guide to telephone terminology  107

DSX-1- Digital Cross Connect level 1 (USA & Canada, primar- ily). Defined as part of the DS1 (T1) specification and is 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 communica- tions 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 automated attendant) at the far end using in-band tones.

E1- A common type of digital telephone trunk widely deployed outside the US and Canada. Has 31 available 64 kbps channels (called DS0s) plus a sync/control channel for a total data rate of 2.048 mbps.

E-1- See E1

ESF- 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 also MSN.

ETSI - European Telecommunications Standards Institute Euro ISDN- See ETS 300.

Exchange- Another name for a Central Office (most often used in European countries). Also used in the USA & Canada to refer to a particular 3-digit prefix of a 7-digit telephone number. See CO.

Extended Superframe. See ESF.

FDL- Facilities Data Link. A bi-directional data link available on T1 circuits when the ESF line format is used. The FDL is primarily used by the Telco to poll the CSU for error statistics.

Four Wire – A circuit path using separate pairs for send and re- ceive. 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. See also Hybrid.

Frame – A unit of data which is defined by the specific communi- cations protocol used. See Line Format, T1.

FXO – Foreign Exchange Office termination. A line or port meant to connect to connect to the POTS output of a Central Office. Such an interface goes off and on hook to signal status and expects to receive ringing current.

FXS – Foreign Exchange Station termination. A line or port mean to connect to a telephone. Such an interface must look for current flow to know when the attached device goes off-hook. It must be able to generate ringing current.

Glare – On a POTS line an incoming call is signaled by periodi- cally applying an AC ring voltage to the line. Since there is a semi random period before the ring, and pauses between rings, it is pos- sible 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 or ISDN trunks are used. See also Ground Start Trunk.

GR-303- See SLC-96

Grade of service- This is simply the ratio of calls blocked to total calls in a decimal form. Therefore, a grade of service of P.08 would represent 8% blocking. Telephone tariffs regulate the acceptable av- erage grade of service which must be provided on public networks. See also Blocking.

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 system use ground start trunks as they are less prone to glare than Loop start trunks. Ground start lines are sometimes used with equipment designed for Loop Start lines. This may or may not work - generally it serves to prevent outgoing calls while incoming calls work normally. Telcos may call these “ground start lines”. See Loop Start Trunk. See also Glare.

HDB3- High Density Bipolar 3. An E1 line coding method. This is the more modern line coding method of the two commonly avail- able. 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 (“hunt- ing”) until an unused channel is found. The term may have origi- nated back in the old manual switchboard days when the operator literally hunted for an unused jack to plug a cord into. This arrange- ment 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 also LDN.

Hybrid – A device which converts from a two-wire signal such as

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Telos NX12 user manual Extended Superframe. See ESF