|
|
Integrated Services
Digital Network
(ISDN) is comprised of digital telephony and data-transport services
offered by regional telephone carriers. ISDN involves the digitalization
of the telephone network, which permits voice, data, text, graphics,
music, video, and other source material to be transmitted over existing
telephone. The emergence of ISDN represents an effort to standardize
subscriber services, user/network interfaces, and network and
internetwork capabilities. ISDN applications include high-speed image
applications (such as Group IV facsimile), additional telephone lines in
homes to serve the telecommuting industry, high-speed file transfer, and
video conferencing. Voice service is also an application for ISDN. This
chapter summarizes the underlying technologies and services associated
with ISDN.
ISDN components include terminals,
terminal adapters (TAs), network-termination devices, line-termination
equipment, and exchange-termination equipment. ISDN terminals come in
two types. Specialized ISDN terminals are referred to as terminal
equipment type 1 (TE1). Non-ISDN terminals, such as DTE, that
predate the ISDN standards are referred to as terminal
equipment type 2 (TE2). TE1s connect to the ISDN network through a
four-wire, twisted-pair digital link. TE2s connect to the ISDN network
through a TA. The ISDN TA
can be either a standalone device or a board inside the TE2. If the TE2
is implemented as a standalone device, it connects to the TA via a
standard physical-layer interface. Examples include EIA/TIA-232-C
(formerly RS-232-C), V.24, and V.35.
Beyond the TE1 and TE2 devices, the next
connection point in the ISDN network is the network
termination type 1 (NT1) or network termination type 2
(NT2) device. These are network-termination devices that connect the
four-wire subscriber wiring to the conventional two-wire local loop. In
North America, the NT1 is a customer premises equipment (CPE)
device. In most other parts of the world, the NT1 is part of the network
provided by the carrier. The NT2 is a more complicated device that
typically is found in digital private branch
exchanges (PBXs) and that performs Layer 2 and 3 protocol functions
and concentration services. An NT1/2 device also exists as a single
device that combines the functions of an NT1
and an NT2.
ISDN specifies a number of reference
points that define logical interfaces between functional groupings, such
as TAs and NT1s. ISDN reference
points include the following:
- R---The reference point
between non-ISDN equipment and a TA.
- S---The reference point
between user terminals and the NT2.
- T---The reference point
between NT1 and NT2 devices.
- U---The reference point
between NT1 devices and line-termination equipment in the carrier
network. The U reference point is relevant only in North America,
where the NT1 function is not provided by the carrier network.
Figure 12-1 illustrates a sample ISDN
configuration and shows three devices attached to an ISDN switch at the central
office. Two of these devices are ISDN-compatible, so they can be
attached through an S reference point to NT2 devices. The third device
(a standard, non-ISDN telephone) attaches through the reference point to
a TA. Any of these devices also could attach to an NT1/2 device, which
would replace both the NT1 and the NT2. In addition, although they are
not shown, similar user stations are attached
to the far right ISDN switch.
Figure 12-1: Sample
ISDN configuration illustrates relationships between devices and
reference points.
The ISDN Basic Rate
Interface (BRI) service offers two B
channels and one D channel (2B+D). BRI B-channel
service operates at 64 kbps and is meant to carry user data; BRI
D-channel service operates at 16 kbps and is meant to carry control and
signaling information, although it can support user data transmission
under certain circumstances. The D channel signaling protocol comprises
Layers 1 through 3 of the OSI reference model. BRI also provides for
framing control and other overhead, bringing its total bit rate to 192
kbps. The BRI physical-layer specification is International
Telecommunication Union Telecommunication Standardization Sector (ITU-T)
(formerly the Consultative Committee for International Telegraph and
Telephone [CCITT]) I.430.
ISDN Primary Rate Interface (PRI)
service offers 23 B channels and one D channel in North America and
Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs
at 64 Kbps). ISDN PRI in Europe, Australia, and other parts of the
world provides 30 B channels plus one 64-Kbps D channel and a
total interface rate of 2.048 Mbps. The PRI physical-layer specification
is ITU-T I.431.
ISDN physical-layer (Layer 1) frame
formats differ depending on whether the frame is outbound (from terminal
to network) or inbound (from network to terminal). Both physical-layer
interfaces are shown in Figure 12-2).
The frames
are 48 bits long, of which 36 bits represent data. The bits of an ISDN
physical-layer frame are used as follows:
Figure 12-2: ISDN
Physical-layer frame formats differ depending on their direction.
Multiple ISDN user devices can
be physically attached to one circuit. In this configuration, collisions
can result if two terminals transmit simultaneously. ISDN therefore
provides features to determine link contention. When an NT receives a D
bit from the TE, it echoes back the bit in the next E-bit position. The
TE expects the next E bit to be the same as its last transmitted D bit.
Terminals cannot transmit into the D
channel unless they first detect a specific number of ones (indicating
"no signal") corresponding to a pre-established priority. If
the TE detects a bit in the echo (E) channel that is different from its D
bits, it must stop transmitting immediately. This simple technique
ensures that only one terminal can transmit its D message at one time.
After successful D- message transmission, the terminal has its
priority reduced by requiring it to detect more continuous ones before
transmitting. Terminals cannot raise their priority until all other
devices on the same line have had an opportunity to send a D message.
Telephone connections have higher priority than all other services, and
signaling information
has a higher priority than non-signaling information.
Layer
2
Layer 2 of the ISDN signaling protocol is
Link Access Procedure, D channel
(LAPD). LAPD is similar to High-Level Data Link Control
(HDLC) and Link Access Procedure, Balanced (LAPB) (see
and "X.25," for more information on these
protocols). As the expansion of the LAPD acronym indicates, this layer
it is used across the D channel to ensure that control and signaling
information flows and is received properly. The LAPD frame format (see
Figure 12-3 ) is very similar to that of HDLC and, like HDLC, LAPD uses supervisory,
information, and unnumbered frames. The LAPD protocol
is formally specified in ITU-T Q.920 and ITU-T
Q.921.
The LAPD Flag and Control
fields are identical to those of HDLC. The LAPD Address field
can be either 1 or 2 bytes long. If the extended address bit of the
first byte is set, the address is 1 byte; if it is not set, the address
is 2 bytes. The first Address-field byte contains identifier service
access point identifier (SAPI), which identifies the portal at
which LAPD services are provided to Layer 3.
Figure 12-3: LAPD
frame format is similar to HDLC and LAPB.
The C/R bit indicates whether the frame
contains a command or a response. The terminal end-point identifier
(TEI) field identifies either a single terminal or multiple terminals. A
TEI of all ones
indicates a broadcast.
Layer
3
Two Layer 3 specifications are used for ISDN
signaling: ITU-T (formerly CCITT) I.450 (also known
as ITU-T Q.930) and ITU-T I.451
(also known as ITU-T Q.931). Together, these
protocols support user-to-user, circuit-switched, and packet-switched
connections. A variety of call-establishment, call-termination,
information, and miscellaneous messages are specified, including SETUP,
CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT.
These messages are functionally similar to those provided by the X.25
protocol (see "X.25," for more information). Figure 12-4 ,
from ITU-T I.451, shows the typical stages of an ISDN circuit-switched
call.
Figure 12-4: An ISDN circuit-switched call moves
through various stages to its destination.
Get
this document in PDF form
|
