telecommunications switching systems 4

7/29/2019 Telecommunications Switching Systems 4

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Transmission Systems, PairGain Systems, FDM

Lecture 4

Open Wire


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Open wire

Relatively low attenuation

Useful for long, rural customer loops

Keeping wires separate is a problem

Large diameter

High copper consumption

Paired Cable


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Paired Cable

Introduced in 1883

6-2700 wire pairs in a single cable.

Underground cable distribution.


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AWG

Lower gauge (Higher diameter) are usedfor longer distances.

Cable pairs are capable of carrying muchhigher frequencies than required by a

telephone quality voice signal (3.4 kHz)


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From economic point of view, subscriberloop lengths should be as large aspossible to cover a large area.

But two factors limit their length: Signalingand Attenuation limits.

Exchanges are designed to accept amaximum loop resistance.


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An exchange uses a -40 V battery to drive subscriber lines. A

resistance of 250 is placed in series with the battery to protect itfrom short circuits. The subscribers are required to use a standard

telephone set which offers a dc resistance of 50 . Themicrophone requires 23 mA for proper functioning. Determine thefarthest distance from the exchange at which a subscriber can be

located if 26 AWG conductor is used.

Solution:

23 mA = 40/(250+50+R); R = 1439

Loop Length = 1439/133.89 = 10.74 km

Farthest Distance = 10.74/2 = 5.37 km

Example

DC resistance constraint is met by

Use of higher diameter (lower gauge) wire

Use of equalized telephone sets.

Unigauge design or Use of higher supplyvoltage.

Loading Coils improve frequency vs.attenuation characteristics.


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Unigauge Design

48 V

72/96 V

Loading Coils

Attenuation constraint is usually overcome bythe use of loading coils.

Typical Inductance value = 88 mH and typicalspacing interval is 1.8 km.

Loading coil std convention: 26-H-88

0.650.21.830.851.71.370.280.920.21Spacing (Km.)

YXHFEDCBALetter Code


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Loading coils

Single Wire Transmission with ground return

Two Wire Transmission


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Single vs. Two wire

Single wire with ground return is too noisyfor customer acceptance.

Balanced pair of wires reject commonmode signal. Thus, provides better circuitquality.

Balanced pair problem

If users on both ends talk simultaneously, theirconversations are superimposed on the wire pair

and can be heard at both sides.

Wire-line transmission is best implemented over

long distances (e.g. between switching offices),

if transmission for both directions are separatedon different wire pairs.


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Long distance requirements

Amplification

Multiplexing

Easy implementation if two directions oftransmission are isolated from each other.

2-wire to 4-wire, copper requirementincreases?

Two-wire versus Four-wire

Two-Wire to Four WireConversion


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Pair Gain Systems

Providing service to rural subscribers hasalways been an expensive proposition dueto length of routes and less number ofsubscribers to support installation andmaintenance cost.

Earlier party lines were used, which involved

sharing of a wire pair among multiplehouseholds

Party Lines

Useful for satisfying new service requestson routes with no spare wires.

Objectionable due to lack of privacy andlack of availability.


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Pair Gain Systems

Concentration

Multiplexing

TASI

A pair gain system is an alternate approachto share pairs of wires that is more

acceptable to users.

Three techniques are used to gain on number

of pairs:

Drawback of multiplexing pair-gain systems?

Concentration

Blocking M


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TASI

Time Assignment Speech Interpolation

Dynamically assigns a channel after sensingvoice activity on channel.

A conversation is active for only 40% of time,which indicates that excess activity can beaccommodate during inactive period in adirection.

If a source becomes active when all channels

are occupied, the speech segment is clippeduntil a channel becomes free.

TASI


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FDM

For long distance telephone connections,20th century telephone companies used L-carrier and similar co-axial cable systemscarrying thousands of voice circuitsmultiplexed in multiple stages.

L-carrier

132,000 voice circuits2 miles2257 MHz1972L-5



360 voice circuits16 miles4840 kHz1942L-2

600 voice circuits8 miles43 MHz1941L-1

Capacity per coaxDistancebetweenrepeaters

Coaxper

cableFrequencyYearSystem


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FDM Hierarchy

FDM is based on channel groups.

The channel groups are based on multiples of the voicechannel, as shown below:

The basic channel is called the Voice Channel (VC) and ithas a bandwidth of 0-4 kHz.

Group is made of 12 Voice Channels.

Supergroup (60 VCs) is made of 5 Groups.

Mastergroup (600 VCs) = 10 Supergroups.

Jumbogroup (3600 VCs) = 6 Mastergroups.

First Level Mux


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Second Level Mux

FDM Hierarchy

Multiplex Level VCs Freq Band(kHz) BW(kHz)

Voice Channel 1 0 - 4 4Group 12 60 - 108 48Supergroup 60 312 - 552 240Mastergroup 600 564 - 3,084 2520Jumbogroup 3600 564 - 17,548 16984

telecommunications switching systems 4

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