13.2.2 Message Switching

Unlike circuit switching, message switching does not establish a dedicated communication path between the sender and receiver. Instead, each message is treated as a complete unit of information that is individually routed through the network using a technique known as store-and-forward.

When a message is transmitted, it is delivered to the first switching node, where it is stored temporarily in memory or on disk. Only after the entire message has been received does the switch examine the destination address, determine the most appropriate route, and forward the message to the next switching node. This process is repeated at every intermediate node until the message reaches its final destination.

The operation is similar to a postal service. A letter is delivered to a local mail center, sorted, transported to another sorting office, and eventually delivered to the recipient. At each stage, the entire letter is stored and processed before continuing its journey. Likewise, every switching node in a message-switched network receives, stores, and forwards the complete message before passing it to the next node.

Because no dedicated communication path is established, network resources are shared much more efficiently than in circuit-switched networks. Individual communication links are occupied only while a message is actually being transmitted, allowing many users to share the same network infrastructure. If a particular communication link becomes congested or temporarily unavailable, a switching node can simply store the message until the preferred route becomes available or select an alternative route through the network. Messages addressed to multiple destinations can also be duplicated automatically by the switching nodes without requiring the sender to transmit the same message repeatedly.

These characteristics made message switching particularly attractive for applications in which immediate delivery was not essential. Early telegraph systems, Telex networks, military communication systems, and some of the earliest computer networks all employed message-switching techniques because they provided efficient utilization of expensive communication links and offered considerable flexibility in routing.

The principal disadvantage of message switching is that every switching node must receive the entire message before forwarding it. Large messages may therefore occupy substantial storage space and experience significant delays while waiting in queues at intermediate nodes. Since the total delay depends upon message length, network loading, and the number of switching nodes encountered, delivery times are often long and highly variable. These characteristics make message switching unsuitable for interactive applications such as voice conversations, video conferencing, or remote computer access, where users expect immediate responses.

Although true message-switched networks are now uncommon, the underlying store-and-forward principle remains widely used. Electronic mail (email), Short Message Service (SMS), multimedia messaging, and many cloud-based messaging systems all store messages temporarily before forwarding them to their recipients. Modern enterprise systems also use message-queuing technologies to exchange information reliably between distributed applications.

The limitations of message switching ultimately led to the development of packet switching. Instead of transmitting an entire message as one large unit, packet switching divides each message into many smaller packets that can be forwarded independently through the network. This significantly reduces storage requirements, decreases transmission delay, and allows multiple users to share network resources much more efficiently while supporting both interactive and delay-tolerant communications.