Glossary of ISDN and Telephone Terminology


Alternate Mark Inversion. A T1 line coding method. This is the older of the two commonly available. See Line CodingT1. See Also B8ZS.

Automatic Number Identification- A system, originally designed for use by Interexchange carriers (IEC’s) which transmits the “billed party number” along with a call. Note that the billed party number is not necessarily the number of the line placing the call. ANI predates SS7 and can operate in with analog as well as digital trunks. See also CLID.

Asynchronous Data
A form of serial data communication which is not clocked. To keep the bit stream synchronized start and stop bits are used. RS-232 computer data is commonly asynchronous data. In contrast to synchronous data.

Bipolar 8 (with) Zero Substitution. A T1 line coding method. This is the more modern line coding method of the two commonly available. See Line Coding, T1. See also AMI

B Channel
Bearer Channel. One of the multiple user channels on an ISDN circuit. Used to carry user’s data; i.e. coded audio data in the case of Zephyr or ZephyrExpress.

Bearer Channel
See B Channel.

Behind the PBX
This is our own definition and refers to when one privately owned phone system is tied to another privately owned phone system. It is a limited Tandem application. See Tandem Switch and Tandem Tie Trunk Switching below.

Bell Labs
The basic research facility which was AT&T’s primary research facility. Bell Labs was spun off with Lucent Technologies. Many very important discoveries have been made at Bell Labs including the transistor, communications theory, and radio astronomy.

BELL COmmunications REsearch. See Telcordia. The research and development organization owned by the RBOCs. Bellcore represents the RBOCs in developing standards for Telco equipment and in testing equipment compliance to those standards. Bellcore also offers educational and training programs open to all interested parties.

1) Bit Error Rate Test- A test for digital lines which involves looping a data path and sending a test pattern. Data returning is compared to the sent data to check for errors. Depending on the “Test Pattern” used, BERTs may or may not uncover problems. A line which only has occasional problems will need a BERT of sufficient time duration to catch that intermittent problem. A five minute BERT of an ISDN BRI circuit will only catch severe problems. 

2) A Bit Error Rate Tester. The test equipment used to perform a Bit Error Rate Test.

Billing Telephone Number
The main phone number which all calls on a PRI are billed to. This information is required when configuring a PRI PBX.

Bit Error Rate Test

Bit Rate
The capacity of a digital channel. ISDN calls are set up at a given bit rate, either 64Kbps or 56Kbps. The bit rate cannot be changed during a call. See Kbps.

When a circuit switched call cannot be completed. The percentage of blocked calls to the number of calls attempted forms the basis of a statistic called “grade of service“. While it is economically infeasible to build a network which would have no blocking, the phone company are held accountable by the utility commissions to keep blocking below tariffed levels. The concept of blocking cannot be applied to packet networks, only circuit switched networks.

Both Way Trunks
See Combination Trunks.

ISDN Basic Rate Interface. The common form of ISDN with 2 Bearer Channels and one D Channel. All three channels are on a single copper pair and encoded with type 2B1Q coding.

Basic Rate Interface Transmission Extension. A technology where ordinary T-1 trunks (or any other digital carrier system) are used to extend ISDN service. See also Repeater.

See Billing Telephone Number.

Business Office
The part of the phone company where you call if they mess up your bill, to report problems, and to order service. Not necessarily technically literate.

Called Party Address
This is the destination phone number of a call delivered to a switch. For instance this could be the CLID of a call delivered to a PBX using DID or two-way trunks. See also DID.

Common Channel Interoffice Signaling. A signaling system where network information such as address and routing information are handled externally to the actual communications (voice) path. SS7 (Signaling System 7) is the internationally standardized CCIS system. Deployment of CCIS increased efficiency since no communications (voice) channels are used merely to report an “all trunks busy” or “far end busy” conditions. It also decreased toll fraud substantially since it removed the potential for access to the signaling information that was inherent to in-band signaling schemes. CCIS also enables CLASS features as well as sophisticated re-routing features for “intelligent network” applications. See also In Band Signaling.

Central Office
See CO.

An actual path you can talk or send data over. This is what you are paying the phone company for. For instance, ISDN BRI lines can be ordered with 1 or 2 active channels and these channels can be configured for voice calls (CSV), data calls (CSD) or both (alternate CSD CSV). A channel does not necessarily have it’s own unique telephone number. See ISDN.

Choke Exchange
A telephone exchange which is assigned to Radio and TV stations, Promoters, and other users which will be receiving large numbers of simultaneous calls. The idea is to group all of these users on a single exchange so when all routes into that exchange are in use “normal” users (on other exchanges) will not experience blocking of incoming or outgoing calls. Trunks from other local exchanges into the choke exchange are deliberately limited to just a few paths so callers will get an “all trunks busy” instead of completely blocking their local exchange. However, when one of the choke exchange users experiences a large number of calls (as when your station runs a contest) the other choke exchange users will be blocked because all trunks into the choke exchange will be busy. See Blocking and Concentration

A physical path through which electrical signals can pass. It consists of a network of conductors and other components, separated by insulators. Technically this term cannot be applied to fiber optic or other “non-metallic” paths.

Circuit Switched Data
See CSD.

Circuit Switching
A system where a dedicated channel is allocated to the users of that call for the duration of that call. That channel is allocated for the duration of the call regardless if information is being transmitted at any given moment. Bandwidth through the channel is fixed, at no time may this bandwidth be exceeded. If this bandwidth is not used it is wasted. While inherently inefficient, the dependable and reliable nature of circuit switching makes it ideally suited to real-time voice and audio/video conferencing applications. This is in stark contrast to systems where statistical multiplexing is used. See Statistical Multiplexing

Custom Local Area Signaling Services. A variety of enhanced features (usually on analog lines) that take advantage of the ability of modern SS7 technology’s ability to transmit information about the calling party. CLASS includes such features as Caller ID, Automatic Callback, Call Trace (initiated by subscriber), Selective Call Rejection, etc.

Competitive Local Exchange Carrier. Your local telephone service provider who is one of the new-generation providers rather than a RBOC or Independent. A CLEC is really just an Independent, albeit one formed after the divestiture of AT&T. See LEC.

Calling Line Identification. This is the ISDN and SS7 equivalent of Caller ID; I.E. the number of the calling party. See also ANI.

Central Office. The Telco facility where your local telephone circuit leads to. Contains Switches and Trunks as well as the local telephone circuits.

COder/DECoder. A device which takes digitized audio and “codes” it in order to reduce the transmission bit rate and which can also simultaneously “decode” such coded audio. Strictly speaking, a codec does not include an ISDN terminal adaptor and related equipment.

Combination Trunk
A trunk (channel) which can both make and receive calls. This generally refers to analog ground start or loop start trunks, although the term can be applied to ISDN BRI or PRI channels as well. Each combination trunk normally has a telephone number, although they are frequently part of a hunt group and only one number may be published for that group. Also called a Both Way Trunk. This is not the same as a Two-way DID trunk. See DID TrunkHunt Group and Trunk

Common Channel Interoffice Signaling

Competitive Local Exchange Carrier

The basic premise is to share facilities wherever possible. For instance, while there may be thousands of customers served by a given Central Office, there will be substantially less than that number of calls which can be handled simultaneously. And even fewer long distance calls can be made simultaneously. The art of Traffic Engineering is to have enough capability that calls are rarely blocked, but not any more than that. See alsoChoke Exchange and Blocking.

Customer Premise Equipment- Customer owned equipment located at his/her facility. In the USA and Canada the NT1 is part of the CPE.


Circuit Switched Data- A dial-up data communications channel which, once established, looks like a transparent data pipe. Also, the type of ISDN service required to utilize this capability of an ISDN circuit. In contrast to CSV.


Channel Service Unit. The NCTE used in the USA & Canada to terminate a T1 line. Typically the CSU must be provided by the end user. See NCTE.

A device which incorporates the functions of a CSU (Channel Service Unit) and a DSU (Data Service Unit) and interfaces between a Switched-56 (or “Dedicated Digital Service”) line and a user’s data equipment such as the Zephyr.

Circuit Switched Voice- A dial-up communications circuit for voice grade communication. Also, the type of ISDN service required to use this capability of an ISDN circuit. In contrast to CSD.

Custom ISDN
An ISDN protocol which pre-dates National ISDN-1. In most cases National ISDN-1 is also available. The Northern Telcom DMS-100 switch supports “Custom DMS ISDN”. The AT&T/Lucent 5ESS switch supports “Custom Point-to-Point” (PTP) and Custom Point-to-MultiPoint (PMP). The ISDN protocol has no relation to where one may call. The Telos Zephyr and TWOx12 do not support PMP.

See Superframe. See also Line Format.

D Channel
Data Channel or Delta Channel (depending on who you ask). The channel which handles ISDN network related data between the user’s equipment and the Telco switch. Used to carry data to set up calls and receive calls. Some Telco’s also allow users to use the D channel to access the packet data network, with appropriate terminal equipment.

Data Communication Equipment. When using serial communications such RS-232, V.35, or X.21, the DCE is the device sending/receiving from the Telco line. ie: a modem or CSU/DSU. In contrast to DTE.

Digital Data System- See Dedicated Digital Service.

Dedicated Circuit
A permanent channel between two locations. As opposed to a Switched Circuit

Dedicated Digital Service
A “Hardwired” or “Nailed Up” digital circuit which is permanently connected between 2 points. Typically 56Kbps or 64Kbps. Dedicated digital lines are frequently cheaper than ISDN for full time service. Also called Digital Data System, or DDS.

Direct Inward Dialing. The ability for an outside caller to dial to a PBX extension without going through an attendant or auto-attendant.

DID Extension or DID Station
A specific phone within a PBX which can be called from the public telephone network without going through an attendant or auto-attendant.

DID Number
A phone number used to route calls from the telephone network to a specific phone in a PBX (the DID extension). DID requires special DID trunks or ISDN PRI “two-way DID” trunks. Blocks of DID numbers (typically 10 or 20) are purchased from the LEC or CLEC for use on the PBX. The number of DID numbers usually substantially exceeds the number of trunks in the system.

DID Trunk
A Direct Inward Dialing Trunk. A trunk (channel) which can only receive calls. A group of telephone numbers (DID numbers) are associated with a given trunk group, however there is no one-to-one correspondence between the individual channels and these numbers. The PBX uses the DID number given it by the phone company to route the channel to the correct DID extension within the PBX extension. This allows some or all PBX stations to receive calls directly without going through an attendant (or auto attendant) Note that there are almost always more DID numbers than there are DID trunks. See DID Number and DID Extension.

Direct Inward Dialing

Directory Number
Your seven digit telephone number (without the area code), as found in the telephone directory. Zephyr generally does not need you to enter the directory number as part of installation.

Directory 1&2 (Zephyr)
The Utility menu on the Zephyr where the 7 digit Directory Numbers can be entered during set up. The Zephyr generally does not require these numbers.

Dialed Number Identification Service- A service, typically offered by a long distance company on 800 lines, that provides the number dialed by the caller. This allows a caller to receive specific treatment depending on the number dialed.

Digital Subscriber Line. Typically refers to an ISDN line or a T-1 line, although the term is also frequently used to mean the next generation beyond ISDN. Sometimes xDSL is used to indicate that the writer is referring to any of a number of emerging DSL technologies.

Data Service Unit. See CSU/DSU.

Data Terminal Equipment- When using serial communications such RS-232, V.35, or X.21, the DTE is the device sending/receiving from a modem or CSU/DSU. In contrast to DCE.

Dual Tone Multi Frequency. The standard tone-pairs used on telephone terminals for dialing using in-band signaling. The standards define 16 tone-pairs (0-9, #, * and A-F) although most terminals support only 12 of them (0-9, * and #). These are also sometimes referred to as “Touch Tones” (actually a copyrighted trade name held by AT&T). Note that while digital data terminals have the same symbols, ISDN uses “common channel signaling” (over the D channel) and therefore does not necessarily generate any tones at all. However many terminals still generate the tones since they will still be used on occasion to access services (such as voicemail or automated attendant) at the far end using in-band tones.

A common type of digital telephone trunk widely deployed outside the US and Canada. Has 30 available 64Kbps channels (called DSø ’s) plus a B channel and a sync/control channel.

Extended Superframe. A type of Line format supported on T1 circuits. The Telco determines the line format and line encoding of your line. See Line Format

ETS 300
The pan European ISDN protocol standardized by ETSI. This protocol is used throughout Europe and has been adopted in many other countries outside the USA & Canada. See MSN

See ETS 300

Another name for a Central Office. See CO

Extended Superframe

Four Wire
A circuit path using separate pairs for send and receive. This term is also used when referring to digital channels that inherently have discrete send and receive paths, regardless of the number of pairs (or other media) used.

A unit of data which is defined by the specific communications protocol used. See Line Format, T1.

On a POTS line an incoming call is signaled by periodically applying an AC ring voltage to the line. Since there is a semi random period before the ring, and pauses between rings, it is possible to seize a line which is “about to ring” (and answer a call) when attempting to place an outgoing call. When this scenario happens it is called glare. Glare is much less likely if Ground Start trunks are used. See Ground Start Trunk.

Grade Of Service
This is simply the ratio of calls blocked to total calls in a decimal form. So a grade of service of P.08 would represent 8% blocking. Telephone tariffs regulate the acceptable average grade of service which must be provided on public networks.

Ground Start Trunk
A type of telephone trunk where the request to make an outgoing call (i.e. request for dial tone) is made by briefly grounding the Tip conductor. Many PBX systems use ground start trunks as they are less prone to glare than Loop start trunks. See Loop Start Trunk. See also Glare

High Density Bipolar 3. An E1 line coding method. This is the more modern line coding method of the two commonly available. See Line Coding, T1. See also AMI and B8ZS

Hunt Group
A group of telephone channels configured so that if the first is busy (engaged) the call goes to the next channel, if that channel is busy it goes to the next channel, etc. Hunt groups may hunt from the highest to the lowest, the lowest to the highest, or on some other arbitrary pattern. But the order of hunting will usually be fixed, beginning with one channel and working through (“hunting”) until an unused channel is found. The term may have originated back in the old manual switchboard days when the operator literally hunted for an unused jack to plug a cord into. This arrangement is very common in business scenarios where a single incoming number (the Listed Directory Number) is given to the public, but multiple incoming channels are supported. See LDN

A device which converts from a two-wire signal such as POTS lines to a four-wire system (separate send and receive paths) such as used in the pro-audio world. While this task is theoretically quite simple, the fact the impedance of most phone lines varies widely across frequency complicates matters. The Telos 10 telephone system was the first practical DSP (Digital Signal Processing) based hybrid and applied the then brand-new technology to this problem

InterExchange Carrier. “Long Distance” carrier. Handles Interlata and interstate calls. Also referred to as IXC.

Incumbent Local Exchange Carrier. A local Exchange Carrier which entered the marketplace before the enactment of the 1996 Telecom act;. i.e. a telephone company which is neither an Indie nor an RBOC. See LECand CLEC.

See Inverse Multiplexing

In Band Signaling
A signaling system where network information such as address and routing information are handled over the communications (voice) path itself. Usually the information is represented in the form of tones, however DC loop current signaling also qualifies as In Band Signaling. See also CCIS.

Incumbent Local Exchange Carrier
See ILEC. See also CLEC & LEC

Any of the phone companies in existence at the time of divestiture that were not affiliated with the Bell System. See RBOCLEC, and CLEC

Interconnect Company
A vendor of telecommunications Customer Premise Equipment (CPE) other than a Bell Operating Company (RBOC) or AT&T. This term was originated by AT&T and was meant to be derisive towards the fledgling industry when the courts said it was OK for end users to buy equipment from someone other than the Bell System. This industry flourished, in spite of AT&T’s disdain, and ironically the RBOCs were not allowed to sell CPE under the terms of the break up of AT&T. With the current state of deregulation, the RBOCs are slowly re-entering this business.

Interexchange Carrier
See IEC.

Inverse Multiplexing
A method of combining and later separating 2 data streams that does not involve the telephone network. Inverse multiplexing is the most common way of achieving this function in Codecs operating at 128kbps or less over ISDN.

IOC Capability Packages
ISDN Ordering Code system. This system was devised by the National ISDN User’s Forum and Bellcore to simplify ordering new ISDN lines in the USA and Canada. Using a single code specifies all line specifications. The Telos Zephyr, ZephyrExpress, TWO, and TWOx12 support IOC package “S”. Search for document NIUF 428-94 at NIUF’s web page for more information.

Integrated Services Digital Network- A relatively new and highly flexible type of telephone service which allows dialing on digital lines with multiple bi-directional “Bearer” channels each with a capacity of 56 or 64 Kbps and a single bi-directional “D channel“. See BRI and PRI.

ISDN Protocol
The “language” used for communication between the Telco’s switch and the customer’s Terminal Adapter. Each ISDN circuit has one protocol, and the protocol has no effect on where or whom one may call. See ETS 300National ISDN, and Custom ISDN.

A term used in Europe for ISDN BRI. See BRI.

A term used in Europe for ISDN PRI. See PRI.

Incoming Service Grouping. See Hunt Group

IntereXchange Carrier- See IEC.

KiloBits Per Second. Measure of digital channel capacity.

Key Telephone System
A system that allows multiple telephones to share multiple pre-determined telephone lines. The system provides indicators to allow the users to understand the status of each line available on a given phone. It is up to the user to provide the intelligence to select an unused line, or answer a ringing line, for example. See also PBX.

Local Access and Transport Area. The area within which calls are routed by your Local Exchange Carrier (LEC). Under the divestiture of the Bell System calls going outside of this area must be handled by an Interexchange carrier (IEC). With the latest round of de-regulation the usual IEC companies are being allowed to compete in the IntraLATA long distance market and LECs are beginning to be permitted to handle InterLATA calls.

Listed Directory Number. When a number of Telco channels share the same hunt group it is customary to give out only one phone number for the group, although generally each channel will have its own number. The number given out is the “Listed Directory Number” since that is the number that would be listed in the Telephone Directory and given to customers. See also Hunt Group.

Local Exchange Carrier. Your local telephone service provider which is either an RBOC or an Independent. In other words, a traditional phone company. In contrast to CLEC or IEC.

An electrical connection between a telephone service provider’s switch (LEC or CLEC) and a telephone terminal or Key system. An electrical connection between a telephone service provider’s switch and another switch is called a trunk. Note that some type of physical lines offer more than one channel. I.E. a BRI circuit has 2 channels, called B channels. This term is a confusing one. See Channel. See also Station Line

Line Card
The circuit in the Telco switch to which your line is connected. On an ISDN circuit the line card performs a role analogous to the NT1 in adapting and equalizing the circuit.

Line Coding, T1
The clock signal for T1 is derived at the far end from the data bits themselves. Therefore T1 lines have certain restrictions as to the data allowed. No more than 15 zeros shall be sent in a row; and average density of 12.5% ones must be maintained. The CSU is responsible to ensure that these requirements are met. The line encoding method, AMI or B8ZS determines exactly how these requirements are met while still allowing recovery of the original data at the far end. Your Telco will determine the method used on a specific circuit. B8ZS is preferred. E1 circuits have similar restrictions. HDB3 is preferred for E1 circuits.

Line Format, T1
Modern T1 circuits usually use either Superframe (sometimes called SF or D4) or Extended Superframe (sometimes called ESF) line formatting. The type of framing used is determined by your Telco. ESF is preferred. See ESF and SF

Listed Directory Number

Local Exchange Carrier
See LEC and CLEC.

Local Access and Transport Area

Long Distance

  1. If your local Telco is a former Bell Operating Company then any call outside of your LATA or any Interstate call is considered long distance and is handled by an IEC
  2. If your local phone company is an independent then only interstate and inter-phone company calls are considered long distance. The above is true regardless of whether you are referring to a dedicated line or a dial up call. 
  3. However, under the current state of deregulation, toll calls within a LATA may now be covered by the IEC, and in some cases RBOCs are being permitted to handle InterLATA calls.

The telephone circuit from the CO to the customer’s premises. Generally refers to a copper cable circuit.

Loop Qualification
Process of actually measuring the loss on a prospective ISDN line to see if it can be used for ISDN service . The actual loss on the line determines whether ISDN service can be offered without a repeater. Generally ISDN is available up to 18,000 feet from the serving Central Office. It may not be available within this range, or may be available further from the CO. Only a loop qualification can tell for sure. Not all Telcos will extend ISDN lines with Repeaters.

Loop Start Line
A plain old telephone line. The telephone terminal signals the “off hook” condition by allowing DC current to flow. See Ground Start Trunk. See also Glare.

Loop Start Trunk
A plain old telephone line connected to a PBX switch. See Loop Start Line. The PBX signals the “off hook” condition by allowing DC current to flow. Ground Start Trunks are generally preferred for use on PBXs to prevent glare. See Ground Start Trunk. See also Glare.

Lucent Technologies
Company which now makes the former AT&T 5ESS switch, as well as various other pieces of Telco gear and semiconductors. Lucent was split off from AT&T in 1996 and owns Bell Labs. As of approximately 1999 Lucent sold their telephone set manufacturing business (and the right to use the AT&T name on telephone sets) to V-Tech. V-Tech is using the AT&T name and line as their high-end line.

Multiple Subscriber Number. This is a telephone number associated with an ETS 300 BRI line. Providers of ETS 300 often give you three MSNs with a BRI, although additional MSNs can be purchased. An ISDN terminal will “ring” (provide an alerting signal) only when calls are made to the MSN (or MSNs) entered in that terminal. If a terminal has no MSNs entered it will “ring” whenever there is a call to any of the MSN’s on that BRI. See ETS 300 and Directory Number.

National ISDN
The first US “standardized” multi-platform ISDN protocol. The first version is National ISDN-1. As of mid 1996 National ISDN-2 has been implemented in some areas and is fully backward compatible with National ISDN-1. 

Network Channel Terminating Equipment. NCTE is a general term that can be applied to a CSU or NT1 or other equipment terminating a digital line at the customer’s premises. In many countries the NCTE is provided by the Telco. The USA is not one of those countries.

Network Channel Terminating Equipment

National ISDN User’s Forum. A user’s group formed under the National Institute of Technology (NIST) in the USA. The NIUF is a neutral forum where the switch manufacturers and Telcos can get input from users and CPE manufacturers regarding the implementation of National ISDN. Among NIUF’s successful projects have been the IOC ordering codes. Their web page is at . See National ISDN.

Northern Telecom (previously Northern Electric). The Canadian company which was once the manufacturing arm of Bell Canada. Manufacturer of the DMS-100 switch as well as many other pieces of telecom equipment.

Northern Telecom
See Nortel.

“Network Termination Type 1”. The termination at the customer premises of an ISDN BRI circuit. The NT1 performs the role of line termination of the “U” interface and Code/Decodes from the line’s 2B1Q coding scheme. The customer end of the NT1 interfaces using the “S” or “T” interface. The NT1 is frequently part of the “Terminal Adapter” and is built-in to Zephyr, ZephyrExpress, Telos TWO and TWOx12 systems sold in the USA & Canada.

Private Branch Exchange. A privately owned switch. Basically a PBX is a private “business” telephone system which also interfaces to the telephone network. Many PBX’s can now offer ISDN BRI service, usually over the S Interface. A few vendors are now offering BRI over the U interface as well. Be wary of these ISDN protocols since they have not been as well tested. They may or may not work with a given piece of CPE.

Primary Interexchange Carrier. This is your default “1+” carrier used for interLATA calls. In some areas you may have two PICs, one for interLATA calls, and one for intraLATA long distance calls (in which case it stands for Primary Intraexchange Carrier). In some areas intraLATA long distance calls are still handled by your RBOC, in others you now have a choice.

AT&T “Custom Point to Point” Custom ISDN Protocol. Not supported by the Zephyr or ZephyrExpress. See Custom ISDN and ISDN Protocol.

Point Of Presence. The local facility where your IEC maintains a switch. This is where your long distance calls get routed so that your IEC can handle them.

This is a pretty general term. Newton’s Telecom Dictionary 10th edition defines a port as “An entrance to or an exit from a network”. Many phone equipment vendors refer to ports as the physical interface between aSwitch and a Line or Trunk. Product literature often refers to the number of ports on a phone system. In this context it refers to the number of phone or lines (or sometimes the combination) the system supports.

Plain Old Telephone. A black, rotary-dial desk phone. Usually a Western Electric model 500 set. Outdated term.

Plain Old Telephone Service. Regular old-fashioned analog loop start phone service.

Primary Rate Interface. A form of ISDN with 23 “B Channels” and one “D channel.” All 24 channels are on a single cable. Functionally related to T1 telephone circuits. In Europe PRI has 30 “B Channels” and one “D Channel”.

The act of configuring an ISDN line. Also refers to the complete line configuration information.

PS2 Power
Power provided on pins 7 and 8 of the “S” interface cable. This power is used so that an NT-1 can provide power to a terminal (usually a phone). In some cases it is used to allow a terminal to power an NT1. The U.S. versions of the Zephyr, ZephyrExpress, and Telos TWO supply PS2 power in the “S” jack. This power arrangement is also used in the Telos TWOx12 and 2101 to power Desktop Directors.

AT&T “Custom Point to Point” Custom ISDN Protocol. Point-to-Point lines have only one incoming phone number which must be dialed twice to connect to both lines. See Custom ISDN and ISDN Protocol.

Regional Bell Operating Company. One of the regional companies formed when AT&T got out of the local telephone business. Each RBOC (or “baby bell”) owns a number of the former “Bell Operating Companies”. The Bell Operating Companies are the traditional local phone companies (pre-1984), except where one’s service is from an “Independent” (non bell) telephone company or a CLEC. Due to their former association with the Bell System RBOCs are regulated by the FCC differently than are independent Telcos. In many cases the Bell Operating Company structure is no longer used. For instance, here in Ohio we now deal directly with the RBOC, Ameritech, while the old Bell Operating Company, Ohio Bell Telephone, no longer exists. Another trend is mergers among the RBOCs (and in some cases the independents as well). See CLEC and LEC.

Receive Data. Data coming from the network, or DCE towards the DTE. Also, a light on a modem or CSU/DSU which lights to indicate presence of this signal.

Regional Bell Operating Company

A device intended to extend ISDN telephone service to site further from the central office than could normally be served. i.e.: beyond 18,000 feet. ISDN repeater technologies include “BRITE“, “Virtual ISDN“, “Lightspan”, and “Totalreach”. Some Telcos do not use repeaters. Compatibility between a given piece of CPE and a repeater is less certain than if that CPE were directly connected to the switch.

Robbed Bit Signaling
A signaling scheme that “borrows” bits on each T1 channel for use as signaling channels. On SF T1’s there are two bits, the A bit and the B bit in each direction. On ESF T1’s there is also a C and D bit in each direction, although they are rarely used. Using these bits, various older analog trunk interfaces can be emulated over a T1. For instance, address signaling using 10 pulse per second (rotary style) digit groups over these bits. Since robbed bit signaling interferes with the least significant bit, only 7 bits can be used for sensitive data applications, leaving only a 56kbps channel for data applications.

See Hunt Group

S Interface
The electrical interface between the NT-1 and the Terminal Adapter or other ISDN equipment. ISDN equipment with built-in NT1’s do not necessarily provide access to the S interface (the Zephyr, ZephyrExpress and Telos TWO do). Multiple devices can share an NT-1 by connecting on the S interface. Also known as the S passive bus.

European term for ISDN BRI. See BRI and ISDN 2.

European term for ISDN PRI. See PRI and ISDN 30.

Sealing Current
Unlike telegraphy, teletypewriter and POTS lines, most digital lines (such as ISDN) use a voltage rather than current mode of operation. Sealing Current allows a controlled amount of current to be passed through a telecom circuit for purposes of “healing” damage caused by corrosion. Bellcore specifies sealing current on the ISDN U interface.

Superframe. A type of Line format supported on T1 circuits. The Telco determines the line format and line encoding of your line. ESF is the preferred Line Format on T1 circuits. See Line Format.

Silence Suppression
See Statistical Multiplexing

A Subscriber Loop Carrier Circuit system manufactured by AT&T (now Lucent). SLC-96 has its own version of T1 framing between it and the CO. This interface is the Bellcore TR-008 or the newer GR-303 interface which are specialized versions of T1 intended to allow transparent transport of analog features such as Caller ID, Call Waiting, etc.


  1. Subscriber Line Interface Circuit, see Line Card.
  2. The equipment used with the AT&T (Lucent) SLCC Subscriber Loop Carrier Circuit, a system used to multiplex a number of subscriber loops onto a single circuit to reduce fixed costs.

Service Profile IDentifier- On the “National ISDN“, “AT&T Custom PMP” and “Custom DMS” ISDN protocols, the Telco switch must receive correct SPID(s) from the CPE before it will allow access to ISDN service. Intended to allow multiple configurations on ISDN lines shared among different types of CPE equipment. While your SPID may include your area code and telephone number, the SPID is distinct from the telephone number. For the National ISDN , Custom PMP, and DMS custom ISDN protocols the Zephyr requires the user to program SPIDs into it. Custom PTP and ETS 300 protocols do not require a SPID.

Station Line
A telephone circuit from a PBX to a telephone on that PBX. Since this is a telephone-to-switch connection it is considered to be a “line”. See Line and Trunk.

Statistical Multiplexing
A method of improving effective bandwidth of a Telco channel. Statistical Multiplexing takes advantage that there are typically many pauses in a conversation. By taking advantage of this fact, and not sending the pauses, improvements in efficiency can be made. Also referred to as silence suppression. See Circuit Switched.

The customer of a Telecommunications company. This term dates back to when a local Telephone Company was formed at the specific request of a group of customers who agreed in advance to “subscribe” to the service.

See SF

Telephone company switching device which “makes the connection” when you place a call. Modern switches are specialized computers. ISDN service is provided from a “Digital” switch, most commonly an AT&T model “5ESS”, Northern Telcom model “DMS-100”, or Siemens model “EWSD”. The switch, and related software running on it, will determine which ISDN protocol(s) will be available to customers connected to it. See also PBX.

Switch Type
(ZephyrExpress)- The utility menu item where the ISDN Protocol is selected. See Telco Setting and National ISDN.

Switched Circuit
A channel which is not permanent in nature, but is connected through a switching device of some kind. The switching device allows a switched circuit to access many other switched circuits (the usual “dial up” type of telephone channels). Once the connection is made however, the complete capacity of the channel is available for use. As opposed to a Dedicated Circuit.

A type of digital telephone service developed in the mid-1980’s which allows dialing on a single 56Kbps line. Each Switched-56 circuit has 1 or 2 copper wire-pairs associated with it. Switched-56 is being rapidly replaced with ISDN, which is cheaper and more flexible. See also CSU/DSU.

Synchronous Data
A form of serial data which uses a clock signal to synchronize the bit stream. SInce, unlike Asynchronous Data, no start and stop bits are use, data throughput is higher than with asynchronous data. ISDN uses Synchronous data.

A proprietary Telos 2.048mbps channelized link. This link uses the DSX-1 electrical protocol and has 30 channels at 64kbps each.

A common type of digital telephone carrier widely deployed within the US, Canada, and Japan. Has 24 64Kbps channels (called DSÆ ’s). The most common framing scheme for T1 “robs” bits for signaling leaving 56kbps per channel available.

The electronic interface between an ISDN device and the NT1. The terminal adaptor handles the dialing functions and interfaces to the user’s data equipment as well as to the NT1 on the “S” or “T” interface.

Tandem Switch
A switch which is between two others. It connects two trunks together. Long distance calls on a LEC line go through a long distance tandem that passes them through to the long distance provider’s switch.

Tandem Tie Trunk Switching
When a PBX switch allows a Tie Line call to dial out of the switch. For example, if switch “A” in Arkansas has a tie line to switch “B” in Boise, Boise could use the tie line to make calls from switch “A”.

Transmit Data. Data coming from the DTE towards the DCE or network. Also, a light on a modem or CSU/DSU which lights to indicate presence of this signal.

Telephone Company. Your local telephone service provider. In the 21st century you generally have a choice of Telcos if you are a business in a major metropolitan area in the USA. Competition is coming to the Telecom industry around the world.

Telco Setting
(Zephyr, TWO, TWOx12)- The menu selection where the ISDN protocol is selected. Choices are Natl I-1 (for National ISDN and DMS-100 Custom Functional ISDN), AT&T Cust or PTP (for AT&T Custom Point-to-Point), and ETS300 (for Euro-ISDN). See also Switch Type.

Telcordia Technologies
Formerly BellCore. The research and development organization owned by the telephone companies. Telcordia represents the phone companies in developing standards for Telco equipment and in testing equipment compliance to those standards. Telcordia also offers educational and training programs open to all interested parties. BellCore was sold to SAIC in 1997. Telcordia is responsive to both RBOCs and independent Telcos. 


Terminal Adapter
See TA

Tie Line
See Tie Trunk

Tie Trunk
A Trunk between two PBXs. Note, a tie line is a dedicated circuit, not a switched circuit. See Trunk

Trouble Ticket
A telco “work order” used to track Customer Repairs within the Telco. If you call someone “inside” the telco’s repair department they will need this number to proceed. It will also be needed whenever you call to check on the status of a repair. Always ask for this number when initiating a repair request.

A communications path between two switching systems. Note that many trunks may be on a single Circuit (if that circuit has multiple Channels). The trunks most users will deal with are between the Telco switch and a PBX. However, a Tie Trunk can connect two PBXs.

Trunk Group
A number of telephone channels which are functionally related. Most common is the Hunt Group. Other common types include Incoming Trunk Groups and Outgoing Trunk Groups. See also Combination Trunks andTwo-way DID Trunks.

Two-way DID Trunk
An ISDN PRI (or T1) line equipped for direct inward dialing. Most trunks are related to a given phone number, either alone or as part of a hunt group. In the case of a “normal” (ie analog) DID Trunk a group of phone numbers are associated with that DID trunk (or group of trunks) and incoming calls include the DID Number, so the PBX can route that call to the correct DID Extension. This is exactly how ISDN PRI functions, with the DID information coming in over the D Channel. There is a big difference between a normal DID Trunk and a Two-way DID trunk over ISDN PRI. For one thing, ISDN PRI is digital. More importantly, you cannot dial out over a true DID trunk and you can dial out over a PRI. See DID

Two Wire
A circuit path where only a single pair of wires is used. A Hybrid is used to convert from two wire to Four Wire circuits. No hybrid is perfect, and those used by the phone company can be pretty bad. But the Telos TWO family is approaching perfection!

U Interface
The interface between the ISDN BRI line and the NT1. This can be considered the ISDN “phone jack” in the USA & Canada and is frequently in the form of a RJ-11 or RJ-45 type telephone jack.

A serial data interface for synchronous data. V.35 uses balanced signal and data lines. The Zephyr models 9201, 9200, 9101, and 9100 support V.35 using part #9812 cable.

The particular protocol (i.e National ISDN-1 or ETS 300) running on a specific switch. Not all variants are valid for a specific switch. The switch brand and model plus the variant defines the ISDN protocol. See ISDN Protocol.

Virtual ISDN
An alternative to repeaters which uses a local Telco Switch to act as a repeater and which then sends the signal onto another switch which supports ISDN. See also Repeater.

Work Order
See Trouble Ticket.

A serial data interface for synchronous data popular in Europe. X.21 uses balanced data and unbalanced signal lines. The Zephyr models 9201, 9200, 9101, and 9100 support X.21 using part #9822 cable.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s