Standard_notes_tsn Unit III 2

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3 Telecommunication Switching and Networks

ECE Study Materials [www.pbtstudies.blogspot.com]

SIGNALLINGIn a telecommunication network, signalling systems are as essential as switching systems and

transmission systems. They must be compatible with the switching systems as they must be able to

transmit all the signals required to operate the switches. They must also be compatible with the

transmission system in order to reach the exchange that they control. Thus, design of signalling systems

is directly influenced by both switching and transmission requirements.

Exchanges usually send signals over the same circuits in the network as the connections which

they control. This is known as channel associated signaling. In SPC, the need for more signals to be

transmitted between exchanges arise. These signals are transmitted between two processors of two

different exchanges over a separate data channel. This is known as common channel signalling (CCS).

Signaling can be classified as follows:

Inchannel versus common channel signalling:

INCHANNEL COMMON CHANNEL

1.

2.

3.

4.

5.

Trunks are held up during signaling

Signal repertoire is limited

Interference between voice and control signals

may occur

Separate signaling equipment is required foreach trunk and hence is expensive.

Signaling is relatively slow

Speech circuit reliability is assured.

Trunks are not required for signalling.

Extensive signalling repertoire is possible

No interference as the two channels are

physically separated.

Only one set of signalling equipment is requiredfor a whole group of trunk circuits, therefore is

inexpensive.

Signalling is significantly fast.

There is no automatic test of the speech circuit.

There is flexibility to change or add signals.

Si nallin

Inchannel Common channel

DC Low frequency Voice frequency PCM

Inband Outband

Associated Non-associated

Fig. Signalling techniques





6.

7.

8.

9.

10.

It is difficult to change or add signals.

It is difficult to handle signaling during speech

period.

Reliability of signaling path is not critical.

Possibility of misuse by customers.

Signals can be handled anytime.

Reliability of signalling path is critical.

Control channel is generally inaccessible to

customers.

Customer line signalling: In a local telephone network, loop/disconnect signaling is used for sending customer’s call and

clear signals to the exchange. Due to maximum permissible line resistance (because of minimum line

current), there is a limit on the maximum length of the line and area served by the exchange.

When dial telephones are used, customers send address information by decadic pulsing which is

received by a relay circuit. However push-button telephones use DTMF revolutionized customer line

signaling.

FDM carrier systems:

Outband signalling:

In Frequency Division Multiplex (FDM) systems, the carriers are placed at intervals of 4kHz and thebaseband is from 300Hz to 3.4kHz. by using channel filters with a sharp cut-off, it is possible to insert a

narrow-band signalling channel above the speech band (3.4kHz to 4kHz). This is known as outband

signalling .





A DC signal on the input lead M at one terminal causes the signal frequency to be sent over the

transmission channel. This is detected on the other terminal to give a corresponding DC signal on the

output lead E. If the repeater station containing the FDM channeling equipment is adjacent to the

switching equipment, it is simpler for the latter to send and receive signals over separate E and M wires

than to extract them from and re-insert them into the speech circuit. The E lead always carries signal

from the signaling apparatus to the switching equipment and the M lead carries signals from the

switching equipment to the signalling apparatus. To use outband signalling successfully in a network, all

routes must use FDM systems with built-in outband signalling.

Inband (VF) signalling:Signals that are placed in the outband region need all routes to be equipped with proper outband

signalling FDM systems. This problem is solved if the signals transmitted are placed in the baseband of

FDM systems. This is known as inband signalling and this will function over any circuit which provides

satisfactory speech transmission. A voice frequency signaling system is shown in the figure below.





The line is split when the signal tone is transmitted in order to confine it to the link concerned.

Consequently, the tone spills over before the receiver has operated but this spill-over is ignored because

its duration is less than the length of the signals used. The unity gain buffer amplifier at the receiving

end prevents transients produced by electro-mechanical switching equipment from reaching the VF

receiver.

Since the voice frequency signals are used, there occurs a possibility of signal imitation which is

undesirable. The following measures are taken to avoid this:

A signal frequency is chosen at which the energy in speech is low (i.e. above 2khz).

The durations of signals are made longer than the period for which the speech frequency is

likely to persist in speech.

Use is made of the fact that the signal frequency is unlikely to be produced in speech without

other frequencies also being present.

In order to make use of the last measure, the receiver contains a signal circuit with a band pass

filter to accept the signal frequency and a guard circuit with a band stop filter to accept all other

frequencies and reject the signal frequency. The outputs of both circuits are rectified and compared. If

the output from the signal circuit exceeds that from the guard circuit, the receiver operates and gives an

output signal, and vice-versa.

Switching

equipmen

t

Switching

equipmen

t

Receive line

split

VF receiver

VF receiver

Buffer amplifier

Buffer amplifier

Transmit line

split

Forward

Backward

Outgoing signal

terminal

Incoming signal

terminal

Transmit line

splitReceive

line split

Four

wirecircuit

fs

~

Fig. Voice frequency(VF) signalling system





PCM signalling: In this, the DC signals associated with the audio frequency baseband circuits in each direction are

sampled and the signal samples are transmitted within the frame of PCM channels. It is therefore

unnecessary to use VF signalling.

The 2Mbits system has 32 8bit time slots, but it provides only 30 channels. Time slot 0 is used for

frame alignment and time slot 16 is used for signaling, as shown above. The 8bits of channel 16 are

shared between the 30 channels by a process of multi-framing. 16 successive appearances of channel 16

form a multi-frame of 8bit time slots. The first contains a multi-frame alignment signal and each of thesubsequent 15 time slots contain 4 bits for each of the two channels. This enables a large number of

signals to be exchanged than is possible with the DC signaling methods. When PCM signaling is used for

common channel signaling, then multi-framing is not needed.

Inter-register signalling: For register-controlled exchanges, a register in the originating exchange receives address

information from the calling customer and sends out routing digits. This goes on till the terminating





exchange is reached. This introduces post-dialing delay which is minimized using inband multi-frequency

signaling systems. This enables an operator to send address information over a junction to an automatic

exchange more rapidly than by dialing.

In inter-register signaling systems, the signal initiates a connection to a register. The register is

released after it has set up a connection through its exchange and sent out routing digits, therefore it

cannot receive answer and clear signals. Consequently line signaling is required in addition to inter-

register signaling.

Either en-bloc or overlap signalling may be used. In en-bloc signaling the complete address

information is transferred from one register to the next as a single string of digits. Thus no signal is sent

until the complete address information has been received. In overlap signaling, digits are sent out as

soon as possible enabling signaling to take place simultaneously on two links.

Also link by link or end to end signaling may be employed. In link by link signaling, information is

exchanged only between adjacent registers in a multi-link connection. In end to end signaling, the

originating register controls the setting up of a connection until it reaches its final destination.

Fig. Link-by-link and End-to-end signalling between registers

Common channel signalling: In common channel signaling, there is a separate data link between the two processors in two

different exchanges. All signals between these two exchanges are transmitted via this data link. It gives

the following advantages:

Information can be exchanged between the processors much more rapidly than when channel-

associated signaling is used.

As a result, a much wider repertoire of signals can be used and this enables more services to be

provided to the customers.

Signals can be added or changed by software modification to provide new services.

There is no longer any need for line signaling equipment on every junction which results in a

considerable cost saving.





Since there is no line-signalling, the junctions can be used for calls from B to A in addition to calls

from A to B.

Signals relating to a call could be sent while the call is in progress.

Signals between two processors can be exchanged for functions other than call processing, for

example for maintenance or network management purposes.

For a common channel signal, the reliability needs to be much greater than channel-associated

signalling because failure of data link could prevent any calls to be made between the two exchanges.

CCS does not provide an inherently checking facility. Therefore a separate means of checking the

functioning of speech circuits must be employed.

In multi-exchange network there will be many CCS links between exchanges and they form a

signalling network. In principle, CCS networks can pass through different routes from the connections

which they control and they can pass through several intermediate nodes in the signaling network. This

is called non-associated signalling. here the messages must include labels containing their destinations.

Switching

network

Switching

network

Processor

Processor

Processor

Processor

Switching

network

Switching

network

Backward

signals

Forward

signals

Exchange A Junction Exchange B

Fig. Channel associated signalling between central processors

Signalling

link

Exchange A Junction Exchange B

Fig. Common channel signalling between central processors





In practice, CCS messages are usually only routed through one intermediate node. This is known as

quasi-associated signalling. The intermediate node is called signal transfer point (STP). Since CCS signals

may be routed via an STP, each message contains a destination point code and also an originating point

code. The transmission bearers used for a CCS network are channels in the main transmission bearer

network.

CCITT signalling system no.7 This was the first CCS system to be standardized internationally. This was used in analog networks

and it used bit rates of 2.4kbits/s and 4.8kbits/s. it used modems to transmit over analog telephone

channels. It used fixed size signal units of 28bits. A later version for use in digital networks added four

padding bits to be compatible with 8bit PCM time slots. However this has now been replaced by the

CCITT signalling system no.7.





High-level data-link control protocol (HDLC):

Flag Address Control Information Check Flag

1 octet 1 or 2 octets 1 octet variable 2 octets 1 octet

fig. Frame structure for high-level data-link control (HDLC) protocol

The level 2 protocol used in the CCITT no.7 signalling uses the international standard known as

high level data-link control (HDLC). Messages are sent by packets contained within frames having the

format shown above.

The beginning and end of each HDLC message is indicated by a unique combination of

digits(01111110) known as a flag. These sequence of digits can occur in the message also and must notbe interpreted as a flag. This is done by 0 bit insertion and deletion which is also called ‘stuffing’ and ‘un-

stuffing’ respectively. When sending digits of a message between two flags, the sending terminal inserts

a 0 after every sequence of five consecutive 1s. the receiving terminal deletes this 0.

The opening flag is followed by bit fields for address and control information followed by the data

field containing the message information. Between the data and the closing fields, there is an error-

check field, which enables the receiving system to detect if the frame is erroneous and request re-

transmission.

Signal messages are passed from the central processor of the sending exchange to the CCS system.

This consists of 3 micro-processor based sub-systems:

1. signalling control subsystem

2. signalling transmission subsystem

3. error control subsystem

The signalling control subsystem structures the messages in the appropriate format and queues

them for transmission. Messages are then passed to the signalling termination subsystem, where

complete signal units (SU) are assembled using sequence numbers and check bits generated by the error

control subsystem. At the receiving terminal, the reverse sequence is carried out.

The system can be modeled as a stock of protocols:

1. Level 1: The physical level

It is the means of sending bit-streams over a physical path. It uses time slot 16 of a

2Mbit/s PCM system or time slot 24 of a 1.5Mbit/s system.

2. Level 2: Data-link level:

It performs the function of error control, link initialization, error-rate monitoring, flow

control and de-lineation of messages.





3. Level 3: Signalling network level:

It provides functions required for a signaling network. Each node in the network has a

signal point code which is a 14 bit address. Every message contains the point code of the

originating and terminating nodes for that message.

4. Level 4: User level:

This must be fully compatible with the level 3 of the model.

Signal units: Information that has to be sent in structures into a signal unit (SU) by the signalling control unit.

The SU is based on the HDLC protocol. SUs are of 3 types:

1) The message signal unit (MSU): This transfers information supplied by a user port (level 4) via

the signaling network level (level 3).

2) The link-status signal unit(LSSO): This is used for link initialization and error control

3) The fill-in signal unit(FISU): This is sent to maintain alignment when there is no signal traffic.

The format of MSU is shown below:

Flag BSN BIB FSN FIB LI Spare SIO SIF Check Flag

8 7 1 7 1 6 2 8 8n 16 8

fig. Message signalling unit

Flag BSN BIB FSN FIB LI Spare SF Check Flag

8 7 1 7 1 6 2 8 or16 16 8

fig. Link status signalling unit

Flag BSN BIB FSN FIB LI Spare Check Flag

8 7 1 7 1 6 2 16 8

fig. Fill-in unit

SF: status field BIB: backward indicator bit

SIF: signaling information field BSN: backward sequence number





SIO: service information octet FIB: forward indicator bit

LI: length indicator FSN: forward sequence number

Fig. Format of signal units in CCITT no.7 signalling system

Messages are of variable length and are sent in 8-bytes as follows:

1) Opening and closing flags are used to delimit signals. They have the code pattern ‘01111110’.

2) The forward indicator bit (FIB), backward indicator bit (BIB), forward sequence number (FSN)

and backward sequence number (BSN) are used for error correction.

3) The length indicator (LI) gives the length of the SU. Value of LI greater than 2 indicates that the

SU is a message signal unit.

4) The service information octet (SIO) indicates the user port appropriate to the message.

5) The signalling information field (SIF) may consist of upto 272 octets and contains the

information to be transmitted.6) The error-check field is immediately before the closing flag. It contains 16 bits generated as a

cyclic redundancy check code.

Traffic.

In telecommunication system, traffic is defined as the occupancy of the server in the network.

There are two types of traffic viz. voice traffic and data traffic. For voice traffic, the calling rate is defined

as the number of calls per traffic path during the busy hour. In a day, the 60 minutes interval in which the

traffic is highest is called busy hour (BH).

Grade of Service.

In telephone field, the so called busy hour traffic are used for planning purposes. Once the

statistical properties of the traffic are known, the objective for the performance of a switching system

should be stated. This is done by specifying a grade of service (GOS). GOS is a measure of congestion

expressed as the probability that a call will be blocked or delayed. Thus when dealing with GOS in traffic

engineering, the clear understanding of blocking criteria, delay criteria and congestion are essential.

Blocking criteria.

If the design of a system is based on the fraction of calls blocked (the blocking probaility), then

the system is said to be engineered on a blocking basis or call loss basis. Blocking can occur if all devices

are occupied when a demand of service is initiated. Blocking criteria are often used for the dimensioning





of switching networks and interoffice trunk groups. For a system designed on a loss basis, a suitable GOS

is the percentage of calls which are lost because no equipment is available at the instant of call request.

Delay criteria.

If the design of a system is based on the fraction of calls delayed longer than a specified length of

time (the delay probability), the system is said to be a waiting system or engineered on a delay basis.

Delay criteria are used in telephone systems for the dimensioning of registers. In waiting system, a GOS

objective could be either the percentage of calls which are delayed or the percentage which are delayed

more than a certain length of time.

Congestion.

It is the condition in a switching center when a subscriber can not obtain a connection to the wanted

subscriber immediately. In a circuit switching system, there will be a period of congestion during which

no new calls can be accepted. There are two ways of specifying congestion.

1. Time congestion.

It is the probability that all servers are busy. It is also called the probability of

blocking.

2. Call congestion.

It is the proportion of calls arising that do not find a free server. Call congestion is a loss system

and also known as the probability of loss while in a delay system it is referred to as the probability of

waiting. If the number of sources is equal to the number of servers, the time congestion is finite, but the

call congestion is zero. When the number of sources is large in comparison with servers, the probability

of a new call arising is independent of the number already in progress and therefore the call congestion is

equal to the time congestion. In general, time and call congestions are different but in most practial cases,

the discrepancies are small.

3.Measure of GOS.

GOS is expressed as a probability. The GOS of 2% (0.02) mean that 98% of the calls will reach a

called instrument if it is free. Generally, GOS is quoted as P.02 or simply P02 to represent a network busy

probability of 0.02. GOS is applied to a terminal-toterminal connection. For the system connection many

switching centers, the system is generally broken into following components.





(i) an internal call (calling subscriber to switching office)

(ii) an outgoing call to the trunk network (switching office to trunk)

(iii) The trunk network (trunk to trunk)

(iv) A terminating call (switching office to called subscriber)

The GOS of each component is called component GOS.

The GOS for internal calls is 3 to 5%, for trunk calls 1-3%, for outgoing calls 2% and for

terminating calls 2%. The overall GOS of a system is approximately the sum of the component grade of

service. In practice, in order to ensure that the GOS does not deteriorate disastrously if the actual busy

hour traffic exceeds the mean, GOS are specified 10% or 20% more of the mean.

TELECOMMUNICATIONS TRAFFIC

In case of telecommunication systems, it is required to design the system in

accordance with the number of calls that are in progress at any point of time and

the total number of subscribers that are connected to the network. Teletraffic

engineering involves the design of the number of switching equipment required

and the number of transmission lines required for carrying telephone calls.

In teletraffic engineering the term trunk is used to describe any entity that

will carry one call. The arrangement of trunks and switches within an exchange is

called it’s trunking.

We can check the number of calls in progress at different intervals of time

for a whole day and then tabulate the results. If a graph is plotted taking the

number of calls in progress on y-axis and the time of the day on x-axis the graph

would look like this:-





fig 1.

The maximum number of calls occurs between 8:00 and 10:00 am for this

particular exchange. This hour which corresponds to the peak traffic of the

exchange is called the busy hour.

The busy hour varies for different exchanges and the teletraffic curve also

varies for different exchanges from what is shown in the fig 1.

Exchanges in which offices and business establishments predominate

usually have a busy hour between 10:00 and 11:00 am. Residential exchangeshave a busy hour normally between 4:00 and 5:00 pm.

The limit of traffic:

The teletraffic intensity or simply the traffic is defined as the average no.

of calls in progress. The unit of traffic is erlang (named after the Danish pioneer in

teletraffic A.K.Erlang).It is a dimensionless quantity.

On a group of trunks, the average number of calls in progress depends on

both the no. of calls which arrive and their duration. The duration of a call is

called it’s holding time because it holds the trunk for that time.

Consider a holding time T for a group of 3 trunks:





Fig. 2

Example of 1 erlang of traffic carried on 3 trunks

Figure 2(a) shows 1 erlang of traffic resulting from one truck being busy for the

holding time T. Figure 2(b) shows 1 erlang of traffic resulting from two trunks with

each trunk being busy for 50% of the time T. Figure 2(c) shows 1 erlang of traffic

being carried by three trunks with each of the trunks being busy for 33.33% of the

time T.

Sometimes the traffic is also expressed in terms of hundreds of call

seconds per hour ( CCS).

1 erlang = 36 CCS

Mathematically traffic can be represented by the following equation:

A=Ch/T (1)

where A=traffic in erlangs

C=average number of calls arriving

during time T

1

2

1

2

1

2

a

b)

c)





h=average holding time

From eqn 1, if T=h, A=C. Thus traffic in erlangs can be defined as mean

number of calls arriving during a period equal to the mean duration of the calls

(average holding time).

A single trunk cannot carry more than one call, therefore A<=1 for a single

trunk. This is called the occupancy of the trunk. The occupancy of the trunk is also

the probability of finding the trunk busy.

Congestion:

Normally in a telecommunication system, the installed equipment will be

enough to carry the busy hour traffic and not the entire traffic that can be

generated by the subscribers since the probability of every subscriber making a

call simultaneously is negligible. In such a design a situation might arise where all

the trunks of the system are busy. The system will not be able to accept any

further calls. This state is known as congestion.

There are two things that could happen to a call that encounters

congestion depending upon the design of the exchange:

1) The call will be unsuccessful i.e. lost. Such a system is called a lost call system.

2) The call will wait in a queue until a trunk frees up. Such calls are delayed and

not lost. Such systems are called delayed systems or queued systems.

GOS-Grade of Service (B):

The proportion of calls lost or delayed due to congestion is a measure of the

service provided.For a lost call system the grade of service is given by,

B= number of calls lost = traffic lost





number of calls offered traffic offered

The traffic carried by a lost call system will always be less than the traffic

offered.

Traffic offered = A erlangs

Traffic lost = AB erlangs

Traffic carried = A(1-B) erlangs (2)

Note: Larger the Grade of service worse is the service given. Ideally B=0.

Traffic measurement:

It is essential to keep a record of the traffic that is offered to a telephone

exchange in order to upgrade the system capacity as and when required.

Initially the number of calls used to be measured manually. Later automatic

traffic recorders were installed in automatic exchanges. In modern SPC systems, aseparate sub-program keeps count of the traffic generated.

Mathematical model:

A mathematical model needs to be developed in order to study

telecommunications traffic. Such a model is based on two assumptions:

a) pure chance traffic

b) statistical equilibrium

a) The assumption of pure chance traffic means that call arrivals and call

terminations are independent random events. It also implies that the





number of sources generating calls is very large. Since call arrivals are

independent random events, the occurrence of calls is not affected by

previous calls, therefore traffic is sometimes called memoryless traffic.

The number of call arrivals in a given time T has a poissonian distribution

given by,

P(n) = μx/x!.e

- μ(3)

where x is the number arrivals in time T

μ is the mean number of call arrivals in time T

i. The intervals Ŧ between calls arrivals are intervals between

independent random events and these intervals have a negativeexponential distribution,

P(x>=t) = e-t/ Ŧ (4)

where Ŧ is the mean interval between call arrivals

ii. The call durations, T are intervals between independent random

events (call termination). Therefore the call durations also have a

negative exponential distribution.P(T>=t) = e

-t/h(5)

where h is the average holding time

b) Statistical equilibrium means that the generation of traffic is a stationary

random process i.e. the probabilities do not change for the period being

considered. Consequently the mean number of calls in progress remains

constant.

Standard_notes_tsn Unit III 2

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