ISDN—Integrated Services Digital Network

ISDN is a digital, worldwide public standard for sending voice, video, data or packets over the public switched telephone network (PSTN). It enables customers to transmit voice, data and video with one or two pairs of wires. As previously mentioned, there are two “flavors” of ISDN: basic rate (BRI), and primary rate (PRI), which are defined in Table 6.3.

Both types of ISDN have these important characteristics:

• Digital connectivity to achieve consistent, high-quality voice.

• Out-of-channel signaling— Calls are set up quickly; each voice/data channel uses all of the bandwidth for user data. The signaling channel is available for packet service such as alarm monitoring.

• A switched service— Users pay long distance or local calling fees for the amount of time they use the line.

• A standard interface— Most users with ISDN can communicate with each other. (There are incompatibilities between Europe, North America and Japan.)

ISDN standards were first published in the mid-1980s, but were finalized in the 1990s. Nortel first demonstrated a public network call over ISDN in 1987. ISDN works over existing copper wiring. It does not require fiber as do higher speed ATM and SONET services. However, central offices need ISDN-capable equipment. BRI ISDN was envisioned as an interim service until DSL became available. However, BRI ISDN never caught on in the United States. It never reached more than 1% penetration in the United States.

Berkshire Connect—Bringing Low Cost Internet Access to Outlying Communities Because T-1 access prices are distance sensitive, customers far from central offices in rural areas pay higher fees than end users in cities. Until recently, commercial organizations in western Massachusetts paid up to three times as much ($2000 and higher monthly) for T-1 Internet access as customers in the Boston area. Businesses in western Massachusetts felt that the lack of affordable telecommunications services was harmful to business development. The Massachusetts government also recognized this fact. The state appropriated $1.5 million to remedy this situation. The money was funneled through the Massachusetts Technology Collaborative, which is an economic development organization made up of state, business and educational representatives. The Collaborative developed an aggregation strategy on the assumption that few network providers serve outlying communities because there is not enough demand to warrant the construction of telecommunications facilities for reasonably priced high-speed service. The Massachusetts Technology Collaborative's first efforts were applied in the western most area of Massachusetts, the Berkshires, the center of which is the town of Pittsfield. The Massachusetts Technology Collaborative helped facilitate the process of bringing high-speed telecommunications services to the Berkshires by sponsoring meetings and hiring a consultant. The plan was to develop a request for proposal for telecommunications service. A group of 30 business people met regularly and the effort became known as Berkshire Connect. The consultant prepared the request for proposal, which was approved and sent to 15 carriers including Verizon, the incumbent local telephone company. Eight carriers responded. The group selected Global Crossing who had local microwave towers built and linked them to Verizon central offices to supply the T-1 service. The construction is mostly in place and many customers are using the service, which should be complete by the fall of 2001. T-1 prices have dropped to less than $500 monthly. Berkshire Connect used only $500,000 of the $1.5 million for the consultant, RFP printing and miscellaneous expenses. None of the money was used to subsidize the T-1 service. According to Donald Dubendorf, a partner in the Williamstown firm Grinell, Dubendorf & Smith, LLP, the group underestimated demand because previous prices were so high. To date, 50 users representing the non-profit, public and business sectors have signed contracts with Global Crossing. As hoped, the effort has had a positive impact on the local economy and attracted new businesses to the area. The Massachusetts Technology Collaborative has sponsored aggregation projects in three additional areas in Massachusetts and one effort also modeled on aggregation is underway in the Monadnock region of New Hampshire.

Table 6.3. Basic Rate and Primary Rate ISDN

ISDN Service	Number and Speed of Channels	Total Speed	Number of Pairs of Wires from Telco to Customer Premise
BRI ISDN	3 total:	144 kilobits per second	1
	2 at 64 kilobits per second
	1 at 16 kilobits per second for signaling or packetized data
PRI ISDN	24 total:	1.54 megabits per second;	2
	23 at 64 kilobits per second;
	1 at 64 kilobits per second for signaling or packetized data
PRI ISDN – Europe	32 total: 30 at 64 kilobits 2 at 64 kilobits per second for signaling	2.05 megabits per second	2

Deployment of BRI ISDN is higher in Europe and Japan than in the United States. France, Germany, Japan and Switzerland are widely acknowledged to have a large base of BRI ISDN customers. In Europe, BRI ISDN is sometimes referred to as ISDN 2 because it has two bearer channels. A user must be within 18,000 feet (3.4 miles) from the central office. According to a Bell employee source, 85% to 95% of telephone lines fit this criterion.

ISDN is a dialup service. To make a connection, the ISDN user dials a phone number. The connection is ended when the caller hangs up.

Telephone companies charge usage fees for ISDN data calls. These fees are either per-minute charges or a combination of usage fees plus flat charges for a set amount of minutes each month. In contrast, cable modems and DSL services have no usage fees. They are available for a flat monthly charge. Moreover, they are “always on.” No dialing is required to establish a connection. The usage fees for transmitting data and the lack of availability have greatly hindered acceptance of ISDN. The difficulty of installing it also is a factor—delays of months and customer confusion of how to connect it to computers is common.

Basic Rate Interface ISDN—Two Channels at 64,000 Bit per Second

Basic rate interface (BRI) consists of two bearer channels for customer voice or data at 64 Kbps. In addition, it has one 16-Kbps signaling channel. Figure 6.5 illustrates the characteristics of a BRI ISDN line. It runs over a single pair of twisted wires between the customer and the telephone company.

The most common BRI ISDN applications are:

• Desktop videoconferencing

• Centrex ISDN multiline telephones (see Chapter 2)

• Backup for Frame Relay connections

Videoconferencing Using ISDN

The price of a desktop videoconferencing system is $2500 and lower. Many organizations purchase these systems as a way to try out videoconferencing without having to buy a full-sized unit. Many desktop video systems are connected to two bearer channels of a BRI ISDN line. Two 64-Kbps lines are bonded together for a combined speed of 128,000 bits per second. Often, these video systems are shipped ISDN-ready. The equipment needed to interface with an ISDN line, terminal adapters, is built into the video system.

ISDN for Data—The Need for ISDN at Both Ends

ISDN lines cannot transmit video or data to analog lines. ISDN circuits only are compatible with other ISDN-equipped services. BRI and PRI (ISDN with 24 channels) ISDN can communicate with each other. BRI and PRI users can use ISDN for voice calls to end users that have plain old telephone service (POTS) lines.

BRI Pricing

Pricing for ISDN varies between telephone companies. Installation is in the range of $150 to $300. Monthly fees are anywhere from $10 to $40 more than the charge for an analog line. Most telcos charge business customers extra for each minute of usage. Some plans for residential and business customers include 140 to 300 hours of data transmission plus per-minute fees for usage over the included amount. Others are flat rate, no charge for usage for residential customers.

Primary Rate Interface ISDN—24 Channels

Primary rate interface (PRI) has 24 channels in the United States and Japan and 30 elsewhere in the world. In the United States and Japan, 23 are bearer channels for user data. Each bearer channel has a bandwidth of 64,000 bps. A 64-Kbps twenty-fourth channel is used for signaling. PRI trunks are used on PBXs, Centrex service and data equipment.

PRI lines are similar to T-1 because they both have 24 channels. However, PRI ISDN has out-of-band signaling on the twenty-fourth channel. On T-1 circuits, the signaling is carried within each channel along with user data. The signaling capability enables the delivery of the calling party's telephone number, as described in using a PBX with a PRI line. On data communications, the signaling channel leaves each of the bearer channels “clear” capacity for all 64,000 bits. PRI does not require any bearer channel capacity for signaling such as call setup or tear-down signals.

PRI ISDN is a trunk connection. It is installed on the “trunk” side of a PBX, or into a multiplexer. BRI ISDN is a line-side connection. It connects to the same ports in PBXs as do telephone sets.

PRI ISDN service is used for the following:

• Videoconferencing at speeds generally from 128 Kbps to 384 Kbps

• Backing up LAN-to-LAN connections

• Backing up dedicated, private lines in case the private lines fail

• ISPs for dial-in from BRI ISDN and 56-Kbps modems

• Corporate sites for remote access from 56-Kbps modems and BRI ISDN sites

When PRI is used by ISPs for modem service, CLECs install PRI ISDN from their switch to the Internet service provider modem rack. The CLEC modem rack often is in the same facility, carrier hotel, as the CLEC switch. It also can be located at a remote site. The signaling channel carries the customer telephone number and the type of modem used. This provides billing information and routing information. Moreover, the modems can handle ISDN as well as analog modem traffic.

PBXs with PRI Trunks

PBXs are used with PRI lines for:

• Call centers, to receive the telephone numbers of callers

• Individual telephone users for call screening

• Videoconferencing units that do not require the use of full-time ISDN service

Large call centers use PRI ISDN to receive the telephone number of the person calling. With ISDN, the telephone number is sent at the same time as the call. However, it is sent on the separate D, or signaling channel. This is significant because it enables the telephone system, the PBX, to treat the telephone number information differently than the call. When used in a call center, it sends the telephone number to a database that matches the telephone number to the customer account number. It then sends the account number to the agent's terminal that the call is sent to. It saves agents' time by eliminating the need to key in account numbers

Many corporations use PRI ISDN for their direct-inward dialing (DID) traffic. (See Chapter 2 for DID.) The local telephone company sends the caller's name and phone number over the signaling channel. The telephone system captures the information and sends it to the display-equipped ISDN telephone. Figure 6.6 illustrates a PRI line for transporting caller ID. Employees who receive heavy volumes of calls from vendors or who only take calls from certain callers use ISDN to screen calls. Calls not taken are forwarded automatically into voice mail.

Some organizations use ISDN-compatible videoconferencing systems as extensions of a PBX. This is called putting the video “behind” a PBX. In this way, the organization does not have to pay for dedicated BRI lines to its telephone company. Rather, the video equipment shares the PRI along with the voice telephone users. When the video is not in use, all of the PRI channels are available for voice. This capability of sharing the line for voice and data is due to the out-of-band signaling on the twenty-fourth channel. The signaling channel sends an identifier to the network telling the network that the video calls are data calls.

Companies with multiple PRI trunks can use non-facility associated signaling (NFAS) to share the twenty-fourth signaling channel among the PRI trunks. For example, an organization with six PRI trunks might have four of them equipped with 24 channels for voice and data. Two of them would have 23 channels for user data and one for the NFAS channel signaling to support all six PRI trunks.

PRI ISDN also is used on private lines that connect PBXs together. The signaling channel carries voice mail signals that identify mailbox numbers and instructions to turn message-waiting indicators on or off. It enables one voice mail system to be shared between multiple sites.

PRI for Bandwidth-on-Demand—Video and Data Devices Sharing a PRI Line

With PRI, all channels can be dynamically used, on-demand for voice or data. Setup signals carried on the signaling channel notify the public network whether calls should be sent over the public network's data network or its voice network. This is significant because all channels are available on-demand for voice or data. The data channels do not have to be reserved ahead of time strictly for video or data. With T-1, channels must be permanently set aside for data or video. During times when no video or data is sent, the channels are idle and cannot be used for voice traffic.

ISDN multiplexers have what is called “bandwidth-on-demand” for applications such as videoconferencing, which requires multiple bearer channels. For example, many companies want video systems capable of transmitting video at 384 Kbps, six bearer channels. However, the video systems are not used 24 hours a day. With bandwidth-on-demand, when the video system is not in use, the six bearer channels can be used by other data applications. Bandwidth-on-demand provides an economical use of the PRI circuit.