Media Access Control and Optimization

In MANET, use of broadcasting and shared transmission media introduces a nonnegligi-ble probability of packet collisions and media contention. In addition, with half-duplex ra-dio, collision detection is not possible, which severely reduces channel utilization as well

L1: Physical Layer

Figure 1.3. MANET network layers and research challenges.

Challenges in each layer Network Layers

as throughput, and brings new challenges to conventional CSMA/CD-based and MAC protocols in general. Among the top issues are the hidden-terminal and exposed-terminal problems.

The hidden-terminal problem occurs when two (or more) terminals, say, A and C, can-not detect each other’s transmissions (due to being outside of each other transmission range) but their transmission ranges are not disjoint [38, 152]. As shown in Figure 1.4, a collision may occur, for example, when terminal A and C start transmitting toward the same receiver, terminal B in the figure.

The exposed-terminal problem results from situations in which a permissible transmis-sion from a mobile station (sender) to another station has to be delayed due to the irrelevant transmission activity between two other mobile stations within sender’s transmission range.

Figure 1.5 depicts a typical scenario in which the exposed-terminal problem may oc-cur. Let us assume that terminals A and C can hear transmissions from B, but terminal A cannot hear transmissions from C. Let us also assume that terminal B is transmitting to terminal A, and terminal C has a frame to be transmitted to D. According to the CSMA scheme, C senses the medium and finds it busy because of B’s transmission, and, there-fore, refrains from transmitting to D, although this transmission would not cause a colli-sion at A. The exposed-terminal problem may thus result in loss of throughput.

1.4. TECHNICAL CHALLENGES AND RESEARCH OVERVIEW 19

A B C D

Figure 1.5. Exposed-terminal problem.

A B C

Figure 1.4. Hidden-terminal problem.

The very early access protocols such as Aloha, CSMA, Bram and, TDMA introduced in the 1970s [7] were primarily intended as solutions to multiaccess channels, such as any broadcast media, similar to early LANs, and quickly proved inadequate to effectively deal with the needs of current-day ad hoc network applications. The first protocols designed specifically for mobile and multihop mobile networks [37, 130, 137] were designed with tactical communication in mind and were based on slotted channels requiring rigid syn-chronization. As recent ad hoc technologies started to take shape, a very large number of new-generation ad hoc protocols such as MACA (multiple access with collision avoid-ance protocol), MACAW (MACA with CW optimization), FAMA (floor acquisition mul-tiple access), MACA/PR and MACA-BI (mulmul-tiple access with collision avoidance by in-vitation protocol) [39–44] have been proposed to resolve the various hidden-terminal, exposed-terminal and similar problems, and improve channel performance in MANET.

The key ideas behind these protocols involve sending RTS (request to send) and CTS (clear to send) packets before the data transmission has actually taken place [38]. When a node wishes to transmit a packet to a neighbor, it first transmits a RTS packet. The receiv-er then consents to the communication by replying with a CTS packet. On hearing the CTS, the sender can transmit its data packet.

For example, a virtual carrier sensing mechanism based on the RTS/CTS mechanism has been included in the 802.11 standard to alleviate the hidden-terminal problem that may occur by using physical carrier sensing only. Virtual carrier sensing is achieved by using two control frames, Request To Send (RTS) and Clear To Send (CTS), before the data transmission actually takes place. Specifically, before transmitting a data frame, the source station sends a short control frame, named RTS, to the receiving station, announc-ing the upcomannounc-ing frame transmission. Upon receivannounc-ing the RTS frame, the destination sta-tion replies by a CTS frame to indicate that it is ready to receive the data frame. Both the RTS and CTS frames contain the total duration of the transmission, that is, the overall time interval needed to transmit the data frame and the related ACK. This information can be read by any station within the transmission range of either the source or the destination station. Hence, stations become aware of transmissions from hidden stations, and the length of time the channel will be used for these transmissions.

However, studies [45, 46] show that when traffic is heavy, a data packet can still expe-rience collision due to loss/collision of RTS or CTS packets. To alleviate this problem, comprehensive collision-avoidance mechanisms have been introduced via a backoff mechanism. In principle, once a transmitting node senses an idle channel, it waits for a random backoff duration (determined by a contention window, and increasing exponen-tially with each reattempt) before attempting to transmit the packet, and congestion trol is achieved by dynamically choosing the contention window based on the traffic con-gestion situation in the network. Besides backoff methods, other mechanisms have also been proposed to address this problem. DBTMA (dual busy tone multiple access) [46]

provides a scheme whereby special signals called busy tones (BTt/BTr) are used to pre-vent other mobile hosts unaware of the earlier RTS/CTS dialogues from destroying the ongoing transmission. The distributed collision resolution protocol EMMCRR [50] uses power control and energy measurement techniques to achieve efficient collision avoid-ance; and in [38], a combination of RTS/CTS, power control and busy-tone techniques are used to further increase channel utilization.

In IEEE 802.11, CSMA/CA (Carrier Sense Multiple Access with Collision Avoid-ance), a variation of the MACA protocol, is used for the MAC layer, and DCF is used to provide collision avoidance and congestion control [47].

Besides collision avoidance, other optimization studies have been done in the MAC layer to improve MANET performance, including MAC improvement and algorithms used to reduce mobile node energy consumption [26, 38] as well as MAC optimizations for improving TCP layer performance.