The MAC layer controls the permission of the access medium channel for a node (or user) to transmit packets in a network, and this therefore allows the nodes to share the same physical medium. The MAC protocols are responsible for detecting or avoiding the collision of data packets, hence this reduces the probability of re- transmissions. Based on the application, the MAC protocols can be classi…ed into many categories. In this section, three main MAC protocols for wireless commu- nications are presented: Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), Distributed MAC Protocol and Central MAC Protocol.
2.3.1
Carrier Sense Multiple Access with Collision Avoid-
ance
CSMA/CA is a wireless network multiple access approach at the MAC layer in computer networks. CSMA/CA is used in the 802.11-based WLAN and can be considered as the original version of the Distributed Coordination Function (DCF) protocol. Three features of CSMA/CA are exploited: carrier sensing scheme, Re- quest_to_Send (RTS) and Clear_to_Send (CTS) exchange schemes, and collision avoidance.
Regarding the carrier sensing scheme, the transmitter gets feedback from the receiver which detects a carrier wave within coverage before transmission. If a carrier is detected (which means that the channel is busy), the transmitter will wait for a period and hold its own transmission. Once the channel within a carrier
sensing range is idle or free, the transmitter is permitted to use the channel for transmissions.
The RTS/CTS handshake scheme is an optional method to implement virtual carrier sensing in CSMA/CA, and it aims to reduce the collisions caused by the hidden-terminal problem in ad hoc networks. When a source node wishes to transmit a data packet, it …rstly sends a RTS packet. If the destination node receives the RTS packet successfully, a CTS packet is replied to the source node to con…rm that the destination node is ready to receive the data packet. On this basis, the source node needs to wait for a certain amount of time which should include both the RTS and CTS packet transmission intervals before trying to access the medium channel for data transmissions. This RTS/CTS handshake scheme is based on the assumption that all nodes have the same coverage in networks.
Another technique used in CSMA/CA is collision avoidance, which is used to reduce the probability of collisions that occur on the channel. If the channel is sensed as busy, the transmitter will be hold for a "random" interval before retrying transmission. Another version of CSMA is CSMA/CD (CSMA with collision de- tection) is also a modi…cation of pure CSMA. CSMA/CD is normally used in the Ethernet to terminate transmission when a collision is detected as to also reduce the probability of another collision occurring.
2.3.2
Distributed MAC Protocols
In the IEEE 802.11, the Distributed Coordination Function (DCF) is de…ned as the fundamental technique at the MAC layer to allow for the sharing of the medium channel between nodes in a network. Based on the CSMA/CA, the DCF employs its binary exponential backo¤ algorithm for nodes to contend channel access permission, and uses the RTS/CTS mechanism to avoid collisions from occurring.
However, DCF does not provide Quality of Service (QoS) (which is the require- ment of users from di¤erent aspects) for users to obtain high and low priority to access the channel early or not. Another limitation of DCF is that, when a node obtains permission to access the medium channel for transmissions, it will take over the channel as long as the duration of the transmission period lasts. If a low data rate is achieved at the transmission link, it will take a long time to send its pack- ets. Other nodes have to endure a long waiting period to obtain the permission for transmissions, resulting in ine¢ cient transmissions and low network throughput. The details of DCF will be presented and discussed in section 5.2.
In 2005, Enhanced DCF (EDCF) was developed in the IEEE 802.11e [13] as the enhancement of the initial DCF and was used to support service di¤erentiation using a distributed control. The EDCF is based on di¤erent priorities where tra¢ c is to be delivered with four access categories (ACs). Each AC achieves a di¤erent channel access, and the di¤erentiation is achieved through varying the length of the contention window and Arbitrary Interframe space (AIFS). Smaller AIFS and contention window sizes are used for high priority users to have a better chance of accessing the channel earlier. The EDCF provides 8 di¤erent tra¢ c classes (or 8 queues) in one station to support 8 user priorities. If there is more than one queue …nishing the backo¤ process at the same time, an internal collision occurs. In order to aviod this, a scheduler inside the station grants the Transmission Opportunity (TXOP) to the AC with the highest priority. Then, the highest priority frame is chosen to transmit and other lower class AC frames increase their backo¤ counters with corresponding contention window sizes. By using this approach, the highest priority AC frame is able to access the channel earlier.
In order to enhance the performance and achieve better throughput, packet bursting and Contention Free Burst (CFB) are used for EDCF in the 802.11e. Once a node is allowed to access the channel, it can send more than one frame without con- tending again. If a collision occurs, the packet bursting will be terminated. EDCF bursting reduces the network overhead, yet it also increases throughput through multiple transmissions.
2.3.3
Central MAC Protocols
The Point Coordination Function (PCF) is an optional MAC protocol used in the IEEE 802.11 to provide QoS for users. PCF is only available when nodes are con- nected to the network through an Access Point (AP). APs send beacon frames at regular intervals, and the PCF de…nes two periods between beacon frames, known as the Contention Free Period (CFP) and the Contention Period (CP).
Within the CP, DCF is adopted to provide distributed MAC control. Within the CFP, the AP sends Contention-Free-Poll (CF-Poll) packets to each node and enables nodes to send a packet. At the beginning of each contention-free period, the AP senses the medium channel. If the medium channel is idle after a PIFS interval, the AP sends a beacon frame that includes the CF Parameter Set element. When nodes receive the beacon, they update their Network Allocation Vector (NAV). After sending the beacon frame, the point coordinator transmits frames after waiting for
at least one SIFS interval. However, although the PCF provides QoS management, the achievable QoS is limited as the Tra¢ c Class (TC) is not de…ned in the PCF.
The Hybrid Coordination Function (HCF) combines the EDCF with generalized PCF features to simplify the 802.11e. Unlike the PCF, HCF uses the EDCF to all nodes during the CP, when the TC and the Tra¢ c Stream (TS) are de…ned. Furthermore, the nodes provide information on the lengths of their queues for each TC, and this information is used to give priority to nodes.