ISDN is a system with digitized phone connections which has been pure analogue connection for a decade. This is the first protocol defines on a digital communications line that allows for the transmission of voice, data, video and graphics, at very high speeds, over standard communication lines simultaneously, carried by bearer channels (B channels) occupying a bandwidth of 64 kbits per second (some switches limit it to a capacity of 56 kb/s). A data channel (D channel) is defined handles signaling at 16 kb/s or 64 kb/s, depending on the service type. ISDN is not restricted to public telephone networks alone; it may be transmitted via packet switched networks, telex, CATV networks, etc. There are two basic types of ISDN service:

• Basic Rate Interface (BRI) - consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. This basic service is intended to meet the needs of most individual users. The U interface provided by the telco for BRI is a 2-wire, 160 kb/s digital connection. Echo cancellation is used to reduce noise, and data encoding schemes (2B1Q in North America, 4B3T in Europe) permit this relatively high data rate over ordinary single-pair local loops.
• Primary Rate Interface (PRI) which is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s. It is also possible to support multiple PRI lines with one 64 kb/s D channel using Non-Facility Associated Signaling (NFAS).

The CCITT (now ITU-T) study group responsible for ISDN first published a set of ISDN recommendations in 1984 . Prior to this publication, various geographical areas had developed different versions of ISDN. The use of nation-specific information elements is enabled by using the Codeset mechanism which allows different areas to use their own information elements within the data frames. Some common nation-specific ISDN variants are: National ISDN used in the USA is by Bellcore. It has four network-specific message types. It does not have any single octet information elements. Other changes are the addition of the SEGMENT, FACILITY and REGISTER message types and the Segmented Message and Extended Facility information elements. Also, some meanings of field values have changed and some new accepted field values have been added.

Due to its limitation of bandwidth and services, this traditional ISDN is called narrowband ISDN, in contrast to the BISDN (Broadband ISDN).

Protocol Structure

Below is the general structure of the ISDN frame:

• Protocol discriminator - The protocol used to encode the remainder of the Layer.
• Length of call reference value - Defines the length of the next field. The Call reference may be one or two octets long depending on the size of the value being encoded.
• Flag - Set to zero for messages sent by the party that allocated the call reference value; otherwise set to one.
• Call reference value - An arbitrary value that is allocated for the duration of the specific session, which identifies the call between the device maintaining the call and the ISDN switch.
• Message type - Defines the primary purpose of the frame. The message type may be one octet or two octets (for network specific messages). When there is more than one octet, the first octet is coded as eight zeros. A complete list of message types is given in ISDN Message Types below.
• ISDN Information Elements - there are two types of information elements: single octet and variable length. Single octet information elements - the single octet information element appears as follows:

Variable length information elements - The following is format and the variable length information element:

The information element identifier identifies the chosen element and is unique only within the given Codeset. The length of the information element informs the receiver as to the amount of the following octets belonging to each information element.

• ISDN Message Types - The possible ISDN message types: Call Establishment, Call Information Phase, Call Clearing, and Miscellaneous.
• Codeset - Three main Codesets are defined. In each Codeset, a section of the information elements are defined by the associated variant of the protocol:
  • Codeset 0 The default code, referring to the CCITT set of information elements.
  • Codeset 5 The national specific Codeset.
  • Codeset 6 The network specific Codeset.

CPE - Customer Premises Equipment - refers to all ISDN compatible equipment connected at the user sight. Examples of devices are telephone, PC, Telex, Facsimile, etc. The exception is the FCC definition of NT1. The FCC views the NT1 as a CPE because it is on the customer sight, but the CCITT views NT1 as part of the network. Consequently the network reference point of the network boundary is dependent on the variant in use.

ISDN Channels B, D and H - The three logical digital communication channels of ISDN perform the following functions:

B-Channel - Carries user service information including: digital data, video, and voice.

D-Channel - Carries signals and data packets between the user and the network

H-Channel - Performs the same function as B-Channels, but operates at rates exceeding DS-0 (64 Kbps). They are implemented as H0 (384 kb/s (6 B channels), H10 (1472 kb/s -23 B channels), H11 (1536 kb/s - 24 B channels), and H12 (1920 kb/s for International -E1 only).

Related protocols: LAP-D, BRI, PRI, Q.920-Q.923, LAP-B, X.25, Q.931, ATM, BISDN

Sponsor Source: ISDN protocol is defined by ITU I-series and G-series documents (Physical Layer) and Q-series documents (Data-link and Network layers).

Reference:
http://www.nationalisdncouncil.com/isdnassistance: The National ISDN Council (NIC) specification
http://www.catcouncil.org/isdn/index : The Council for Access Technologies (CAT formerly NIC)specification and documents.