Analog Interfaces, Intelligent Networks, DNHR

Lecture (7)

Analog Interfaces

• The design, implementation and maintenance of any large and complex system require partitioning of the system into subsystems. • Associated with each subsystem is an interface that defines the inputs and outputs independent of internal implementations. • Well established interfaces are fundamental requirements to maintain compatibility between old and new equipment.

1 Principal Analog Interfaces:

• Subscriber Loop interface • Loop-Start Trunks • Ground Start Trunks • Direct-Inward-Dial Trunks • E&M trunks

Subscriber Loop Interface

• The most common interface involves 2- wire connection of individual telephone lines to end office switches. • Fundamental characteristics of this interface are: BORSCHT • Per-line SLIC s allow implementation of per-line BORSCHT functions.

SLIC : Subscriber Loop Interface Circuit

2 BORSCHT

• Battery Feed: 48V normally • Over-voltage Protection • Ringing: 20Hz, 86 V rms, 2 sec ON & 4 sec OFF. • Supervision: Detection of ON-Hook or Off-Hook • Coding *: A/D or D/A • Hybrid *: 2- to 4- wire conversion • Test: Access to line test in either direction.

* Digital Switch Interface Requirements

LS Trunks

• Loop start circuit is monitored based on its DC current status for signals and on- and off-hook conditions. • Loop Start lines have immediate dial tone. • 24 × 7. • Less expensive, easy deployment. • Residential, Small and Medium Business Services.

3 Loop-Start Trunks

• The two wires in the phone line (tip and ring) are bridged (shorted) together through a resistor (say 1K) when the handset is taken off-hook. • The CO detects the current in the loop. • By operational point of view, identical to SLI.

Drawback

• With 2-way LS trunks, difficulty arises when both ends of the line seize the line at the same time – ‘glare’ situation. • Glare condition may be recognized by timing out on the wait of dial tone. • So, LS trunks are used only as one way trunks: either incoming or outgoing.

4 GS Trunks

• Used for PABX/PBX connections to CO. • No immediate dial tone. • PBX sends a signal (ground) to close a relay at CO. CO sends dial tone. • This methods prevent 'glare' situations. • Medium to Large -size businesses

Ground Start Trunks

• When originating a call: The CO grounds the tip lead of dedicated lines, and wait for PBX to acknowledge the seizure by drawing loop current. • When PBX originates a call: The ring lead of the line (tip and ring) is momentarily connected to ground. (The CO does not apply battery during an idle state as it does in LS interface).

5 • The CO acknowledges the connect request by applying battery to the tip/ring pair and momentarily applying ground to the tip.

Advantages (GS Trunks)

• A GS protocol prevents simultaneous seizure unless originations occur within a few hundred milliseconds of each other. • Ability of CO to signal ‘network disconnects’ to the PBX by removing the battery. • CO immediately signals an incoming call abandonment by removing ground from the tip lead.

6 Direct-Inward-Dial (DID) Trunks

• The purpose of DID trunks is to allow a large block of numbers to be assigned to users on customer side of PABX from the CO, while connection between PABX and CO is smaller than the number assigned.

• These trunks are designed to give external callers direct access to individuals or departments within large corporations.

DID Trunks

• The trunk circuit gains power from PABX and look for number forwarded by CO.

• The number is compared to a table and a number conversion is done depending on circuit connection ratio.

7 DID Trunks

• No attendant’s assistance.

• The trunk is an incoming only trunk that is handed off in the form of an analog interface to customer-provided equipment.

DID Trunks

• In contrast to LS and GS trunks, the PBX end of a DID trunk provides battery voltage so that CO can signal an incoming call by merely closing the loop to draw current. • After PBX reverse battery momentarily to signify it is ready to receive digits, CO either generate dial pulses or DTMF tones to send extension number.

8 DID

• After the designated station answers, PBX reverse batty again to signify connected state and hold the state for the duration of call.

• DID trunks – “loop reverse-battery supervision ” trunks

DID signaling