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Physical Details of Leased Lines

The leased line service delivers bits in both directions, at a predetermined speed, using fullduplex
logic. In fact, conceptually it acts as if you had a full-duplex crossover Ethernet link
between two routers, as shown in Figure 3-2. The leased line uses two pairs of wires, one
pair for each direction of sending data, which allows full-duplex operation.
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Of course, leased lines have many differences compared to an Ethernet crossover cable. To
create such possibly long links, or circuits, a leased line does not actually exist as a single
long cable between the two sites. Instead, the telco installs a large network of cables and
specialized switching devices to create its own computer network. The telco network creates
a service that acts like a crossover cable between two points, but the physical reality is
hidden from the customer.

Leased lines come with their own set of terminology as well. First, the term leased line
refers to the fact that the company using the leased line does not own the line, but instead
pays a monthly lease fee to use it. However, many people today use the generic term service
provider to refer to a company that provides any form of WAN connectivity, including
Internet services.

Given their long history, leased lines have had many names. Table 3-2 lists some of those
names, mainly so that in a networking job, you have a chance to translate from the terms
each person uses with a basic description as to the meaning of the name.
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Leased-Line Cabling
To create a leased line, some physical path must exist between the two routers on the ends
of the link. The physical cabling must leave the buildings where each router sits. However,
the telco does not simply install one cable between the two buildings. Instead, it uses what
is typically a large and complex network that creates the appearance of a cable between the
two routers.

Figure 3-3 gives a little insight into the cabling that could exist inside the telco for a short
leased line. Telcos put their equipment in buildings called central offices (CO). The telco
installs cables from the CO to most every other building in the city, expecting to sell services
to the people in those buildings one day. The telco would then configure its switches
to use some of the capacity on each cable to send data in both directions, creating the
equivalent of a crossover cable between the two routers.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 3-3 Possible Cabling Inside a Telco for a Short Leased Line
Although what happens inside the telco is completely hidden from the telco customer,
enterprise engineers do need to know about the parts of the link that exist inside the customer’s
building at the router .
First, each site has customer premises equipment (CPE), which includes the router, serial
interface card, and CSU/DSU. Each router uses a serial interface card that acts somewhat
like an Ethernet NIC, sending and receiving data over the physical link. The physical link
requires a function called a channel service unit/data service unit (CSU/DSU). The CSU/DSU
can either be integrated into the serial interface card in the router or sit outside the router
as an external device. Figure 3-4 shows the CPE devices, along with the cabling.
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Figure 3-4 Point-to-Point Leased Line: Components and Terminology
The cabling includes a short serial cable (only if an external CSU/DSU is used) plus the cable
installed by the telco for the leased line itself. The serial cable connects the router serial
interface to the external CSU/DSU. (Many cable options exist; the cable just needs to match
the connector of the serial interface on one end and the CSU/DSU on the other end.) The
four-wire cable from the telco plugs in to the CSU/DSU, typically using an RJ-48 connector
that has the same size and shape as an RJ-45 connector (as shown in Figure 2-7 in Chapter 2,
“Fundamentals of Ethernet LANs”).

Telcos offer a wide variety of speeds for leased lines. However, you cannot pick the exact
speed you want; instead, you must pick from a long list of predefined speeds. Slower-speed
links run at multiples of 64 kbps (kilobits per second), while faster links run at multiples of
about 1.5 Mbps (megabits per second).

Building a WAN Link in a Lab
On a practical note, to prepare for the CCENT and CCNA Routing and Switching exams,
you can choose to buy some used router and switch hardware for hands-on practice. If you
do, you can create the equivalent of a leased line without a real leased line from a telco, and
without CSU/DSUs, just using a cabling trick. This short topic tells you enough information
to create a WAN link in your home lab.

First, the serial cables normally used between a router and an external CSU/DSU are called
data terminal equipment (DTE) cables. To create a physical WAN link in a lab, you need
two serial cables: one serial DTE cable, plus a similar but slightly different matching data
communications equipment (DCE) cable. The DCE cable has a female connector, while the
DTE cable has a male connector, which allows the two cables to be attached directly. The
DCE cable also does the equivalent task of an Ethernet crossover cable by swapping the
transmit and receive wire pairs, as shown in Figure 3-5.
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Figure 3-5 Serial Cabling Uses a DTE Cable and a DCE Cable
The figure shows the cable details at the top, with the wiring details inside the cable at
the bottom. In particular, at the bottom of the figure, note that the DCE cable swaps the
transmit and receive pairs, whereas the DTE serial cable does not, acting as a straightthrough
cable.

Finally, to make the link work, the router with the DCE cable installed must do one function
normally done by the CSU/DSU. The CSU/DSU normally provides a function called
clocking, in which it tells the router exactly when to send each bit through signaling over
the serial cable. A router serial interface can provide clocking, and the more recent router
software versions automatically supply clocking when the router senses a DCE cable is
plugged into the serial port. Regardless of whether a router has an older or newer software
version, you will want to know how to configure serial clocking using the clock rate
command. The section “Bandwidth and Clock Rate on Serial Interfaces,” in Chapter 17,
“Operating Cisco Routers,” shows a sample configuration.

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