Dedicated Circuit Switched Connections

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Leased-line serial connections typically connect to a transport service provider through a DCE device, which provides clocking and transforms the signal to the channelized format that is used in the service provider network. These point-to-point dedicated links provide a single, preestablished WAN communications path from the customer circuit-switched premises, through a carrier network, to a remote network. Dedicated lines through T3/E3 rates are frequently described as leased lines. The established path is permanent and fixed for each remote network that is reached through the carrier facilities. The service provider reserves the full-time private use of the customer circuits through the transport network.

Synchronization of timing and data-link control is preserved end to end. These dedicated connections are made using the synchronous serial ports on the router with bandwidth of up to 34 Mbps over a service provider E3 transport link and 45 Mbps over T3. Different encapsulation methods at the data-link layer provide flexibility and reliability for user traffic. Typical connections on a dedicated network WAN connection employ 56-kbps, 64-kbps, T1, E1, T3, and E3 data rates.

These synchronous serial standards are supported on Cisco routers through serial interfaces:

In North America, the connecting device is called a CSU/DSU. The CSU connects to the service provider network, while the DSU connects to the network device serial interface. The CSU/DSU is a device (or sometimes two separate digital devices) that adapts the media format from a serial DTE device, such as a router, to the media format of the service provider equipment, such as a WAN switch, in a switched carrier network. The CSU/DSU also provides signal clocking for synchronization between these devices. The figure shows the placement of the CSU/DSU.

It is increasingly common to have direct connections to the carrier transport network using fractional or complete T1/E1 circuits. In this case, a CSU provides demarcation and logical termination between the service provider network and the customer network. Direct T3/E3 and Synchronous Digital Hierarchy/SONET (SDH/SONET) connectivity may also be available for organizations requiring higher data rates.

The private nature of a dedicated connection allows better control over the WAN connection. Dedicated connections also offer high speeds beyond T3/E3 levels using SDH/SONET. Dedicated connections are ideal for high-volume environments with steady-rate traffic patterns or high-peak demands of critical traffic. However, because the line is not shared, dedicated connections tend to be more costly.

As a general rule, dedicated connections are most cost-effective in these situations: ■ Long connect times Short distances

Critical traffic requirements that must be guaranteed

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