An ISDN Issue

Posted On 2008-03-01 by FortyPoundHead
Keywords: ISDN Networking
Tags: Networking Tutorial 
Views: 1656

ISDN has been a much-maligned telecommunications service. Saddled with secondary meanings of "I Still Don't Need it", and "I Still Don't Know", ISDN may be regaining some respect.

ISDN - Integrated Digital Services Network - has had limited success as a telecommunications service offering in the United States, primarily because of incompatibility and tariffing problems. Various ISDN switch manufacturers implemented different versions of the Consultative Committee for International Telephony and Telegraphy's 1988 ISDN specification, making one vendor's equipment unable to communicate with another's. The best you could hope for was pockets of ISDN. Service coverage had been limited to large metropolitan areas, and when available, ISDN equipment and installation costs, as well as tariffs, are higher than those for the public switched telephone network and packet-switched networks.

So while the Regional Bell Operating Companies (RBOCs) have proposed ISDN for more than a decade, it has not been widely deployed. In its stead have come less-expensive and more-available services from the RBOCs, local exchange carriers, and public-switch network providers, including Switched 56, fractional T1, and frame relay. And X.25, while geriatric, provides worldwide coverage for data communications.

But with the November 1992 ratification of National ISDN-I, a single specification finally exists for ISDN service across most parts of the United States. But national coverage still has limitations. In the first phase, only Basic Rate service is offered, meaning users have a maximum of 144Kbits/sec of bandwidth. AT&T and MCI were initially the only long-distance carriers. Some RBOCs, notably US West and Southwestern Bell, initially opted not to upgrade to ISDN-I. Compatible ISDN will be available in isolated islands across most of the country.

Over Here, Over There

The Corporation for Open Systems proposed National ISDN-I in February 1991 to stop the squabbling among the ISDN switch vendors. The November, 1992 issue represents its first fruits. Meanwhile, in Europe and Japan, vendors haven't been bickering; they've been installing ISDN to upgrade their aging X.25 packet-switched networks.

According to Frost & Sullivan (New York), the total European ISDN market was $4.62 billion in 1992, and the market research firm expected the figure to be $5.91 billion by 1994. By 1997, the ISDN market should be $10.23 billion. Germany leads the world market with the largest installed base of ISDN, holding a 30-percent share. The United Kingdom maintains a 25-percent share and France holds 20 percent. Frost & Sullivan expects to see the most growth in Spain and Italy. A standard ISDN service that will become available in several European countries starting this year will encourage market growth.

Declining prices remain key to the growth of the market. By 1997, the cost of ISDN Customer-Premises Equipment (CPE) will fall to one-twentieth of its 1992 cost. An ISDN PC terminal adapter that costs $1,000 today will cost $50 by 1997. While the greatest market growth will come in ISDN-capable PBXs, Frost & Sullivan expects to see significant growth in the PC workstation market as well as the PC board market because of the use of LANs and fax cards.

An Application Push

Manufacturers often avoid the term ISDN, because of the negative connotation that ISDN has in the United States, instead referring to the service as "bandwidth on demand". But the manufacturers' interests are piqued. Sun Microsystems (Mountain View, CA) shipped SunLink ISDN, an adapter card that provides Basic Rate Interface service to Sun SPARCstation 10 workstations running TCP/IP. The card is certified in France, Germany, the United Kingdom, Japan, and the United States. Sun also plans to OEM the card. Although Sun also offers a frame-relay card, it contends that the real global connectivity will be performed using ISDN.

Vendors such as Ascend Communications (Alameda, CA) offer LAN bridges that will connect LANs using ISDN. Ascend's Multiband LAN Service Unit can use ISDN in addition to leased 56Kbit/sec lines, full and fractional T1, and Switched 56 to interconnect LANs.

Despite the product-development and marketing efforts of the vendors, ISDN's utility will determine its success. ISDN was once positioned as the way to provide simultaneous digital-voice and data connectivity to the desktop. Stock brokers, engineers, and the like were supposed to benefit from the full integration of voice and data on the same terminal. ISDN was also positioned to deliver videotext, facsimile and advanced call management services. But ISDN did not combine the telephone and computer into one device. And LANs exploded. ISDN has been repositioned to provide services to residential users, people who work at home, and small businesses. For example, Microsoft is testing ISDN as a way to connect its employees who work at home back to the corporate LAN.

ISDN's technical details have not supported image, video, and telemetry. But its applicability has mutated. ISDN will also prove useful for users at remote sites who need real-time access to corporate databases and services. Document conferencing is also possible if a shared workspace is provided.

In addition to providing connectivity for people working at home and for small businesses, ISDN should prove useful for providing desktop videoconferencing, dial-up data services, and LAN or host interconnection. ISDN can be used to provide connections between LANs as they are needed - a feat leased lines cannot perform. Because of this flexibility, ISDN can be used to accommodate periods of heavy traffic flow, for network backup, and for disaster recovery, where ISDN connections can be set up for short periods of time or according to traffic patterns. For example, a large credit-card company uses ISDN to provide for overflow bandwidth. Instead of leasing a dedicated line to accommodate its two days of peak demand - the day after Christmas and the day after Thanksgiving - the company can use ISDN to provide bandwidth as needed.

A Tough Market

In this "bandwidth-on-demand" market, ISDN faces some stiff competition. Switched 56 and frame relay also provide similar services. Switched 56 is inexpensive and widely deployed. Because it is a switched service, it is not suitable for voice traffic, but it can carry data and digitized video.

Frame relay, ironically, is a byproduct of work on ISDN. Frame relay is still quite new-equipment is expensive and service coverage is limited-but it has gained momentum. Still, frame relay only operates at speeds up to T1 (1.544Mbits/sec), whereas ISDN can provide for greater bandwidth.

Switched Multimegabit Data Service (SMDS) is another potential competitor. Currently, it suffers from limited service coverage and overall newness of service and equipment, but SMDS is capable of carrying voice, video, and data at 45Mbits/sec.

Which technology succeeds will be determined by service coverage, service cost, product availability, and ease of use.

In general, a switched service such as Switched 56 or ISDN will make sense over a leased-line service such as fractional or full T1 when you use the link between five hours and eight hours per day.

According to Ascend Communications, a manufacturer of customer premises switched-digital access equipment, a full T1 between New York and San Francisco will cost $22,400 per month and a 384Kbits/sec fractional T1 will cost about $7,500 per month. A six-channel 56/64Kbit/sec switched service will cost about $5,000 per month for five hours of usage per day and about $7,500 to $11,000 for eight hours of usage per day, making switched services fiscally wise. For an international link, the break-even point with a 384Kbit/sec fractional T1 line is between five hours and seven hours of usage per day.

Walk on the Technical Side

ISDN consists of a Basic Rate ISDN (BRI) and a Primary Rate ISDN (PRI) service. BRI is two "Bearer" (B) channels for user-data transfer plus a "Data-link" (D) channel for control and signaling information. BRI provides a total of 144Kbits/sec of throughput. In the United States, the Primary Rate access is equivalent to a 1.544Mbit/sec T1 circuit, and it supports 23 B channels plus one 64Kbit/sec D channel - usually written as 23B+ D. In Europe, the PRI is 30 B channels plus one D channel, since the European equivalent to T1 is a 2.048Mbit/sec E1 circuit. The European PRI is typically written as 30B+D. ISDN service can be carried out over a circuit-switched or packet-switched network, although most will be circuit-switched.

Three types of ISDN channels are defined: B, D, and H. The B channel is a 64Kbit/sec clear channel that can carry any digitized data, voice, text, image, or video. A clear channel means no signaling information is sent; it is an open communications line.

The D channel, used for signaling information, can operate at either 16Kbits/sec or 64Kbits/sec. The D channel can be used for common-channel signaling, telemetry, monitoring, alarm signals, videotext, and telephone-to-telephone text messages.

There are three H channels, all providing higher-speed transmissions than the D channel. The HO channel operates at 384Kbits/sec and can be used for switched-video conferencing, high-speed fax, or packet-switched data. Up to four HO channels can be multiplexed into a single H1 channel, which operates at 1.544Mbits/sec. The H1 channel can be used for high-speed data communications or LAN interconnection. The H2 channel, which operates at 1.9Mbits/sec, is available only in Europe.

Two types of services are defined: bearer services and teleservices. Bearer services correspond to a basic service that operates over one or both of the B channels and provides lower-layer functions that are equivalent to those of OSI Layers 1 to 3, such as establishing, holding, and releasing a telecommunications path. Teleservices are the higher-layer services, such as X.400 message handling, videotext, and fax. ISDN services are separated into different layers, as is the OSI model, which provides for greater flexibility for product designers and users.

ISDN, once positioned as the way to provide simultaneous voice and data service to the desktop, is now positioned at the low end, where it will provide services to residential users, home offices, and small businesses. ISDN can provide videotext, dial-up data services, desktop teleconferencing, and advanced call management to the many users who work at home or out of small business offices. It can also deliver bandwidth to LAN users who do not need the constant connection provided by a leased-line service. Although ISDN-I has resolved the problems of incompatibility, and ISDN-2 will resolve the limited geographical availability, ISDN has to fight for its share in a market crowded with WAN service offerings.

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