Figures 1 and 2.) Every circuit slot in the system can be used for trunk cards or voice/data station cards. All these attributes allow the client to add future capability without breakage and re-engineering of existing equipment. Thus, the client is able to minimize initial investment while not restricting future growth.

Over time, the type of tools and facilities that a business utilizes changes. It is important that a PBX provide support for the full set of Telco network options over its installed life, even when only a subset is initially used. Trunks link two switching systems, such as System 25 and the local Central Office or System 25 and another PBX. System 25 supports five different telephone company trunk interfaces to provide desired connectivity at minimum expense. Thus the opportunity exists to select the best trunk types, depending on tariffs and customer needs.

Loop Start (LS) trunks for public network access at minimum tariff. These trunks handle outgoing and incoming attendant calls, incoming DGC calls, outgoing pooled

facility calls, and personal line calls.

Ground

Start

(GS) trunks for

public network

access. These trunks handle the

same

type

of

calls

as

LS trunks. They provide protection against call reorigination without

toll

restriction,

more reliable

automatic route

selection, virtual facilities, SMDR

and

CAS. Simultaneous incoming and outgoing call seizure of the same trunk under heavy traffic conditions is essentially eliminated with ground start trunks. GS t r u n k s s h o u l d u s u a l l y b e s e l e c t e d i n p r e f e r e n c e t o L S t r u n k s u n l e s s t a r i f f considerations are overriding. Note, however, that Centrex Service requires LS trunks.

Direct Incoming Dial (DID) trunks for dialing a station directly from outside

(attendant assistance not required). Outside dial access to stations, trunks (optional), and answering groups (Direct Group calling) is provided.

Tie Trunks

for linking PBXs

with dedicated private circuits for high volume calling.

Dial access to stations, other

trunks, answering groups (Direct Group Calling) and an

Electronic

Tandem Network

endpoint capability are provided.

Off-Premises Stations (OPS) allow a single-line voice terminal to be located remotely and connected to System 25 via arrangements with the local Telco. This service is used to provide users at secondary sites (or their residences) many of the same features as an on-premises single-line station.

To enhance the usage and control of the above set of network facilities, System 25 provides the rich set of access features outlined in the Outgoing Business Communications Section. In addition, System 25 can support networking between systems by:

. Serving as

an endpoint on an electronic tandem network (ETN)

using

its

tie trunks

and flexible

dialing plan.

 

 

 

. Serving as an off-network or on-network access point with its dial access/transfer

between tie-trunks and Telco trunks (LS/GS/DID). This allows

usage

of

tie trunks

to reach a distant System 25 and then connect through that System 25 to local Telco

facilities to complete the call.

 

 

 

To support efficient

utilization of trunks, they can be grouped together

(up to

16

groups) if

all trunks in the group perform the same function. This resource pooling provides better service with a given number of trunks, and simplifies administration and calling.

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AT&T AT&T manual