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The following section describes the common Media Access Control layer used by the 802.11 family of standards.
The 802.11 family uses a MAC layer known as CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) NOTE: Classic Ethernet uses CSMA/CD - collision detection). CSMA/CA is, like all Ethernet protocols, peer-to-peer (there is no requirement for a master station).
In CSMA/CA a Wireless node that wants to transmit performs the following sequence:
802 11 also offers a polling mode (known as PCF - Point Co-ordination Function) which is fairly classic polling scheme e.g. 3270 bi-sync!! As with all polling protocols a single master (Base Station) is required.
To improve efficiency additional features are employed:
At the end of every packet the receiver, if it has successfully received the packet, will return an ACK packet (if not received or received with errors the receiver will NOT respond i.e. there is no NACK). The transmit window allows for the ACK i.e. CONTENTION period starts after the ACK should have been sent.
If no ACK is received the sender will retry the transmit (using the normal CSMA/CA procedures) until either successful or the operation is abandoned with exhausted retries.
Bit error rates on wireless systems (10**-5, 10**-6) are substantially higher than wire-line systems (10**-12). Large blocks may approach the number of bits where the probability of an error occurring may = 1 i.e. every block could fail including the re-transmission. To reduce the possibility of this happening large blocks may be fragmented by the transmitter and reassembled by the receiver node e.g. a 1500 byte block (12,000 bits) may be fragmented into 5 blocks of 300 bytes (2,400 bits). While there is some overhead in doing this - both the probability of an error occurring is reduced and, in the event of an error, the re-transmission time is also reduced.
The hidden node problem occurs in a point to multi-point network and is defined as being one in which three (or more nodes) are present. Node A, Node B and Node C. It is possible that in this case Node B can hear Node A (and vice versa) and Node B can hear Node C (and vice versa) BUT Node C cannot hear Node A. In a CSMA/CA environment Nodes A and C would both properly transmit (they cannot hear each other on the 'listen' phase so could both simultaneously and properly transmit a packet) but Node B would get corrupted data. Nodes A and C are said to be 'hidden' from each other.
Hidden Nodes are solved by the use of a RTS (request to send)/CTS (clear to send) protocol prior to packet transmission. In our three node network above Node A sends a small RTS packet which is heard by Node B which send a small CTS packet which is heard by both Nodes A and Node C. Node C will not transmit in this case.
CSMA/CA with RTS/CTS
Each node in a 802.11 network is identified by its MAC address (exactly the same as Ethernet a 6 byte - 48 bit value). Receiving nodes recognize their MAC address.
The following defines the format of an 802.11 packet (for 802.3 packet format see here)
|Frame Control||Duration ID||Address1
|2||2||6||6||6||2||6||0 - 2,312||4|
NOTE: Bits are numbered right to left (i.e. bit number is same as 2**n)
|15 - 14||Protocol version. Currently 0|
|13 - 12||Type|
|11 - 8||Subtype|
|7||To DS. 1 = to the distribution system.|
|6||From DS. 1 = exit from the Distribution System.|
|5||More Frag. 1 = more fragment frames to follow (last or unfragmented frame = 0)|
|4||Retry. 1 = this is a re-transmission.|
|3||Power Mgt. 1 = station in power save mode, 1 = active mode.|
|2||More Data. 1 = additional frames buffered for the destination address (address x).|
|1||WEP. 1 = data processed with WEP algorithm. 0 = no WEP.|
|0||Order. 1 = frames must be strictly ordered.|
|Duration ID||15 - 0||For data frames = duration of frame. For Control Frames the associated identity of the transmitting station.|
|Address 1||47 - 0||Source address (6 bytes).|
|Address 2||47 - 0||Destination address (6 bytes).|
|Address 3||47 - 0||Receiving station address (destination wireless station)|
|Sequence Control||15 - 0|
|Address 4||47 - 0||Transmitting wireless station.|
|Frame Body||0 - 2312 octets (bytes).|
|FCS||31 - 0||Frame Check Sequence (32 bit CRC). defined in P802.11.|
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