Chapter # 2Telecommunication

Transmission

Introduction

• Transmission system provide link between nodes

• Information carried by them is classified as:-– Analog (varying signal e.g. ?)– Digital (discrete values e.g. ?)

• Bandwidth– Digital system (bits per sec vs bauds per sec)– Analog system (range of frequency which can be

carried by a channel i.e. (Fh-FL)

Introduction

• Analog signal can be transmitted over analog or digital channel e.g. ?

• Digital signal can be transmitted over digital or analog channel e.g. ?

• Advantage of digital signal over analog signal is immunity to noise and ease of detection

• Disadvantage of digital signal is ?

Introduction• If the capacity of link is more then the

requirement of signal to be transmitted then it can be shared between users

• At sender side signals of users are combined to make composite signal with wider bandwidth also called broad band channel or bearer channel i.e. multiplexing

• At receiver side they are separated i.e. de- multiplexing

• Two main techniques used for this purpose are :-– FDM (Spatial sharing)– TDM (Temporal sharing)

Decibels (dB)• It is ratio of two electrical quantities

such as watt, volts and amperes• If we pass a signal through a device

then it will suffer loss or achieve gain

• Input Signal Output Signal• Ratio = Output / Input

– If O/P is > I/P then Gain– If O/P is < I/P then Loss– Gain is represented by +– Loss is represented by -

Decibel (dB) • It was invented by Bell telecomm USA • Input >-- Amp #1 --- Amp #2 --> Output A1 = 275 .. A2 = 55• In the example, the total gain factor At = 275 x 55 =

15,125• Aim was to use logs for ease of calculation i.e. Log

A1 + log A2 thus bell was introduced i.e. gain in Bell = Log A

• Using Bels meant working to at least two decimal places rounding things off to one decimal place, meant introducing error since 4.179 bels is a power gain of 15,101 while 4.2 bels is a power gain of 15,849, yielding an error of about 5%.

• Solution was to express power gain in units which were equal to one-tenth of a Bel, or in decibels. This simply meant that the gain in Bels would be multiplied by 10 i.e. Gain in decibels (dB) = 10 log A

Decibel (dB)• Useful Rules for calculations in dB :-

• Log is exponent thus :-– Log 1(10 0) = 0– Log 10(10 1) = 1 Log 1/10(10 -1) = -1– Log 100(10 2) = 2 Log 1/100(10 -2) = -2– Log 1000(10 3) = 3 Log 1/1000(10 -3) = -3

• Double / Half increase of power is represented by +/-3 dB – 6mw +9db o/p in mw ?– 15mw +7db o/p in mw ?

dBm & dBW

• dBm is decibel value with reference to one milliwatt (1 mw)

• dBw is decibel value with reference to one watt (1 W)

• dBm = 10 log (P / 1 mw) • dBW = 10 log (P / 1 W)

– 1 mW = ? dBm– 1 W = ? dBW– +30 dBm = ? dBW = ? W– -30 dBW = ? dBm = ? mW

dB for Voltage/ Current

• P =V I• P = I 2/ R• P =V 2 /R• If resistance of circuit is constant then dB will be

defined as :-• 10 Log (output V/ input V) 2 OR 10 Log (output I/

input I) 2

• 20 Log (output V/ input V) or 20 Log (output I/ input I)

dBr

• It is relative level of signal at any point with respect to reference point

• It would be algebraic sum of gains and losses between that point and reference point

• dBmo = dBm (Absolute level) – dBr (Reference level)

2 to 4 Wire Conversion

• Telephone sets are 4 wire devices i.e. 2 wires for microphone and two wires for ear phone but extension of 4 wires from subscriber to exchange is not cost effective solution therefore 2 wires are used

• The connection between exchanges is 4 wire as all the repeaters are unidirectional

• We need a device for conversion of 4 wires signals to 2 wire and vice versa

• A Hybrid TransformerHybrid Transformer is used to convert a 2-wire circuit to a 4-wire system

• Hybrid provides isolation between send and receive path

2-wires 2-wiresHybrid Hybrid

Amplifier or Regenerator

2-wires 2-wires

receivetransmit

Amplifier

2-wires 2-wires

receive transmit

Hybrid

Hybrid

• Hybrid is a transformer with four separate windings

• One is used for 4 wire transmit path

• One is used by 4 wire receive path

• One is used for 2 wire side

• One is used for balancing circuit which is used for balancing the 2 wire line impedance

2 to 4 Wire Conversion

Balance Network (ZB)

2-W Line (ZLine)4-wire portion

Send Amplifier

Receive Amplifier

Working of Hybrid

• Output from receive amplifier causes equal voltage to be induced in secondary windings of T1

• Matching NW winding are connected in opposite • Thus equal but opposite current flows in secondary

of T2• If impedance is exactly matched then nothing will be

coupled to transmit path• Input power has been equally shared by 2-wire and

matching NW i.e. 3 db loss has occurred• Once signal is applied from 2 wire side again power

will be equally divided between send and receive amplifier i.e. 3db loss

Echo

• If impedance matching NW impedance dose not matches with impedance of 2 wire then part of received signal will be coupled/reflected back to sender through trans path

• This reflected energy is called echo and it can be classified as :-

• Talker echo :- Talker echo :- when talker hears his/her own voice

• Listener echo :- Listener echo :- when listener hears talker’s voice twice

• Talker echo is more troublesome as it is louder

Talker Listener

Be+6dB

L

Talker Echo:

Loss = Be + 2L

Listener Echo:

Loss = 2Be + 2L

Listener/Talker Echo

Echo

• Effect of echo depends on relative echo level and on the delay– The stronger the echo and the longer the

delay, the more troublesome the echo

• The echo can be controlled by– Adding loss on 4 wire side– Improving hybrid return loss – Reducing gain of amplifiers

Echo

• There is a limit to which the loss of connection can be increased

• Increasing further loss will decrease loudness of signal

• Maximum value set for this is normally 40 ms

Return trip delay (ms)

Loss (dB)

Echo• For longer circuits echo suppressors or echo

cancellers are used• Echo suppressor

– is fitted in one path of 4 wire & is operated by speech signals of other side

• Echo canceller– echo canceller employs a digital adaptive filter

to set up a model or characterization of the voice signal

– As a voice path passes back through the cancellation system, the echo canceller compares the signal and the model to cancel existing echo dynamically

Stability• If the balance return loss is small and

gain is high then net gain may exceed 0 and singing will occur

• Singing can be regarded as excessive echo

Reflection(ZB ZL)

Stability• The net loss of singing path Ls = 2 (B + Lp) where B is

balance return loss due to reflection at hybrid and Lp is sum of losses between 2-W to 2-W

• Thus singing path loss equals– Balance return loss– Sum of 2W to 2W loses in both direction

• Ls > 0 , B + Lp >0 ,G < B• Singing point defines maximum gain w/o singing = B• Stability margin defines maximum amount of

additional gain that can be introduced on both sides = B + Lp

1020log B LS

B L

Z ZB

Z Z

Multiplexing• n Lines share bandwidth of one link• Multiplexer combines n inputs into single stream

(many to one)• De multiplexer separates them back in to

individual components (one to many)• Channel carries stream

FDM• Signal generated by each device modulate

different carrier frequencies• Modulated signals are then combined into one

composite signal• Channels are separated by unused bandwidth

called guard bands

FDM (MUX)

FDM (De - MUX)

FDM

Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands.

FDM

1.116 to 3.396 MHz

FDM

60 to 108 KHz

420 to 612 KHz

TDM

TDM (Synchronous)

TDM (Interleaving)

PCM

PCM

Non Uniform Quantization

• Quantization steps can be uniform or non uniform

• Uniform quantization will work well with large amplitude signal

• Small amplitude signal will only range over few steps and result into more quantization error

• Solution is to use small steps for small input signal and large steps for large input signals i.e. companding

• Two standard laws for non uniform quantization are A Law and mu Law

PCM (Sampling Frequency)

PCM Primary Multiplex group

• Telephone channels are combined using TDM to form 30 channel or 24 channels called primary multiplex channels

• Building block for making higher mux blocks i.e. PDH or SDH

• Length of frame is 125 micro sec • One speech sample for each channel &

additional digits for signalling / synchronization• Two widely used standards are:-

– 30 channel E-1 system in Europe– 24 channel T-1 system in USA/Japan

E -1 (30 Channels)

• We have total of 32 channels

• Channel # 0 is used for synchronization/frame alignment

• Channel # 16 is used to carry signalling information

• Total bit rate would be 8000 x 8 x 32 = 2.048 Mbit/s

T -1 (24 Channels)

• We have total of 24 channels

• First bit is used for frame alignment

• 24 (eight bit) Channels are used to carry speech information

• Total bit rate would be 8000 x (8 x 24 +1) = 1.544 Mbit/s

T 1 (24 Channels)

PDH (Plesiochronous digital

hierarchy)• Plesio- means “almost” but truth is that

each PDH island has its own clock

• First digital multiplexing network

• Based on 64kbps PCM encoded speech

• Transmission lines with 24 or 32 time slots

• ITU-T recommendation G.702/703

PDH

PDH Frame• Bit interleaving is used which dose not allow

Add/Drop facility • Overhead bits are used for two purposes :-

– Frame alignment (E-1/Block,T-1/Distributed)– Justification (If I/P rate is slow then justification

bits added)

SDH (Synchronous digital hierarchy)

• Motivations– Digital networks– high degree of flexibility; various bit rates can be combined

in one trunk circuit– truly worldwide standard

• ITU introduced SDH multiplexing hierarchy• Synchronous Optical Network (SONET) is American

version of the ITU (SDH)• SDH composite bit rates are chosen to be identical

to three of the eight lower SONET bit rates• These bit rates have become worldwide the most

widely used ones

SDH (Synchronous digital hierarchy)

• SONET can take payload of north American PDH hierarchy (1.5/6/45 Mbps + 2 Mbps) with in 51.84Mbps

• SDH std defined 1.5/2/6/34/45/140 Mbps with in 155.52 Mbps

• Primarily for Fiber Optics but now it supports radio also

SONET (Synchronous Optical NW)

SDH (Synchronous digital hierarchy)

• SDH frame consist of 9 equal length segments • It is depicted as 9 rows containing 270 columns where each

column size is one byte• First nine bytes of each segment (SOH, section overhead) are

used for overhead like framing , error monitoring etc • Rest 261 columns carry payload• The basic frame is known as STM-1(Synronous transport

module 1)and consists of 2430 bytes• Rest of the levels are multiple of 4• Data Rate?• Each column contains 9 bytes (rows) , with each byte having 64

kbps capacity– Three columns can hold 1.5 Mbps– Four columns can hold 2 Mbps – STM -1 can hold 6/45 American or 8/34/140 rates European – 6 Mbps ?

SDH (Synchronous digital hierarchy)

SDH (Multiplexing Scheme)

• Bytes from tributaries are assembled into container and path overhead is added to make virtual container

• VC travels through NW as one package until it is de-multiplexed

• A pointer may be added with VC to give its starting point this pointer + VC are called TU (Tributary Unit)

• STM-1 can contain # of TU s• TU s or VC s are then placed inside AU

(Administrative Units) • STM-1 data are simply byte interleaved with other

STM-1 data streams to make up a higher transmission data rate

• The de-multiplexer receives all STM-1 frames independently

Overall Loudness Rating (OLR)• Loss accumulated from speaker’s mouth and listener’s ear• Because of 4 wire conversion there is a need to introduce

some loss to avoid singing if # of 4 wire conversions increase then it can effect OLR therefore maximum allowable loss is given by formula L = 4 + 0.5 n dB where n is # of 4 wires in tandem

• OLR = SLR + CLR + RLR

SLR – Send Loudness Rating

CLR – Circuit Loudness Rating

RLR – Receive Loudness Rating

Mouth to

Interface

Loss

Interface to

Interface

Loss

Interface

to Ear

Loss

OLR Good/Excellent Poor/Bad

5-15dB 90% 1%

20dB 80% 4%

25dB 65% 10%

30dB 45% 20%

Overall Loudness Rating (OLR)

• Transmission loss engineering

• To prepare transmission loss plan which should take care of following :-– Control singing– Keep echo levels with in limits tolerable

to the subscriber– Provide an acceptable OLR

Digital Transmission

• Digital transmission links impairment :-– Bit error rate (percentage of bits with errors

relative to total number of transmitted bits)– Slip (loss of consecutive digits due to failure of

sync)– Jitter (short-term variations of digital signal from

their ideal positions in time)– Wander (long-term variations of digital signal

from their ideal positions in time )• Errored second (ES)— An errored second is any second

in which one or more bits are in error• Severely errored second (SES)— A severely errored

second has an error rate of 10-3 or higher• Degraded minutes— A degraded minute (DM) occurs

when there is a 10 -6 or worse bit error rate during 60 seconds

Digital Transmission Performance Parameters

• Degraded minutes - should be < 10%

• Severely erroded seconds – should be < 0.2%

• Erroded seconds (seconds with at least one error) should be < 8%

Transmission Systems• Customer Lines

– POTS, for Plain Old Telephone Service (Smallest wire guage to provide minimum resistance)

– Multi-party line offered– ISDN

• There are two basic types of ISDN service: Basic Rate Interface consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. This basic service is intended to meet the needs of most individual users

• PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s

– Digital subscriber line • 1.5M- to 9M-bps downstream (to the user) transmission, and 16K- to 800K-bps

upstream transmission – WLL (Wireless Local Loop)

• Junction Circuits• Trunk Circuits

Transmission Systems

• Junction Circuits– Cables with large no of conductors– FDM carrier system introduced– PCM primary multiplexer systems used on routes

requiring large number of circuits

• Trunk Circuit– Latest trend is to use digital communication on

fiber optics– Satellite communication systems for long haul

communication– Submarine cables an alternative

• Type Distance Bandwidth  Voice Channels  • Copper  2.5 km 1.5 Mb/s  24 • Fiber  200 KM 2.5+ Gb/s 32,000 +