telecommunications switching systems 7

7/29/2019 Telecommunications Switching Systems 7

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Analog Interfaces, Intelligent Networks,DNHR

Lecture (7)

Analog Interfaces

The design, implementation and maintenance ofany large and complex system require

partitioning of the system into subsystems.

Associated with each subsystem is an interface

that defines the inputs and outputs independentof internal implementations.

Well established interfaces are fundamentalrequirements to maintain compatibility betweenold and new equipment.


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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 telephonelines to end office switches.

Fundamental characteristics of thisinterface are: BORSCHT

Per-line SLICs allow implementation ofper-line BORSCHT functions.

SLIC: Subscriber Loop Interface Circuit


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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 itsDC current status for signals and on- andoff-hook conditions.

Loop Start lines have immediate dial tone.

24 7.

Less expensive, easy deployment. Residential, Small and Medium Business

Services.


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Loop-Start Trunks

The two wires in the phone line (tip andring) are bridged (shorted) togetherthrough a resistor (say 1K) when thehandset is taken off-hook.

The CO detects the current in the loop.

By operational point of view, identical toSLI.

Drawback

With 2-way LS trunks, difficulty ariseswhen both ends of the line seize the line atthe same time glare situation.

Glare condition may be recognized bytiming out on the wait of dial tone.

So, LS trunks are used only as one waytrunks: either incoming or outgoing.


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GS Trunks

Used for PABX/PBX connections to CO.

No immediate dial tone.

PBX sends a signal (ground) to close arelay 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 groundsthe tip lead of dedicated lines, and wait forPBX to acknowledge the seizure bydrawing loop current.

When PBX originates a call: The ring leadof the line (tip and ring) is momentarily

connected to ground. (The CO does notapply battery during an idle state as itdoes in LS interface).


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The CO acknowledges the connectrequest by applying battery to the tip/ringpair and momentarily applying ground tothe tip.

Advantages (GS Trunks)

A GS protocol prevents simultaneousseizure unless originations occur within afew hundred milliseconds of each other.

Ability of CO to signal networkdisconnects to the PBX by removing thebattery.

CO immediately signals an incoming callabandonment by removing ground fromthe tip lead.


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Direct-Inward-Dial (DID) Trunks

The purpose of DID trunks is to allow alarge block of numbers to be assigned tousers on customer side of PABX from theCO, while connection between PABX andCO is smaller than the number assigned.

These trunks are designed to give external

callers direct access to individuals ordepartments within large corporations.

DID Trunks

The trunk circuit gains power from PABXand look for number forwarded by CO.

The number is compared to a table and anumber conversion is done depending on

circuit connection ratio.


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DID Trunks

No attendants assistance.

The trunk is an incoming only trunk that ishanded off in the form of an analoginterface to customer-provided equipment.

DID Trunks

In contrast to LS and GS trunks, the PBXend of a DID trunk provides batteryvoltage so that CO can signal an incomingcall by merely closing the loop to drawcurrent.

After PBX reverse battery momentarily to

signify it is ready to receive digits, COeither generate dial pulses or DTMF tonesto send extension number.


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DID

After the designated station answers, PBXreverse batty again to signify connectedstate and hold the state for the duration ofcall.

DID trunks loop reverse-batterysupervision trunks

DID signaling

CO

Directory

PBX


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Summary (DID)

One Way trunks-incoming only w. r. t PBX

They allow a CO to forward extension no. ofincoming calls. So a PBX can immediately routethe call to destination without attendantassistance

The calling party dials a number that is listed inpublic telephone directory which includes thenumber of the called station to establish a directconnection

PBX provides the talk battery for called line andcontrols the answer supervision for line

E&M trunks E&M Signaling is commonly referred to as "ear & mouth"

or "recEive and transMit", but its origin comes from theterm earth and magnet.

Earth represents electrical ground and magnetrepresents the electromagnet used to generate tone.

E & M trunks connects multiple PABXs separated inwide area conditions.

The advent of lower priced long distance services anddirect connect T1 carrier services has eliminated E&Mcircuits.


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E&M Trunks

E&M interfaces: Type I, II, III

Type I: for electromechanical switching;two leads.

Type II: For Electronic Switching; FourLeads.

Type III: Hybrid of Type I and II

Type IV Type V

E & M Interface E (Ear or Earth)Signal wire from trunking (CO) side to signaling

side.

M (Mouth or Magnet)Signal wire from signaling side to trunking(CO) side.

SG (Signal Ground)Used on E & M Types II, III, IV.

SB (Signal Battery)Used on E & M Types II, III, IV.

T/R (Tip/Ring)T / R leads carry audio between the signaling unitand the trunking circuit. On a two-wire audio operation circuit, thispair carries the full-duplex audio path.

T1/R1 (Tip-1/Ring-1)Used on four-wire audio operation circuitsonly. The four-wire implementation provides separate paths toreceive and send audio signals.


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Type II

Intelligent Networks (IN)


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IN

As the capabilities of NCPs (SCPs) areexpanded from being data base servers toprocessing service requests, concept of INdeveloped.

Switching machines provide only switchingfabric, connection commands come froman SCP.

IN platform


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Motivations to develop IN

To enable the deployment of network-basedfeatures such as citywide Centrex mentionedpreviously

To allow the introduction of new features withouthaving to change the hardware or software ofswitching vendors

To speed up the design/deployment cycle ofnew features because only the SCP softwareneeds upgrading

To allow customizing of services by theoperating companies according to the needs oftheir customers.

The obvious disadvantage of such centralized control isThe vulnerability of the network to a failure in SS7 network orSCP.

Software in the switching machines recognizes specialservice situations as software triggers to involve an SCP.

Leaving basic services with switching machines reduces loadon SS7 links and on processing speed in SCP


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Dynamic Nonhierarchical Routing(DNHR)

Limitations of conventional routing


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Specific Features of DNHR

The combination of large digital switches,large bandwidth (fiber-optic) transmission links,

and development of the SS7 network

has stimulated a transition from

hierarchal to flat networks.


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Voice Digitisation

TDM


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T1-Carrier Systems

Digital Pair Gain SystemsSubscriber Loop Multiplexer

Subscriber Loop Carrier


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Digital Switching

Spatial Translation: Space switchingTime Translation: Time SwitchingCombined Together: Two dimensional Switching

Digital Network Evolution

T1: 24 Voice CircuitsT2: 96 Voice Circuits


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DN Evolution

DPBX: Digital PBXDTO: Digital Toll OfficeDEO: Digital End OfficeMDR: Microwave Digital Radio

DN Evolution

ISDN (BR: Basic Rate 2B+D)ISDN (PR: Primary Rate 23B + D or 30B + D)DLC: Digital Loop Carrier

telecommunications switching systems 7

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