Transmission & Switching SystemTransmission & Switching System

2-Wire Circuit2-Wire Circuit

• Tx in both direction are carried on the same pair of wires called as 2-Wire circuit

• Application

• For short distances

• E.G Telephone set in our home connected with local exchange office

CONT:CONT:

•PB ?????

Four-Wire CircuitFour-Wire Circuit

• A circuit which required separate channel for tx and separate for rx called as 4-wire circuit

• IT Cannot be connected directly to 2-wire

• WHY???

• A Hybrid TransformerHybrid Transformer is used to convert a 2-wire circuit at the phone/terminal end to a 4-wire system in the switching network:

The Concept and Implications of The Concept and Implications of “two-wire” (2W) to “four-wire” (4W) “two-wire” (2W) to “four-wire” (4W) conversionconversion

2-W to 4-W Conversion (2)2-W to 4-W Conversion (2)

2-wires 2-wiresHybrid Hybrid

Amplifier or Regenerator

2-wires 2-wires

receivetransmit

Amplifier

2-wires 2-wires

receive transmit

repeater

Telephone Repeater System:

Crosstalk:BA

DC

Electromagnetic Induction

Telephone C hears telephone B

on a different circuit.

2-W to 4-W Conversion (3)2-W to 4-W Conversion (3)

• Balance network has a balance impedance of ZZBB.

• If ZZBB=Z=ZLineLine then no reflection occur

• If ZZBBZZLineLine then create “sidetonesidetone”.– Reflections from the C.O. return to the station set.– Talker hears his/her own voice.– No sidetone makes the line feel “dead” and unnatural (IP

telephony often sounds like this since there’s no sidetone).

Concept of Hybrid Return Loss Concept of Hybrid Return Loss

Echo return loss (ERL) = average attenuation of power reflected at the 2W-4W interface

Singing Return Loss (SRL) = minimum attenuation to reflected power at any frequency

coming back from the 2W-4W interface

Network Loss PlanningNetwork Loss Planning

• Stability or Oscillation Control: “Singing”– Manage reflections that can result if there’s a poor

mismatch of the 2-wire line impedance and the hybrid balance impedance.

– Singing can result.

• Echoes– Talker should not hear his/her own voice reflected back

(with a significant enough delay).

StabilityStability

• Long distance connections all have 2-W to 4-W to 2-W conversion (as do most local connections).

• If there’s a poor mismatch of the 2-W line impedance with the hybrid balance impedance, signal energy passes across the hybrid reflecting from one 4-W direction into the other.

2-wires 2-wiresHybrid Hybrid

Amplifier

2-wires 2-wires

receivetransmit

Amplifier

2-wires 2-wires

receive transmit

Reflection(ZB ZL)

Stability (2)Stability (2)

• Reflection at the hybrid re-inserts the signal with “balance balance return lossreturn loss” (BRLBRL or BBSS) into the return side of the 4-W loop.

1020log B LS

B L

Z ZB

Z Z

Minimum return loss seen atthe hybrid in any frequency

in the voice-band

• Additional 3+dB loss at hybrid when converting 4-W signal to 2-W signal, and another 3+dB going from 2-W to 4-W (6db total).

• Total trans-hybrid loss of returned signal:3 3STHL dB B dB

6STHL B dB Ideal loss

7STHL B dB Loss in practice (~3.5 db splitting loss)

Stability (3)Stability (3)

3dB

3dB

BS+6dB

G

Net Gain of one side of 4-W loop(total amplifier gain minus line losses)

T2-W to 2-W total attenuation

6T dB G

Stability (4)Stability (4)

• Total round-trip closed loop loss (“singing marginsinging margin”):

2( 6 )Sm B dB G 2( )Sm T B

• Stability margin define as max amount gain introduce equally in both direction define as

• M=B+T db

• Otherwise, singingsinging may result.– out of control runaway oscillation in the loop.

Echo-Delay PhenomenaEcho-Delay Phenomena

• If the reflection at the hybrid is strong enough, telephone users will hear it.

• Talker echoTalker echo is when talker hears his/her own voice.

• Listener echoListener echo is when listener hears talker’s voice twice.

Talker Listener

Be+6dB

TTalker Echo:

Loss = Be + 2T

Listener Echo:

Loss = 2Be + 2T

Echo-Delay (2)Echo-Delay (2)

• Recall Bs:– Balance Return Loss– Minimum return loss seen at any voice-band frequency

• What is Be?– Hybrid Echo Return LossHybrid Echo Return Loss– Average return loss in voice-band.

Frequency

B(f)

Return Lossat Frequency f

10

( ) ( )( ) 20log

( ) ( )B L

B L

Z f Z fB f

Z f Z f

Be (echo)

BS (stability)Why BWhy Bee and not B and not BSS??