Data Communications Networking Devices - 4th Ed.pdf

5.4 LAN SWITCHES _______________________________________________________________ 603invented the switchboard was developed to enable multiple simultaneousconversations to occur without requiring telephone wires to be installed in acomplex matrix between subscribers. Later, telephone of®ce switches were developedto route calls based upon the telephone number dialed, followed in a similarmanner by the development of bridges in a LAN environment. Bridges can beconsidered to represent an elementary type of switch due to their limited number ofports and simplistic switching operation. That switching operation is based uponwhether or not the destination address in a frame `read' on one port is known toreside on that port.Akey limitation of a bridge is its common inability to perform more than oneframe-forwarding operation at any point in time. Recognizing this performancebottleneck, equipment designers developed a product that can perform multiplesimultaneous frame-forwarding operations. That device, which has evolved into amulti-billion dollar business, is the LAN switch. The LAN switch is based uponmatrix switches which for decades were successfully employed in telecommunicationsoperations. By adding buffer memory to store address tables, frames ¯owingon LANs connected to different ports could be simultaneously read and forwardedvia the switch fabric to ports connected to other networks.Basic componentsFigure 5.44 illustrates the basic components of a four-port intelligent switch. Like abridge that reads frames ¯owing on a network to construct a table of sourceFigure 5.44 Basic components of an intelligent switch. An intelligent switch consists ofbuffers and address tables BAT), logic and a switching fabric which permits frames enteringone port to be routed to any port in the switch. The destination address in a frame is used todetermine the associated port with that address via a search of the address table, with theport address used by the switching fabric for establishing the cross-connection