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Telephony System• Public switched telephone network

•The public switched telephone network (PSTN) is the concentration of

the world's public circuit-switched telephone networks, in much the same

way that the Internet is the concentration of the world's public IP-based

packet-switched networks

• Originally PSTN is a fixed-line analog system

• Now, the PSTN is now almost entirely digital, and now also

includes mobile cellular network as well as fixed telephones. (e.g.

DSL, ISDN, cable system)

• Analogue telephony system will be covered 1st,as the introduction. (Basic ISDN system will be covered, if time permitted)

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A Simple Telephone

Off hook now !!

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A More Realistic Telephone

On hook now !!

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Signal From To Function Off-hook User LE Signal intention to call Dial tone LE User Signal readiness for call Pulses & tones

User LE Signal destination

Ringback tone

LE User Signal destination phone is ringing

Ringing voltage

LE User Cause phone to ring

Busy tone LE User Signal busy destination On-hook User LE Signal action to end call Call waiting tone

LE User Signal another call on the way

Howler tone LE User Phone off hook alert

Signalling in Telephone Network

LE: Local Exchange (i.e., Central Office)

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Dialling in Telephone Network

Rotary dialling(Pulse dialling)

Dual-tone Multi-frequency (DTMF)(Touch-tone dialing )

Dual tones for each key

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Why DTMF ?

• allows dialling numbers via microphone

• allows in-band signalling

• allows end-to-end signalling between two telephones

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Telephone Network

Point-to-point telephone connection

Mesh network

Resource utilisation ?

Scalability ?

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n n connectio

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Telephone Network

Star network

•More efficient resource utilisation

•Scalable

•Allows hierarchical routing

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Telephone Network Routing – Circuit Switching

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Routing in Telephone Network•Hierarchical routing paradigm

• Intra-LATA services are offered by common carriers (Telephone

Companies) within a LATA. Handled by local exchange carrier (LEC).

• Inter-LATA services between LATAs are handled by interexchange

carriers (IXCs) or (long distance companies).

: LATA – Local access transport

areas:

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National/International Routing

POP – Points of Presence: A switching office that handles carriers exchange btw LEC &

IXC

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Voice Communication Service Offerings•Wide Area Telecommunication Services (WATS)

non-standard billing for businesses with high call volume

•Inward WATS business agrees to pay for all INCOMING long-distance calls from certain areas (e.g., 0800, 0870)

•Foreign exchange phone numbers in distant cities where your customers can call your company at local charges

•Leased lines permanent point-to-point connection

annual rent

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PART – C

Customer Premises Equipment

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Customer Premises Equipment

• Centrex– Central Office is acting as the customer’s

exchange or switch

• Private Branch Exchange (PBX)– Acts as a mini-central office for a company,

dedicated to customer who owns it (not owned by Central-Office).

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Customer Premises EquipmentCentrex service:

• Very Dependable & Reliable -because services are maintained by Central-office (full-time maintenance & troubleshooting staff).

• Inflexible – additional lines can only be fitted by central-office.

• Inefficient – Calls within company are routed all the way through Central Office and back.

• Hence expensive – regular fees for each month

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Customer Premises EquipmentPrivate Branch Exchange (PBX):

• Suitable for large business – provide thousands of lines.

• VERY flexible – can add/remove a line anytime

• Efficient – internal calls are routed within PBX without going through Central-office, hence provide quick access.

• Maintenance is CRUCIAL – need a team to always maintain it.

• Does not need local loop for each employee. Only a few local loops are needed for external call.

• Internal calls are free using ext. assigned to each one.

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For Information Only (FIO)

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Regulatory BodiesOFCOM

(Office of Communications)

ITC(Independent Television

Commission)

Radio Authority

Radio Communications Agency

OFTEL(Office of Telecommunications)

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OFTEL's functions•Promoting the interests of consumers.

•Maintaining and promoting effective competition.

•Making sure that telecommunications services are provided in the UK to meet all reasonable demands, including:

Regulatory Bodies

emergency services;

public call boxes;

directory information; and

services in rural areas

20Tutorial

 Describe the signaling procedure for making a call through telephone network.

List three signals that your phone sends to local exchange List three signals that local exchange sends to your phone Describe how rotary dialing and touch-tone dialing is made.

What is a mesh network? What is a star network? What is local loop and Central Office?

 

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Why Telephone network uses star network?

Describe how your local call is routed.  Describe how your international call is routed.

Briefly describe two types of Customer Premises Equipment

Describe the advantages and disadvantages of using a Centrex Service? Describe the advantages and disadvantages of using a private branch exchange (PBX)?