lecture 1 -telecommunication overview

7/30/2019 Lecture 1 -Telecommunication Overview

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Introduction to Telecommunication

Concepts

Lecture 1

BTN 302 Broadband Network

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Content

Basic concepts PCM and Voice Coding

Primary Level Multiplexing

Multiplexor and Transmission Hierarchy concepts

Telecom transmission concepts Switching and traffic engineering

Signaling and billing

Data

Enterprise networks

Cell based communications Packet based communications

Regulatory and business consideration


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Basic Concepts

Subscriber connection to the PSTN

A subscribers handset or Terminal Equipment (TE) is

connected to the Public Switched Telecommunication Network

(PSTN) by means of a twisted pair made up of two insulated

copper wires twisted together.

PSTN is represented by a cloud for simplicity

The purpose of twisting the insulated copper wires is to reduce

cross-talk, a type of electrical interference caused by proximityto similar signal carrying conductors

The infrastructure made up of the copper twisted pairs to each

subscribers premises is know as the local loop.


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Basic Concepts

Hybrid Circuits The twisted pair is connected to a hybrid circuit at the local

telephone exchange

The hybrid circuit takes the twisted pair from the local loop

and presents two pairs of wires, that is a 4 wires to theexchange

One pair carries the transmit signal (Tx), the second pairscarries the receive signal (Rx)

A circuit that utilizes two separate paths for Tx and Rx is called

a full-duplex circuit. The purpose of a hybrid is to reduce the cost of copper in the

local loop and to provide some isolation of exchangeequipment from the local loop.


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Basic Concepts

Voice PSTN was solely designed to carry voice, which is an analogue signal.

It has been determined through experiments that acceptable tollquality voice us possible by utilizing a bandwidth of less than 4khz

Humans perceives frequencies in terms of pitch High pitch tones will be composed of higher frequencies and low

pitch tones will be composed of lower frequencies

Human ears is particularly sensitive to certain frequencies

To allow for greater intelligibility certain parts of the voice signal are

amplified more than others The weighting given to each frequency is called psophometric

weighting


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Digitizing Voice

In a digital system the voice signal is digitized by ananalogue to digital converter at the exchange

Digitizing the signal allows it to be exactly replicated at thefar end of a network

The A to D converter uses Pulse Code Modulation (PCM) toconvert the signal from analogue to digital

It requires 8000 samples a second each sample being 8 bitsto accurately depict a human voice

The digitized voice signal requires a bit rate of 64 kbps


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Multiplexing

There are two types of multiplexing in common use:

Frequency Division Multiplexing (FDM) each analogue voice

message is carried on a different frequency within the carrier wave

Time Division Multiplexing (TDM) each 64 kbps message is

positioned in a different, individual time slot

The reason for multiplexing is to allow one transmission line

carry several signals, thereby reducing the cost and

complexity of the PSTN

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Time Division Multiplexing (TDM)

TDM is a technique for interleaving data from severalusers onto a single serial channel

Each user is given a distinct time period in which theirdata can be carried

Each distinct period of time is called a Time Slot (TS)

TDM is the method used throughout the PSTN to carrydigitized voice.


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Consider multiple signal sources each producing an 8 bitquantized sample every 125 us as per a PCM signal

In order that the far end receiver can extract theinformation in an intelligible form each quantized samplemust be presented to the far end every 125us

Whatever format for our TDM channel we decide on interms of number of voice channels, the format must repeat

every 125us or 8000 times a second


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TDM signal is made up of distinct signals each containedin separate TS, each TS contains 8 bit quantized sample.

Now if the TDM contains 32 TSs of 8 bits and therequired repetition rate is 8000 times per second, werequire a TDM signal with a bit rate of 2,048,000 or2.048Mbps


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E1

Primary rate for TDM is at 2.048Mbps (Europeanstandards).

This contains frame structure of 32 distinct TSs

This format is more commonly known as an E1

In an E1 TS0 is used for synchronization, alarm transportand to transport international bits

TS16 is used for signaling purposes This leaves 30 TSs to carry voice or data traffic at 64 kbps.


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T1

In North America and countries where TDM was adoptedearly, TDM frame consists of 24 8 bits TSs plus a specialbit called single framing bit

The frame repeats 8000 times a second or every 125us

The bit rate of a T1 signal is 1.544 Mbps

One bit from each TS is reserved for signaling and voice isquantized using 7 bits

Voice is carried over 56 kbps instead of the 64 kbps in anE1


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Telephone Numbers

In order to connect the calling party to the called party it

must be possible for the PSTN to automatically route calls

through a network

The calling party provides the routing information to the

PSTN when dials a telephone number

A telephone number is constructed hierarchically as

follows:

International access code (optional)

Country codes and regional codes (optional) Called parties telephone number (mandatory)

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Switches

The routing machines that makes routing decisions are

called switches

Switches are arranged in hierarchy to make call processing

easier and quicker Each switch is aware of the national and international

numbering plan.

Numbering plans are regulated at a national and an

International telecommunications Unions (ITU)

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PCM and Voice Coding

Why digitize voice?

Digitizing the signal allows it to be exactly replicated at

the far end of the network

Quality is not affected by the number of elements in the

network as the signal is regenerated at each node in the

network

Cross-talk which is caused by adjacent cables is almost

eliminated

Manual adjustment of analogue equipment is eliminated

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PCM and Voice Coding

How many samples are required?

In order to represent an analogue signal we need to

sample at twice the highest frequency contained in the

analogue signal, this requirements is known as Nyquists

theorem.

Sampling at higher rate does not add any useful

information

Given that the PSTN is based around a voice signal

where the highest frequency components is less than 4khz it is adequate to sample the signal 8000 times a

second for one sample every 125us.

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Telecom Transmission Concepts

Key transmission Technologies Twisted pairs

Coaxial Transmission Lines

Microwave Radio

Fiber Optics

Satellite communications

Each technology has a specific role to play in thedesign of the core network, the back-haul and thelocal loop.


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Coaxial Cable

Coaxial Cable consists of an inner conductor surrounded bya dielectric with an outer conductor, which operates as ashields

It is an unbalanced transmission line

Coaxial cables are generally confined to the back-haulnetwork and for connecting multiplexors at all levels of themux hierarchy

Before the advent of the fiber optic technology, coaxial

cables were used in the backbone network and for theintercontinental undersea cables

Coaxial cable transmission systems require significantlymore repeaters than the equivalent fiber optic system


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An optical fiber consists of a strand of silica glasswhich has been doped to modify the refractive indexof the glass

Light travels down the center of the fiber, the center

of the fiber is the core Transmitter sends light down the fiber, it received at

the other end by a detector

There are two types of fiber optic cable in commonuse: Single mode very small core

Multimode has a larger core

Fiber Optic Cable System


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Microwave Radio

Microwave radio transmission system are used in thebackbone, the backhaul and the local loop, they are flexible,economic and can carry high capacity traffic.

Used radio wave in the 1Ghz to 50Ghz range

One of the main characteristics of microwave radio is thatLine of Sight (LOS) between the transmitting antenna andthe receiving antenna is required

Generally higher frequency systems are more prone

to rain and atmospheric attenuation Microwave systems have to be carefully planned to

avoid RF interference and rain attenuation


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Satellite System

Satellite transmission system use radio frequencies calledmicrowave

A ground station communicates with a spacecraft, thespacecraft appears to be stationary from the earth but

moves through space in sync with the rotation of the earth Only spacecraft at a distance of 36,000 km from the earth

can orbit in this manner, this orbit is called geosynchronous

The spacecraft carries several radio relays, called

transponder, satellite communication can cover vastdistances at a fixed cost.

It is used primarily for communicating internationally orwith extremely remote communities


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Switching and Traffic Engineering

Circuit Switching

First circuit switches were manual and required operator tomanually connect calls with jumper cables

An undertaker called Almon B. Strowger invented the firstautomatic circuit switching equipment

Example of switches: Crossbar switches

Space Division Switches

Time Division Switches


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Probability Blocking

The interest of the network designer is the number of trunkcircuits needed to satisfy the call demand placed upon thelocal exchange

Although every subscriber is connected to the local

exchange, the same number of egress (outgoing) lines froma switch will not be required

The number of egress lines is chosen such that there is asmall probability of blocking

The probability of blocking is not the same at 3 am as 3 pm.Probability of blocking is calculated based on a busy-hour


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Erlangs

Calls arriving at a rate lambda during the busy hour, withan average holding time T, offers a traffic load of lambdaTErlangs in the absence of blocking

Erlangs are the units in which the average busy hour traffic

load is expressed If we know the traffic load offered in Erlangs and the

number of trunks, the probability of blocking can bedetermine through the Erlang B formula

We can also design a system with a predetermined

probability of blocking, known as the grade of Service(GOS), by providing sufficient trunks for the traffic load inErlangs.


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Enterprise Networks

Voice and PABX Centrex Services

WANs and IP

Data and Voice Convergence Service Provider Networks

ATM

All IP Networks


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Voice and PABX

A large corporation campus will normally have its own

wired network for telecommunications

The wired voice network will be connected to a Private

Automatic Branch Exchange (PABX) PABX will route calls internally to users telephone

extensions and may require an operator on site.

The term PBX is also used instead of PABX

The PABX will then connect with the PSTN of choice

PSTN operator will ensure that all voice calls are processed

in as efficient a manner as possible


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PBX Advance Features

Some of the common feature includes:

Direct dialing (DDD or DDI)

Customized Abbreviated dialing (speed dialing)

Follow-me

Call forwarding on absence

Call forwarding on busy

Call transfer

Music on holdAutomatic ring back

Call distribution (ACD)


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Centrex Service

Some public toll switches can offer feature sets similar to

those on a PBX

These services are commonly marketed by PSTN under

the name of Centrex (CTX) services

Centrex CO indicates that all equipment except the

attendants position and station equipment is located in

the central office

Centrex CU indicates that all equipment, including the

dial switching equipment, is located on the customers

premises


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Photonics systems roadmap www.isa.se

WANs and IP

Leased Lines play an important part in enterprise data

networks and comprise part of what is commonly known as

a Wide Area Network (WAN)

Enterprise leases a fixed capacity connection between twoor more points and runs either Frame Relay, ATM or X.25

IP runs at a higher level and takes care of addressing of

data and delivery of data


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Data and Voice Convergence

As Enterprise data networks were traditionally designed forthe delivery of data packets, latency or delay was of

minimal concern

A reasonable delay between packet transmission and

reception was deemed acceptable However there is a desire on the part of enterprise to

recoup some of their investments in their networks and one

of the obvious means is to ensure that the same links can

carry data and real time voice and video This move towards a network that can accommodate the

two very different requirements is know as convergence


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Service Provider Networks

each Enterprises data network is designed for therequirements of that particular enterprise

As such, reliability and robustness requirements, are a

purely individual decision

Service provider network (SPN) requirements are different.

As SPNs sell services to other parties normally with some

Service Level Agreement (SLA) robustness becomes a

more important consideration.

SPNs normally design their networks within availabilityfigures recommended by the ITU

Equipment sold to SPNs will normally have high levels of

redundancy with a high Mean Time Between Failure

(MTBF)


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ATM

Asynchronous Transfer Mode (ATM) has been designedwith the requirements of convergence in mind.

It is a connection-oriented protocol that conditioned traffic

according to a traffic contract

The traffic contract can guarantee that delay variation is

within acceptable constraints and can provide bandwidth on

demand

ATM represents a good compromise for PSTN operators

because it can support the traditional cash cow oftelecommunications i.e. voice and accommodate newer

opportunities in the data communication world.


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All IP Networks

advocates of an all IP networks predict that trunks androuters will become so fast in the future that latency will be

only a minor issue that can be overcome by a set of quality

of service labels which will give preference to real time

requirements An all IP network represents a step into the unknown

It also requires huge investments in equipment with

uncertain payback.


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End of Lecture 1

- Telecommunication Overview -

lecture 1 -telecommunication overview

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