MAC Protocols

1. Introduction
In ad hoc networks, transmitters use radio signals for communication. Generally, each node can only be a transmitter (TRX) or a receiver (RX), one at a time. Communication among mobile nodes is limited within a certain transmission range. And nodes share the same frequency domain to communicate. So, within such ranges, only one transmission channel is used, covering the entire bandwidth.
Medium access control (MAC) protocols play an important role in the performance of the mobile ad hoc networks (MANETs). A MAC protocol defines how each mobile unit can share the limited wireless bandwidth resource in an efficient manner. The source and destination could be far away and each time packets need to be relayed from one node to another in multihop fashion, a medium has to be accessed. Accessing media properly requires only informing the nodes within the vicinity of transmission. MAC protocols control access to the transmission medium. Their aim is to provide an orderly and efficient use of the common spectrum. These protocols are responsible for per-link connection establishment (i.e., acquiring the medium) and per-link connection cancellation (i.e., releasing the medium).
One of the fundamental challenges in MANET research is how to increase the overall network throughput while maintaining low energy consumption for packet processing and communications. The low throughput is attributed to the h arsh characteristics of the radio channel combined with the contention-based nature of MAC protocols commonly used in MANETs.
The following performance measures should be considered in the MAC protocol for a wireless mobile ad hoc network:
Throughput and delay: Throughput is generally measured as the percentage of successfully transmitted radio link level frames per unit time. Transmission delay is defined as the interval between the frame arrival time at the MAC layer of a transmitter and the time at which the trans mitter realizes that the transmitted frame has been successfully received by the receiver.
• Fairness: fairness measures how fair the channel allocation is among the flows in the different mobile nodes. The node mobility and the unreliability of radio channels are the two main factors that impact fairness.
• Energy efficiency: is measured as the fraction of the useful energy consumption (for successful frame transmission) to the total energy spent.
• Multimedia support: This is the ability of a MAC protocol to accommodate traffic with different service requirements, such as throughput, delay, and frame loss rate.

2. Design goals of the MAC protocol
• The operation of the protocol should be distributed.
• The protocol should provide QoS support for real-time traffic.
• The access delay, which refers to the average delay experienced
by any packet to get transmitted, must be kept low.
• The available bandwidth must be utilized efficiently.
• The protocol should ensure fair allocation of bandwidth to nodes.
• Control overhead must be kept as low as possible.
• The protocol should minimize the effects of hidden and exposed terminal problems.
The protocol must be scalable to large networks.
• The protocol should have power control mechanisms.
• The protocol should have mechanisms for adaptive data rate control.
• The protocol should try to use directional antennas.
• The protocol should provide synchronization among nodes.

3. Hidden- and exposed-terminal problems
As illustrated in Figure 1, node B is within the range of nodes A and C, but A and C are not in each other’s range. Let us consider the case where A is transmitting to B. Node C, being out of A’s range, cannot detect a carrier and may therefore send data to B, thus causing a collision at B. This is referred to as the hidden-terminal problem, as nodes A and C are hidden from each other.
Figure 1 Illustration of hidden- and exposed-terminal problems.
Let us now consider another case where B is transmitting to A. Since C is within B’s range, it senses a carrier and decides to defer its own transmission. However, this is unnecessary because there is no way that C’s transmission can cause any collision at receiver A. This is referred to as the exposed-terminal problem, since B’s being exposed to C caused the latter to defer its transmission needlessly. MAC schemes are designed to overcome these problems.

4. Classification of MAC Protocols
This section describes the classification of MAC protocols and the various factors considered for classification. Various MAC schemes developed for wireless ad hoc networks can be classified as shown in Figure 2.

Figure 2 Classification of MAC protocols.

4.1. Contention-Based MAC Protocols
These protocols concentrate on the collisions of transmitted data. This includes two categories: random access and dynamic reservation/collision resolution protocols.
1. With random access-based schemes, such as ALOHA, a node may access the channel as soon as it is ready. Naturally, more than one node may transmit at the same time, causing collisions. ALOHA is more suitable under low system loads with large numbers of potential senders and it offers relatively low throughput. A variation of ALOHA, termed slotted ALOHA, introduces synchronized transmission time slots similar to TDMA. In this case, nodes can transmit only at the beginning of a time slot. The introduction of time slots doubles the throughput as compared to the pure ALOHA scheme, with the cost of necessary time synchronization. The CSMA-based schemes further reduce the possibility of packet collisions and improve the throughput.

2. Dynamic reservation/collision resolution protocols: To solve the hidden- and exposed-terminal problems in CSMA, researchers have come up with many protocols that are contention based but involve some forms of dynamic reservation/ collision resolution. Some schemes use the request-to-send/
clear-to-send (RTS/CTS) control packets to prevent collisions (e.g., multiple access collision avoidance [MACA] and MACA for wireless LANs [MACAW]).