UNIT-IV Signalling Techniques

Three forms of signalling

• Subscriber loop signalling

• Intraexchange or register signalling

• Interexchange or inter register signalling

Signalling Techniques

• In Channel Signalling

• Common Channel Signalling

Modes of operation of CCS

In Channel Signalling vs. CCSInchannel Signalling

Trunks are held up during signallingInterference between voice and

control signals may occurSeparate signalling equipment is

required for each trunk and hence is expensive

It is difficult to handle signalling during speech period.

Difficult to change or add signals.

In Channel Signalling vs. CCSCommon Channel Signalling

Trunks are not required for signalling.No interference as the two channels are

physically separate.Only one set of signalling equipments is

required for a whole group of trunk circuits and therefore CCS is economical.

There is freedom to handle signals during speech

Flexible to add or change signals.

Inchannel Signalling

• Inchannel Signalling Systems are SS1,SS2,SS3,SS4,SS5,SS5 bis

• International signalling systems SS4,SS5,SS5 bis adopt in band signalling.

• SS5,SS5 bis use Multifrequency signalling for interregister signalling

• Better talk off performance.

E and M signalling control

• This is the standard method of transferring signaling information between switching equipment and the signalling equipment.

• Three types of E and M• Type-I has two leads, one for each

direction of transmission.• The M lead carries d.c. signals from

the incoming part of the signalling terminal to the switching terminal.

E and M signalling control

• E lead carries d.c. signals from the incoming part of the signalling terminal to the switching equipment.

• Signals from exchange A are carried on M line and received on E lines to exchange B and similarly from exchange B To A.

• Type-II E and M interface is a 4-wire fully looped interface and is preferred for Electronic Switching Systems.

• Type-III is compromise between I and II.

Outband signalling cheme with E and M control

24 Channel PCM structure

• 24 channel system is organized on a 4-frame multiframe basis with bit 1 of the 8 bits in each time slot used for frame synchronization and signalling

• Bit 1 of each 8-bit time slot in frames 1 and 3 contains the signalling information.

• Bit 1 of every time slot in frame 2 is unused.First bit positions of the first 16 timeslots in frame 4 are used for frame synchronization with the last eight first bit positions being unused.

24 Channel PCM structure

Common Channel Signalling

• Signalling is completely separate from switching and speech transmission.

• Signalling is done over a channel that is different from the one which carries the voice or data.

• CCS is digital in nature modems are used for carrying digital data over analog lines.

• Since the channels are dedicated for signalling they are capable of carrying signalling information for a group of circuits.

Common Channel Signalling

• The group size is determined by the capacity of a signalling channel.

• CCS network is basically a store and forward network where signalling information travels on a link by link basis along the route.

• When signalling information is received at a node,it os stored,procesed and forwarded to the next node in the route.

Common Channel Signalling

CCS signalling message formats

• Signalling information is transferred as message of varying length usually defined as one or more fixed length signalling units(SUs).

• A message of one signal unit length is called single unit message (SUM).

• One with multiple signal units as multiunit message(MUM).

• Each SU is of size 28 bits of which the last eight bits are check bits.

CCS signalling message formats

Architecture of SS7

• SS7 is suitable for operation over analog channels and at speeds less than 64 kbps.

• SS7 is suitable for operation over both terrestrial and satellite links.

• SS7 has 4 levels, the lower three levels referred as message transfer part(MTP).

• There are 3 types of signalling units defined in SS7 Message Signal Unit, Link Status Signal Unit, Fill In Signal Unit.

Architecture of SS7

Formats of signalling units• All the SUs begin and end with a flag field which

has unique bit pattern 01111110.• A common flag may be used as the closing flag

for one SU and the opening flag for the next.• It is possible that the unique flag bit pattern

appears inside the SU,thus destroying synchronization.

• To avoid this problem, a technique known as bit stuffing is used.

• Transmitter inserts an extra 0 whenever it comes across 5 consecutive ones.

• Receiver on detecting 5 consecutive ones deletes the zero following it.

• All the SUs in SS7 contain 16-bit error checking field (CRC).

Formats of signalling units

• Backward Sequence number (BSN) and the Backward Indicator (BI) bit together permit acknowledgement of the SUs received.

• A negative acknowledgement is indicated by inverting the Backward Indicator(BI) bit.

• Forward Sequence number (FSN) identifies the SU uniquely using modulo 128 count

• Retransmission is indicated by inverting the Forward Indicator (FI).

Formats of signalling units

Network Traffic Load and Parameters

Busy Hour Peak Busy Hour Time Consistent Busy Hour Call completion rate (CCR)is the ratio number of

successful calls to the number of number of calls attempt.

Number of calls attempt in busy hour is called Busy Hour Call Attempts (BHCA).

Busy Hour Calling Rate defined as the average number of calls originated by a subscriber during the busy hour.

Busy hour calling rate=Average busy hour calls/Total number of subscribers.

Average busy hour calls=BHCA x CCR.

Network Traffic Load and Parameters

Network Traffic Load and Parameters

Day to Busy Hour Traffic Ratio is busy hour calling rate to average calling rate for the day.

Typically this ratio may be over 20 for a city business area and around 6 or 7 for rural areas.

Traffic Intensity is the ratio of period for which a server is occupird to total period of observation.

Period of observation generally taken as 1 hour.

Network Traffic Load and Parameters

Centum Call Second CCS another method for calculating traffic intensity.

One CCS may mean one call for 100 seconds or 100 calls for one second.

Call Seconds (CS) and Call Minutes (CM) are also used as e measure of traffic intensity.

1E=36 CCS=3600CS= 60 CM

Grade of Service and Blocking Probability

Grade of service=A-Ao/AWhere A=Offered traffic

Ao=Carried Traffic

A-Ao=Lost TrafficTrunks in a group are busy and can not

accept any call, this state is defined as Call Congestion or Loss Probability.

Blocking Probability is called Time Congestion.

Grade of Service and Blocking Probability

The probability that a call experiences a delay, termed Delay probability.

The technique of maintaining the stable operation is called Flow Control.

Subscriber view pointGOS=call congestion = loss probability.

Network view pointBlocking probability = time congestion