6.4 Switch

Switches are network devices with basic frame store-and-forward capabilities that can support multiple simultaneous transmissions. Switches are also called "multiport bridges" (Figure 11) . Switches have the ability to read the target MAC addresses of the frames and forward them only to the appropriate port associated with the target device. Device may be directly attached to the switch, or may be connected to a hub that connects to the switch.

Switches may use several methods of operation:

• Cut-Through: A cut-through switch quickly reads the MAC address of the frame and quickly flows the frame through the switching matrix bit by bit
• Store and Forward: A store-and-forward switch temporarily buffers, or stores, the frame as it is presented to the incoming switch port, examining the entire frame for errors through a CRC check before forwarding it through the switching matrix to the output port. While cut-through switching is faster and less expensive, it carries with it the risk of the propagation of errored data and the resulting potential for negative impact on overall throughput, as errored frames ultimately have to be retransmitted. Store-and-forward switching generally is preferred over cut-through switching.
• Fragment Free: A third, and less common, method is fragment-free switching, which is similar to cut-through except for the fact that the switch stores the first 64 octets of the frame before forwarding it. As most errors occur at the beginning of a frame, this approach eliminates the possibility that runt frames, that is, truncated frames, will be transmitted.

Switches operate at the Physical and Data Link Layers of the OSI Reference Model Layers 1 and 2, respectively. Switches read the destination addresses of the frames, filtering and forwarding as appropriate, based on MAC addresses (Layer 2 address). Switches are fast and relatively inexpensive. Some switches make routing decisions based on IP addresses (Layer 3 address). Layer 3 switching involves a combination of switching and routing. This involves more complex routing decisions that are made in the context of the network as a whole yet not at the level of complexity that characterizes a router.

A switch does a great deal to reduce congestion and in a number of ways. First, a switch can support multiple simultaneous transmissions. Second, switches serve to segment a network through filtering, as they forward traffic only to the port associated with the link to which the target device is connected. Thereby, that traffic does not contribute to congestion on other links or segments. Third, a switch can be equipped to buffer incoming frames until internal bus resources are available to process them. A switch also can be equipped to buffer outgoing frames until such time as the link to the next switch becomes available. Fourth, a switch can exercise a flow control mechanism, whereby it can advise a device to stop transmitting when its buffers are in danger of overflowing and then advise the device to resume transmission when the pressure on resources has been relieved. Fifth, store-and-forward and fragment-free switches variously eliminate or reduce the number of errored frames. Finally, a switch supports full-duplex transmission, thereby eliminating data collisions associated with CSMA in an Ethernet environment, assuming that the station is directly connected to the switch rather than through a hub. This approach is the current best practice.

The cost of switches has dropped dramatically in recent years to the point that they often compete effectively against hubs. But switch costs are sensitive to factors such as the type and speed of the transmission media interfaces, the number and speed of the ports, the number and size of the buffers, the number and speed of the internal buses, the complexity of the internal switching matrix, and the complexity of the switching or routing logic.

Figure 11: 48-port Fast Ethernet Cisco Switch