AT&T DEFINITY 7200 series - Manual (Page 168)

8-14 LOCAL AREA NETWORK CONNECTIONS
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LAN TRANSMISSION MEDIA
The transmission media through which LAN nodes are linked may be fiber optic cable, coaxial cable,
twisted-pair, or a combination of these. Each transmission medium has a maximum data rate, and distance
and capacity limits that characterize it:
Table 8-3. LAN Transmission Media Characteristics
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Transmission Maximum Distance Node
Medium Data Rate Limit Capacity
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Coaxial cable 50 Mbps 100 km 10,000
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Twisted pair 16 Mbps 5 km 100
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Fiber optic cable 1.7 Gbps 200 km 100
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Diverse transmission media along LAN communications paths are linked through adapters or connectors.
Adapters allow the transmission media along the same LAN to change, for example, from fiber optic cable
to twisted pair. Connectors can provide changes in the transmission media, but their major purpose is to
provide LAN access to outside networks and terminals.
LAN CONNECTORS
Generally, three types of connectors — bridges, routers, and gateways — link LANs to outside terminals or
networks. These connectors provide external access and translate messages between protocols. The kind of
connector that is appropriate for a configuration depends upon where the differing protocols fall in the Open
System Interface (OSI) hierarchy. (See appendix D for an explanation of the OSI hierarchy.)
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Bridges translate data-link layer (layer 2) messages.
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Routers translate network layer (layer 3) messages.
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Gateways translate messages at layers higher than the network layer.
Most often, the installation of a circuit-board, along with specialized communications software, transforms
a LAN node into a LAN connector that functions as a bridge, router, or gateway for outside access to the
LAN.
ADVANTAGES OF A COMMUNICATIONS SYSTEM IN A LAN ENVIRONMENT
Linking a LAN to a communications system can offer three distinct advantages to the LAN user:
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First, it can let LAN nodes and outside (both on-premises and off-premises) DTEs communicate with
each other. This means: