EA C451 (Internetworking Technologies) Vishal Gupta Department of Computer Science and Information Systems Birla Institute of Technology and Science, Pilani

Medium Access Control Protocol IEEE 802.11 Medium Access Control Protocol

Coordination function It is the logical function that determines when a station (STA) operating within a BSS is permitted to transmit protocol data units (PDUs) via the wireless medium (WM). The coordination function within a BSS may have one hybrid coordination function (HCF), or it may have one HCF and one point coordination function (PCF) and will have one distributed coordination function (DCF). A quality of service (QoS) BSS will have one DCF and one HCF.

MAC Protocol The 802.11 MAC protocol is built with the help of two coordination functions: Distributed Coordination Function (DCF) for traffic without QoS, referred to as asynchronous services Point Coordination Function (PCF) for traffic with QoS requirements, referred to as synchronous services.

Distributed Coordination Function A class of coordination function where the same coordination function logic is active in every station (STA) in the BSS whenever the network is in operation. The fundamental access method of the IEEE 802.11 MAC is a DCF known as carrier sense multiple access with collision avoidance (CSMA/CA). The DCF shall be implemented in all STAs, for use within both IBSS and infrastructure network configurations. CSMA/CA Procedure. The CSMA/CA protocol is designed to reduce the collision probability between multiple STAs accessing a medium, at the point where collisions would most likely occur. This is because CSMA/CA uses random back off procedure. The duration of this random time is determined as a multiple of a slot duration (aSlotTime). Each station maintains a so-called Contention Window (CW), which is used to determine the number of slot times a station has to wait before transmission.  To further strengthen, it uses RTS/CTS frames.

RTS/CTS Besides its basic use, it has following advantages: It performs a transmission path check by detecting CTS. The RTS/CTS mechanism may also improve operation in a typical situation where all STAs can receive from the AP, but may not be able to receive from all other STAs in the BSA.

IFS The time interval between frames is called the IFS. A STA shall determine that the medium is idle through the use of the Carrier Sense function for the interval specified. Five different IFSs are defined to provide priority levels for access to the wireless media. These are: a) SIFS: short interframe space b) PIFS: PCF interframe space c) DIFS: DCF interframe space d) AIFS: arbitration interframe space (used by the QoS facility) e) EIFS: extended interframe space

IFS aSlotTime The duration aSlotTime is used to calculate the IFSs. As the name indicates, aSlotTime is used during the Collision Avoidance (CA). In 802.11a, aSlotTime is 9 μs. As the name indicates, aSlotTime is used during the Collision Avoidance (CA). SIFS The SIFS is used to: prioritize the immediate Acknowledgment (ACK) frame of a data frame the response (Clear To Send (CTS) frame) to a Request To Send (RTS) frame a subsequent MPDU of a fragmented MSDU any frames of the AP during the Contention Free Period (CFP). SIFS is 16μs for 802.11a.

IFS PIFS  The PIFS is used by stations operating under the PCF to obtain channel access with highest priority. PIFS is calculated as PIFS = SIFS + aSlotTime and is 25 μs for 802.11a. DIFS  The DIFS is used by stations operating under the DCF to obtain channel access for frame exchanges. DIFS is calculated as: DIFS = SIFS+2. aSlotTime. DIFS is 34 μs for 802.11a. EIFS  The EIFS is used instead of DIFS whenever multiple stations initiated frame exchanges at different starting times. This occurs typically when these stations are hidden from each other. The EIFS is an extended interframe space resulting in a longer deferral from channel access, which gives other stations clearly a higher priority in medium access. EIFS is around 200μs for 802.11a.

IFS

Random backoff time It minimizes collisions during contention between multiple STAs that have been deferring to the same event. Backoff Time = Random() × aSlotTime where Random() = Pseudo-random integer drawn from a uniform distribution over the interval [0,CW], where CW is an integer within the range of values of the PHY characteristics. aCWmin ≤ CW ≤ aCWmax. aSlotTime = The value of the correspondingly named PHY characteristic.

The CW shall take the next value in the series every time anunsuccessful attempt to transmit an MPDU causes either STA retry counter to increment, until the CW reaches the value of aCWmax.

Fig: Increase of contention window size after unsuccessful frame exchanges.

NAV – Network Allocation Vector An indicator, maintained by each station (STA), of time periods when transmission onto the wireless medium (WM) will not be initiated by the STA whether or not the STA’s clear channel assessment (CCA) function senses that the WM is busy. It is virtual carrier sensing mechanism used with wireless network protocols such as IEEE 802.11 andIEEE 802.16 (WiMax). It is a logical abstraction which limits the need for physical carrier sensing at the air interface in order to save power. The MAC layer frame headers contain a Duration field that specifies the transmission time required for the frame, in which time the medium will be busy. The stations listening on the wireless medium read the Duration field and set their NAV, which is an indicator for a station on how long it must defer from accessing the medium. The NAV may be thought of as a counter, which counts down to zero at a uniform rate. When the counter is zero, the virtual CS indication is that the medium is idle; when nonzero, the indication is busy.

DCF access procedure Basic access The CSMA/CA access method is the foundation of the DCF. Basic access Basic access refers to the core mechanism a STA uses to determine whether it may transmit.

DCF access procedure Backoff procedure for DCF The backoff procedure shall be invoked for a STA to transfer a frame when finding the medium busy as indicated by either the physical or virtual CS mechanism. The backoff procedure shall also be invoked when a transmitting STA infers a failed transmission

DCF access procedure Setting and resetting the NAV STAs receiving a valid frame shall update their NAV with the information received in the Duration field for all frames where the new NAV value is greater than the current NAV value, except the NAV shall not be updated where the Receiver address is equal to the receiving STA’s MAC address.

Hidden Stations and RTS/CTS This problem arises when a station is able to successfully receive frames from two different stations but the two stations cannot detect each other. When stations cannot detect each other, a station may sense the channel as idle even when other hidden stations are transmitting. It may initiate a transmission while the other station is already transmitting. This may result in a collision. To reduce throughput reduction owing to hidden stations, 802.11 specifies as an option the exchange of Request-to-Send/Clear-to-Send (RTS/CTS) frames. Before transmitting a data frame, a station may transmit a short RTS frame, which must be followed by a CTS frame transmitted by the receiving station. Consecutive frames in the sequence of RTS, CTS, data, and ACK are spaced by an SIFS duration (16us for 802.11a)

Hidden Stations and RTS/CTS Timing of frame exchanges and NAV settings of the 802.11 DCF. Station 6 cannot detect the RTS frame of the transmitting station 2, but can detect the CTS frame of station 1.

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