06.621 Data Communications and Networks 2
Week 12 – ISDN

By the end of this session the student should be able to analyse and evaluate a selection of data communication services. In this session the student will look at the features of ISDN and its application areas.

Integrated Services Digital Network (ISDN)
ISDN is a suite of services based on a set of technologies, including transmission, switching, and signaling and control. ISDN is a set of international standards recommendations which will allow the provisioning of a wide range of services which are intended to be available on a ubiquitous basis. Additionally, the ISDN network is accessible through a standard set of interfaces—one for low-bandwidth applications and another for high-bandwidth.

The specific characteristics of ISDN include its entirely digital nature. Three channel types are identified, including B-channels (Bearer channels) that carry the information, D-channels (Data channels) for signaling and control, and H-channels (High-speed channels) for channel aggregation in order to accommodate bandwidth-intensive applications. The User Network Interface (UNI) protocols include Basic Rate Access (BRA) for low-speed termination, and Primary Rate Access (PRA) for high-speed access.

Announced to the world with great fanfare, ISDN quickly captured the imagination of carriers, manufacturers and user organizations worldwide. ISDN offers the compelling advantages of improved bandwidth, flexibility, error performance, reliability, availability, and interconnection to a wide range of services. Unfortunately, it has stalled. For the past 25 years, ISDN has progressed at such a slow pace that it has become legendary. Among the many reasons for its slow development are slow standards development, lack of adherence to standards, lack of availability, regulatory hurdles, circuit and equipment costs, and poor marketing.

Rates for ISDN access have not been tariffed at attractive levels compared with the cost of basic services. Again, the regulators were, and still are, largely responsible. The charges typically are higher for a data call than for a voice call.

Equipment costs were high, as the manufacturers were constantly investing in R&D to maintain ISDN compliance with developing standards. Additionally, the limited demand for ISDN caused the manufacturing runs to be small—lower costs are achieved through increased volume.

While ISDN has frustrated the industry, in general, there currently are over 200,000 ISDN access lines worldwide. Some industry pundits forecast that more than 750 million will be installed by the year 2000—the author does not share that level of enthusiasm. ISDN is much more mature in Europe and certain parts of the Pacific Rim than it is in the United States. In those regions, deployment was encouraged by the regulators and even subsidized by the governments. Additionally, marketing was much more effective, focusing on PRA, rather than BRA.

Standard Interfaces and Channel Types
The current version of ISDN is Narrowband ISDN (N-ISDN).  ISDN currently is available in essentially two interface varieties BRA (2B+D) and PRA (23B+D in North America, and 30B+D in Europe and many other countries).

Figure  ISDN BRI and PRI interfaces.

Basic Rate Access (BRA)
Basic Rate Access (BRA) is also known as 2B+D and Basic Rate Interface (BRI). BRA provides 2 B channels (Bearer, or information-bearing channels), each operating at the clear-channel rate of 64 Kbps by virtue of SS7 nonintrusive signaling. Each B channel can carry digital data, digitized voice (PCM-encoded at 64 Kbps or a lower rate), or a mixture of low-speed (subrate) data as long as it all is intended for the same destination. BRA also provides a D (Data) channel at 16 Kbps, which is used for control, messaging, and network management. The D channel also generally is made available for packet data transmission and low-speed telemetry when not in use for signaling purposes. Cost-effective applications include credit card authorization, which involves very small bursts of data [8-30]. BRA is primarily used for residential, small business, Centrex, and telecommuting applications that are not particularly bandwidth-intensive. The B channels can be aggregated or bonded, to provide up to 128 Kbps to a given conversation, such as a videoconference; additionally, multiple BRIs can be bonded for even greater capacity. Whether bonded or not, ISDN BRA provides multiple channels over a single physical loop, which is a great advantage.

A single BRA line can support up to eight devices, which might be in the form of telephones, facsimile machines, or computers. Additionally, up to 64 individual telephone numbers can be supported. While BRA supports as many as three simultaneous calls, only one can be a voice conversation.

Primary Rate Interface (PRA)
Primary Rate Access (PRA) is also known as 30B+D. PRA offers 30 B (Bearer) Channels, plus 1 D (Data) channel. Both the B and D channels operate at 64 Kbps. The individual B channels can be used as discussed in the case of BRI; the D channel is reserved exclusively for signaling. As the standards provide for a D channel to support up to five PRI connections, numerous carriers have recently embraced this concept, thereby yielding additional usable bandwidth for user application.

PRA provides a full duplex (FDX) point-to-point connection through a NT2-type intelligent CPE switching device (e.g., a PBX or router) device for protocol interface with the carrier CO exchange switch.

While designed for transmission over a standard T1 trunk, PRA is a significant improvement over T1, because the channels can be allocated dynamically. Each channel can act as an incoming, outgoing, combination, or DID trunk as the need arises. The nature of the channel can be determined as required or as specified, varying based on user-definable parameters. Additionally, multiple B channels can be aggregated to serve bandwidth-intensive applications, such as videoconferencing.

ISDN Equipment
ISDN hardware, at the end user side of the connection, includes Terminal Equipment (TE), Terminal Adapters (TAs), and Network Terminations (NTs). The carrier requires digital COs that are equipped with ISDN and SS7 software.

• Terminal Equipment (TE) Terminal Equipment (TE) is the term for a functional device that connects a customer site to ISDN services. Examples include computers, telephones, facsimile machines, and videoconferencing units. TE1 has a built-in ISDN interface, while TE2 devices do not have native ISDN compatibility.
• Terminal Adapters (TAs) Terminal Adapters (TAs) are interface adapters for connecting one or more TE2 (non-ISDN) devices to an ISDN network. TAs act as ISDN DCE, serving a function equivalent to protocol or interface converters. Applied to equipment which does not have ISDN capability built within it; the TAs must be exactly tuned to the specific CPE/DTE. TAs can be in the form of either standalone units or printed circuit boards fitting into an expansion slot of a PC.
• A key function of the TA is that of rate adaption, which effectively throttles down the transmission rate from 64 Kbps to the rate at which the non-ISDN device is capable [8-32]. As an example, a non-ISDN PC might be capable of only 19.2 Kbps through the serial port. Another example might involve a connection supported at only 56 Kbps (non-ISDN or Pacific Bell “ISDN”) on the receiving end; the device would throttle down to that rate, rather than 64 Kbps. Rate adaptation is accomplished in North America through the ITU-T V.120 protocol; the European standard is V.110.
• Network Terminations (NTs) Network Terminations 0(NTs) are Network Termination devices, NT1s and NT2s.
• NT2s NT2s (Network Termination type 2) are intelligent devices responsible for the user’s side of the connection to the network, performing such functions as multiplexing, switching or ISDN concentration. A NT2 device would likely be in the form of a PABX, LAN router or switching hub.
• NT1s NT1s (Network Termination type 1) physically connect the customer site to the carrier side of the connection, performing such functions as signal conversion and maintenance of the local loop’s electrical characteristics. In a PRI environment, these functions are similar to those provided by Data Service Units (DSUs) and Channel Service Units (CSUs). In a BRI environment, these devices are TE1 devices.
• Inverse MUXs, Inverse MUXs, offered by some manufacturers, allow multiple BRIs to be bonded, or linked, for greater aggregate transmission over a BRI circuit(s). For example, four BRIs can be linked to support a 512 Kbps data transmission. Such an approach competes effectively with Fractional T1 (FT1).
• D-Channel Contention Devices D-Channel Contention Devices, offered by some manufacturers, allow as many as eight devices to share a BRI circuit, contending for access to the B-channel. The individual devices identify themselves to the network through contention for the D channel.

ISDN Characteristics And Benefits
ISDN is unusual, if not unique, as it is undoubtedly the most carefully planned, well coordinated, and best documented network technology in history. Despite this fact, it will be at least a decade before it is widely available and implemented. The key characteristics of ISDN include its end-to-end digital nature, unusual for a circuit-switched network.

ISDN’s reliance on SS7 offers a number of advantages, including faster call setup and nonintrusive signaling and control. Additionally, SS7 (either with or without ISDN) makes possible a number of interesting CLASS services, including Caller ID, Name ID, call trace, selective call forwarding, and selective call blocking.

ISDN also is interoperable with X.25, Frame Relay, SMDS and ATM. In fact ISDN standards were developed specifically with these services in mind. Frame Relay, in particular, is closely aligned with ISDN link-level protocols. Broadband ISDN (B-ISDN), although many years in the future, is dependent on ATM network technology.

ISDN Characteristics and Drawbacks
ISDN is not without its drawbacks, which include limited availability, standards variations, channel application, and high cost. As discussed earlier, availability is limited in most nations, because ISDN and SS7 software are costly for the carriers to deploy. While ISDN capability can be extended to non-ISDN CO exchanges, that incremental cost is not trivial. As discussed previously, standards vary by carrier and manufacturer, and BRI limits the user to a single voice channel.

Not surprisingly, cost/benefit considerations dictate the success or failure of technologies, regardless of how compelling they appear at first glance. The cost of an ISDN BRA circuit is more than twice that of an analog line—installation costs can range as high as $578.00 per BRA. These costs tend to discourage ISDN to a considerable extent, particularly in voice-intensive environments—although ISDN provides three channels, only one can be used for voice at any given time.

Additionally, many carrier tariffs impose a usage surcharge in the form of a flat rate per minute. The usage charge may apply to local, as well as long distance, calls. Additionally, data calls often carry a higher usage rate than do voice calls. While this additional usage charge tends to discourage ISDN usage, the call connect time may be reduced significantly through ISDN.

Hardware Costs are additional in support of ISDN. In a BRA environment, such additional equipment might include Terminal Adapters (TAs), Inverse MUXs, and BRI contention devices. In a PRA application, ISDN software for PBXs and routers is an additional cost. Older systems might require replacement in order to effect ISDN compatibility.

ISDN Applications
The applications for ISDN are broad in range. While ISDN was long described as “a technology in search of an application,” it recently has been opened to applications developers, and aggressively so. There is no killer ap for ISDN, rather there are a number of applications that, in total, promise to ensure the future of ISDN. A host of applications which benefit from the improved quality of digital networking and that are bandwidth-intensive are well served by ISDN. Further, ISDN offers an affordable circuit-switched alternative to DDS, Switched 56, and T-carrier, which simply cannot be cost-justified in many cases.

Personal office internetworking, remote office internetworking, and telecommuting (or Telework) all are facilitated by the increased bandwidth and error performance offered by ISDN BRA. In such applications, file transfers and facsimile transmission are accomplished much more quickly and with much greater clarity; the improved quality of the voice communications are an added benefit.

ISDN also is used for access to X.25 packet networks, with users benefiting from the faster call setup and teardown time made available by virtue of SS7. As either the B channels or D channel can be used for packet data transfer in a BRI implementation, ISDN offers additional flexibility and bandwidth utilization. Additionally, some manufacturers of Terminal Adapters offer built-in X.25 PADs for end-to-end error correction.

As a replacement or backup for dedicated digital services, ISDN performs well for data and image networking, whether in a host-to-host, LAN-to-LAN, or remote LAN access application. Intensive users of the Internet and World Wide Web find ISDN bandwidth to be of great advantage, as the speed of call setup and file transfer are much increased compared to dialup analog connections.