The term switch means to shift from one thing to another. The switches in your house change a circuit from on to off, for example. This is one kind of change, but it is not the kind of change being talked about in telecommunication networks. In telecommunication networks, switches are more like their counterparts in the railroad system. On a railroad track, a switch is a mechanical device that determines which track a train is going to be placed on when it reaches a junction point.
But it is not just this capability that makes a switch. If it were, then a switch would be indistinguishable from a multiplexer or a digital cross-connect system (DCS). What truly differentiates a switch is the ability for it to make that decision differently from one moment to the next, based on information provided to the switch for control purposes.
In the context of information networks, we can distinguish two general classes of switches:
- Circuit switches: Circuit switches are most commonly found in voice networks, most notably the public switched telephone network (PSTN). Circuit switches establish a path through a network and dedicate bandwidth to the connection for the duration of the need. In a circuit-switched voice network, for example, the voice switches create a 64 kbps end-to-end path through the network to connect the two telephones at either end. If there are no available facilities to create this end-to-end circuit, the call is rejected from the network. Once established, the bandwidth along the path is dedicated to this communication and unavailable for other transmissions. Circuit switches are typically interconnected with channelized trunks, making it possible to dedicated particular channels on particular trunks to the establishment of an end-to-end circuit. In the language of the OSI Reference Model (OSI-RM), a circuit switch would be considered a Layer 1, or Physical Layer device if it was supporting a data network.
- Packet switches: Packet switches have historically been most commonly found in data networks, but they are becoming the default switching model for all telecommunication networks. Unlike circuit switches, packet switches are typically interconnected with unchannelized transport facilities. Rather than dedicating resources end-to-end for a communication, the packet switch is capable of receiving a transmission on any interface, and forwarding it on any other interface based on the information contained within the packet. The term packet is being used loosely in this description. This model for switching can be implemented at many layers in the OSI-RM, this model for switching can occur at any layer above the Physical Layer. Today we have Layer 2 switches, Layer 3 switches, Layer 4 switches, and even switches that work at the Application Layer. Each has slightly different characteristics and capabilities, but they all operate on a shared-bandwidth model.
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