merely the location where all the lines coming into your premises are separated into individual
pairs. Once the decision has been made regarding where to put the cable and the cable is put in
place, the installer attaches an eight-pin modular (RJ-45) jack to the cable and attaches the jack
to the wall.
circuit identifier number. This information is necessary if a call for service is needed in the
future. In North America, this jack is the point of demarcation (demarc), where responsibility
for the line changes hands. The equipment on your side of the point of demarc is known as
customer premise equipment (CPE). The jack that the telco installs is a direct interface from the
local central office switch to your customer premise equipment.
similar to a channel service unit/data service unit (CSU/DSU), which is used in serial
connections. The NT1 terminates the local loop.
is attached to the router's BRI interface. The U interface is attached to the telco jack. Many of
Cisco's BRI-capable routers are now available with an integrated NT1. These interfaces are
labeled "BRI U". If this feature is not available on the chassis, the interface is labeled "BRI S/T"
and an external NT1 is necessary. This native ISDN interface is the router's TE1 interface.
your router. In such cases, it is still possible to use ISDN. However, another piece of hardware
known as a terminal adapter must be used. The terminal adapter is a device that contains the
BRI that your router is missing. In recent ISDN hype, telecommunications manufacturers
marketed terminal adapters as ISDN modems. Terminal adapters are not modems. They do not
modulate and demodulate signals. What they do is interface your router's universal I/O serial
port. The terminal adapter interfaces the NT1 with a native BRI.
the TE2 and the TA is known as the R interface. It is important to note that a non-native ISDN
interface (more specifically, a solution that lacks a D channel) requires you to use the dialer
in-band command to issue signaling requests to the ISDN switch.
bandwidth for signaling. Therefore, the bandwidth available per B channel becomes 56,000
bps. In some cases, ISDN facilities are available only at 56,000 bps per B channel, regardless
of whether the interface is native ISDN. Check with the telco provider for details for a particular
installation. Figure 6-3 shows the ISDN reference points.