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ISDN Primer

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Introduction to ISDN

The Integrated Services Digital Networks standard is governed by ITU-T (formerly CCITT). The standard defines a general digital telephone network specification and has been in existence since the mid 1980s. Although ISDN offers a completely different method for organizing information being transported from point to point, (that is, the use of a digital signal vs. an analog signal) one of its major strengths is its compatibility with much of the in-place international telephone/communications infrastructure. Existing switches and most in-place wiring may be used to hook up both residential and business customers. Although ISDN is designed to operate with special digital terminal equipment, 'standard' telephones and fax machines can be attached to ISDN with adapters.


Basic Rate (BRI) vs Primary Rate (PRI) Interfaces

An ISDN "call" is a 64 kbps end-to-end channel controlled by standardized protocol. A Basic Rate Interface (BRI) is two 64K bearer ("B") channels and a single delta ("D") channel. The B channels are used for voice or data, and the D channel is used for signaling and/or X.25 packet networking. This is the variety most likely to be used in residential services. An organization may have several BRIs linked to a CO with all switching operations for those BRIs accomplished at the CO.

The ISDN Primary Rate Interface in NorthAmerica and Japan consists of 24 channels, usually divided into 23 B channels and 1 D channel, running over the same physical interface as T1. In other countries where E1 is the standard and bandwidth is greater, the PRI has 31 user channels, divided into 30 B channels and 1 D channel. The PRI is typically used for connections such as between a PBX and a central office (CO) of a local or long distance telephone company. In contrast to an organization with several BRIs and CO switching, the PRI offers the opportunity for a PBX or other controlling device to distribute the B channels as needed throughout the organization.


Basic Rate (2B+D) ISDN Configuration

An ISDN BRI U-Loop is 2 conductors with a maximum length of 18,000 feet running from the CO to the customer residential or office premises . The ISDN pairs may be the same wire pairs used for regular telephone service. In North America 90% of existing telephone lines need no conditioning in order to be used for ISDN BRI service.

The TA - To interoperate with existing terminal equipment standards such as RS-232 and V.35 ISDN requires a Terminal Adapter (TA) at the point of connection. Usually TAs are small modules, computer bus cards or interface cards that plug into PBXs or other communications equipment. Today's digital to analog signal converting modems are not needed in the ISDN circuit, and are 'replaced' by the TAs.

Local area networks may be connected using an ISDN channel to transport data. Bridges and Routers for this purpose often provide features such as demand dialing and data compression.

The NT1- At the customer premises the U-loop is terminated by an NT1 (network termination device). The connection between the NT1 and customer supplied equipment is called 'point T'. ISDN data terminals, telephones, FAX machines, etc. can connect directly to NT1 at 'point T'. If a PBX is used, the connection between the PBX and the terminal is referred to as 'point S'. 'Point S' is physically similar but logically different from 'point T' allowing the central office equipment to distinguish the PBX from other terminal equipment (TE).

The S/T bus - The NT1 drives a 4-conductor S/T-bus (named after points S and T) which may be expanded to 8 conductors to provide for emergency power. The North American implementation however provides for no emergency mode of operation in the event of power failure.

The S/T bus can be implemented using the same cable and connectors as in 10 base T Ethernet, with up to 8 devices on the S/T bus. Unlike 10 base T Ethernet, the S/T bus is not limited to a star configuration and may be connected with splitters and T connectors. The D channel is used to control the connection of the one to eight devices to the two B channels. No two devices connect to the same B channel at the same time.

TE Arbitration - A major function of the NT1 is to allow more than one device to have access to the 2 B channels provided by the ISDN BRI. An ISDN telephone, an ISDN FAX and an ISDN computer interface attached to the BRI will each listen for calls and only connect to a B channel when it identifies a message requesting a service it can provide. In this way a FAX terminal will not answer an incoming voice call or computer data call.

Encoding - The NT1 also performs other functions; it translates the bit encoding scheme used on the lines between it and the telephone company (the U loop) to the encoding used between it and the devices. These schemes are different because the device-to-NT encoding was designed to enable channel sharing whereas the NT-to-CO encoding was designed to allow transmission across long distances.

The SPID - SPIDs are Service Profiles IDs. SPIDs are used to identify the services and features the switch is to provide to the ISDN device. Currently they are used only for circuit-switched service (as opposed to packet-switched). Annex A to ITU recommendation Q.932 specifies the (optional) procedures for SPIDs. They are most commonly implemented by ISDN equipment used in North America.

When a new subscriber is added, the CO personnel allocate a SPID just as they allocate a directory number. In many cases, the SPID number is identical to the (full ten digit) directory number.

Subscribers must configure the SPID into their telephone, computer, and other ISDN terminals before they will be able to connect to the central office switch. Once the SPID is configured, the terminals go through an initialization/identification state in which the terminal sends the SPID to the network. After confirmation the SPID is not sent to the switch again.


International Equipment Compatibility Issues

Voice Encoding - ISDN telephone equipment designed for use in North America and Japan uses mu-law encoding when converting from analog to digital, whereas the rest of the world uses A-law. To operate properly in both North America and Europe, for example, the equipment must have some option to select either of these encoding types.

Homologation - In most countries the manufacturer of ISDN (and other network) terminals must obtain certifications of approval from various governing agencies before the equipment may be connected to the network. These approvals affect both interoperability and operational safety of the equipment and the network.

Basic Configuration Differences - In the US the TA and NT1 are both provided by the customer, while in Europe the NT1 is provided by the telephone company. Because of this, base level equipment (with the NT1 incorporated) in the U.S. attaches to 'point U' whereas in Europe base level equipment plugs into 'point T'.

NIUF - To remedy this situation the bNational ISDN User's Forum/b is defining standards that increase the uniformity of ISDN services. In Europe, a new standard called NET3 is being developed to the same end.

TE/Switch Compatibility - The National ISDN-1 standard specifices how terminal equipment communicates with the central office through the call setup dialogue, although in practice different switching equipment may require different setup dialogues. To compensate for this, carefully designed ISDN TE equipment can be configured to communicate with all vendors switching equipment.


ISDN Service Installation and Features

The ISDN line may act like two independent phone lines with two numbers. Features available to the customer depend on the capabilities of the equipment at the CO. BRI ISDN phones can support key-set features (for example, conferencing, forwarding, voice mail) similar to that of a PBX. As with 'regular' telephone systems, switching for larger customers may be accomplished at a CO where several BRIs maybe linked together.

Data Calls - Without the overhead of D/A conversion through a modem, data calls between a computer or other interface equipment and the ISDN BRI service using all 56kbps or 64kbps available in the central office may be placed. The availability of increased bandwidth facilitates the transmission of high fidelity audio, video signals and various types of secure encrypted data.

Audio/Video over ISDN - Video transmission over narrowband ISDN is governed by a suite of ITU-T (formerly CCITT) interoperability standard H.320. H.320 is an "umbrella" standard; it specifies H.261 for video compression, H.221, H.230, and H.242 for communications, control, and indication, G.711, G.722, and G.728 for audio signals, and several others for specialized purposes.

Resolution - An H.320-compatible terminal can support audio and video in one B channel resulting in 16 kb/s in audio bandwidth and 46.4 kb/s for video. The resolution of this video image is either 352x288 or 176x144 with speeds up to 30 frames/second. The resulting image looks comparable to a VHS videotape picture. In video phone applications, the rate of 15 frames/second is adequate at best.


Optimizing Narrow Band ISDN : Bonding

An inverse multiplexing method of the Bandwidth ON Demand INteroperability Group specifies a set of protocols that allow splitting of communications over sets of separate channels as if their bandwidth were combined into a single channel. In this way single 384 kb/s data stream may be transmitted over six 64 kb/s channels. The specification defines a way of calculating relative delay between multiple network channels so that the split data stream may be properly recombined.

Between Bandwidth on Demand Controllers, ISDN channels are dialed only as needed for overflow or out-of-service conditions.


ATM and the Future of Broadband ISDN

ATM (Asynchronous Transfer Mode) is a switching/transmission technique where data is transmitted in small, fixed sized cells (5 byte header, 48 byte payload). The cells lend themselves both to the time-division- multiplexing characteristics of the transmission media, and the packet switching characteristics of data networks.

Hardware Switching = Speed - In ATM, slower software switching is avoided. At each switching node, the ATM header identifies a "virtual circuit" for which the cell contains data, enabling the switch to forward the cell and data to the correct neighboring trunkline. The "virtual circuit" is set up through the appropriate ATM switches when two endpoints wish to communicate. This scheme allows ATM switching to be implemented in hardware which is fast enough to support transmission rates to 1Gb/s.

Broadband ISDN over ATM - One use of ATM is to serve as the core technology for a new set of ISDN offerings known as Broadband ISDN (B-ISDN) .

B-ISDN Services - When they become available B-ISDN services will be categorized as either Interactive Services or Distribution Services. The two methods utilizing the bandwidth of ATM have the potential to complete the promise of true telephonic multi-media including audio/video conversation and conferencing, enhanced email, on-line shopping, news, education, and television.


Technical References and Literature

"ISDN In Perspective"
Fred R. Goldsteinbr Addison-Wesley
ISBN 0-201-50016-7

"ISDN: Concepts, Facilities, and Services, Second Edition"
Gary Kessler
McGraw-Hill, 1993 (2/e).
ISBN 0-07-034247-4

"Integrated Services Digital Networks: Architectures / Protocols / Standards"
Hermann J. Helgert
Addison Wesley
ISBN 0-201-52501-1

The Basic book of ISDN (second edition)
Motorola University Press
Addison-Wesley Publishing Company Inc.
ISBN 0-201-56374-6

"Sensible ISDN Data Applications"
Jeffrey Fritzbr West Virginia University Press

"ISDN and Broadband ISDN" (2nd edition)
William Stallings
Macmillan
ISBN 0-02-415475-X

"Networking Standards: A Guide to OSI, ISDN, LAN and MAN Standards"
William Stallings
Addison-Wesley

"A Catalog of National ISDN Solutions for Selected NIUF Applications"
North American ISDN User's Forum

ISDN Sourcebook
Information Gatekeepers Inc.
214 Harvard Ave,
Boston, MA 02134


Belllcore Documents

Bellcore National ISDN Specifications
SR-NWT-001953
SR-NWT-002361
SR-NWT-002120 (National ISDN-2)

Bellcore ISDN Availability Report
WR-NWT-2102

Bellcore ISDN Deployment Data
Special Report (SR) 2102


Northern Telecom Documents

NTP 297-2401-100 ISDN System Description
NTP 297-2401-010 ISDN Product Guide


ATT Documents

ATT Technical Journal special issue on ISDN
(Volume 65, Issue 1) January/February 1986

"5ESS(rg.tm) Switch National ISDN Basic Rate Interface
Specification - 5E8 Software Release"
ATT document number 235-900-341

"5ESS(rg.tm) Switch ISDN Basic Rate Interface
Specification - 5E7 Software Release" {Custom BRI}
ATT document number 235-900-331

"5ESS(rg.tm) Switch ISDN Primary Rate Interface
Specification - 5E7 Software Release"
ATT document number 235-900-332

"5ESS(rg.tm) Switch Interface Specification to a
Packet Switched Public Data (X.75) Network -5E8 Software Release"
ATT document number 235-900-317

"5ESS(rg.tm) Switch X.75' Intranetwork Interface
Specification - 5E8 Software Release"
ATT document number 235-900-325

"5ESS(rg.tm) Switch Documentation Description and Ordering Guide"
ATT document number 235-001-001

ATT Customer Information Center Order Entry
2855 N. Franklin roadbr Indianapolis, IN 46219


Credits for Source Materials and Information:

Dave Cherkus,
UniMaster, Inc.

Pacific Bell (publication # PB 2027-2 11/94)
2600 Camino Ramon,
San Ramon, CA 94583

Advanced Computer Communications
315 Bollay Dr.
Santa Barbara, CA 93117

See Also