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A glossary of Telephone, Telecommunications and Wide Area Network words and terms.

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Telecom Words

Telecom, short for Telecommunications, includes anything and everything that is transmitted over wire, fiber or microwaves owned by a telephone company or other common carrier.

Leased Line - a dedicated telephone line leased from the telephone company on a monthly basis for a flat fee. It is always connected whether or no there is traffic. Commonly a 56kbs line, but 128kbs, T1 and higher speeds are available.

Multiplex - combining several voice and/or data streams into a single signal at one end, and sorting them out into separate signals at the other end. By this means a single wire, fiber or microwave signal can carry many simultaneous conversations or data transmissions. The signal must be very high in frequency (high bandwidth) compared to the voice / data streams it carries, or transmission speed of each stream will be negatively impacted.

There are many multiplexing schemes, but the simplest is to timeslice the carrier signal, giving each stream a slice when its turn comes. More sophisticated systems are demand sensitive, allowing no slice to an idle stream - and from there it starts getting rather complex.

  • TDMA - Time Division Multiple Access (same as TDM).
  • TDM - Time Division Multiplexing (same as TDMA).
  • DWDM - Dense Wavelength Division Multiplexing.

Dial-up Line - a plain switched telephone line or an ISDN line. Charged by the minute only when actually connected (dialed).

WAN - Wide Area Network - is grouped here under telecom because the most easily understood definition is that a WAN is the part of a company's data network that passes over a connection not owned by the company - generally a link owned by a telephone company or other common carrier. A seldom used subset of WAN is MAN - Metropolitan Area Network.

Local Loop - The telephone wire from your demarc, phone, KSU or PBX equipment to the local Central Office Often called a CO line.

Loop Start - A method of initiating a telephone connection (getting a dialtone) used for single line and ksu phones. PBXs generally prefer Ground Start. Loop start shorts tip to ring through a resistance.

Ground Start - A method of initiating a telephone connection (getting a dialtone) used for PBXs. Single line phones prefer Loop Start. Ground start shorts tip to ground through a maximum of 550 ohms of resistance.

Tip & Ring - the two wires of a standard dialup telephone line. Derived from the plugs used in manual switches (see pictures of old telephone offices). The plug shaft ended in a ball (the tip). Right behind the ball was a ring, insulated from the ball on one side and the shaft on the other. Tip is the wire that goes to the tip, Ring is the wire that goes to the ring.

VAN - Value-Added Network - A privately operated network originally devoted to EDI transactions. VANs now provide other services as well, including translation of EDI transactions for Internet tranmission (EDI-INT).

PDN - Public Data Network - switched or leased lines maintained by a telephone company or other carrier and available for a fee to organizations needing Wide Area (WAN) or Metropolitan Area (MAN) networking.

PSDN - Public Switched Data Network - a switched (connection based) data network similar to a telephone company, but offering data services instead of voice services. Some PSDN carriers are: Compuserve, SprintNet and Tymnet.

PSTN - Public Switched Telephone Network - your local and long distance telephone service.

PTT - Postal, Telephone & Telegraph - International - generally designating a state owned monopoly.

QOS - Quality of Service - a promised or contracted level of speed and reliablility for data and voice communications services, generally from a common carier. QOS is largely mythical at this time.

CO, Central Office - The telephone company switching equipment to which your telephone system is wired by the Local Loop.

Demarc - Demarcation point between the telephone company's equipment and your equipment. Today, the demarc is as close to the edge of your facility as possible. You are responsible for all wiring and equipment on your side of the demarc.

Hunt Group - A group of telephone numbers that looks like a single number to the outside world. If the main number is dialed, but buisy, the ring will be routed to an available number in the hunt group. Numbers in the hunt group can also be individually dialed, where only the dialed number will ring, and outgoing calls can be placed on any of them.

Telecom Business Units

BOC, RBOC - Regional Bell Operating Company - USA - "Local" phone companies created by the court mandated breakup of the AT&T monoply. Notworthy for defining the concept of "monopoly" on a regional basis. Now they are all merging to form a new national monopoly. The Telecommunications Reform Act at work.

LEC, ILEC, CLEC - Local Exchange Carier, Incumbent Local Exchange Carrier, Competetive Local Exchange Carrier - Created by deregulation. The ILEC is your old telephone company before deregulation, the CLEC is a new carrier selling you services over the ILEC's lines. CLECs generally specialize in a particular type of service.

LATA - Local Access Transport Area - The Bell divestiture divided the U.S. into 161 local exchanges. Calls with both ends within the LATA are the provence of the LEC (Local Exchange Carier), and InterLATA calls, where the two ends are in different LATAs) are fair game for long distance carriers.

Telecom Equipment

DSLAM - DSL Access Multiplexor - This is the device that must be within a certain distance from your site to provide DSL services at a particular speed (or at all). The DSLAM multiplexes a number of DSL feeds into a single high speed line connecting to the server operated by your ISP. The high speed side is usually an ATM line.

PBX - Private Branch Exchange - a customer owned telephone switching system used by larger companies and organizations. It is similiar in function to the switching systems at the telco's central office, but on a much smaller scale. A PBX is much more complex and capable than a KSU.
PABX - (Private Automatic Branch Exchange) - but since all PBX's are automatic now the A has been dropped.
CBX - (Computerized Branch Exchange) - a term for PABX used by Rholm and IBM.
EPABX - (Electronic Private Automatic Branch Exchange) - (give me a break!).

KSU - Key Service Unit - Telephone equipment for small offices. A typical KSU is served by 6 regular dial-up phone lines and fans them out to 24 desk phones (though thay do get larger). It also usually provides paging, intercom, music on hold and other local services. Next up from the KSU is the PBX.

Terminal Equipment - Device that terminates a transmition line. For instance an ISDN "modem" includes the terminal equipment for an ISDN line.

MDF - Main Distribution Frame - the wiring termination frames where incoming cables are punched down, and from where they are patched to telecom equipment.

CPE - Customer Premise Equipment - the equipment that terminates a transmission line at the customer demarc. Often, but not always, the CPE will be owned by the carrier. Notable exceptions are ISDN and DSL termination equipment (modems, bridges and routers) which are generally installed by the carrier but owned by the customer.

OLT / ONU - Optical Line Terminator / Optical Network Unit. The OLT is the device handling a fiber optic line at the CO (Central Office) end. The ONU is at the subscriber end and converts the optical signal to whatever the "last mile" format is (DSL, Ethernet, etc.).

Telecommunications Services & Protocols

POTS - Plain Old Telephone Service - the regular 2-line voice telephone service.

  • DS-0 - Digital Services, International name for 64-kbs digital data service.
  • DS-1 - Digital Services-1, T-1, 24 DS-0, 1.544-mbps
  • DS-3 - Digital Services-3, T-3, 24 DS-1, 44.736-mbps.
  • E-1 - European version of T-1, 2.048-mbs
  • ISDN - Integrated Service Digital Network
  • T-1 - DS-1, 24 DS-0, 1.544-mbps
  • T-3 - DS-3, 24 DS-1, 45-mbps

ATM - Asynchronous Transfer Mode - a former "latest and greatest" high speed network transport and protocol. "ATM to the desktop" was the battle cry. Unfortunately ATM is very complex to implement and quite costly, and incompatible with common LANs, so it never got as far as the desktop. It is used for campus backbones, Internet backbones and WANs.

When setting up a router to talk to an ATM circuit, you must set VPI (Virtual Packet Identifier), VCI (Virtual Circuit Identifier), and DLCI(Data Link Connection Identifier), which will be included in your packet headers. These numbers are provided by your service provider.

DSL, xDSL, ADSL, CDSL, IDSL, SDSL, G.Lite, G.SHDSL, HDSL-2, VDSL, RADSL, - Digital Subscriber Line - a high data rate digital voice data service over copper telephone lines. DSL is an "always-on" service like a dedicated line. There are two (incompatible) DSL protocols in use, called DMT (Discrete MultiTone) and CAP (carrierless Amplitude Phase). DMT is expected to predominate.

Alas, poor Germany. They did such a Teutonically efficient job of ripping out their obsolete telephone wiring and replacing it with fiber. Now everyone wants DSL, which only goes over copper wire.

  • ADSL - Asymmetric DSL. Downstream (to you) speed is considerably higher than upload speed. This is the variety of DSL being deployed to homes and small offices. Usually 345-kbps to 1500-kbps downstream (depending on provider) and about 64-kbps to 800-kbps upstream. Maximum distance from your site to telephone company's central office is 18,000 feet (3.5 miles). Sites closer than 12,000 feet can sometimes get speeds exceeding 1.6-mbps. 1-pair, can share a line with voice.
  • ADSL-Lite - Asymmetric DSL - 64-kbps to 384-kbps upstream, 1-mbps to 1.5-mbps downstream. To 25,000 feet. 1-pair, can share a line with voice.
  • CDSL - Consumer DSL. A version of ADSL that supports regular 56K V.90 modems if ADSL is not available. 128-kbps upstream, 1-mbps downstream. To 18,000 feet. 1-pair, can share a line with voice.
  • G.Lite - a variety of ADSL which requires no splitter at the subscriber end to separate voice and data. Phones may, however, need a low pass filter on their line.
  • G.SHDSL - Global Symmmetric High-bit-rate Digital Subscriber Line - a newer global standard expected to replace SDSL
  • HDSL - High Bit Rate DSL. At under 12,000 feet 768-kbps to 2.048-mbps. 2-pair except 2.048-mbps requires 3-pair, cannot share a line with voice.
  • HDSL-2 - High-speed DSL-2 1.5-mbps, and 2.408-mbps. 1-pair, to 12,000 feet, cannot share a line with voice.
  • IDSL - ISDN speed DSL, 128-kbps to 144-kbps. Slower than ADSL but with a range of up to 40,000 feet (7 miles) from the central office. 1-pair. Unlike standard ISDN, it is an "always on" connection rather than dial-up and does not support voice.
  • RADSL - Rate Adaptive DSL - adjusts speeds based on signal quality. 128-kbs to 1024-kbps upstream, 600-kbps to 700-kbps downstream. To 25,000 feet, can share a line with voice.
  • SDSL - Symmetric DSL. Has the same speed in both directions. About 184-kbps at 15,000 feet. At less than 10,000 feet 384-kbps to 768-kbps. 1-pair, cannot share with voice.
  • VDSL - Very High Bit Rate DSL - expected to be used for short drops from fiber to facilities requiring above T1 bandwidth. 1.6-mbps to 6.4-mbps upstream, 13-mbps to 52-mbps downstream. 1 pair, to 4,500 feet. can share a line with voice.
    A symetrical version of VDSL does 26-mbps both directions. 1-pair, to 4,500 feet.
  • xDSL - DSL, not specific as to type. Any of the above.

DWDM - Dense Wavelength Division Multiplexing - describes transmission by the use of multiple laser transmitters and receivers simultaneously on the same fiber. Each transmitter / receiver pair uses a different color (wavelength). Multiplexing allows several carrier streams to simultaneously use the same fiber. In most cases each of these streams will be multiplexed to carry a number of simultaneous voice or data streams.

Frame Relay - A telecomunications protocol for economical transport of digital material. It is used for transmission between two sites on a LAN, or from a LAN into the Internet. Frame can run over various transmission media. Most common are 64-kilobit/sec and 128-kilobit/sec leased lines. Over high speed media, Frame Relay can simultaneously carry both voice and data communications.

PON - Passive Optical Networking - a transmission scheme where a fiber transmission line uses splitters to fan out to multiple end points. Passive means the system has no active components such as amplifiers, repeaters, hubs or switches. This transmission method is limited to about 12 miles. The fiber can have up to 32 splits (64 end points).

Transmission speed is 155-Megabits/sec both ways, or 622-Megabits/sec downstream and 155-Megabits/sec upstream. PON is a shared line, so actual subscriber performance depends on how many subscribers are on the line and how heavy their traffic is.

PON is a relatively simple method of getting high speed connections out to more end users. The last link to the user is usually DSL, but instead of fanning out from a CO (Central Office), the fan out is from a number of "neighborhood gateways" or "mini-COs". This means subscribers need only be within 18,000 feet of a local mini-CO rather than within 18,000 feet of a full Central Office. SBC (Southern Bell Companies - including Pacific Bell) is the most agressive deployer of PON at this time (Project Pronto).

PVC / SVC - Permanent Virtual Circuit / Switched Virtual Circuit. A PVC looks like a permanent "Leased Line" to the subscribers at both ends of what is actually a packet switched, multiplexed network. A SVC looks to the subscriber as if it were a regular dial-up line even though it is actually a packet switched, multiplexed network.

Async - Asynchronous Communications - a protocol for transmission of binary data in which framing information defines the beginning and ending of each character transmitted. This makes it possible to transmit such data over long links or between differing systems where it is not possible to synchronize the two participants with timing signals.

BSC, BISYNC - Binary Synchronous Communications - an IBM protocol for transmission of binary data. There is no framing information as with Asynchronous (standard modem and serial) communications, nor is it needed as the sending and receiving machines are synchronized by timing signals.

ISDN - Integrated Services Digital Network - consists of one D-channel (Data channel) for signalling and low speed non-voice telemetry, and 2 or more B-channels (Bearer channels) for voice and data transmission.

  • Basic Rate ISDN consists of 1 D-channel (16-kbs) and 2 B-channels (64-kbs each, or 128-kbs combined)
  • Primary Rate ISDN consists of 1 D-channel (64-kbs) and 23 B-channels (16-kbs each, or 1.544-mbs combined).

X modem, Y modem, Z modem

- Protocols to assure error free data transfer ovr unreliable connections, particularly modems. These protocols include CRC (Cyclical Redundancy Checking) and retransmission features.
  • X modem - One of the earliest protocols to assure error free transfer of data over telephone lines. Simple with CRC checking. Slow.
  • Y modem - Faster and more sophisticated than X modem, Y modem is particularly designed to handle batches of files.
  • Z modem - Faster and more sophisticated than Z modem.

CTI - Computer Telephone Integration

Computer Telephone Integration referrs to the equipment on your premises. At the telco, this integration was done long ago. Telephone switches are now just big computers. The most familiar uses of CTI are IVR (Interactive Voice Response - also the most annoying) and Voice Mail.

IVR - Interactive Voice Response - "For cotton doilies, please press 5, for polyester doilies, please press 6, for paper doilies, please press 7".

Voice Mail - A system by which a computer answers the telephone and stores your message in a voice mailbox (on its hard disk) for later retrieval by the person who is pretending not to be there. The advantage of voice mail over an answering machine is that many individuals can have their own private mailboxes and not have to hear each other's messages.

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©:Andrew Grygus - Automation Access
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