or not. This feedback packet from Yoshi, denoted by
, can, in principle, carry a single bit of information, either acknowledging (ACK) the packet reception or sending a negative ACK (NACK). The latter is sent in the case there is a collision and thus an incorrect reception. The feedback packet is, generally, much shorter than the data packet.
Figure 2.5(a) shows the transmission of the feedback packet from Yoshi for the case when the channel is slotted. Differently from the slotted model used before, where the slot duration was equal to the packet duration, here the slot duration is longer than the duration of Zoya's packet. The remainder of the slot, after the end of Zoya's packet, is reserved for Yoshi to transmit the ACK message , while Zoya is in a listening mode. Referring to Figure 2.4(a), Yoshi receives successfully and sends ACK. The packet is not received correctly and Yoshi sends NACK. If we assume that all nodes are in each other's communication range, then at the same time when Walt sends NACK to Xia, collision occurs at the receiver of Zoya, as well as that of Xia. As a result, Zoya does not receive the NACK. To avoid this type of situation, one can use randomized contention for sending the feedback packets, which is a contention that occurs in addition to the contention used for data packets. Nevertheless, in our communication model, Zoya can receive NACK also implicitly, through the absence of ACK. In other words, the mere existence or absence of can be understood as a single bit of information sent from Yoshi to Zoya. If Yoshi does not receive the data packet correctly, he “sends” NACK by not transmitting .
from Yoshi to Zoya for (a) a slotted channel, (b) carrier sensing.
Figure 2.5(b) shows how the transmission of a feedback packet would operate in a CSMA setting. As in CSMA the basic time unit is a minislot and not a slot; we cannot say that the end of the slot is reserved for transmission of a feedback packet. Note that Yoshi, after receiving , does not wait for an idle minislot and he immediately sends to Zoya. Xia, and potentially other contending transmitters, will detect that the medium is busy and postpone her transmission to start after is followed by an idle slot. We can interpret this as if the feedback packet has a higher priority over a data packet sent by another node.
This observation reveals the inherent capability of the CSMA mechanism to introduce different priority classes. In the simplest case, there can be two classes of traffic: high and low importance, respectively. Then the protocol can be designed such that a high-priority data uses a single minislot for carrier sensing, while low-priority data uses two minislots for carrier sensing. Let, for example, Zoya send a high-priority packet that corresponds to some data for critical control system managed by Yoshi, while Xia sends to Walt data from, for example, an entertainment service. After the busy medium is released, then Zoya will always be the first to start a transmission after a single idle minislot, while Xia will defer her transmission. On another note, this property of CSMA can be misused by a malicious user. For example, the CSMA protocol can be specified such that the minimum idle time before the transmission is . 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
