| Dedicated connections, referred to
as leased lines, provide links that are continuously available; they are
effective in environments where a steady flow of traffic must cross the
WAN. Leased lines are used primarily to provide full-time WAN
connectivity between LANs. These connections are referred to as
point-to-point because the source and destination are exclusively tied
to each other. The use of leased lines is not cost-effective when the
link isn’t used on a full-time basis.
As shown in Figure ,
a point-to-point dedicated link provides a single, preestablished WAN
path from the customer premises, through a carrier network (the
telephone company), to a remote network. The established path is permanent and fixed for each remote
network that is reached through the carrier facilities. Point-to-point
links are reserved full-time by the carrier company for the private use
of the customer.
The private nature of a dedicated leased-line connection allows a
corporation to maximize its control over the WAN connection. Leased
lines also offer high speeds up to T3/E3 levels (45 Mbps (Megabits per
second)/34Mbps). They are ideal for
high-volume environments with steady-rate traffic patterns. However,
because the line is not shared, they tend to be more costly.
As a general rule, leased-line connections are most cost-effective
when the following situations occur:
- Long connect times
- Shorter distances
Some WAN services, such as T1, provide a fixed fee for local-loop
access for both locations, and then provide a distance fee for linking
those two locations.
Dedicated leased lines typically require synchronous serial
connections. The dedicated connections are made by using the router's
synchronous serial ports with bandwidth of up to 34 Mbps on an E3 and 45
Mbps on a T3, available through the use of a channel service unit/data
service unit (CSU/DSU). Different
encapsulation methods at the data link layer provide flexibility and
reliability for user traffic. Typical connections on a dedicated network
employ 56 kilobits per second (Kbps), 64 Kbps, T1 (1.544 Mbps), E1
(2.048 Mbps), T3, and E3 technologies.
The synchronous serial standards that are supported on most routers
are as follows (EIA/TIA stands for Electronic Industries
Association/Telecommunications Industry Association):
- EIA/TIA-232
- EIA/TIA-449
- V.35
- X.21
- EIA-530
As shown in Figure ,
a CSU/DSU is a digital interface device (or sometimes two separate
digital devices) that adapts the physical interface on a data terminal
equipment (DTE) device (such as a terminal) to the interface of a data
communications equipment (DCE) device (such as a switch) in a switched
carrier network. The CSU/DSU also provides signal timing for
communication between these devices. The graphic shows the placement of
the CSU/DSU. Different encapsulation methods at the data link
layer provide flexibility and reliability for user traffic.
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