Telephone Terminology A-D

You’ll get better results from the Telco if you understand and speak the lingo, so we have tried to include the most typical and commonly-used acronyms used by Telco personnel here with their definitions.

Numerical

1MB – USOC term for a single Measured Business line in the US. See USOC.

1FB – USOC term for a single Flat Rate Business line in the US. See USOC.

1FR – USOC term for a single Flat Rate Residential line in the US. See USOC.

5ESS - The 5ESS Switch is a Class 5 telephone electronic switching system sold by Alcatel- Lucent. This digital central office telephone circuit switching system is used by many telephone companies. It is one of the most common Central Office switches used in the US.

A

Analog Terminal Adapter - A device used to connect one or more standard analog telephones (POTS) to a digital telephone system (such as a Voice over IP network) or a proprietary telephone system such as a PBX or Key System.

ADSL – Asymmetrical Digital Subscriber line. The most common type of telephone company delivered Internet access. The download/downstream direction is usually at a higher rate than the upload/upstream direction. Most often installed as “Line Share DSL”, that is the DSL “rides on top of ” the POTS line. “Dry Pair DSL” is usually available for customers desiring only the data service. Service availability is limited by loop length and cable type.

AMI – Alternate Mark Inversion. A T1 line coding method. See Line Coding, T1. Also see B8ZS.

ANI – Automatic Number Identification. A system originally designed for use by Interexchange carriers (IXCs) that 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 with analog as well as digital trunks. See CLID. Also see Caller ID.

Asynchronous Data - A form of serial data communication that is not clocked. To keep the bitstream synchronized, start and stop bits are added which cuts down on throughput. RS-232 computer data is commonly asynchronous data. In contrast to synchronous data.

ATA – See Analog Terminal Adapter.

B

B8ZS- Bipolar 8 (with) Zero Substitution. A T1 line coding method. This is the more modern and most common line coding method of the two commonly available. See Line Coding, T1. Also see AMI.

Behind the PBX- This is our own term and refers to when one privately owned phone system is tied to another privately owned phone system. The most common application is when a key system is connected to the analog ports of a PBX. When it involves one PBX behind another, it is a limited Tandem application. See Tandem Switch. Also see Tandem Tie Trunk Switching.

Bell Operating Company. Most often called “LEC’ (Local Exchange Carrier) at the time of this writing. Few companies use the Bell name in the U.S any longer, though it is used in Canada. See BOC. See also RBOC.

Bellcore- Bell Communications Research. 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. Now Telcordia. See Telcordia.

BERT - Bit Error Rate Test. A test for digital lines involving 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 that only has occasional problems will need a BERT of sufficient time duration to catch the intermittent problem. A five-minute BERT of an ISDN BRI circuit will only catch severe problems. BERT may also refer to a Bit Error Rate Tester, the test equipment used to perform a Bit Error Rate Test.

Billing Telephone Number - The main phone number to which all calls on hunt group or a PRI are billed. This information may be required when configuring a PRI PBX.

Bit Error Rate - The basic measure of errors on digital transmission paths, usually expressed as the number of errors per number of bits.

Bit Error Rate Test - See BERT.

Bit Rate - The capacity of a digital channel. See Kbps.

BLEC - Building Local Exchange Carrier. An LEC who covers the occupants of a single building (or a small group of buildings) only. Often Telecom services are provided by a BLEC as a service or incentive to potential tenants. If a BLEC offers Long Distance Service it is covered by the same regulations as any other LEC.

Blue Alarm - Also called an Alarm Indicating Signal (AIS). A keep-alive signal sent if a problem occurs mid-span in a T-carrier system. The blue alarm signal is required because in some cases T-1 repeaters will become unstable if inadequate 1’s density is not maintained.

BOC - Bell Operating Company. One of the regional telephone companies that were owned by AT&T before divestiture in 1984 (i.e. New England Telephone, Ohio Bell, etc). The 22 BOCs were divided among the Regional Bell Operating Companies at divestiture. See RBOC.

Both Way Trunks - See Combination Trunks.

BTN- See Billing Telephone Number.

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

C

Caller ID - A CLASS feature on an analog line that provides the number of the calling line as a burst of FSK data (Bell 202 modem tones) following the first ring. Also called Calling Line Identification. See CLASS.

Calling Line ID - See CLID. Also see Caller ID.

Calling Party Control - See CPC.

Call Progress Tones – Tones used in the telephone network to indicate call status or progress through the network. This includes dial tone, ring-back tone, and re-order (fast busy) tone, Special Information Tones (SIT’s), etc. The Telos Hx can be set to listen for these tones and disconnect when one is heard if CPC is not available or is unreliable.

CAS - Channel Associated Signaling. A bit-based signaling method used on digital lines (such as T1) that is periodically inserted into the low order bit that is also used for the audio transmission. See Robbed Bit Signaling.

CCIS - 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 “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. Also see In-Band Signaling and SS7.

Central Office - See CO.

Centrex - Central Exchange Service. An enhanced business telephone service intended to offer most of the features of a PBX but where the lines are all from the LEC out of a public switch. Offers CLASS-like features for business users such as 4-digit “inside” dialing, hold, transfer, attendant, etc.

CEPT- Conference on European Posts & Telecommunications. This is a European standards body that formerly set the standards for telephone interfaces for 26 countries.

CEPT Format - The usual rate and frame format for E1 circuits (2.048 mbps). Also see E1.

CEPT Rate- See CEPT Format. Also see E1.

Channel Associated Signaling - See CAS. Also see Robbed Bit Signaling.

Channel Bank - A device that multiplexes or demultiplexes a group of communications channels, such as analog or digital telephone lines, into one channel of higher bandwidth or higher digital bit rate such as a DS-1 (T1) circuit. Also see Pair Gain.

Choke Exchange - A telephone exchange assigned to Radio and TV stations, promoters, and other users that 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.

In the modern network, using CCIS signaling such as SS7, actual trunks are not used to convey “busy” or “all trunks busy” conditions. Thus blocking due to a station contest should not occur as the busy status in response to a call attempt is conveyed over the separate SS7 network. Therefore, the need for choke exchanges has pretty much disappeared. Nonetheless, many Telcos still insist that broadcasters use special choke lines for call-in lines. Much of the need for the “choke network” is historical, but few current telephone company employees understand this concept and only seem to believe that it is “required”.

Unless very aggressive contesting is planned, these arrangements should probably best be avoided. See blocking and concentration.

Circuit - 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. Also see Channel.

Circuit Switching - A system where a dedicated channel is allocated to the users of that call for the duration of that call and regardless if the information is being transmitted at any given moment. Bandwidth through the channel is fixed, and 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.

When over-loaded Circuit Switched networks will respond “all circuits are busy… try again later”. This is in stark contrast to Packet Switched networks or to systems where statistical multiplexing (Statmux) is used. See Statistical Multiplexing. Also see Packet Switching.

CLASS - 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 Screening, etc.

Class 4 Switch - A Class 4, or Tandem, telephone switch is a U.S. telephone company central office switch used to connect local exchange (Class 5) central offices for long distance communications in the Public Switched Telephone Network. See Class 4/5 switches.

Class 4/5 Switch – Also called a “Hi/Lo” switch. This is a U.S .telephone company central office switch that provides both local and long distance service. Many CLEC’s are configured in this way. See Class 4 or Class 5 switches.

Class 5 Switch - A Class 5 telephone switch is a telephone switch or telephone exchange in the Public Switched Telephone Network located at the local telephone company’s central office, directly serving subscribers. Services provided include basic dial-tone, calling features, and additional digital and data services to subscribers using the local loop. It is considered a “local switch’.

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

CLI - Calling Line Identity. European term for CLID. See CLID.

CLID - Calling Line Identification. This is the ISDN and SS7 equivalent of Caller ID, that is, the number of the calling party. Also see Caller ID and ANI.

CO- Central Office. The Telco facility to which your local telephone circuits lead. Contains “Switches” and “Trunks” as well as the local telephone circuits.

Codec- COder/DECoder. A device that takes digitized audio and “codes” it in order to reduce the transmission bit rate. It can also simultaneously “decode” such coded audio.

Strictly speaking, a codec does not include an ISDN terminal adapter and related equipment. Simple codecs are also used in digital telephony that employ simple companding schemes to reduce channel noise.

COL - COnnected Line number. A European term referring to the number to which you have connected. This may not be the number you dialed if call forwarding is used.

Combination Trunk - A trunk (channel) that 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 Trunk, Hunt Group, and DID Trunk.

Common Channel Interoffice Signaling - See CCIS.

Competitive Local Exchange Carrier- See CLEC.

Concentration- 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 fewer calls that can be handled simultaneously, and even fewer long-distance calls. The art of Traffic Engineering is to have enough capacity that calls are rarely blocked, but no more than that number. See also Choke Exchange and Blocking.

CPC - Calling Party Control. Sometimes referred to as “CPC Wink” or “disconnect supervision”. A call supervision feature on an analog loop start line that provides the ability for a CO (Central Office) to signal the called party when the calling party hangs up. CPC allows the PBX, key system, or telephone answering device to reset the line so that it is ready to accept or initiate another call. CPC is accomplished by either a loop current drop or reversal. With some CO equipment, it is also provided if the called party drops the call. See also MCLD.

CPE - Customer Premise Equipment. Customer-owned equipment located at his/her facility, such as a CSU or terminal. In the U.S. and Canada, the ISDN NT1 is part of the CPE.

CPN - Called Party Number. European term for the number that has been dialed. See Called Party Address.

CSU - Channel Service Unit. The NCTE used in the U.S. & Canada to terminate a T1 line. Typically the CSU must be provided by the end-user. See NCTE, NIU, and DSX1.

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

D

D4 - See Superframe. See also Line Format.

DCE - 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. i.e.: a modem or CSU/DSU. In contrast to DTE.

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

Demarc – The point of “Demarcation," that is, where the telephone company’s responsibility ends and the customer's begins. Most often located in “the main phone closet” or basement terminal. At one time, a special block was required for a Demarc, one for each customer, with a simple way to disconnect the customer's wiring from the Telco’s wiring at the demarc point. This practice has been abandoned. When you order service today, it will be “to the demarc” only and the telco will (usually) provide a tag or “cable and pair” number so that you can locate the circuit in question. Residential phones still have a demarc in the form of a box (usually gray) mounted to the side of the house. It most often provides remote line disconnection facilities (for remote testing by the phone company) and a customer-accessible modular jack to allow the customer to verify proper operation of the line at the side of the house before the house wiring.

DID - Direct Inward Dialing. The ability for an outside caller to dial to a PBX extension without going through an attendant or auto-attendant. See also DID Number and DID Trunk.

DID Extension or DID Station - A specific phone within a PBX that 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, sometimes higher) 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) that 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 by the phone company to route the channel to the correct DID extension within the PBX. This allows some or all PBX stations to receive calls directly without going through an attendant or auto-attendant. Note that there are usually more DID numbers than there are DID trunks. See DID number and DID extension.

Direct Inward Dialing - See DID.

Directory Number (U.S. & Canada)- Your seven-digit telephone number (without the area code) as found in the telephone directory.

DMS-100 – Nortel’s very popular central office switch that provides local POTS service and connectivity to the public switched telephone network. It is used widely throughout the world.

DNIS - 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.

DP - Dial Pulse. A method of sending address information by either causing brief inter- ruptions in loop current, or causing brief changes of state of a bit on a digital circuit using Channel Associated Signaling. In other words, “rotary” or “pulse” dialing. See also DTMF and CAS.

Dry Loop or Dry Pair DSL – DSL service offered without a POTS line to carry it. These circuits often cost less because many of the taxes, “regulatory recovery fees” and other nonsense charges apply only to POTS phone service. Also, many customers wish to use VoIP with their DSL and have no use for the expensive POTS phone once required to be ordered with DSL service.

DSØ - Digital Signal Level Zero. At 64 kbps, he smallest unit of measure of the standard rate hierarchy used by the Telcos (i.e. all other rates are a multiple of the DSØ rate). For example, the T1 rate is 24 times the DSØ rate and the E1 rate is 32 times the DSØ rate. See also B channel.

DS1- Digital Signal Level 1. The second level up the digital rate hierarchy used by the Telcos. This is 24 times the DSØ rate for a total of 1.544 mbps. See DSØ. See also T1.

DS2 -Digital Signal Level 2. Provides a data rate of 6.312 mbps (4 times the DS1 rate). See DSØ and DS1.

DS3 -Digital Signal Level 3. Provides a data rate of 43.232 mbps (28 times the DS1 rate). See DSØ and DS1.

DSL - Digital Subscriber Line. Traditionally refers to an ISDN circuit or sometimes a T1 line, although the term is also frequently used to mean the next generation beyond ISDN. Sometimes xDSL is used to indicate a reference to any of a number of emerging DSL technologies.

DSLAM – Central Office or wire center-based Digital Subscriber Line Access Multiplexer, aka what's on the “other end” of your DSL line. These are sometimes fed by HICAP (High Capacity or Fiber) facilities to remote wire centers (“remotes” or “vaults”) where “Pair Gain” (carrier) systems reside so that DSL service may be provided to customers who otherwise could not be served due to loop length limitations.

DSU - Data Service Unit. See CSU/DSU.

DSX-1 - Digital Cross Connect Level 1 (U.S. & Canada, primarily). Defined as part of the DS1 (T1) specification and a closely related signal. The type of signal switched by a Digital Cross- Connect System (DACS). The FDL is stripped off at the DACS interface. DSX-1 is also the type of signal that arrives at the user side of a CSU on a T1 line. A DSX-1 cable is limited to 655 feet (200 meters).

DTE - 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.

DTMF - 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”. 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 an automated attendant at the far end using in-band tones. The extra 4 tones were originally used in the U.S. military “Autovon” phone network.