Once you have ISDN service, you need to know which ISDN interface your equipment expects. There are two ISDN interfaces. The U-Interface carries ISDN signals over a single pair of wires between your location and the central office. This interface is designed to carry ISDN signals over long distances. The Subscriber/Termination (S/T) Interface uses two pairs of wires to deliver the signal from the wall jack to your ISDN adapter or other ISDN equipment.

If your equipment supports the S/T-Interface, you need to get a device known as a Network Termination 1 (NT-1) which converts between the U-Interface and the S/T-Interface. The NT-1 has a jack for the U-Interface from the wall and one or more jacks for the S/T Interface connection to the PC, other ISDN or analog devices, as well as an external power supply

Some ISDN adapters sold in North America connect directly to a U-Interface. If the PC is the only equipment to be connected to an ISDN line, this type of adapter is the easiest to install. Manufacturers may describe this feature as a "built-in NT-1" or simply as a U-Interface ISDN adapter.

Wiring and Jacks

ISDN service from the phone company officially ends at what is called the demarcation point ("demarc") usually just inside the building. You are responsible for the "inside wiring" from the demarc to your ISDN equipment, including the wall jacks. The telephone company or an electrical contractor will install and maintain the inside wiring for an additional charge.

If you are just connecting your PC to the ISDN line, the wiring requirements may be very simple. Many homes and offices are wired with extra sets of telephone wires and one of those sets can be used for your ISDN line. There are a number of possible wiring pitfalls however:

• your “extra” wires may already be in use for analog line(s)
• your “extra” wires are being used to power lighted phone buttons
• your “extra” wires are not connected directly to the demarc
• the wiring may be “daisy-chain” rather than “home-run'”

Direct wiring between the ISDN wall jack and the demarc (also known as a 'home run') is recommended. For more information on wiring issues, consult your telephone company or an electrical contractor.

There are two types of jacks associated with ISDN: RJ11 - the standard analog phone jack. The RJ11 has 4 wires. The wire from the wall to the NT-1 will usually have RJ11 jacks. RJ45 - this jack is slightly wider than the RJ11, and has 8 wires. The wire from the NT-1 to the ISDN adapter will usually use RJ45 jacks.

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Connecting Multiple Devices to an ISDN Line

If you do not plan to connect anything except a single PC ISDN adapter to your ISDN line, you can ignore this section.

It is possible to connect up to eight devices to a single ISDN line. These devices can include network routers and bridges, Group 4 ISDN fax machines, ISDN telephones as well as traditional analog telephone devices. ISDN is intelligent enough to arbitrate the use of the two B channels between these devices (up to two devices can be in use simultaneously) and route incoming calls to the appropriate device.

Instead of connecting the ISDN line to a single PC, it is possible to connect an ISDN line to a LAN so all the PCs on the LAN can share the ISDN line. This requires an ISDN network bridge or router.

It is possible to connect several ISDN devices to a single ISDN line. For example, you might wish to have an ISDN adapter in your PC, an ISDN telephone to make voice calls and a Group 4 ISDN fax machine all connected to the same ISDN line. Incoming data calls would go to the PC, voice calls to the telephone and fax calls to fax machine. To support this configuration, you need an NT-1 that supports multiple S/T Interface connections. Each device would be connected to the NT-1. Each device would also need its own Service Profile Identifier (SPID) to ensure the telephone company can route calls to the appropriate device.

In addition to ISDN devices, some NT-1s or ISDN adapters also support analog telephone devices like phones, data modems, Group 3 fax machines and answering machines. The NT-1 or the ISDN adapter converts the analog signal into ISDN and vice versa.

ISDN CPE

There are two major types of National ISDN CPE. They are:

• Terminal Adapters
  
• ISDN Terminals (an actual ISDN telephone)

There is a 3rd major type of National ISDN CPE called a Network Termination Interface (NT1). There must be an NT1 in the ISDN loop before the terminal adapters or ISDN terminals will work. The NT1 converts the ISDN from a 2-wire to an 8-wire circuit. The NT1 can be a stand-alone unit or be built into the terminal adapter or ISDN terminal. Most National ISDN terminal adapters have the NT1 built into the unit.

NOTE: This document isn't intended to recommend any particular piece of ISDN CPE. While the basic functions of the CPE are pretty consistent (circuit switched data and circuit switched voice) the actual capability of any particular piece of ISDN CPE is as varied as there are manufacturers. For example, while this document will look at terminal adapters that are stand alone pieces of equipment, a terminal adapter can also be a card that is placed inside a computer.

Terminal Adapters and Routers

From the most basic standpoint, a terminal adapter (TA) could be describe as a circuit switched data device that can do some voice, and an ISDN terminal as a voice device that can do some circuit switched data. However, there are some terminal adapters that are capable of some pretty sophisticated voice, and some ISDN terminals that can do some pretty sophisticated circuit switch data, such as bonding the 2 B channels together for 128 K/bps of bandwidth.

ISDN routers are also available. Think of an ISDN router as a smart terminal adapter. Routers are targeted toward the circuit switched data users but they can also deliver voice features. Routers require the use of a Network Interface Card (NIC) and the NIC connects to the router as a 10BASE-T connection. For the purposes of this document, we'll consider the router as a type of terminal adapter.

Speed

A terminal adapter is a physical piece of equipment in which the ISDN line is terminated. Terminal adapters allow for 64 K/bps of circuit switched data, and most terminal adapters allow the user to bond together both B channels for 128 K/bps of circuit switch data. By adding a high-speed serial port to a computer and using compression, the throughput on a terminal adapter can reach speeds of 230 K/bps.

Bonding is a function of the CPE. From a network standpoint, there are two separate 64 K/bps data paths. The CPE at the receiving and sending ends bonds the 2 channels together for a 128 K/bps data stream. There are some settings in the switch that allow the CPE to be notified of an incoming call so one of the B channels can be dropped to accept the call, but the actual bonding function is a function of the CPE.

To get true 128 K/bps data speed (bandwidth) a user must be using a router. A terminal adapter, because of conversion considerations between the computer serial ports and terminal adapter, will provide 115.2 K/bps of data speed. With compression, a router may generate throughput of 512 K/bps and a terminal adapter, with a high-speed serial port, may generate throughput of 230.4 K/bps. The actual speed received can be impacted by other factors, such as Internet connections or number of people accessing the server.

AutoSPID

Automatic Service Profile Identification (AutoSPID) is a function of the terminal adapter and the ability of the switch to deliver the SPIDs. A SPID is the telephone number with four 1's after it. A SPID uses one of the B channels to deliver data or voice and normally two SPIDs are entered for each ISDN line. Once the ISDN line is plugged into the CPE and the power is turned on, the necessary communications between the switch and CPE will take place. The AutoSPID function uniquely identifies the terminal's service profile and initializes the terminal by downloading the SPIDs into the CPE. Not having to manually enter the SPIDs reduces the start-up problems. Cerzán only offers this service in Lucent 5ESS switches, today.

Features

Terminal adapters can only have Primary Directory Numbers. The normal configuration is 2 PDNs (2 different SPIDs). Terminal adapters cannot have Secondary Directory Numbers or Shared Call Appearances. The diagram below (INSERT HERE) shows the use of an analog phone plugged into the analog jack on the back of the terminal adapter. Some ISDN features that can be used with the analog phone are conference and transfer, call forwarding, speed calling and voice messaging. Call waiting functionality is available with the feature called Additional Call Offering Service.

Connections

The ISDN line terminates into a jack in the back of the TA. A cable connects the TA to the computer for 64 K/bps or 128 K/bps of digital circuit switched data. The cable may be for a USB port or a serial port on the computer, depending on which is shipped with the TA.

Most terminal adapters have 2 analog ports in the back. Standard analog telephones or FAX machines can be plugged into these ports (using standard telephone cords) and can make and receive voice calls. Depending on the CPE, most voice features can be assigned to these analog phones. Most terminal adapters are smart enough to know that if someone picks up one of the analog phones, the person wants to make a voice call, the terminal adapter will drop one of the circuit switched data channels to free up a B channel for voice.

Here's a picture (insert here) of a very normal configuration. The user's terminal adapter plugs into the computer for circuit switched data and an analog telephone plugs in the back of the terminal adapter for voice services.

ISDN Terminals

ISDN terminals are actual ISDN telephones; they have buttons for call appearances and features. ISDN terminals support Secondary Directory Numbers, Shared Call Appearances and Analog Call Appearances. ISDN terminals may also be referred to as ISDN sets or ISDN telephones.

Most ISDN terminals have a LED Display that gives Calling Line ID, Outgoing Line ID, time / date, and displays features and functions that have been built into the terminal by the manufacturer.

Some ISDN terminals have a data port in the back. The data port can run circuit switched and/or packet switched data. Some ISDN terminals even have an analog port in the back, that could be used, for example, by a fax machine. ISDN terminals can use an external or internal NT1.

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