The entry level interface to ISDN is the Basic Rate Interface (BRI), a 128 kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s payload rate is broken down into two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signaling channel ('D' channel or data channel).
The interface specifies the following network interfaces:
The U interface is a two-wire interface between the exchange and a network terminating unit, which is usually the demarcation point in non-North American networks.
The T interface is a serial interface between a computing device and a terminal adapter, which is the digital equivalent of a modem.
The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on a Network termination 1 (NT1).
The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.
BRI-ISDN is very popular in Europe but is much less common in North America. It is also common in Japan — where it is known as INS64.
The other ISDN access available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s.
In North America PRI service is delivered on one or more T1 carriers (often referred to as 23B+D) of 1544 kbit/s (24 channels). A PRI has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1). Inter-changeably but incorrectly, a PRI is referred to as T1 because it uses the T1 carrier format. A true T1 (commonly called "Analog T1" to avoid confusion) uses 24 channels of 64 kbit/s of in-band signaling. Each channel uses 56 kb for data and voice and 8 kb for signaling and messaging. PRI uses out of band signaling which provides the 23 B channels with clear 64 kb for voice and data and one 64 kb 'D' channel for signaling and messaging. In North America, Non-Facility Associated Signalling allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B". D-channel backup allows for a second D channel in case the primary fails. NFAS is commonly used on a T3.
PRI-ISDN is popular throughout the world, especially for connecting private branch exchanges to the public network.
Bearer channels
The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.
Each one of these channels is known as a DS0.
Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines. This was commonplace in the 20th century, but has since become less so.
Signaling channel
The signaling channel (D) uses Q.931 for signaling with the other side of the link.
X.25
X.25 can be carried over the B or D channels of a BRI line, and over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale (credit card) terminals because it eliminates the modem setup, and because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines.
X.25 was also part of an ISDN protocol called "Always On/Dynamic ISDN", or AO/DI. This allowed a user to have a constant multi-link PPP connection to the internet over X.25 on the D channel, and brought up one or two B channels as needed.
Frame Relay
In theory, Frame Relay can operate over the D channel of BRIs and PRIs, but it is seldom, if ever, used.
Sample call
The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPD and the Q.931/Network message intermixed (i.e. exactly what was exchanged on the D-channel). The call is originating from the switch where the trace was taken and goes out to some other switch, possibly an end-office LEC, who terminates the call.
The first line format is <time> <D-channel> <Transmitted/Received> <LAPD/ISDN message ID>. If the message is an ISDN level message, then a decoding of the message is attempted showing the various Information Elements that make up the message. All ISDN messages are tagged with an ID number relative to the switch that started the call (local/remote). Following this optional decoding is a dump of the bytes of the message in <offset> <hex> ... <hex> <ascii> ... <ascii> format.
The RR messages at the beginning prior to the call are the keep alive messages. SETUP message indicate the start of the call. Each message is acknowledged by the other side with a RR.
10:49:47.33 21/1/24 R RR
0000 02 01 01 a5 ....
10:49:47.34 21/1/24 T RR
0000 02 01 01 b9 ....
10:50:17.57 21/1/24 R RR
0000 02 01 01 a5 ....
10:50:17.58 21/1/24 T RR
0000 02 01 01 b9 ....
10:50:24.37 21/1/24 T SETUP
Call Reference : 000062-local
Bearer Capability : CCITT, Speech, Circuit mode, 64 kbit/s
Channel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive
Calling Party Number : 8018023000 National number User-provided, not screened Presentation allowed
Called Party Number : 3739120 Type: SUBSCRB
0000 00 01 a4 b8 08 02 00 3e 05 04 03 80 90 a2 18 03 .......>........
0010 a9 83 85 6c 0c 21 80 38 30 31 38 30 32 33 30 30 ...l.!.801802300
0020 30 70 08 c1 33 37 33 39 31 32 30 0p..3739120
10:50:24.37 21/1/24 R RR
0000 00 01 01 a6 ....
10:50:24.77 21/1/24 R CALL PROCEEDING
Call Reference : 000062-local
Channel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive
0000 02 01 b8 a6 08 02 80 3e 02 18 03 a9 83 85 .......>......
10:50:24.77 21/1/24 T RR
0000 02 01 01 ba ....
10:50:25.02 21/1/24 R ALERTING
Call Reference : 000062-local
Progress Indicator : CCITT, Public network serving local user,
In-band information or an appropriate pattern is now available
0000 02 01 ba a6 08 02 80 3e 01 1e 02 82 88 .......>.....
10:50:25.02 21/1/24 T RR
0000 02 01 01 bc ....
10:50:28.43 21/1/24 R CONNECT
Call Reference : 000062-local
0000 02 01 bc a6 08 02 80 3e 07 .......>.
10:50:28.43 21/1/24 T RR
0000 02 01 01 be ....
10:50:28.43 21/1/24 T CONNECT_ACK
Call Reference : 000062-local
0000 00 01 a6 be 08 02 00 3e 0f .......>.
10:50:28.44 21/1/24 R RR
0000 00 01 01 a8 ....
10:50:35.69 21/1/24 T DISCONNECT
Call Reference : 000062-local
Cause : 16, Normal call clearing.
0000 00 01 a8 be 08 02 00 3e 45 08 02 8a 90 .......>E....
10:50:35.70 21/1/24 R RR
0000 00 01 01 aa ....
10:50:36.98 21/1/24 R RELEASE
Call Reference : 000062-local
0000 02 01 be aa 08 02 80 3e 4d .......>M
10:50:36.98 21/1/24 T RR
0000 02 01 01 c0 ....
10:50:36.99 21/1/24 T RELEASE COMPLETE
Call Reference : 000062-local
0000 00 01 aa c0 08 02 00 3e 5a .......>Z
10:50:36.00 21/1/24 R RR
0000 00 01 01 ac ....
10:51:06.10 21/1/24 R RR
0000 02 01 01 ad ....
10:51:06.10 21/1/24 T RR
0000 02 01 01 c1 ....
10:51:36.37 21/1/24 R RR
0000 02 01 01 ad ....
10:51:36.37 21/1/24 T RR
0000 02 01 01 c1 ....