Exposed terminal problem
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In wireless networks, the exposed node problem occurs when a node is prevented from sending packets to other nodes due to a neighboring transmitter. Consider an example of 4 nodes labeled R1, S1, S2, and R2, where the two receivers are out of range of each other, yet the two transmitters in the middle are in range of each other. Here, if a transmission between S1 and R1 is taking place, node S2 is prevented from transmitting to R2 as it concludes after carrier sense that it will interfere with the transmission by its neighbor S1. However note that R2 could still receive the transmission of S2 without interference because it is out of range from S1.
IEEE 802.11 RTS/CTS mechanism helps to solve this problem only if the nodes are synchronized. When a node hears an RTS from a neighboring node, but not the corresponding CTS, that node can deduce that it is an exposed node and is permitted to transmit to other neighboring nodes. If the nodes are not synchronised the problem may occur that the sender will not hear the CTS or the ACK during the transmission of data of the second sender.
MEDIUM ACCESS CONTROL TECHNIQUES
The medium access problem for wireless networks has received a great deal of attention in recent years; Chandra, Gummalla, and Limb is a good review of protocols proposed in the literature.
ALOHA
The ALOHA system is generally described as the first wireless computer communication system employing random access. The ALOHA network uses a star topology, consisting of a central computer and several remote user sites, and employs two physical channels: one for outbound transmissions and one for inbound transmissions. The remote sites accessed the central computer on the inbound channel in a completely asynchronous manner. Packet collisions, when they occurred, were handled by retransmissions after a random backoff time. If the offered traffic is assumed to be of a Poisson distribution, it can be shown that the channel throughput rate is Ge−2G, where G is the mean offered traffic rate (in packets per unit time); ALOHA thus has a maximum throughput of 1/(2e) ≇ 0.184.
This relatively low value has lead to criticism of ALOHA for its poor channel efficiency, and a search for protocols, such as slotted ALOHA, having higher throughput. However, this statement is usually made in the context of an infinite channel signal-to-noise power ratio. Abramson, considering the more realistic case of finite power ratios, notes that
[W]e see that the channel efficiency of an ALOHA channel approaches one for the important case of small values of throughput and small values of the signal-to-noise power ratio. In other words, under these conditions it is not possible to find a multiple access protocol which has a higher capacity for a given value of average power and a given bandwidth.
