ISDN-2

The early phone network consisted of a pure analog system that connected telephone users directly by a mechanical interconnection of wires. This system was very inefficient, was very prone to breakdown and noise, and did not lend itself easily to long-distance connections. Beginning in the 1960s, the telephone system gradually began converting its internal connections to a packet-based, digital switching system. Today, nearly all voice switching in the U.S. is digital within the telephone network. Still, the final connection from the local central office to the customer equipment was, and still largely is, an analog Plain-Old Telephone Service (POTS) line.
 * __HISTORY__**

A standards movement was started by the International Telephone and Telegraph Consultative Committee (CCITT), now known as the International Telecommunications Union (ITU). The ITU is a United Nations organization that coordinates and standardizes international telecommunications. Original recommendations of ISDN were in CCITT Recommendation I.120 (1984) which described some initial guidelines for implementing ISDN.

Local phone networks, especially the regional Bell operating companies, have long hailed the system, but they had been criticized for being slow to implement ISDN. One good reason for the delay is the fact that the two major switch manufacturers, Northern Telecom (now known as Nortel Networks), and AT&T (whose switch business is now owned by Lucent Technologies), selected different ways to implement the CCITT standards. These standards didn't always interoperate. This situation has been likened to that of earlier 19th century railroading. "People had different gauges, different tracks... nothing worked well."

In the early 1990s, an industry-wide effort began to establish a specific implementation for ISDN in the U.S. Members of the industry agreed to create the National ISDN 1 (NI-1) standard so that end users would not have to know the brand of switch they are connected to in order to buy equipment and software compatible with it. However, there were problems agreeing on this standard. In fact, many western states would not implement NI-1. Both Southwestern Bell and U.S. West (now Qwest) said that they did not plan to deploy NI-1 software in their central office switches due to incompatibilities with their existing ISDN networks.

Ultimately, all the Regional Bell Operating Companies (RBOCs) did support NI-1. A more comprehensive standardization initiative, National ISDN 2 (NI-2), was later adopted. Some manufacturers of ISDN communications equipment, such as Motorola and U S Robotics (now owned by 3Com), worked with the RBOCs to develop configuration standards for their equipment. These kinds of actions, along with more competitive pricing, inexpensive ISDN connection equipment, and the desire for people to have relatively low-cost high-bandwidth Internet access have made ISDN more popular in recent years.

Most recently, ISDN service has largely been displaced by broadband internet service, such as xDSL and Cable Modem service. These services are faster, less expensive, and easier to set up and maintain than ISDN. Still, ISDN has its place, as backup to dedicated lines, and in locations where broadband service is not yet available.

 Integrated Services Digital Network (ISDN) was designed in the mid-1980s to provide a worldwide public telecommunications network that would support telephone signals, data, and a multitude of other services for both residential and business users. With ISDN connections, no carrier signal is present. Only the actual user data is transmitted and it transfers at a fraction of the time compared to analog service. Therefore, implementing ISDN switches can actually reduce traffic overhead for service a provider which reduces their service costs. ISDN provides services such as facsimile; teletex; alarm systems; and modern telephone services, including call transfer, caller identification, caller restriction, call forwarding, call waiting, hold, conference call, and credit-card calling. All ISDN services are provided simultaneously, at varying data transfer rates, and users are charged only according to the capacity used, not the connection time. ISDN defines three kinds of channels: B channel, D channel, and H channel. The B channel, which stands for bearer??? channel, is a 64-kbps full-duplex channel, capable of carrying digital data, pulse code modulated voice, or a mixture of lower-data-rate traffic, such as home burglar alarm systems and emergency services. Three types of connections can be established using a B channel: circuit-switched connections (dial-up telephone); packet-switched connections into the Internet or a public data network; and semi-permanent connections, or leased lines. The second type of channel supported by ISDN is the D channel. It is a 16- or 64-kbps full-duplex channel that can carry the signaling information for the B channel. The D channel may also be capable of carrying packet-switched data or low-speed data such as burglar alarms and emergency services information. The third channel is the H channel, which stands for high-speed data channel. The H channel is a full-duplex channel that can support three different data rates: 384 kbps, 1.536 Mbps, or 1.92 Mbps. It is capable of carrying high-speed data for applications such as facsimile, video, high-quality audio, or multiple lower-speed data streams. Different combinations can be created by the ISDN provider for varying user requirements. For example, a Basic Rate Interface (BRI) package can be created, which consists of two full-duplex 64-kbps B channels plus one full-duplex 16-kbps D channel. A BRI package is standard for most residential services and many small businesses. It allows the simultaneous transfer of voice as well as a number of data applications. Since most ISDN subscribers already have a standard voice telephone line, they commonly tie their two B channels for a combined data transfer rate of 128 kbps. For users requiring larger data capacity, a Primary Rate Interface package is good. It consists of twenty-three 64-kbps B channels plus one 64-kbps D channel, for a combined data transfer rate of 1.544 Mbps, which is the same speed as a T-1 line. ISDN standards define different types of devices that have certain functions but may not represent physical devices so they called functional devices. The devices communicate between each other through interfaces and special protocols. These interfaces are called reference points. Shown in the next figures
 * __Implementation of ISDN__**



Figure 4



Figure 5

 In the U.S., the Telephone Company will be providing it's BRI customers with a U interface. The U interface is a two-wire (single pair) interface from the phone switch. It supports full-duplex data transfer over a single pair of wires, therefore only a single device can be connected to a U interface. This device is called an Network Termination 1 (NT-1). The situation is different elsewhere in the world, where the phone company is allowed to supply the NT-1, and thereby the customer is given an S/T interface. The NT-1 is a relatively simple device that converts the 2-wire U interface into the 4-wire S/T interface. The S/T interface supports multiple devices (up to 7 devices can be placed on the S/T bus) because, while it is still a full-duplex interface, there is now a pair of wires for receive data, and another for transmit data. Today, many devices have NT-1s built into their design. This has the advantage of making the devices less expensive and easier to install, but often reduces flexibility by preventing additional devices from being connected. Technically, ISDN devices must go through an Network Termination 2 (NT-2) device, which converts the T interface into the S interface (Note: the S and T interfaces are electrically equivalent). Virtually all ISDN devices include an NT-2 in their design. The NT-2 communicates with terminal equipment, and handles the Layer 2 and 3 ISDN protocols. Devices most commonly expect either a U interface connection (these have a built-in NT-1), or an S/T interface connection.

Devices that connect to the S/T (or S) interface include ISDN capable telephones and FAX machines, video teleconferencing equipment, bridge/routers, and terminal adapters. All devices that are designed for ISDN are designated Terminal Equipment 1 (TE1). All other communication devices that are not ISDN capable, but have a POTS telephone interface (also called the R interface), including ordinary analog telephones, FAX machines, and modems, are designated Terminal Equipment 2 (TE2). A Terminal Adapters (TA) connects a TE2 to an ISDN S/T bus. Going one step in the opposite direction takes us inside the telephone switch. Remember that the U interface connects the switch to the customer premises equipment. This local loop connection is called Line Termination (LT function). The connection to other switches within the phone network is called Exchange Termination (ET function). The LT function and the ET function communicate via the V interface. an immediate response. Unfortunately, due in large part to the telephone companies’ earlier lack of preparation, the acceptance of ISDN among users has been slow to spread, and other technologies are quickly leaving it behind. In fact, a 128-kbps digital telecommunications line into the home is looking fairly old-fashioned when compared with newer technologies such as digital subscriber line and cable modems.

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