Telephone Switching

8/3/2019 Telephone Switching

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Ral [email protected]

Telephone Switching

Basics and Back to the Future (Voice

over Internet Protocol, VoIP)

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Why is VoIP important?

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Data Growth Relative Volume vs. Revenue

Bits per second per dollar invested


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History of Telephone

Switching

Users would ring an operator and askto be connected to a given line

The operator would then run a cableconnecting his/her line to the called-partys line

Operators may barge-in to let youknow of and incoming call (VERY earlycall-waiting signal)

Limited number of lines able toconnect

Challenging calling queue foroperators

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Fairness of Attention to Callers

Allmon Strowgers switch

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Dialing to reach a party

A pulse advances the relay one step

Train of pulses constitute the phone

number of the party

Beginnings of a dialing plan (P6-5000)

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Switch bank


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Call Aggregation Trunking

Impractical to build, house and maintain

bigger switches

Need to interconnect switches within a region

(urban sprawl)

Scale-up the volume of calls (make more

revenue)

Tap into the Long Distance market

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DS0 Voice Sampling

Voice band: 20Hz to 4000HzNyquist sampling rate: sample at

least twice as fast as the highest

frequency in the band

Hence, sample at 8000Hz

But, do it for several

conversations (channels), somultiplex

Add framing information (In-

band signaling)

8000*193=1.54MHz

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Time Division Multiplexing (TDM)


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E1 T1 Signaling Comparison

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8000*256=2.048 MHz

8000*193=1.544 MHz


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Digital Signal Hierarchy

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Meanwhile, at the Central Office (CO)

Telcos (Telephone companies) started selling newservices, suitable for small business start-ups,banks and financial institutions, temporarylocations, to name a few.

It wasnt necessary for any switching equipmentto be installed on the customer premises (noPremises Equipment, PE).

Stations inside the service group could call each

other with 3, 4 or 5 digits, depending on howlarge the group, instead of an entire telephonenumber.

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Kinds of Centrex Services

Call Transfer

Call Divert on no reply /on busy

Call Waiting

Three Party Conference

Call Pick Up (Group) Ring Back

Reminder / Alarm call

Last Number Redial

Centrex Hotline (non-dialed connection)

Centrex Warm Line (delayed Hotline) Centrex Hunt Groups, with optional bypass numbers,

AND AND

Automatic Call Distribution (ACD)

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Miniaturization

In the early seventies, the advent ofthe microprocessor unveiled a

plethora of switches targeted to

Government offices, hotels,

universities and Fortune 500

companies.

Depending on the customers

requirements, they were connected by

one or more T1 trunks.

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For Smaller Customers

A key system or key telephone system is amultiline telephone system typically used in small

office environments. They usually offered the

following functions:

Answering machine functions

Remote supervision of the entire system

Automatic call accounting

Speed dialing

Caller ID

Station-specific limitations (such as no long

distance access or no paging)

Selection of signaling sounds

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Key Service Unit (KSU)


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TELEPHONE SWITCHING PART 2

Now back to our regular programming

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Circuit Switching vs.

Packet Switching

Circuit switching is part of an ordinaryphone call

Each user has sole access of a circuit

during network use

3 Phases: Set-up, use, disconnect

Set-up is achieved by dialing

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Inefficient use of resources when

circuit is used to transfer information


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Packet switching

Breaks messages into packetsPackets can traverse the network

individually

Packet == Datagram == PDU

Protocol Data Unit (PDU). More on

PDUs later

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Role of Network Protocols

Protocols establish actions on processes occurringin different network devices

The format or structure of the message

The method by which networking devices shareinformation about pathways (trunks, links) withother networks

How and when error and system messages arepassed between devices

The setup and termination of data transfersessions

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Layering and Models

Using a layered model to describenetwork operations can aid in

Protocol design.

Allowing products from different

vendors work together.

Preventing any changes in one layer

from affecting other layers above and

below.Providing a common language to

describe networking functions and

services.

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Internet Protocol (IP)

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Message Encapsulation

As application data is passed down theprotocol layers on its way to be

transmitted across the network various

protocols add information to it at each

level.

On the receiving computer, the

opposite process reassembles thePDUs into the necessary data for the

peer application to run.

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A brief digression

The Internet Protocol grew up as part of theinter-networking efforts of the labs working forthe Defense Advanced Research Program Agency(DARPA).

At the same time the InternationalTelecommunications Union (ITU) were developinga set of protocols and interfaces definitions forthe Integrated Services Digital Network (ISDN) so

digitized voice and packet data would seamlesslyintegrate within the Public Switched TelephoneNetwork (PSTN)

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Integrated Services Digital

Network

A circuit-switched telephone networksystem.

Also provides access to packet

switched networks.

Designed to allow digital transmission

of voice and data over ordinary

telephone copper wires.

Bearer (B) channels carry voice ordata streams at DS0 rate.

Delta (D) channel carries signaling

and control information.

Out-of-band signaling

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ISDN Promises

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Virtual Circuits

For packet switching networks (X.25, Frame

Relay), even though the packets are

independent units, routing is assured for them

Those constitute Virtual Channels

In Asynchronous Transfer Mode (ATM) packets

are reduced in size down to 53 bytes (48

payload)

Virtual Channels now become Virtual Circuits

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Demise of Broadband ISDN (B-ISDN)

Fiber optic deployment among Telcospromised to provide greater throughput tocustomers

The problem with ATM was in porting servicesand applications into their own services

ATM is, nevertheless, used as the premier

packet transport mechanism, for instance inDigital Subscriber Loop (DSL) and othernetworks

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TELEPHONE SWITCHING PART 3

Once again, back to our regular programming

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What is a Carrier?

The 1996 Telecommunications Actsought to open up markets to

competition.

Key fact: Interconnectedness

entrants could interconnect their

networks with those of the incumbent

carriers.

Competing Local Exchange Carriers

(CLECs) connect their equipment at

the main distribution frame where the

plain old telephone systems (POTS)

exchange equipment connects with

the outside plant (user telephone

lines).

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Main Distribution Frame (MDF)


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A much briefer digression

CLECs connect their networks at thelocal exchange (LE) or class 5 of the

Switching Hierarchy.

Local Number Portability (LNP) was

mandated by the 1996 Act, allowing

subscribers to change service to a

different provider and keep the same

phone number.

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Switching Hierarchy and Numbering Plan


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Success factors in VoIP

Standards

For interconnection to the PSTN

For the interconnection of IP-based components

that emulate and replace the functionality of thelegacy equipment: signaling, call setup and tear

down, number portability, etc.

A CLEC doesnt have to spend millions on PE,but can provide some of the telephone

network functions at competitive prices

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Ral Hazas

VoIP Topology

The Media Gateway is a router withtrunk capabilities; converts PSTN DS0

streams into IP packets.

A class 5 Service Switching Point (SSP)

is responsible for call setup,

management, and termination with

other SSPs (SSP==CO). Provide dial

tone to subscribers.

In order to provide LNP, phone

companies subscribe to the services of

an independent administrative entity

who holds the management rights for

up to 5 years.

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Telephone Switching

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