Petar Popovski - Wireless Connectivity

The circuit-switched allocation has an additional degree of flexibility, since Basil decides when to set the transmission timing by sending the headers at appropriate instants. Even more, the circuit-switched allocation is “realistic” in a sense that it does not start in the infinite past, but instead it starts with a link establishment and ends with a link termination.

The reader can easily extend the system design to address the case of multiple base stations that have overlapping communication areas, as for the example depicted in Figure 1.3(a). In this case, a frame header should also contain an address that identifies the base station sending the header. There is again the issue of collision between the frames, observed at a device that is in the communication range of both base stations. As discussed in the problem of link establishment, this issue can be solved by randomization.

The system design presented above is not a proper one that can thrive in real life, but rather a sketch of a system that works reasonably well under the assumed collision model. Let us first define Basil's cell to be the area around the base station Basil in which a terminal is in the communication range of Basil. Using our simple collision model, the cell is a circular area.

We can now state that a condition for correct operation of the described TDMA scheme is that any terminal connected to Basil should remain in Basil's cell until Basil decides to terminate the connection. This condition is somewhat strange with respect to the way a link is terminated, since it does not consider the wishes of the terminal. In other words, Basil may decide to terminate the connection to Walt, although Walt may have more data to send in the uplink. However, this is not critical for the overall system operation, as Basil can continue to use the same TDMA structure, substituting Walt with another terminal. What is critical is the case in which Walt has an active link with Basil and Walt walks out of Basil's cell. With the protocol specified above, this leads to a rather fatal system error: the slot allocated to Walt will remain unused forever, as Basil has not terminated the connection and the slot for Walt stays reserved, potentially forever. A practical fix to this situation could be to introduce a certain timeout mechanism : if Basil does not hear from Walt for a certain time and several consecutive time slots allocated for uplink transmission to Walt are silent, then Basil considers Walt to be out of the network and makes Walt's logical channel available to another terminal.

This is still not sufficient to ensure a system design that is robust in practice. Take the following situation: Walt walks temporarily out of reach of Basil but he is back after the timeout has passed. Now Walt does not know that his slot has been allocated to someone else, which may lead to collision in the uplink transmissions made by Walt. The system design can be further patched in different ways in order to deal with this challenge. One solution is that Walt also uses a timeout mechanism, such that if Walt does not send anything to Basil for a time longer than the timeout, then both Basil and Walt claim the link to be non-existent. With this, Walt now knows that he needs to go again through the link establishment procedure.

Alas, this patch is still not sufficient. Recall that the collision model is only a model of reality, but does not fully grasp the practical conditions. One such practical condition is that, even in the absence of collision, the packet is not always received correctly by a receiver that is in the communication range. For example, several consecutive transmissions of Walt may be received incorrectly by Basil due to random noise. In such a situation, Basil starts the timeout for deciding link termination, but Walt does not. This can lead to inconsistent perception of the link between them, since Basil thinks the link is terminated, but Walt thinks the opposite. Yet another patch to the system design can be to use a mechanism based on two-way transmissions between Basil and Walt to check if the link is alive.

We could largely broaden this discussion by spotting other practical deficiencies and finding out suitable patches to the system design. The objective here is not to make a full real-life protocol, but rather illustrate how a simple protocol specification can operate under certain assumptions. However, this protocol needs to be enriched in order to be robust to other practical issues, even for ones that have a very low probability of occurrence.

The introduction of a frame header for the MAC protocol relaxes, to some extent, the strict constraints of the circuit-switched operation. Note that the usage of the communication channel as a system resource is not deterministic between the moment the link is established and the moment it is terminated. This is because Basil has the freedom to determine when a certain frame should start, after which Zoya reads from the frame header which of the slots that follow that frame header are allocated to her. Zoya does not have a predefined, absolute, time instant to use a communication resource and needs to receive some form of command from Basil associated with a particular physical slot in which she will send or receive data. Therefore, the transmissions of Basil should contain control information or control signaling, which we have also termed metadata . This type of operation can be characterized as packet-switched operation , which stands in contrast to a circuit-switched operation.

In the described rudimentary system, one can already spot the main trade-off between circuit-switched and packet-switched operation. In circuit-switched operation, signaling is minimized at the expense of losing flexibility. On the other hand, frequent transmission of control signaling or metadata in packet-switched operation introduces overhead , which can be considered a waste, since it does not represent data that is of use to the end user. However, the metadata can be used to describe changes in the operation mode, such as a new allocation of the resources. This enables Basil to adapt the allocation of the slots to the current traffic demand from the users and thus offers flexibility advantage over the circuit-switched operation. Note that here we speak about circuit- and packet-switched operation in order to describe the possible ways in which the MAC protocol can use the shared communication channel. Nevertheless, the concepts of circuit-switched, which stands for inflexible but low-overhead operation, and packet-swtiched operation, which stands for flexible, but high-overhead operation, is universal and applicable to all communication protocols.

Strictly speaking, the usage of a frame header in the MAC protocol of the previous section is not packet-switched, since there is no separate control signaling sent for each packet, but the header is valid for all the packets transmitted in a frame. This is useful to note, because it illustrates, in a simple way, the main principle that can be used to get the desired trade-off of signaling and flexibility. Namely, sending a common frame header is a kind of a hybrid design, by which a portion of control signaling is used as a metadata for several packets. On the one hand, the signaling overhead is decreased by increasing the number of packets associated with a signaling information. On the other hand, more flexible allocation requires control signaling to be used more frequently, which would increase the overall signaling overhead.

The portion of the TDMA channel allocated to a device does not need to be constant from the instant of link establishment and until the disconnection of the device. Furthermore, there is no need to divide the total amount of communication resources equally among the users. The latter one has already been illustrated in Figure 1.5(c), where Zoya, Yoshi, and Xia get different amounts of resources in a frame. It needs to be noted that there is no header in Figure 1.5(c), but, based on the previous discussion, the reader can easily add it. The common feature of these examples is that, once established, the allocation of communication resources is fixed for all the frames.