The following section describes the common Media Access Control layer used by the802.11 family of standards.

The 802.11 family uses a MAC layer known as CSMA/CA (Carrier Sense MultipleAccess/Collision Avoidance) NOTE: Classic Ethernet uses CSMA/CD - collisiondetection). CSMA/CA is, like all Ethernet protocols, peer-to-peer (there is norequirement for a master station).

In CSMA/CA a Wireless node that wants to transmit performs the followingsequence:

I want to sell online some of the electronic stuff that I don't need anymore, such as my ASUS Wi-Fi router, and I'm wondering when I upload photos of it should I leave its serial number, MAC address, and pin code written on the back of the device visible or should I photoshop it out. The MAC address (short for media access control address) is the worldwide unique hardware address of a single network adapter. The physical address is used to identify a device in computer networks. Since MAC addresses are assigned directly by the hardware manufacturer, they are also referred to as hardware addresses.

1. Listen on the desired channel.
2. If channel is idle (no active transmitters) it sends a packet.
3. If channel is busy (an active transmitter) node waits until transmission stops then a further CONTENTION period. (The Contention period is a random period after every transmit on every node and statistically allows every node equal access to the media. To allow tx to rx turn around the contention time is slotted 50 micro sec for FH and 20 micro sec for DS systems).
4. If the channel is still idle at the end of the CONTENTION period the node transmits its packet otherwise it repeats the process defined in 3 above until it gets a free channel.

CSMA/CA Protocol

Key:

1. D = DCF Inter Frame Space (DIFS)
2. S = Short Inter Frame Space (SIFS)
3. CW = Contention Window
4. MPDU = MAC Protocol Data Unit
5. A = Ack

802 11 also offers a polling mode (known as PCF - Point Co-ordinationFunction) which is fairly classic polling scheme e.g. 3270 bi-sync!! As with allpolling protocols a single master (Base Station) is required.

To improve efficiency additional features are employed:

1. Positive Acknowledgement (ACK)
2. MAC level retransmission
3. Fragmentation

ACKing

At the end of every packet the receiver, if it has successfully received thepacket, will return an ACK packet (if not received or received with errors thereceiver 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.

MAC level retransmission

If no ACK is received the sender will retry the transmit (using the normalCSMA/CA procedures) until either successful or the operation is abandoned withexhausted retries.

Fragmentation

Bit error rates on wireless systems (10**-5, 10**-6) are substantially higherthan wire-line systems (10**-12). Large blocks may approach the number of bitswhere the probability of an error occurring may = 1 i.e. every block could failincluding the re-transmission. To reduce the possibility of this happening largeblocks may be fragmented by the transmitter and reassembled by the receiver nodee.g. a 1500 byte block (12,000 bits) may be fragmented into 5 blocks of 300bytes (2,400 bits). While there is some overhead in doing this - both theprobability of an error occurring is reduced and, in the event of an error, there-transmission time is also reduced.

The Hidden Node Problem

The hidden node problem occurs in a point to multi-point network and isdefined as being one in which three (or more nodes) are present. Node A, Node Band Node C. It is possible that in this case Node B can hear Node A (and viceversa) and Node B can hear Node C (and vice versa) BUT Node C cannot hear NodeA. In a CSMA/CA environment Nodes A and C would both properly transmit (theycannot hear each other on the 'listen' phase so could both simultaneously andproperly transmit a packet) but Node B would get corrupted data. Nodes Aand C are said to be 'hidden' from each other.

Use of RTS and CTS

Hidden Nodes are solved by the use of a RTS (request to send)/CTS (clear tosend) protocol prior to packet transmission. In our three node network aboveNode A sends a small RTS packet which is heard by Node B which send a small CTSpacket which is heard by both Nodes A and Node C. Node C will not transmit inthis case.

CSMA/CA with RTS/CTS

Key:

1. D = DCF Inter Frame Space (DIFS)
2. S = Short Inter Frame Space (SIFS)
3. CW = Contention Window
4. MPDU = MAC Protocol Data Unit
5. A = Ack

Node Identification

Each node in a 802.11 network is identified by its MAC address (exactly thesame as Ethernet a 6 byte - 48 bit value). Receiving nodes recognize their MACaddress.

Access Points

MAC Packet Format

The following defines the format of an 802.11 packet (for 802.3 packet formatsee here)

Values:

Why Is The Mac Address Required For Sender And Receiver For Wireless Microphone

Note: Bits are numbered right to left (i.e. bit number is same as 2**n)

Why Is The Mac Address Required For Sender And Receiver For Wireless Subwoofer

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