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Then the network encounters an error and simultaneously sends reset indications to A and B, which both respond to. Finally, B requests a disconnection, which is
indicated to A, and terminates the connection.
4.2. Switching Methods
Switching is the generic method for establishing a path for point-to-point communication in a network. It involves the nodes in the network utilizing their direct
communication lines to other nodes so that a path is established in a piecewise fashion see Figure 4.38. Each node has the capability to ‘switch’ to a neighboring
node i.e., a node to which it is directly connected to further stretch the path until it is completed.
Figure 4.37 A ‘switched’ path.
One of the most important functions of the network layer is to employ the switching capability of the nodes in order to route messages across the network.
There are two basic methods of switching: circuit switching and packet switching. These are separately discussed below.
4.2.1. Circuit Switching
In circuit switching, two communicating stations are connected by a dedicated communication path which consists of intermediate nodes in the network and the
links that connect these nodes. What is significant about circuit switching is that the communication path remains intact for the duration of the connection, engaging the
nodes and the links involved in the path for that period. However, these nodes and links are typically capable of supporting many channels, so only a portion of their
capacity is taken away by the circuit.
Figure 4
.
38 shows a simple circuit switch which consists of a 3×3 matrix, capable of connecting any of its inlets a, b, and c to any of its outlets d, e, and f.
Each crosspoint appears as a circle. A hollow circle means that the crosspoint is off i.e., the two crossing wires are not connected. A solid circles means that the
crosspoint is on i.e., the crossing wires are connected. The switch can support up to three simultaneous but independent connections. Although we have used an
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equal number of inlets and outlets here, in general, this need not be the case. Switches may also have more inlets than outlets, or more outlets than inlets.
Figure 4.38 A simple circuit switch.
a b
c d
e f
off crosspoint connects b and f
on crosspoint
Figure 4.39 shows a simple circuit-switched network built using the switch in Figure 4.38. When the two hosts shown in the figure initiate a connection, the
network determines a path through the intermediate switches and establishes a circuit which is maintained for the duration of the connection. When the hosts disconnect,
the network releases the circuit.
Figure 4.39 Circuit switching.
Circuit switching relies on dedicated equipment especially built for the purpose, and is the dominant form of switching in telephone networks. Its main advantage lies
in its predictable behavior: because it uses a dedicated circuit, it can offer a constant throughput with no noticeable delay in transfer of data. This property is important in
telephone networks, where even a short delay in voice traffic can have disruptive effects.
Circuit switching’s main weakness is its inflexibility in dealing with computer- oriented data. A circuit uses a fixed amount of bandwidth, regardless of whether it is
used or not. In case of voice traffic, the bandwidth is usually well used because most of the time one of the two parties in a telephone conversation is speaking. However,
computers behave differently; they tend to go through long silent periods followed by a sudden burst of data transfer. This leads to significant underutilization of circuit
bandwidth.
Another disadvantage of circuit switching is that the network is only capable of supporting a limited number of simultaneous circuits. When this limit is reached, the
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network blocks further attempts for connection until some of the existing circuits are released.
4.2.2. Packet Switching
