 2009)
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CECS 474 Computer Network Interoperability

Wireless (802.11) Supplement

Wireless (802.11) The most popular wireless LAN technology is the IEEE 802. standard. Physical Properties 802.11 was designed to run over three different types of physical media :

  1. Spread spectrum using frequency hopping
  2. Spread spectrum using direct sequence
  3. Infrared signals Spread spectrum spreads the signal over a wider frequency band than normal so that interference from other devices will not have as great an impact. 1. Frequency Hopping The signal is transmitted over an (apparently) random sequence of frequencies…first one, then a second, then a third, etc. The sequence of frequencies is computed algorithmically using a pseudorandom number generator. The sender and the receiver use the same seed value to calculate synchronized frequencies. 802.11 defines a physical layer standard that uses frequency hoping (over 79 1-Mhz-wide frequency bandwidths). It runs in the 2.4 Ghz frequency band of the elecromagnetic spectrum.

2. Direct Sequence Each bit in the frame is represented by multiple bits in the transmitted signal. The sender sends the XOR of the bit it wants to send and n random bits. This is known as an n -bit chipping code. The sender and receiver random number streams match. This spreads the signal across a frequency band that is n times wider than is required. FIGURE 1: Example 4-bit chipping sequence. 802.11 defines a physical layer standard that uses direct sequence (using an 11-bit chipping sequence). It runs in the 2.4 Ghz frequency band of the elecromagnetic spectrum. 3. Infrared Signals The third physical layer standard for 802.11 uses infrared signals that are diffused (the sender and receiver do not have to be aimed at each other). This technology works for about 10 m inside buildings only. 802.11 MAC Protocol: Collision Avoidance 802.11 uses a modified Ethernet protocol. Two problems arise which make the Ethernet modifications necessary:  The hidden node problem (A cannot see D in Figure 2) Using the standard Ethernet protocol, the hidden node problem will result in collisions that the sender cannot recognise.

Multiple Access with Collision Avoidance (MACA) 802.11 uses CA to solve these two problems. The sender and receiver exchange control frames before the sender actually transmits data so that all nearby nodes know that transmission is about to begin.  Sender sends a request to send (RTS) frame to the receiver. The frame includes the length of time the sender needs to transmit its data frame (or “hold the medium”).  Receiver replies with a clear to send (CTS) frame that echoes the length field back to the sender and anyone else who receives it.  If a node sees the CTS frame, it must not transmit during the time interval indicated in the length field (solving the hidden node problem).  If a node sees the RTS frame but not the CTS frame, it is free to transmit (solving the exposed node problem).  The receiver sends an ACK to the sender after successfully receiving a frame. All nodes wait for this ACK before trying to transmit.  If two senders transmit RTS frames at the same time, a collision happens. Collisions cannot be detected, but because the RTS frames are destroyed no CTS frames are ever sent. The sender’s time-out while awaiting the CTS frames, and must retransmit.

Ad hoc vs. Infrastructure Modes In ad hoc mode, all nodes communicate directly with each other. In infrastructure mode, nodes communicate via an access point (AP). FIGURE 3: Access points connected to a distribution network. Access points are connected to a wired network infrastructure. Access points are connected to each by a distribution system. The distribution system runs at layer 2 of the ISO architecture.

802.11 Frame Format FIGURE 5: 802.11 frame format.  Source Addr—48 bits  Destination Addr–48 bits  Data—up to 2312 bytes  32-bit CRC  Addr3—48 bits (described below)  Addr4—48 bits (described below) Not shown three subfields:  6-bit Type Field that specifies whether the frame carries data, RTS, CTS, or is being used by ToDS or FromDS  1-bit field called ToDS  1-bit field called FromDS DS Field Examples:If ToDS=0 and FromDS=0, the node is being sent directly from the source to the destination. (Addr1=target node; Addr2=source node)  If ToDS=1 and FromDS=1, the node is being sent through the distribution system (DS). (Addr1=ultimate destination; Addr2=immediate sender; Addr3=intermediate destination; Addr4=original source)