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ISDN Glossary

2B+D : The Basic Rate Interface (BRI) in ISDN. A single ISDN circuit is divided into two 64kbps digital channels for voice or data and one 16kbps channel for low speed data and signaling. In ISDN, 2B+D is carried on one or two pairs of wires (depending on the interface), the same wire pairs that today bring a single voice circuit into your home or office.

Analog : An electrical circuit that is represented by means of continuous, variable physical quantities (such as voltages and frequencies), as opposed to discrete representations (like the 0/1, off/on representation of digital circuits).

ANSA (Alternate Network Service Agreement) : Under ANSA, customers who reside in areas where the central office switch does not support ISDN can be serviced from a neighboring central office at no additional charge. From the customer's perspective, ISDN is readily available and affordable, but the customer must agree to migrate to the local central office if and when service becomes available. In most cases this will involve a change in phone number. This agreement pertains to Bell South customers only.

AT&T 5ESS : A digital central office switching system made by AT&T.

B Channel : An ISDN communication channel that bears or carries voice, circuit, or packet conversations. The B-channel is the fundamental component of ISDN interfaces. It carries 64,000 bits per seconds in either direction.

BRI (Basic Rate Interface) : The most common ISDN interface available in the US. BRI contains two B channels, each with 64 kbps capacity, and a single D channel (16 kbps) which is used for signaling and call progress messages.

CO (Central Office) : A facility that serves local telephone subscribers. In the CO, subscribers' lines are joined to switching equipment that allows them to connect to each other for both local and long distance calls.

D-Channel : An ISDN communication channel used for sending information between the ISDN equipment and the ISDN central office switch. The D-channel can also carry "user" packet data at rates up to 9.6 Kilobits.

DMS : The name of digital central office switches from Northern Telecom. Model numbers start with BCS.

Digital : The use of a binary code to represent information, such as 0/1, or on/off.

Foreign Exchange : If your local central office is not scheduled to have ISDN for a while, it may be possible to obtain ISDN service from a nearby central office. This is called Foreign Exchange. There are additional charges associated with this type of service.

Inside Wiring : Wiring that is done from the point of demarcation to the jack in the wall where the line terminates.

Jack Type : Different types of jacks (RJ-11, RJ-45, or RJ-48) can be used for an ISDN line. The RJ-11 is the most common and is most often used for analog phones, modems, and fax machines. RJ-48 and RJ-45 are essentially the same, as they both have the same 8-pin configuration. An RJ-11 jack can fit into an RJ-45/RJ-48 connector; however, an RJ-45/RJ-48 cannot fit into an RJ-11 connector.

Loop Qualification : A test done by the phone company to make sure the customer is within the maximum distance of 18,000 feet from the central office that services that customer. Note however that ISDN service could be available at a longer distance than that with a mid-span repeater.

LATA (Local Access and Transport Area) : A geographic territory used primarily by local telephone companies to determine charges for intrastate calls. As a result of the Bell divestiture, switched calls that both begin and end at points within the LATA (intraLATA) are generally the sole responsibility of the local telephone company, while calls that cross outside the LATA (interLATA) are passed on to an Inter eXchange Carrier (IXC).

LEC (Local Exchange Carrier) : The local phone companies - either a Regional Bell Operating Company (RBOC) or an independent phone company (such as GTE) - that provide local transmission services.

Mid-span Repeater : A device that amplifies the signal coming or going to the central office. This device is necessary for ISDN service if you are outside the 18,000 feet distance requirement from the central office.

NI1 (National ISDN 1) : A specification for a "standard" ISDN phone line. The goal is for National ISDN 1 to become a set of standards to which every manufacturer can conform. For example, ISDN phones that conform to the National ISDN 1 standard will work, regardless of the central office to which the customer is connected. Note: Future standards, denoted as NI2 and NI3, are currently being developed.

NT-1 : The NT-1 (Network Termination 1) is a device that is required to connect ISDN terminal equipment to an ISDN line. The NT-1 connects to the two-wire line (twisted pair copper wiring) that your telephone company has assigned for your ISDN service. Your ISDN service will not work if the NT-1's plug is not connected to a working electrical outlet.

PBX : Private Branch eXchange is a small version of the phone company's larger central switching office. A PBX is a private telephone switch. It is connected to groups of lines from one or more central offices and to all of the telephones at the location served by the PBX.

Point of Demarcation : The physical point where the phone company ends its responsibility with the wiring of the phone line.

POP (Point Of Presence) : A long distance carrier's office in your local community. A POP is the place where your long distance carrier, or IXC, terminates your long distance lines just before those lines are connected to your local phone company's lines or to your own direct hookup. Each IXC can have multiple POPs within one LATA. All long distance phone connections go through the POPs.

POTS (Plain Old Telephone Service) : The basic telephone service - standard single line telephones, telephone lines, and access to the public switched network. There are no added features, such as call waiting or call forwarding, with POTS.

RBOC (Regional Bell Operating Company) : There are currently seven regional telephone companies that were created by the AT&T divestiture.

SON (Service Order Number) : The SON is the number issued by the local exchange carrier to confirm the order for the ISDN service. It provides a matching number for cross referencing the order to the phone company.

SPID (Service Profile Identifier) : The ISDN switch needs to have a unique identification number for each ISDN set to which it sends calls and signals.

S/T-interface : A four-wire ISDN circuit. The S/T interface is the part of a ISDN line that connects to the terminal equipment.

SVN (Subscriber Verification Number) : The SVN is the number issued by the long distance carrier to confirm the order for long distance service.

Switched 56 : Digital service at 56 Kbps provided by local telephone companies and long distance carriers. Similar to ISDN, Switched 56 traffic can travel over the same physical infrastructure that supports ISDN. Switched 56, however, is an older technology with decreasing significance.

Twisted Pair : Two insulated wires, usually copper, twisted together and often bound into a common sheath to form multi-pair cables. In ISDN, the cables are the basic path between a subscriber's terminal or telephone and the PBX or the central office.

U-interface : A two-wire ISDN circuit - essentially today's standard one pair telephone company local loop made of twisted-wire. The U interface is the most common ISDN interface and extends from the central office.

Virtual ISDN : An alternate way for a customer to get ISDN service. A customer can be serviced out of a nearby central office that has ISDN capabilities but not charged the extra mileage charges as they would with a foreign exchange. The phone company does not add charges because the costs are recouped from the large volume of customers serviced out of the CO. A customer will usually have to change phone numbers if the CO where they receive their POTS service becomes ISDN capable.