4. • At the subscriber end Drop wires are taken to distribution point
• The drop wires are the individual pairs that run to the subscriber
premises
• The drop wires are connected to distribution cable through DP.
• These DCs are connected to Feeder Point, in turn they are
connected to Main feeder cable.
• Main Feeder cable typically consists of 100-2000 wire pairs,
distribution cable typically 10-500 pairs.
• The Main Feeder Cable is terminated at MDF.
• The subscriber pair and exchange pair are interconnected at the
MDF by means of Jumpers.
• With Jumper interconnection changes the user can be permitted
to shift from one place to another place with out changing his
number under the same exchange.
5.
6.
7. • DC signalling is used for subscriber lines eg.
Off-hook signal and dial pulses.
• exchanges are designed to accept a max loop
resistance of 1300 ohms.
• Microphone in the subscriber set requires
about 25 mA as bias current
• Bias current and loop resistance limits the
loop length for a given gauge wire.
11. • Obviously, this criterion is somewhat subjective.. In
this system, Reference Equivalent (RE) of a telephone
set or a subscriber loop is arrived at by comparing its
performance with a standard. The standard setup
consisting of a telephone transmitter, receiver and
network is established in the ITU laboratory in Geneva.
The NOSFER setup is illustrated in Figure 9.2.
• A rating system standardised by CCITT to grade
customer satisfaction is known as reference
equivalent(RE).
• Tests are conducted for both transmit and receive
qualities and the corresponding RE’s are known as
Transmit-receive Equivalent(TRE) and receive reference
equivalent (RRE) respectively.
12.
13.
14. The DC Resistance constraint
• Special techniques are used to meet the resistance
1. Use of higher diameter (lower gauge) wire,
2. Use of equalised telephone sets, and
3. Unigauge design or use of higher supply voltage.
• A significant portion (30–40%) of the cost of a telecommunication network comes
from the cost of copper in the subscriber lines. Hence the first technique of using
higher diameter copper wire is not recomended.
• Equalised telephone sets require about 8–12 mA of DC bias current as against 22–
30 mA required by a normal telephone. As a result, a larger value of loop resistance
is acceptable.
• A typical unigauge design is shown in Figure 9.3. This design attempts to use a wire
with as small a diameter as possible while retaining the resistance and attenuation
limits. For both long and short distances, the same gauge wire is used and hence the
name unigauge design.
15. Attenuation constraint
• It can be usually overcome by the use of loading coils.
• Loading coils are identified by standard convention
19–D–44, 24–A–88, etc.
• The first number indicates the wire gauge with which
the coil is used
• The letter specifies the spacing between the coils
• The number at the end gives the inductance value in
mH.
• Table gives the standard letters and their associated
spacing. The most commonly used spacings are B, D
and H.
16.
17. • In rural areas, subscribers are generally dispersed. It is both
unnecessary and expensive to provide a dedicated pair for every
subscriber.
• Three techniques are used to gain on the number pairs (Reducing the
wire pairs)
1. Party lines
2. Concentrators
3. Carrier systems.
• two or more subscribers are connected to one line which is termed
party line. This scheme is not used commonly as it has a number of
drawbacks. Only one subscriber at a time can use the line. Selective
ringing is difficult and privacy is not maintained. Dialling between
two subscribers on the same line is not possible
• In the second technique, a concentrator expander (CE) is used near
the cluster of users and another one at the exchange end as shown in
Figure. Typically, a ratio of 1:10 between the junction lines and the
subscriber lines is used
• Carrier systems employ multiplexing techniques over a single line.
Analog FDM or digital TDM systems are used.
18.
19. • Signalling and voice transmission on the subscriber lines requires that
the exchange performs set of functions.
• Functions are performed by an interface at the exchange end known
as subscriber loop interface.
• Some functions are required in analog networks, some in digital
networks and others in both.
• The set of functions are known by an acronym BORSCHT, stands for:
B = battery feed (48V dc)
O = overvoltage protection
R = ringing
S = supervision
C = coding
H = hybrid
T = test
• Functions B and R are well known.
20. •Over voltage protection deals with equipment and personnel
protection from lightning strikes and power line surges.
• Ringing: Detection of off-hook condition is a supervisory
function. (90Vrms at 20 Hz))
•Supervision means monitoring specfic function to provide the
required services, function is continues i.e. Even when the call is
not made.
•Functions C and H are exclusive to digital switch interfaces. As
we know, digital switching demands that analog-to-digital (A–D)
and digital-to-analog (D–A) conversions and some form of
coding/decoding be done.
•Hybrid combines the signal from telephone transmitter with
received signal to receiver
•Hybrid provides separate transmit and receive signals.
21. Switching Hierarchy and Routing
• Telephone networks require some form of interconnection of
switching exchanges to route traffic effectively and
economically.
• Exchanges are interconnected by groups of trunk lines, usually
known as trunk groups that carry traffic in one direction.
• Two trunk groups are required between any two exchanges.
• Three basic topologies are adopted for interconnecting exchanges:
Mesh, Star, and Hierarchy.
• Mesh is a fully connected network. The number of trunk groups
in a mesh network is proportional to the square of the
exchanges being interconnected. As a result, mesh connections
are used only when there is heavy traffic among exchanges, as may
happen in a metropolitan area.
22.
23. • A star connection utilises an intermediate exchange called
a Tandem exchange through which all other exchanges
communicate.
• A star configuration is shown in Figure 9.7(a).
• Star networks are used when the traffic levels are
comparatively low.
• Many star networks may be interconnected by using an
additional tandem exchange, leading to a two-level star
network as shown in Figure 9.7(b).
• An orderly construction of multilevel star networks leads to
hierarchical networks.
24. • Hierarchical networks are capable of handling
heavy traffic where required.
• and at the same time use minimal number of
trunk groups.
• A 5-level switching hierarchy is recommended
by CCITT as shown in Figure. In a strictly
hierarchical network, traffic from subscriber A
to subscriber B and vice versa flows through the
highest level of hierarchy, viz. quaternary
centres in Figure.
25. • A traffic route via the highest level of hierarchy is known as the
final route.
• If there is a high traffic intensity between any pair of
exchanges, direct trunk groups may be established between
them as shown by dashed lines in Figure.
• These direct routes are known as high usage routes.
• Wherever high usage routes exist, the traffic is primarily routed
through them. Overflow traffic, if any, is routed along the
hierarchical path.
• No overflow is permitted from the final route.
• In Figure, the first choice routing for traffic between
subscribers A and B is via the high usage route across the
primary centres. The second and the third choice routes and
the final route are also indicated in Figure 9.7(c).
• A hierarchical system of routing leads to simplified switch
design.
26. Routing: Three methods are commonly used for
deciding the route for a particular connection:
1. Right-through routing : In right-through routing the originating
exchange determines the complete route from source to
destination. No routing decisions are taken at the intermediate
exchanges
2. Own-exchange routing : Own-exchange routing or distributed
routing allows alternative routes to be chosen at the
intermediate nodes. Another advantage of distributed routing is
that when new exchanges are added, modifications required in
the switch are minimal.
3. Computer-controlled routing : Computers are used in networks
with common channel signalling (CCS) features to control traffic.
In A strictly hierarchical network suffers from one serious
drawback, i.e. its poor fault tolerance feature.
27. Transmission Plan
• The transmission quality and efficiency of the operation
are limited by the number of circuits (exchanges) in
Tandem exchange (Intermediate Switch).
CCITT has recommend certain guidelines
The maximum number of circuit to be used in an
international call is 12.
No more than 4 international circuits be used in tandem
between the originating and the terminating international
switching centres.
In exceptional cases and for a low number of calls, the total
number of circuits may be 14, but even in this case, the
international circuits are limited to a maximum of 4.
28. • The transmission loss is associate with Line loss, Switch
loss, end equipment loss (may be in terms of TRE, RRE,
ORE), and Echo signal, Control singing .
• So, the transmission loss budget has to be planned by
considering all these.
• The Hybrid circuit is needed to convert 2- wire to 4- wire
between the subscriber and digital exchange.
• Exchanges are interconnected by 4 wires through Amplifiers
and Repeaters.
• The important function of hybrid is to ensure that the received
signal is not coupled at another port.
• The coupling is Zero only when 2-wire circuit and 4- wire
circuit impedances are perfectly matched.
29. Two wire to Four Wire conversion / Hybrid transformer
Need to Four wire terminating set
To transmit the signal beyond the
local subscriber loop, the receive
and transmit path are normally
separated to give two one way
paths. It requires two wire pair.
The conversion from two to four
wires and back is done by hybrid
transformer or four wire
terminating set. The hybrid
transformer provides total
isolation from input to output in
4 wire circuit.
The input signal is electromagnetically coupled to Q to U winding
equally. If the impedance Z exactly matches that of 2 wire circuit,
the effect of input signal of the output winding S is completely
nullified.
30. When impedance mismatches, the part of the incoming speech
signal will couples in outgoing circuit, which returns to the speaker as
Echo. The echo may me loud enough to annoy the speaker as it is
amplified like other signal in the return path.
31. • Short delay Echoes are controlled by using
attenuators and long delay Echoes are by echo
suppressors or echo cancellers.
• CCITT recommended to use echo suppressors if
delay exceeds 50 ms.
• It is mandatory in satellite communication as the
delay will be several hundred milliseconds.
• The operation of echo is illustrated in the following
figure.
• Echo suppressors are voice activated attenuators.
• Normally echo suppressors remain in deactivated
state, i.e. the attenuators are bypassed.
• Speech in one channel activates the echo
suppressor in return path.
32.
33. • From the figure A’s speech activates the echo
suppressors EB and B’s speech activates the
echo suppressors EA
• From the figure, it can be noticed that B is talking
and A is silent. Should A attempt to talk at this
juncture, his talk is also attenuated. He can, however,
turn off the echo suppressor by interrupting B loudly.
34. The amount by which the reflected signal is attenuated is
known as Return Loss (RL). This is given by
35.
36. NUMBERING PLAN
• The objective of numbering plan is to uniquely identify every
subscriber connected to a telecommunication network.
• In the early days, exchanges were identified by the names of the
towns in which they were located.
• As the numbers of subscriber increase, there are more than one
exchange available in one town.
• Generally, a large centrally located exchange called Main Exchange
serves business area. Smaller exchanges known as Satellite
exchanges serves residential locations.
• The main exchange and satellite exchanges together is called multi-
exchange area
• With common numbering scheme the multi-exchange area
(Numbering Area) can be identified, some times it is called Linked
numbering scheme.
37. • Such kind of multiexchange area are identified uniquely
nationwide, called national numbering plan , and can be called
with Subscriber Trunk Dialling (STD) or Direct Distance
Dialling (DDD).
• The International Subscriber Dialling (ISD) makes international
numbering plan.
Types of numbering Plans
• A numbering plan may be Open or Semi-open or Closed.
• Open Numbering Plan: Also named as non uniform scheme,
permits wide variation in no. of digits to identify the subscriber.
----- No existence
• Semi Open Numbering Plan: It permits number length differs by
almost one or two digits, it is most commonly used including
India, Sweden, Switzerland, and the United Kingdom.
• Closed Numbering Plan: Number is fixed. It is also known as
Uniform numbering scheme. This scheme is used by a few
countries which include France, Belgium and the countries in the
North America (USA, Canada, Hawaii, etc.).
38. For international numbering plan world is divided in to
zones as shown in the figure
- Each Zone is given with a single digit code
39. Every international number consists of country code and national
number.
Eg. Country code for India is 91, and Maldives is 960.
National number consists of three parts
40. • Area or Trunk code: It identifies the particular multiexchange area or
numbering area .
- A number area in a region always has the region code as the first
digit of the STD code . Country is divided in to 8 regions
• Exchange code: It identifies the particular exchange (satellite
exchanges)within the numbering area.
• Line Number: The subscriber number is the number assigned to a line
connected to one customer's equipment.
• Eg. 918572248040
41. • The term Local call means a call within the numbering areas.
• National call is a call between two different numbering areas
(Trunk call) within the same country.
Basically four possible approaches to dialing procedures.
• 1. Use single uniform procedure for all calls (Local, national,
international)
• 2. Two different procedures. One for Local, national and other
for International calls
• 3. Three different procedures. One for International calls, second
for national trunk calls, and third for local calls.
• 4. Use four different procedures, three procedure are same as
given in 3 above and a fourth procedure for calls in the
adjacent numbering areas.
42. • Usually single digit ‘0’ prefix to the number
differentiates local call and a national call.
• A two digit ‘00’ or ‘a three digit ‘010’ prefix to
the number differentiate national call or
international call.
43. CHARGING PLAN
• To meet the capital & operational expenses.
• The capital cost includes that of line plant, switching systems, buildings and
land.
• Operating costs include staff salaries, maintenance costs, water and
electricity charges and miscellaneous expenses.
• A telecommunication administration receives its income from its
subscribers.
• A charging plan provides for recovering expenses from subscribers.
• The cost of dedicated resources like the telephone instrument and the
subscriber line must be recovered from individual customers.
• The operating costs must be worked out depending on the quantum of
resources used in providing a service and the duration for which these
resources are used.
• Taking all these factors into account, a charging plan for a
telecommunication service levies three different charges on a subscriber:
1. An initial charge for providing a network connection
2. A rental or leasing charge
3. Charges for individual calls made.
44. • We use the term ‘meter’ in the ensuing discussions to denote
the instrument or the register.
• The Count in the meter represents the number of charging
units.
• A bill is raised by assigning a rate to the charging unit.
• The count is incremented by sending a pulse to the meter.
• Charging methods for individual calls fall under two broad
categories:
Duration independent charging
Duration dependent charging.
• Local calls within a numbering area are usually charged on
a duration independent basis.
• The charging meter is incremented once for every successful
call, i.e. whenever the called party answers.
45. • In the case of duration dependent charging, a periodic
train of pulses from a common pulse generator
operates the calling subscriber’s meter at appropriate
intervals.
• This method is called periodic pulse metering.
• In this case, the charge for a call is proportional to its
duration.
• In order to restrict the peak demand and encourage off-
peak demand, it is common to make the metering rate
vary with the time of day.
• This is done by suitably changing the pulse repetition
frequency under the control of a time-of-the-day
clock.
46. The table shows tariff variation during 24 hours period.
The pulse repetition rate is reduced to half and one-
fourth of the normal rate during off-peak hours
Period of the Day Meter pulse Repetition rate
08 – 19 hours
19 – 22 hours
22 – 06 hours
06 – 08 hours
X
X/2
X/4
X/2
47. When STD facility is used to establish a long distance call,
charging is usually accounted for pulsing the meter at an
appropriate rate. Depending on the time of the day and distance
involved between the stations the meter pulsing frequency
varies. The following table shows Distance Vs. Metering pulse
Rate in India during normal rate hours.
Distance (Km) Metering Pulse Rate (Pulses/min.)
20-50
50-100
100-200
200-500
500-1000
> 1000
1.67
5.00
7.50
15.00
20.00
30.00
The other charging methods like automatic message accounting, public telephone
booths, coin operated boxes also used
48. Signalling is used between user and the network, or between
two network elements to exchange various control information.
Signalling system links the variety of switching systems,
transmission systems and subscriber equipment in a
telephone Network to enable the network function as a
whole.
Three forms of signalling are involved in a telecommunication
Network:
1.Subscriber Loop signalling
2. Intra exchange or Register signalling
3. Inter exchange or Intersegiste signalling
SIGNALLING TECHNIQUES
49. The subscriber loop signaling depends upon the type of
telephone instrument used.
The intra exchange signaling refers to the internal portion
of a switching system that is heavily dependent upon
the type and design of a switching system, which varies
depending upon the model.
The inter-exchange signaling takes place between
exchanges. This helps in the exchange of address digits,
which pass from exchange to exchange on a link-by-
link basis. The network-wide signaling that involves end-to-
end signaling between the originating exchange and the
terminating exchange is called the Line signaling.
50. In-Channel Signaling
In-Channel Signaling is also known as Per Trunk
Signaling. This uses the same channel, which carries
user voice or data to pass control signals related to
that call or connection. No additional transmission
facilities are needed, for In-channel signaling.
Common Channel Signaling
Common Channel Signaling uses a separate common
channel for passing control signals for a group of
trunks or information paths. This signaling does not use
the speech or the data path for signaling.
53. In-channel Signaling
This type of signaling is used to carry voice or data and pass
control signals related to a call or connection.
There are different types of In-channel Signaling, as seen in the
above figure. The D.C. signaling is simple, cheap and reliable
even for unamplified audio circuits.
However, for amplified audio circuits, low frequency A.C.
signaling may be adopted.
The Voice Frequency signaling is used when FDM
(Frequency Division Multiplexing) transmission systems are
used, because low frequency signaling and D.C. signaling
cannot be provided.
This Voice Frequency signaling may be In-band or Out-band.
54. In-band Signaling
In-band voice frequency uses the same frequency band as the
voice, which is 300-3400 Hz, which has to be protected against
false operation by speech.
One such instant took place when a lady’s voice which has
generated a tone at around 2600Hz lasting for a duration of 100ms
was detected as the line disconnect signal due to which her calls
were frequently being disconnected in the middle of her
conversation. Such problems precluded the in-band signaling
during speech phase.
The advantages of In-band signaling are −
The control signals can be sent to every part where a speech signal
can reach.
The control signals will be independent of the transmission
systems as they are carried along with the speech signals.
The Analog to digital and Digital to analog conversion processes
will not affect them.
55. Out-band Signaling
The out-band signaling uses frequencies which are above the
voice band but below the upper limit of 4000 Hz of the
nominal voice channel spacing.
The signaling is done throughout the speech period and thus
continuous supervision of the call is allowed.
Extra circuits are needed to handle the extremely narrow band
width of this signaling, due to which it is seldom used.
Both of these in-band and out-band voice frequency signaling
techniques have limited information transmission capacity.
In order to provide enhanced facilities, common channel
signaling is used.
56. Now a days, PCM systems are widely used and signalling in
theses systems is of considerable interest.
With built-in PCM signalling, in addition to the bits required
to transmit speech and ensure frame synchronisation, bits
are required to carry signalling information which is usually
binary coded at the transmitting end and reconverted to
original form at the receiver end.
The signalling information pertaining to particular speech may
be carried in the same time slot as the speech or in a separate
time slot.
The former is known as In-slot signalling and later as out-
slot signalling
57. The inslot system was originally developed by bell
system as bell D2 24-channel system.
The outslot system by CEPT as CEPT 30- CHANNEL
system
71. The traffic is defined as the occupancy of the server.
The basic purpose of the traffic engineering is to determine the
conditions under which adequate service is provided to
subscribers while making economical use of the resources
providing the service.
The functions performed by the telecommunication network
depends on the applications it handles.
Some major functions are switching, routing, flow control,
security, failure monitoring, traffic monitoring, accountability
internetworking and network management.
72. Traffic engineering provides the basis for analysis and design of
telecommunication networks or model.
The developed model is capable to provide best accessibility and
greater utilization of their lines and trunks.
Also the design is to provide cost effectiveness of various sizes
and configuration of networks.
The traffic engineering also determines the ability of a telecom
network to carry a given traffic at a particular loss probability.
73. Network Traffic Load and Parameters
• There is a large peak around mid-forenoon and mid-
afternoon signifying busy office activities.
• The afternoon peak is, however, slightly smaller.
• The load is low during the lunch-hour period, i.e.12.00-14.00
hours.
• The period 17.00-18.00 hours is characterised by low
traffic signifying that the people are on the move from offices
to their residences.
74.
75. • 1. Busy Hour: Continuous 1-hour period lying
wholly in the time interval concerned, for which the
traffic volume or the number of call attempts is
greatest.
• 2. Peak Busy Hour: The busy hour each day; it
usually varies from day to day, or over a number of
days.
• 3. Time Consistent Busy Hour: The 1-hour period
starting at the same time each day for which the
average traffic volume or the number of call
attempts is greatest over the days under
consideration.
76. • Call completion rate (CCR):
Defined as the ratio of the number of successful calls
to the number of call attempts.
• The number of call attempts in the busy hour is called
busy hour call attempts (BHCA)
77. • A measure is called the traffic intensity which is defined
as
• The period of observation is taken as one hour. Ao is
obviously dimensionless.
• It is called erlang (E) to honour
• The Danish telephone engineer A.K. Erlang,
who did pioneering work in traffic engineering.
• His paper on traffic theory published in 1909 is
now regarded as a classic.
78.
79.
80.
81.
82. Traffic Statistics
Calling rate: This is the average number of requests for connection that
are made per unit time.
The calling rate (λ) is also referred as average arrival rate. The average
calling rate is measured in calls per hour.
Holding time: The average holding time or service time ‘h’ is the average
duration of occupancy of a traffic path by a call. The reciprocal of the
average holding time referred to as service rate (μ) in calls per hour is
given as:
Distribution of destinations: Number of calls receiving at a exchange may
be destined to its own exchange or remoted exchange or a foreign
exchange.
User behavior: The statistical properties of the switching system are a
function of the behavior of users who encounter call blocking.
Average occupancy: If the average number of calls to and from a terminal
during a period T seconds is ‘n’ and the average holding time is ‘h’
seconds, the average occupancy of the terminal is given by
DIT
83. Example:
If a group of 20 trunk carries 10 erlangs and the average call duration is
3 minutes, calculate (a) average number of calls in progress (b) total
number of calls originating per hour.
For the present day networks which support voice, data and many
other services, erlang is better measure to represent traffic intensity.
DIT
84. Grade of Service
(GOS)
The grade of service refers to the proportion of unsuccessful calls
relative to the total number of calls.
GOS is defined as the ratio of lost traffic to offered traffic.
The smaller the value of grade of service, the better is the service. The
recommended GOS is 0.002, i.e. 2 call per 1000 offered may lost. In a
system, with equal no. of servers and subscribers, GOS is equal to
zero.
GOS is applied to a terminal to terminal connection. But usually a
switching centre is broken into following components
An internal call (subscriber to switching office)
An outgoing call to the trunk network (switching office to
trunk) The trunk network (trunk to trunk)
A terminating call (switching office to subscriber).
There are two possibilities of call blocking, which are Lost system and
Waiting system.
DIT
86.
Example
During a busy hour, 1400 calls were offered to a group of
trunks and 14 calls were lost. The average call duration
has 3 minutes. Find (a) Traffic offered (b) Traffic carried
(c) GOS and (d) The total duration of period of
congestion.
DIT
87. Blocking Probability and
Congestion
The value of the blocking probability is one aspect of the telephone
company’s grade of service.
The basic difference between GOS and blocking probability is that
GOS is a measure from subscriber point of view whereas the
blocking probability is a measure from the network or
switching point of view.
Based on the number of rejected calls, GOS is calculated, whereas
by observing the busy servers in the switching system, blocking
probability will be calculated.
The blocking probability, B is defined as the probability that all
the servers in a system are busy.
88. Congestion theory deals with the probability that the offered
traffic load exceeds some value. Thus, during congestion, no
new calls can be accepted.
There are two ways of specifying congestion, which time
congestion and call congestion.
Time congestion is the percentage of time that all servers in a
group are busy.
Call congestion is the proportion of calls arising that do not find
a free server.
In general GOS is called call congestion or loss probability and
the blocking probability is called time congestion.
89. Traffic Engineering Problems
• On the average, during busy hour, a company makes 120 outgoing calls of average
duration two minutes. It receives 240 incoming calls of average duration three minutes.
Find
1. Outgoing Traffic
2. Incoming Traffic
3. Total Traffic
• During busy hour, on an average ,a customer with a single telephone line makes three calls
and receives three calls. The average call duration is two minutes. What is the probability
that a caller will find the line engaged?
• During the busy hour,1200 calls were offered to a group of trunks and six calls were lost.
Average call duration was 3 minutes .Find
1. The traffic offered
2. Traffic carried
3 Traffic lost
4. Grade of service
5. Total duration of period of congestion
• Observations were made of the number of busy lines in a group of junctions at intervals of
6 minutes during busy hour. The results were :
11,13,8,10,12,14,16,9,7
Calculate the total traffic carried during busy hour.
• During busy hour an Exchange is designed to receives 240 calls. If average
holding time per call is 3 minutes, what is the traffic intensity?
If average call holding time is limited to 2 minutes per call, calculate the maximum number of
simultaneous calls that could be accommodated by the switching system.
