Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

44 CHAPTER 3 Network Devices
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ISDN Interfaces There are two basic types of ISDN service:
■ BRI consists of two 64 Kbps B channels and one 16 Kbps D channel for
a total of 144 Kbps. Only 128 Kbps is used for user data transfers. BRIs
were designed to enable customers to use their existing wiring. This provided
a low-cost solution for customers and is why it is the most basic
type of service today intended for small business or home use. To use
BRI services, you must subscribe to ISDN services through a local telephone
company or provider. By default, you must be within 18,000 ft.
(about 3.4 miles) of the telephone company’s central office for BRI
services.
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PRI requires T1 carriers to facilitate communications. Normally, the channel
structure contains 23 B channels plus one 64 Kbps D channel for a
total of 1536 Kbps. This standard is used only in North America and
Japan. European countries support a different kind of ISDN standard for
PRI. It consists of 30 B channels and one 64 Kbps D channel for a total
of 1984 Kbps. A technology known as Non-Facility Associated Signaling
(NFAS) is available to enable you to support multiple PRI lines with one
64 Kbps D channel.
ISDN devices The standard refers to the devices that are required to connect
the end node to the network.
ISDN reference points They are used to define logical interfaces. They are,
in effect, a type of protocol used in communications. The following list
contains the reference points:
■ R defines reference point between a TE2 device and a TA device.
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S defines reference point between TE1 devices and NT1 or NT2 devices.
T defines reference point between NT1 and NT2 devices.
U defines reference point between NT1 devices and line termination
equipment. This is usually the central switch.
ISDN identifiers They use five separate identifiers when making a connection.
The provider assigns two of these when the connection is first set up:
the service profile identifier (SPID) and the directory number (DN). These
are the most common numbers used because the other three are dynamically
set up each time a connection is made. The three dynamic identifiers
are terminal endpoint identifier (TEI), bearer code (BC), and service access
point identifier (SAPI).
Channel Service Unit/Data Service Unit
Channel service unit/data service unit (CSU/DSU) is a device that takes a signal
from a digital medium and multiplexes it. Characteristics of a CSU/DSU include
the following: