Supervisor: Kaisa Sere
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2.3 DiscussionThere are basically two types of MAC protocols: The first one solves shared medium access by letting terminals compete asynchronously and if we look globally at the network, it is a random terminal that is allowed to transmit when two or more nodes are competing. The second type of MAC protocol is such that it divides the medium so that it does not have to be competed for. This can be done by dividing transmission time (slots), a frequency band or codes between terminals and requiring them to synchronize in order to ensure that they never use the same slot, frequency or code. In ad hoc networks only terminals in neighborhood have to be synchronized, this is because terminals outside the neighborhood cannot interfere with each other. In [Bao+02] Garcia-Luna-Aceves and Bao name the first type “contention based” and the second “conflict free” protocols. The RTS-CTS based schemes are contention based and xDMA conflict free. As the amount of traffic increases in contention based protocols the amount of collisions will also increase. PAMAS and DBTMA take this into account by providing a different channel for control messages to avoid terminals interference, thus improving throughput. It is also important to differentiate between sender initiated and receiver initiated contention based protocols. To the group of receiver-initiated protocols belongs MACA-BI and MARCH, in which terminals “invite” senders to transmit at suitable time. Both of these try to make informed guesses about who has data to send, based on either meta data included in previous packets or on higher level routing information. Asynchronous, contention based protocol types have the following unwanted property: As network traffic increases, the amount of collisions will also increase, which in turn means that throughput degrades dramatically when more and more terminals are backing off and trying again. When terminals that have backed off try again, their transmission might collide with new traffic, adding to the number of terminals that are backed off and are waiting for access. This is the main problem that conflict free protocols tries to solve. There is a positive side to contention-based protocols and that is simplicity, they are relatively easy to implement. We can also say that conflict-free protocols are deterministic because we know for certain that a terminal will be allowed access at a certain point in time. Contention based protocols are, on the other hand, indeterministic due to the randomizing methods - we cannot always know which terminal will transmit first. We have not discussed many conflict free protocols because they have not been implemented for ad hoc networks and research is still on going. The conflict-free protocols can be both centralized and distributed. For instance, a Bluetooth piconet uses a conflict free centralized MAC protocol in which time slots are divided among terminals by the master. Also, cell based, single hop, mobile phone networks belong to this group. Interestingly, protocols which employ clusters that contain controlling masters are also said to be centralized [Lin+95]. Therefore, Bluetooth Scatternet would also qualify as a conflict free centralized MAC protocol. Distributed collision free protocols do not have a centralized master but terminals belonging to a neighborhood must agree on which xDMA resources to use, so that no terminal use the same. Therefore, the terminals are not globally synchronized, but inside a neighborhood they must be. Assigning xDMA properties to nodes in a network based on a collision free protocol is the equivalent of a well studied problem in graph theory called “the edge coloring problem” in which no two edges (links) may have the same color if they share a common vertex. Therefore, the analogue problem is that two neighbors adjacent of a common node must not use the same spreading code. A protocol called WAMIS (Wireless Adaptive Mobile Information System) is presented in [Lin+95]. It is also based on clusters (fully connected, i.e. every terminal has a link to every other terminal), but without cluster leadership (thus distributed). This protocol assigns a common spreading code to every cluster and if a terminal belongs to two clusters it is assigned two codes. Therefore, the second thing that must be ensured is that no two terminals ever transmit at the same time in a cluster. Intercluster conflicts will never arise since these use different codes. Inside a cluster a round-robin medium access scheme is enforced in order to provide transmission time fairly. A carrier sense method (CSMA-Round Robin) can be effectively used since no hidden terminal problems can exist inside a fully connected cluster (all terminals one hop away). WAMIS uses a clever prioritization scheme to enable time critical traffic to be scheduled first (i.e. voice calls). The first packet of a call uses CSMA-RR, but this packet can have a reservation control packet piggybacked, telling other terminals that it will be this terminals turn to transmit after a maximum time that can be tolerated with voice traffic. When link utilization is high, this protocol will resemble TDMA but without the overhead of being time synchronizing. Additionally, we mention that this protocol belongs to the group of conflict free, distributed protocols. Someone might argue that this is not a synchronized protocol, but this element can be found in the fact that clusters have to be synchronized in such a way that these never use the same code. Code assignment is a difficult issue in for mobile ad hoc networks and it can be done in the following ways [Hu93]:
Fig. 2 8: Classification of MAC protocols. To the group of hybrid MAC protocols we place HAMA. HAMA utilizes a contention-based scheme in the neighbor protocol and a conflict free protocol for data transmission, thus this is a hybrid protocol. A taxonomy graphic of MAC protocols is shown in figure 2-8. Download 298.62 Kb. Share with your friends:
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