Читать книгу Wireless Connectivity - Petar Popovski - Страница 26

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.