switching principle

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Switching Principles

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DIGITAL SWITCHING

If we look back for a moment at the time when telephony was young,our exchanges were operated manually. Nowadays we have completely automatic switching systems based on electronic technology. Here we shall only look at systems for digital switching.

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In "Digital Switching Systems", two types of switches will be presented i.e. the time switch and space switch, together with combinations of these. To round it off, two main principles for connection of subscriber lines to the switching network in the exchanges will be covered.

Traffic between different exchanges in the network presumes that these can send and receive signalling information (inter-exchange signalling). The demands on the switching systems are extremely high, switching of calls must be fast and error-free. In order to achieve this, control systems are needed which are both efficient and reliable.

Switching PrinciplesDigital exchanges establish connections by re-arranging the 8 bit

binary codes of various telephone signals as specified by the call request

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Digital Switching Network

Digital switching networks comprise the time, space-time and space switches .

These switches can be used individually or in any desired combination. A common combination is the time/space/time configuration. The combination actually used depends on the switching system concept and the size of the exchange.

Time stage Space stage Time stage

3 3

3 3

3 3

3 3

3 3

3 3

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There are two types of Switching devices:

i) The Time Switch ii) The Space SwitchTime SwitchThe time switch can switch any 8 bit PCM binary code from the incoming highway to any time slot on the outgoing highway (full availability). For this, the incoming binary codes are written into the data memory cyclically. As specified by the call request stored in the control memory, the code words are read out again.

TIME SWITCHING

Memory Memory location 1location 1




A3A3

A4A4

2413

A4A4 A3A3 A2A2

A4A4 A3A3A2A2 A1A1

tE4

tE3

tE2

tE1 tA2

tA4

tA1

tA3Control memory

Control addresses( no. of data memory location)=̂

tA1tA2tA3tA4

125 s period (outgoing time slots)

tA4 tA3 tA2 tA1

Outgoing highway

8-bit PCM words

Incoming highway

8-bit PCM words

125 s period (Incoming time slots)

tE4 tE3 tE2 tE1

Cyclic Write-in Arbitrary controlled read-out

A1A1

A1A1 A2A2

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Time Switch Features- Switching process:

The PCM word time slots are interchanged.

- Non- blocking:

All incoming PCM words can be forwarded when the number of time slots on the incoming highway <

the number of time slots on the outgoing highway.

- Full availability:

Any incoming PCM word can be forwarded to any outgoing time slot.

- Efficient and small:

The memory and control elements consist of large-scale integrated semiconductor components.

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Space SwitchThe space switch can switch any 8 bit PCM binary code from the incoming highways to any outgoing highway without change of time slot.

&&&&&&

A3 A2 A1

t3 t2 t1Electronic switch

&&&&&&

B3 B2 B1

&&&&&&

C3 C2 C1

t1

t2

t3

Space stage

1

2

3

Control line

Number of incoming highway

Control memory

1 2 3Outgoing Highway

2

3

3

2

1

1

3

2

1

Incoming Highway

8

&&&&&&

A3 A2

t3 t2 t1

&&&&&&

B3 B2

&&&&&&

C3 C2

t1

t2

t3

1

2

3

1 2 3Output

2

3

3

2

1

1

3

2

1

Input

The through-connection is performed by means of electronic switches (AND gate). The exact sequence in which the AND gates are to be turned ON or OFF, is indicated in the control memory. For each time slot (t1 - t3), the AND gate is switched again.

A1C1B1

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SPACE SWITCH

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Space Switch Features- Switching process:

The PCM word retain same time slot, but can be allocated to any outgoing highway.

- Non- blocking:

In an arrangement with m incoming and n outgoing highway, where n >m .

- Full availability:

Any incoming PCM word can be forwarded to any outgoing highway.

- Efficient and small:

The space switch consists of large-scale integrated electronic circuits.

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Space-time Switch The space-time switch is a high-speed variation of the time Switch. Because of its high operating speed, it can switch an 8 bit PCM binary code from several incoming highways to any time slot of several outgoing highways. To achieve this, PCM binary codes of the incoming highways must be combined (multiplexed) and extended to the data memory. This means that the highway from the multiplexer to the data memory has a bit rate which is several times higher than that of the incoming highway.

In the example shown, the highway from the multiplexer to the data memory has a bit rate which is four times as high as the bit rate of a single incoming highway. Following the switching, the demultiplexer distributes the PCM binary codes to four outgoing highways operating at the original bit rate.

In all other respects, the space-time switch works on the same principles as the time switch. It also enables that any PCM binary codes on the incoming side is switched to any time slot on any of the four outgoing highways without blocking (full availability).

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SPACE TIME SWITCH

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SPACE TIME SWITCH

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Call example in PSTNFollowing steps are involved in the establishment and subsequent release of a call on PSTN:

1. DETECTION OF HOOK OFF

2. PROVISION OF DIAL TONE

3. DIALING OF DIGITS

4. RESERVATION OF RESOURCES FOR CALL

5. RINGING OF CALLED PARTY AND RING BACK TO CALLING

PARTY

6. DETECTION OF ANSWER FROM CALLED PARTY

7. NECESSARY CHARGING INITIATION

8. SPEECH

9. INITIATION OF CALL TERMINATION FROM EITHER END

10. CALL END, RELEASE OF ALL RESERVED RESOURCES

11. NECESSARY ACTION REGARDING BILLING

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1 Off Hook

2 Dial Tone

3 Digits Dial

4 Siezure 5a Ringing

5b Ring Back

6 Off Hook

7 Charging Started8 Conversation / Speech

9 Call End

10 Release Resources 10 Release Resources10 Release Resources

11 Charging Stopped

GENERAL CALL SETUP

Local Exchange

Local ExchangeA B

switching principle

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