Methods of switching and data transmission
In large networks it is impossible to provide each pair of subscribers with its own communication channel for permanent ownership. Therefore, when organizing a channel, any method of temporary connection of communication lines between individual nodes of the network, implemented with the help of switches, is always used. Subscribers are connected to the switches by individual communication lines, each of which is used at any time by only one subscriber assigned to it. Between the switches (nodes of the network), the lines are separated by several subscribers, i.e. are used together. There are two common methods:
• Circuit Switching, in which the channel is created for the duration of one communication session between two subscribers;
• Packet Switching, when a channel is created to transfer one or more data packets.
We reveal the features of the channel switching method using the network example (Figure 9.9), accompanying the presentation with explanations from the telephony area. In this method of switching, a composite physical channel between two subscribers is formed by the serial connection of individual transmission lines using switches, for which, first, one of the subscribers performs the connection establishment procedure.
Let's assume that the subscriber A 1 wants to establish communication with the subscriber B1. In this case, the following sequence of actions is performed:
• the subscriber (node) A sends a request to establish a connection to the switch K1 with the destination address of the subscriber B (call by phone number);
• Switch K1 selects a route to create a composite channel and sends the request to the next switch, for example, K2, which transmits the request to switch K4, which sends it to subscriber B1;
Fig. 9.9. Illustration of the principle of circuit switching
• Subscriber B1, having accepted the connection request, sends a response to the already established channel to the original node A1, after which the connection establishment procedure is completed and the nodes A1 and 51 can exchange data over it (conduct a telephone conversation).
Thus, to create a channel, the request must itself go through the entire route from A1 to 51 to make sure that all necessary communication lines, as well as the end node 51 are free. When the route passes in each of the switches K1, K2, K4, the connection of the input to the required output is made and information is stored that the corresponding communication line is allocated (reserved) for connection of the subscribers L1 and L1. For example, in switch K1, input 1 is connected to output 2, and in switch K2, depending on the operation algorithm, connection of input 5 to outputs 2 or 3 is possible.
An important feature of circuit switching is the possibility of failure in establishing a connection. Failure is fixed if the called subscriber is busy or all possible routes. If after the communication between subscribers A1 and B2 is established, two requests from subscribers A2 and A3 to establish connections with subscribers B2 and B3 are received, then they can be satisfied, but not via switch K2, but only through a short-circuit switch. If, then, the request for connection of subscribers A4 and B4 follows, then it will not be executed because of the shortage of communication lines. There are only three bonds between K1 and K4: one bond K1 → K2 → K4 and two connections K1 → K3 → K4. Thus, the more currently installed in the network connection, the higher the probability of failure in the request to establish a new connection.
After the connection of subscribers A and B is established, a channel with a fixed bandwidth arrives at their full disposal. Subscribers can not transfer data to the network at a speed exceeding the capacity of the line. At a lower transmission speed, the bandwidth of the channel is underutilized.
As the networks with the switching in question allow simultaneous data transfer of several subscriber channels between the switches (K1 and K4), multiplexing can increase the bandwidth of these channels. For example, by combining three communication lines into one channel, you can triple the transfer rate between subscribers of switches K1 and K5. Multiplexing in circuit-switched networks has features, due to the fact that all the connected communication lines must go along the same route. Currently, two technologies are used to multiplex subscriber channels (Figure 9.10): frequency division multiplexing (FDM) and time division multiplexing (TDM).
Fig. 9.10. Illustration of the principle of the frequency (a) and temporary (b ) multiplexing (splitting) channels
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