Telephone Terminology E-H

E

E1 - A common type of digital telephone trunk widely deployed outside the U.S. and Canada. Has 31 available 64 kbps channels (called DSØs) 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. See Line Format.

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

Extended Superframe - See ESF.

F

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 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. See also Hybrid.

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

FXO – Foreign Exchange Office termination. A line or port meant 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 meant 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. An FXS port will “run a telephone set”.

G

Glare – On a POTS line, an incoming call is signaled by periodically applying an AC ring voltage to the line. Since there is a semi -andom period before the ring and pauses between rings, it is possible to seize a line that is “about to ring” and thereby answer a call when attempting to place an outgoing call. This scenario 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 average 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.

GTD-5 - The GTD-5 EAX (General Telephone Digital Number 5 Electronic Automatic Exchange) is the Class 5 telephone switch developed by GTE Automatic Electric Laboratories. This digital central office telephone circuit switching system is used in the former GTE service areas and by many smaller telecommunications service providers. It does not support ISDN.

H

HDSL - High-Data-Rate Digital Subscriber Line. HDSL delivers 1.544 Mbps of bandwidth each way over two copper twisted pairs. Because HDSL provides T1 speed, telephone companies have been using it to provision local access to T1 services whenever possible. The operating range of HDSL is limited to 12,000 feet and repeaters can be installed to extend the service. HDSL requires two twisted pairs, so it is deployed primarily for PBX network connections, digital loop carrier systems, interexchange POPs, Internet servers, and private data networks. DC voltage used to power the network Interface Unit (NIU) is “phantomed” between the two pairs. See NIU.

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 in 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 also LDN.

Hybrid – A device which converts from a two-wire signal such as POTS lines or a 2-wire intercom to a four-wire system with 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 based hybrid and applied the then brand-new technology to this problem.