Data Communications Networking Devices - 4th Ed.pdf

5.4 LAN SWITCHES _______________________________________________________________ 607storage of a full frame in memory as well as the necessity for error checking.Concerning the latter, switches should operate error-free, so a store-and-forwardswitch only removes network errors which should be negligible to start with.When a switch removes an errored frame, the originator will retransmit theframe after a period of time. Since an errored frame arriving at its destinationnetwork address is also discarded, many persons question the necessity of errorchecking by a store-and-forward LAN switch. However, ®ltering capability, ifoffered, may be far more useful as you could use this capability, for example, toroute protocols carried in frames to destination ports far more easily than by framedestination address. This is especially true if you have hundreds or thousands ofdevices connected to a large switch. You might set up two or three ®lters instead ofentering a large number of destination addresses into the switch.Figure 5.46 illustrates the operation of a store-and-forward LAN switch. Notethat a common switch design is to use shared buffer memory to store entire frames,which increases the latency associated with this type of switch. Since the minimumlength of an Ethernet frame is 72 bytes, the minimum one-way delay or latency, notcounting the switch overhead associated with the look-up table and switching fabricoperation, becomes:96 s ‡…72 bytes 8 bits=byte 100 ns=bit†ˆ…9:6 10 6 †‡…576 100 10 9 † secondsˆ 67:2 10 6 secondsFigure 5.46 Store and forward switching. A store and forward LAN switch reads the framedestination address 1) as it is placed in buffer memory 2). As the entire frame is being readinto memory, a look-up operation 3) is performed to obtain a destination port address.Once the entire frame is in memory, a CRC check is performed and one or more ®lteringoperations may be performed. If the CRC check indicates the frame is error-free, it isforwarded from memory to its destination address 4), otherwise it is disregardedHere 9.6 s represents the Ethernet interframe gap, while 100 ns/bit is the bitduration of a 10 Mbps Ethernet LAN. Thus, the minimum one-way latency of astore-and-forward Ethernet switch is 0.000 0672 seconds, while a round-tripminimum latency is twice that duration. For a maximum length Ethernet frame