CSMA/CA

IEEE 802.11 standard for WLAN defines a distributed coordination function (DCF) for sharing access to the medium based on the CSMA/CA protocol Collision detection is not used since a node is unable to detect the channel and transmit data simultaneously A node listens to the channel before transmission to determine whether some one else is transmitting The receiving node sends an acknowledge packet (ACK) a short time interval after receiving the packet If an ACK is not received, the packet is considered lost and a retransmission is arranged Information

DCF basic access

DCF consists of a basic access mode as well as an optional RTS/CTS access mode In basic access mode the node senses the channel to determine whether another node is transmitting before initiating a transmission If the medium is sensed to be free for a DCF inter-frame space (DIFS) time interval the transmission will proceed If the medium is busy the node defers its transmission until the end of the current transmission and then it will wait an additional DIFS interval and generate a random backoff delay uniformly chosen in the range [0, W − 1] where W is called the backoff window or contention window (CW)

DCF basic access

The backoff timer is decreased as long as the medium is sensed to be idle for a DIFS, and frozen when a transmission is detected on the medium, and resumed when the channel is detected as idle again for a DIFS interval When the backoff reaches 0, the station transmits it packet For IEEE 802.11 time is slotted in a basic time unit which is the time needed to detect the transmission of a packet from any other station The initial CW is set to W = 1, if two or more nodes decrease their backoff timer to 0 at the same time a collision occur, at this situation the CW is doubled for each retransmission until it reaches a maximum value

Exponential backoff scheme

Whenever a backoff occurs the backoff time is uniformly chosen in the range [0, W − 1] After each unsuccessful transmission the backoff windows size is doubled, up to a maximum value Once the backoff window size reaches its maximum value it will stay at that value until it is reset The value of W will be reset after every successful transmission of a data or RTS packet, or when a retry counter reaches its limit Since W is used to control the backoff counter its value will affect the performance of the DCF protocol, improvements can be made by choosing better update rules than in IEEE 802.11 standard

WLAN operating modes

The IEEE 802.11 standard specifies two operating modes, either ad hoc mode (peer-to-peer) or infrastructure mode (peer-to-AP) The former is used when connecting a number of wireless nodes, e.g. for a temporary network at a meeting or for connecting a few wireless appliances The latter have one special node, the Access point (AP) which is an Ethernet switch and provides access to a subnet with wire access In infrastructure mode all communication between nodes or between a node and the wired network go through the AP.

WLAN operating modes

A basic service set (BSS) are a collection of nodes that have recognized each other and established communication It is also possible to have an extended service set (ESS) consisting of a series of overlapping BSSs (each containing an AP) and the APs are connected together by means of a Distribution System (DS) which in practice is an Ethernet LAN but could be any type of network All nodes within a BSS are synchronized by periodic transmission of time stamped beacon packets, in infrastructure mode the AP is the timing master and generates all timing beacons.

Maximum throughput 802.11

Assuming one peer communicating with an AP using TCP protocol (e.g. transferring file with ftp, fetching web-page with http). Assume that each TCP data packet is followed by a TCP ACK packet. To transfer the data segment there will be Silence during at least one DIFS slot, signaling that the medium is available. More than one if the node is executing back-off. The data frame containing the TCP data. A SIFS gap between data frame and 802.11 ACK frame. The 802.11 ACK frame.

Maximum throughput 802.11

To transfer the TCP ACK packet there will be Silence during at least one DIFS slot, signaling that the medium is available. More than one if the node is executing back-off. The data frame containing the TCP ACK. A SIFS gap between data frame and 802.11 ACK frame. 802.11 ACK frame. In addition to the payload data, the data frame has additional 36 bytes of data, out of those 28 bytes are 802.11 MAC header for various control and management, error detection and addressing. The other 8 bytes are a header to identify network layer protocol. Information Networks

Maximum throughput 802.11

To transfer 1460 bytes of payload, we have an packet with 1500 bytes of data including the TCP/IP headers, and 1536 bytes including the MAC and SNAP header. For the TCP ACK packet of 40 bytes TCP/IP header, there is also an additional 36 bytes for MAC and SNAP header, so total 76 bytes for the TCP ACK. We neglect media contention and backoff times and retransmissions