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The lower sub layer of the MAC is the Distributed Coordination Function (DCF) [34]. DCF is the basic and most prominent method in the MAC protocol to access the shared medium. It is a random access scheme based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism and uses a contention algorithm to provide access to the channel.

DCF employs two techniques for packet transmission: Basic Access (BA) mechanism and Request-To-Send/Clear-To-Send (RTS/CTS) mechanism. Basic Access mechanism is the default mechanism which uses binary exponential backoff rules for the management of hosts and retransmission of collided packets.

BA is a two-way handshaking technique and is characterized by the immediate transmission of a positive acknowledgement (ACK) by the destination station, upon successful reception of a packet transmitted by the sender station.

Source Destination Other stations DATA SIFS ACK DIFS Waiting Time Contention window (randomized back-off mechanism) Medium Busy DIFS Direct access if Medium is free ≥ DFS DIFS DATA Slot time

Figure 2.2: The Basic Access mechanism of DCF

A station with a new packet to transmit monitors the channel for a period of time. If the channel is idle for duration of Distributed Inter-Frame Space (DIFS), as shown in Figure 2.2, then the station transmits, otherwise if the channel is sensed busy (either immediately or during the DIFS duration) then the station continues to monitor the channel until it is idle for DIFS duration. When the situation is right, the station generates a random backoff interval before transmitting. This is the Collision Avoidance (CA) feature of the protocol. The backoff procedure also helps to avoid channel capture by allowing the stations to wait a random backoff time between two consecutive new packet transmissions, even if the medium is sensed idle in the DIFS time. The time immediately following the idle DIFS is slotted and

stations are allowed to transmit at the beginning of each time slot, which is dependent on the physical layer.

At each packet transmission the backoff time is uniformly chosen from the range of [0, 𝑊𝑖 − 1], where 𝑊 specifies the current Contention Window (CW) size and 𝑖

indicates the backoff stage. The value of 𝑊 depends on the number of transmission failures for a packet. At the beginning of transmission and in the first attempt the value of 𝑊 is set equal to the minimum contention window size. After each unsuccessful transmission the value of the contention window is doubled up to a maximum value of 𝑊𝑚 = 2𝑚𝑊 , where 𝑚 represents the backoff stage. When

reaching the maximum, size of the contention window remains at the value of 𝑊_𝑚 until the transmission is successful or the retransmission attempts reach a retry limit. As long as the channel is sensed idle, the backoff counter is decreased by one for each time slot. When a transmission is detected on the channel, the backoff counter is “frozen” and then reactivated when the channel is sensed idle

again for a period of DIFS. The station transmits when the channel counter reaches zero [34]. At the same time, other stations which are in the hearing distance of the transmission of the frame, update their Network Allocation Vector (NAV) to the expected period of time in which the wireless channel will be busy. This is known as Virtual Carrier Sensing mechanism. The stations start their backoff procedure either when the virtual carrier sensing or the physical carrier sensing indicate that the channel is busy [34].

Successful packet reception is signalled by the destination station via sending an ACK frame immediately after a Short Inter-Frame Space (SIFS) from complete reception. The SIFS duration plus the propagation delay is shorter than the DIFS

duration, therefore no other station is able to detect the channel idle for a DIFS until the end of the ACK. If the source does not receive the ACK frame within a specified ACK-Timeout period, or detects the transmission of a different packet on the channel, it reschedules for a retransmission of the packet based on the backoff procedure. The retransmission process continues until the retransmission limit is reached and by that time the packet is dropped.

DCF does not include a collision detection function (i.e., CSMA/CD) because collision detection is not practical on a wireless network. Another consideration in wireless networks is the problem of hidden terminals [35], which is the situation when a station is unable to detect a potential competitor for the channel because they are not within the transmission range of each other. The RTS/CTS shown in Figure 2.3 is a four-way handshaking technique implemented by DCF to help overcome the problem of hidden terminals. A station wanting to transmit a packet listens to the channel until it is sensed idle for a DIFS, then it follows the exponential backoff rule and then transmits a short frame called Request-To-Send (RTS). When the receiving station detects the RTS frame it responds after a SIFS with a Clear-To-Send (CTS) frame. The transmitting station then sends its packet if the CTS frame is received. The RTS and CTS frames carry information about the length of the packet to be transmitted. The listening stations can read this information and update their Network Allocation Vector (NAV). When detecting one frame among the RTS and CTS, a hidden station can delay further transmissions and thus avoid collision [8, 28, 36].

Source _RTS CTS Destination Other Stations NAV (RTS) DATA SIFS ACK NAV (CTS) Defer Access DIFS SIFS DIFS DATA Contention SIFS

Figure 2.3: The RTS/CTS mechanism of DCF