Yoshi where both of them use half-duplex devices and their roles in the protocol are not predefined.
But what if Zoya and Yoshi are not synchronized in using the slotted channel; can that help or make things even more difficult? In fact, the synchronous case can be seen as the worst case: if both Zoya and Yoshi decide to transmit in a certain slot, then their packets are completely overlapping. On the other hand, the lack of synchronism can be helpful in breaking the symmetry between Zoya and Yoshi. For example, we can have the situation in which both Zoya and Yoshi decide to transmit, but the transmission slot of Yoshi starts slightly later than Zoya's slot. Yoshi starts to receive the invite packet and postpones his transmission in order to complete the reception of Zoya's packet. In this example, Yoshi adjusts its clock to Zoya and, after the invite packet is received, Yoshi sends back a reply using Zoya's slot timing. It can be seen that, when the devices are not in synchrony, the symmetry can be broken without using randomization, as some form of randomization is already embedded in the asynchronism. However, recalling our discussion on a proper protocol design and controlled randomness, asynchronism can facilitate the rendezvous protocol, but it should not be the definitive solution; the protocol should always have the opportunity to rely on an intentional randomized choice.
Through the problem of first contact and link establishment, we have introduced randomization as the key idea used in breaking the symmetry among different wireless devices that want to access the same shared medium. In the next chapter we will see that randomization can also be useful for efficient medium access after the links have been initially established.
1.2.3 Rendezvous with Full-Duplex Devices
The problem of first contact becomes easier if the mobile devices are equipped with full-duplex capability. If Zoya and Yoshi both transmit simultaneously an invite packet to each other, then each of them will simultaneously receive the invite packet from the other device. Furthermore, in the next slot both Zoya and Yoshi transmit an invitation_accepted packet and, again, Zoya receives this packet from Yoshi and vice versa. With this, the link can be considered as being established. Therefore, full-duplex avoids the need for randomized assignment of transmit/receive roles and thus speeds up the procedure of link establishment. This may appear to be one of the most important advantages of full-duplex technology in scenarios in which fast link establishment and device discovery is of high importance.
We note that, even with full-duplex, there is still the problem of interference due to collision. Hence, full-duplex cannot help to solve the problem of colliding invite packets in Figure 1.3, as Zoya still does not get either of the two invite packets.
1.3 Multiple Access with Centralized Control
Referring again to Figure 1.2, let us assume that each of the mobile devices has established a link with Basil and we turn to the problem of actual data communication. Since in some examples we have to refer to a set of
1.3.1 A Frame for Time Division
Let us first consider downlink traffic, such that Basil has data to transmit to each of the devices. Then a straightforward idea is the one in which Basil divides the time into slots, where each slot has a duration of
(1.1)
This very simple fact is, surprisingly, often neglected in practice. For example, a wireless system may have a nominal speed of, e.g.,
(1.2)
Most of the time
