Читать книгу Wireless Connectivity - Petar Popovski - Страница 48

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.