alt="images"/>. However, a malicious user can set his device to wait for a time less than and in this way always gain an advantage in accessing the shared medium.
Transmission of feedback has a special role if we assume that the devices have the full-duplex capability. Namely, full-duplex enables the use of CSMA with collision detection (CD), which was widely used in the early days of wired ethernet. Consider the collision of and from Figure 2.4(b) and let us assume that a single minislot is sufficient for the receiver to determine whether the received signal is a single transmission or collision among multiple packets. Then after the first minislot in which and are overlapping, Yoshi and Walt start to transmit a signal termed busy tone. Since Zoya and Xia can receive while transmitting, they will both detect a busy tone. To be precise, each of them will detect a collision of the two busy tones sent by Yoshi and Walt. The introduction of busy tone is another enrichment of our communication model, as it is a special signal that only tells that the medium is busy and does not carry any additional information. This enables us to assume that a collision of busy tones is a special case of collision that does not lead to error, but it can be again treated as a busy tone signal. Therefore, at the end of the first minislot during which and are colliding, both Zoya and Xia detect a busy tone and stop their transmission. This saves two minislots that are otherwise wasted in collision in Figure 2.4(b); instead, Zoya and Xia can already, after receiving the busy tone, make a random choice on which minislots to retransmit their packets. It can thus be concluded that the capability of collision detection, unleashed by full-duplex, enables early termination of the collisions and thus brings performance gain in addition to the gain brought by carrier sensing.
2.4 Random Access and Multiple Hops
The scenario from Figure 2.4 in which all nodes are in range of each other is rather limiting. Carrier sensing works well in that scenario because each transmitter and the corresponding receiver are able to receive the same signals from external transmitters; for example, both Zoya and Yoshi are in range of Xia. However, if the positions of the communication nodes are changed, while still applying the simple model based on a strict definition of a communication range , then we arrive at a completely new setting from a system viewpoint. Some of the possible communication/interference configurations that can be obtained in this way are depicted in Figure 2.6. There are other possible configurations that are not depicted in the figure, but, due to symmetry, they can be analyzed in an analogous manner. Compared to the all-in-range scenario, also known as single-hop, in each of the scenarios in Figure 2.6 there are at least two nodes that are outside the communication range, such as Zoya and Xia in Figure 2.6(a). The name multi-hop attached to the scenarios in Figure 2.6 comes from the following. If Zoya would like to communicate with Xia, Zoya sends first her packet to Walt and then Walt sends it to Xia. If we say that one link corresponds to one transmission hop, then Zoya can reach Xia through multiple hops.
Figure 2.6 Different multi-hop configurations that can arise if not all nodes are in range. (a) The hidden terminal problem for two interfering links. (b) The classical hidden terminal problem. (c) Zoya and Xia are hidden from each other, but only Walt can experience collisions due to the hidden terminal problem. (d) The exposed terminal problem. (e) Multi-hop structure in which carrier sensing still works well.
In the discussion that follows we stick to the communication model for which the communication range and the interfering range are the same. Figure 2.6(a) depicts the basic difficulty posed in carrier sensing in a multi-hop spectrum sharing setting, termed the hidden terminal problem. Zoya and Xia are not in range and thus they cannot sense each other's carriers. If Zoya starts a transmission to Yoshi, Xia thinks that the medium is still idle and starts transmission to Walt, collisions occur at Yoshi and Walt, and both packets are lost. Carrier sensing does not help here. Figure 2.6(b) depicts the standard defining scenario for a hidden terminal, where both Zoya and Xia try to send to the same receiver Yoshi, but they are hidden from each other. In Figure 2.6(c), Zoya and Xia are hidden from each other, and Yoshi is outside the range of Xia. Zoya and Xia cannot apply carrier sensing with respect to each other and can thus end in a situation where they transmit simultaneously. However, in that case only Walt experiences a collision, but not Yoshi.
One can extend the communication model by assuming that the carrier sensing range is larger than the communication range, as already mentioned before when the carrier sensing was introduced. In a model where the carrier sensing range is larger than the communication range, the hidden terminal problem can be mitigated. Furthermore, if the carrier sensing range is sufficiently larger than the communication range, then it can happen that even if the terminals are in a multi-hop setting with respect to the communication range, they are still in a single-hop setting with respect to carrier sensing.
Nevertheless, it is not only the absence of carrier sensing that can cause problems in a multi-hop setting. Figure 2.6(d) illustrates what is known as the exposed terminal problem. Zoya and Xia are in range, such that they can inhibit each other by using carrier sensing. Let us look at the example in Figure 2.4(b): when arrives, Zoya is already transmitting , such that Xia defers her transmission until one idle slot after the transmission of is over. However, if the physical positions of the nodes are the ones depicted in Figure 2.6(d), then Xia can start transmitting while Zoya sends and both and will be received correctly. In short, the exposed terminal problem is manifested by unnecessary inhibition of a terminal that can, in fact, transmit.
Not all multi-hop settings exhibit problems with carrier sensing; the reader can verify that it works correctly for Figure 2.6(e), although Yoshi cannot be interfered by Xia.
alt="images"/>. However, a malicious user can set his device to wait for a time less than 