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IEEE 802.5


4 or 16Mbps


Token


Passing


Physical star, Logical ring


STP


IEEE 802.11b


1 or


11Mbps


CSMA/CA


Cellular


None


(Wireless)


FDDI


200Mbps


Token


Passing


Physical star, Logical ring


Fiberoptic (UTP

implemented as CDDI)


TABLE


Data Link Layer Devices


Three main devices manipulate data at the Data Link layer:


■    Bridges


■    Switches


■    NIC


They are more complex than their Physical layer counterparts and thus are more expensive and more difficult to implement. But they each bring unique advantages to the network.

The Bridge


A bridge is a network device, operating at the Data Link layer, that logically separates a single network into two segments, but it lets the two segments appear to be one network to higher layer protocols. The primary use for a bridge is to keep traffic meant for stations on one side of the bridge and not let it pass to the other side. For example, if you have a group of workstations that constantly exchange data on the same network segment as a group of workstations that don’t use the network much at all, the busy group will slow down the performance of the network for the other users. If you put in a bridge to separate the two groups, however, only traffic destined for a workstation on the other side of the bridge will pass to the other side.


All other traffic stays local. Figure 2.14 shows a network before and after bridging.

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