Wireless Connectivity. Petar Popovski

both packets are wasted. With full-duplex devices, the time consumed by a collision is at most a single packet duration, as Zoya and Xia interrupt transmission immediately after a busy tone is sent to them. If the RTS packet is much shorter than the data packet, then it is still useful to limit the maximal duration of the collision to be short, even if the devices have full-duplex capability. Full-duplex operation can also be beneficial to address the exposed terminal problem, Figure 2.6(d). As discussed above, Xia can start to transmit after receiving an RTS from Zoya and no CTS from Yoshi. When Zoya has finished transmitting, she sends a signal to issue a command to Xia to temporarily switch off her transmission, until Zoya receives an ACK from Yoshi. If Xia has a full-duplex device, then while still transmitting to Walt, Xia can detect the command sent by Zoya, suspend her transmission while Zoya receives an ACK, and continue transmitting afterwards.

2.5 Chapter Summary

      In this chapter we have used the dark room analogy to depict a situation in which the same wireless channel needs to be used for coordination and control of transmissions, as well as for the transmission of the actual data. This problem is addressed through the broad class of random access protocols. Two different paradigms for random access have been presented: protocols based on ALOHA and the tree-splitting protocols based on probing. By extending the communication model to introduce minislots, we have introduced the widely used mechanism of carrier sensing. Finally, the chapter presented some challenges and possible solutions to random access problems applied in a wireless multi-hop setting.

2.6 Further Reading

      The history of random access protocols is very rich, but also surprisingly vital in identifying new models, aspects and associated problems, for example related to the recent developments in massive communication for the IoT. It has started with the paper on ALOHA Abramson [1970], while the paradigm based on probing and splitting tree was introduced later on in Hayes [1978], Tsybakov and Mikhailov [1978] and Capetanakis [1979]. Detailed analysis of random access protocols can be found in Bertsekas and Gallager [1992] and Rom and Sidi [2012]. A beautiful example of modeling and analysis of random access protocols can be found in Bianchi [2000].

2.7 Problems and Reflections

      1 Random access over multiple channels. Let us consider a scenario in which a number of devices attempt to communicate with the base station Basil through random access. Assume that there are available communication channels. At a given instant a device or Basil can be active (transmit or receive) on only one channel. All devices and Basil are half-duplex. Propose a design of random access protocols for the following two cases:All channels are used for data transmission.Part of the channels are reserved for random access and coordination of the devices, while the remaining channels are used for data transmission.

      2 The room is not dark. In problem 1(b) it seems that we are departing from the dark room analogy, as there is a dedicated channel for reservation/signaling. Compare this to a classroom in which the students reserve a speech channel by raising a hand through the visual channel. Hence, this classroom scenario has different channels. Explain how the model from assignment 1. should be changed in order to represent correctly the communication model in the classroom.

      3 Detecting packet multiplicity. Consider an ALOHA type protocol with a single channel, but let us upgrade the communication model by assuming that, when more than one device transmits simultaneously and there is a collision, Basil can perfectly detect how many packets are present in the collision, but he cannot decode the packets. Propose a random access protocol that can utilize this upgraded model to improve the overall throughput when:Basil knows only the number of the packets involved in the collision, but not the identities of the devices that transmitted the packets.For each collision, Basil knows the identities of the devices that have transmitted the packets that constitute the observed collision.

      4 Errors beyond collisions. In order to make the collision model more realistic, let us assume that even when Zoya is the single device that transmits to Basil, her packet can be received with errors due to, e.g., noise, such that she needs to resend the packet. To make the things more challenging (and even closer to reality), assume that Basil cannot distinguish between a collision and a single packet that is in error due to noise. Analyze how the introduction of error in single packets affects:ALOHA type protocols.Probing and splitting-tree protocols.For both cases suggest a suitable re-design of the protocols.

      5 Longer sensing range. Consider the cases of multi-hop communication depicted in Figure 2.6. How would their operation change if we assume that the sensing range is:Two times longer than the communication range.Three times longer than the communication range.

Note

      1 1 One may object to this by noting that the successful reservation by Yoshi can be memorized for a future frame, where Basil allocates the data slot to Yoshi regardless of the actual outcome of the reservation slots in that particular future frame.