Packet switching is used in the x25 public packet

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Unformatted text preview: m the source to the destination either by store-and-forward method or broadcast method. As shown in Figure 17.17, in the store-and-forward method, the message is transmitted from the source node to an intermediate node. The intermediate node stores the complete message temporarily, inspects it for errors, and transmits the message to the next node based on an available free channel and its routing information. The actual path taken by the message to its destination is dynamic as the path is established as it travels along. When the message reaches a node, the channel on which it came is released for use by another message. In Figure 17.16, if a message is to be transmitted from station A to station B, it may take either path 1-2-3-4 or 1-5-4 depending on the availability of a free output path at that particular moment. As shown in Figure 17.18, in the broadcast method, the message is broadcast over a common medium known as broadcast channel. All the stations check the destination address of each message as they pass by and accept only those addressed to them. The routing delays inherent in store-and-forward method are eliminated in this method. However, this method requires that all stations must be connected to the broadcast channel. Advantages 1. In message switching, no physical connection is required between the source and the destination as required in case of circuit switching. 2. As channels are used only when, messages are transmitted, this method uses the channels very effectively. Disadvantages 1. As the message length is unlimited, each switching node must have sufficient storage to buffer messages. In many cases, this storage space may be underutilized. 2. In message switching, a message is delayed at each node for the time required to receive the message plus a queuing delay waiting for an opportunity to retransmit the message to the next node. 3. The method is too slow for interactive/real-time applications. Hence, it is used when message delays are not critica...
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