Cabling the WAN
Interconnecting Cisco Network Devices
Author: Stephen McQuerry
Publisher: Cisco Press (53)
to connect our networks to other remote networks, it is sometimes necessary
to utilize WAN services. WAN services provide different connection methods,
and the cabling standards differ from those of LANs. It is therefore important
to understand the types of cabling needed to connect to these services. Figure 2-10 illustrates the cabling in a typical
This section discusses the following topics:
Many physical implementations carry traffic across the WAN. Needs vary,
depending on the distance of the equipment from the services, the speed, and
the actual service itself. Figure 2-11 lists
a subset of the physical implementations that support some of the more common
WAN solutions today. The type of physical layer you will choose depends on
the distance, speed, and what type of interface in which you need to connect.
Serial connections are used to support WAN services such as dedicated
leased lines running Point-to-Point Protocol (PPP), High-Level
Data Link Control (HDLC), or Frame Relay encapsulations at Layer 2. The speeds
of the connections typically range from 56 Kbps to T1/E1 (1.544/2.048 Mbps).
Other WAN services, such as ISDN, offer dial-on-demand connections or dial
backup services. An ISDN BRI (Basic Rate Interface) is composed of two 64
Kbps Bearer channels for data, and one Delta channel at 16 Kbps used for signaling
and other link-management tasks. PPP is typically used to carry data over
the B channels. You will learn more about the data link operation, ISDN, and
Frame Relay in Chapters 11, 12,
transmission is a method of data transmission in which bits of data are transmitted
over a single channel. This one-at-a-time transmission contrasts with parallel
data transmission, which passes several bits at a time. For long-distance
communication, WANs use serial transmission. To carry the energy represented
in bits, serial channels use a specific electromagnetic or optical frequency
Frequencies, described in terms of their cycles per second (or Hertz),
function as a band or spectrum for communicationsfor example, the signals
transmitted over voice-grade telephone lines up to 3 kHz (kilo, or thousand,
Hertz). The size of this frequency is called bandwidth.
Several types of physical connections allow us to connect to serial
WAN services. Depending on the physical implementation you choose, or the
physical implementation imposed by your provider, you will need to select
the correct serial cable type to use with your router. Figure
2-12 shows the different serial connector options available. Note that
serial ports on most Cisco routers use a proprietary 60-pin connector. Therefore,
the router-ends of most port adapter cables use a male 60-pin connector, while
the network ends of the adapter cable must match the specific WAN service
Another way to express bandwidth is to specify the amount of data in
bits per second (bps) that can be carried using two of the physical layer
implementations shown in Figure 2-12. Table 2-5 compares the physical standards for
WAN serial connection options.
In addition to determining
cable type, you will need to determine if you need data
terminal equipment (DTE) or data circuit-terminating equipment (DCE) connectors
for your equipment. The DTE is the endpoint of the user's device on the WAN
link. The DCE is typically the point where responsibility for delivering data
passes into the hands of the service provider.
As shown in Figure 2-13, if you are
connecting directly to a service provider, or to a device that will perform
signal clocking, the router is a DTE and needs a DTE serial cable. This is
typically the case for routers.
There are cases, however, where the router will need to be the DCE.
For example, if you are performing a back-to-back router scenario in a test
environment, one of the routers will be a DTE, and the other will be a DCE.
When you're cabling routers for serial connectivity, the routers will
have either a fixed or modular port. The type of port being used will affect
the syntax you use later to configure each interface.
Figure 2-14 shows an example of a router with
fixed serial ports (interfaces). Each port is given a label of port type and
port number, such as “serial 0.” To configure a fixed interface,
you specify the interface using this convention.
Other routers have modular ports. Figure 2-15
shows examples of routers with modular serial ports. Usually each port is
given a label of port type, slot number (the location of the module), and
port number. To configure a port on a modular card,
you will be asked to specify the interface using this convention:
An example would be serial 1/0.
Figure 2-15. Modular Serial Ports on a 1603 Router and a 3640 Router
Depending on the type of router you have, port designation convention
might change. For example, some high-end routers such as a 7500 series device
can have a virtual interface processor. Designation of these ports would also
include the VIP slot:
<port type> <slot number>/<port adapter number>/<port number>
An example would be serial 1/0/0.
The 1603 router shown in Figure 2-15
has both fixed and modular serial interfaces. Even though the serial port
shown is a modular interface, you configure it as though it were fixed, using
a label of port type and port number, such as serial 0.
With BRI, there are two
types of interfaces that you can useBRI S/T and BRI U. To determine
which interface type you need, you must determine whether you or the service
provider will provide an NT1 device.
device is an intermediate device between the router and the service provider's
ISDN switch (the cloud) that is used to connect four-wire subscriber wiring
to the conventional two-wire local loop. NT1 refers to a network termination
type 1 device. In North America, the NT1 is typically provided by the customer,
while in the rest of the world, the service provider provides the NT1 device.
If you need to provide the NT1 device, you can use an ISDN BRI with
a U interface; a U interface indicates that the NT1 device is built in. If
you are using an external NT1 device, or if your service provider uses an
NT1 device, the router needs an ISDN BRI S/T interface. Because routers can
have multiple ISDN interface types, you need to determine which interface
you need when you purchase the router. You can determine which type of ISDN
connector the router has by looking at the port label. Figure
2-16 shows the different port types for the ISDN interface.
To interconnect the ISDN BRI port to the service provider device,
you will use a UTP category 5 straight-through cable.
It is important to insert a cable running from an ISDN BRI port only
to an ISDN jack or an ISDN switch. ISDN BRI uses voltages that can seriously
damage non-ISDN devices.
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