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7/29/2019 Telecommunication system engineering Network Design Procedures & Traffic Routing
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Irfan Khan
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Exchange placement (Toll
-
center placement):
Rather than base the placement decision on subscriber density and
their calling rates, the basic criterion is economy, the most cost-effective optimum.
Traffic matrix/Routing:
The design procedure is to construct the familiar traffic matrix,where cost ratio studies are carried out to determine whether
routing will be direct or tandem.
The tendency is to use tandem working and direct routes with
overflow
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Comparison of local versus long-distance networks
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We are moving away from the hierarchical concept (though slowly)
to one using more direct routes.
In Pakistan we Prefer hierarchical structure with almost three to
four levels in the hierarchy .
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LINK LIMITATION
ITU-T Organization recommends that there be
, except for verylarge countries where 14 links may be acceptable
On an international connection, the
, each 4 links in tandem as follows:
1. National connection of country originating call
2. International portion
3. National connection of country terminating the call.
link in this context is defined as the connectivity from one
exchange to an adjacent exchange serving the international
connection Irfan Khan
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ITU-T places this link limitation in the transmission plan to ensure
some minimum transmission quality and to provide efficient
operation of signaling ,end-to-end
LINK LIMITATION
An international connection to illustrate the nomenclature adopted and the maximum number of
links in tandem for an international connection. From ITU-T Rec.G.101 Irfan Khan
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INTERNATIONAL NETWORK
International Telephone Routing Plan is contained in ITU-T Rec.
E.171. Some of its highlights are:
It is not hierarchical.
Direct traffic should be routed over final (fully provided) or high
usage circuit groups.
No more than four international circuits in tandem should be
involved between originating and terminating ISCs
Advantage should be taken of the noncoincidence of
international traffic by use of alternative routings to effect circuit
economies and provide route diversity.
The routing of transit switched traffic should be so planned to
avoid circular routings (ring-around-the-rosy). Irfan Khan
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When a group consists of both terrestrial and satellite circuits,
the choice of routing should be governed by:
INTERNATIONAL NETWORK
Total delay of connectivity (
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EXCHANGE LOCATION (TOLL/LONG-DISTANCE NETWORK)
Toll Areas
Assignments of toll exchanges regarding numbering &Impact of
numbering on routing a call and on accounting equipment.
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Maximum size of a toll exchange
For 0.003erlangs (see next slide) per subscriber line; thus a 4000-
line toll exchange could serve just under a million subscribers
maximum
The exchange capacity should be dimensioned to the forecast
long-distance traffic load 10 years after installation.
we must have at least two levels:
1.Local area
2.Toll area.
Factors leading to more than two levels are:
Geographical size
Telephone density, usually per 100 inhabitants
Toll traffic trends
Political factors
Toll Areas
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There are many choices open to the system engineer to establish
the route-plan hierarchy
Figure B is a three-level hierarchy with a four-to-five fan-out at each stage.
For a two-level hierarchy, two possibilities are suggested:
Figure C has low initial fan-out, and Figure D has a high one. The choice between C and
D may depend on traffic intensity between nodes or availability of routes
For national networks, the fan-out in Figure D may be most economical because traffic is
brought to a common point more quickly
Principal city
1
2
1
234
Low fan-out
High fan-out
Irfan Khan
Fan out:The maximum number of
devices that can be safelydriven by the output from a
logic gate or logic device
(which have only a limited
ability to drive other
devices from their output
terminals). If the fan-out is
exceeded the voltage levels
corresponding to a logic 1
and a logic 0 become more
similar and errors are more
likely.
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Erlang =
Considering a group of circuits, traffic intensity in erlangs is the
number of call-seconds per second or the number of call-hoursper hour
A group of 10 circuits had a call intensity of 5 erlangs, we would
expect half of the circuits to be busy at the time of measurement.
Erlang
Calls carried x Mean holding time
Observation time period
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NETWORK DESIGN PROCEDURES
The attempt to attain a final design of an optimum national
network is a major cut-and-try process.
Simple logic demands that the design must first take into account
the existing network.
Major changes in the network require a large expenditure
Technology advances are galloping along.
Ten years age of a switch might be the very outside.
Even a 5-year-old switch may have to be replaced because of
Obsolescence.
Signaling on the national and international networks has been
standardized on CCITT Signaling System No. 7. But every country or
administration has its own national variant of SS No.7
Factors considered for Network Design:
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NETWORK DESIGN PROCEDURES
Design Process
Starting from the local exchange, there are now three bases to workfrom:
1. There are existing local areas, each of which has a toll exchange.
2. There is one or more ISCs placed at the top of the network
hierarchy.
3. There will be no more than four links in tandem on anyconnection to reach an ISC.
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Class 4 exchange/ Primary center
T is a tandem exchange with a fan out of four
Four local exchanges, A, B, C, and D homing on T
The entire national geographic area will be made up of small
segments, as shown in Figure and each may be represented by a
single exchange such as T.
Design Process
Areas and exchange relationships Irfan Khan
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The next step is to to and from each T.
Design Process
This information is organized and tabulated on a
Toll Traffic Matrix (Sample) (in Erlangs)
The convention used here is that values are read from the exchange in the left-hand column to the exchange in
the top row. For example, traffic from exchange 1 to exchange 5 is 23 erlangs
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It is recommend that a hierarchical structure be established.
At the top of a countrys hierarchy is the international switchingcenter.
A typical hierarchical network. The
example illustrated here is the North
American network circa 1990.
Dashed lines show high-usage trunks.
Note how the two highest levels are
connected in mesh.
The earlier AT&T network in the United
States was a five-level hierarchy.
Design Process
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An example of a hierarchical network with alternative
routing.
The lowest level is not shown in the
figure, that of the local exchange.
Routing structure
Design Process
The routes in the set will always betested in the same sequence
although some routes may not be
available for certain call types
The last choice route is the finalroute in the sense that no traffic
streams using this route may
overflow further.
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Suppose a country had
and
could be divided into fourareas around each center.
A sample network design
We define a as a
route from which no traffic
can overflow to an alternativeroute
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Irfan Khan
TRAFFIC ROUTING IN THE NATIONAL NETWORK
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TRAFFIC ROUTING IN THE NATIONAL NETWORK
Objective of Routing
The objective of routing is to establish a successful connectionbetween any two exchanges in the network.
The function of traffic routing is the selection of a particularcircuit group, for a given call attempt or traffic stream, at an
exchange in the network.
The choice of a circuit group may be affected by information onthe availability of downstream elements of the network on a
quasi-real-time basis.
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Network Topology
A network comprises a number of nodes (switching centers)
interconnected by circuit groups.
Direct route consists of one or more circuit groups connecting
adjacent nodes
Indirect route as a series of circuit groups connecting two nodes providingan end-to-end connection via other nodes
Network Architecture
Hierarchy of switching centers (e.g., local area, regional trunk, and
international) with each
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A simplified network with circuit groups connecting pairs of nodes with one-way
and both-way (two-way) working.
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Routing Scheme
There are fixed routing schemes and dynamic routing schemes
Routing patterns in a network may be fixed, in that changes in routechoices for a given type of call attempt require manual intervention.
Such changes may be time-dependent, state dependent and/or event-
dependent.
The updating of routing patterns may take place periodically or
aperiodically, predetermined, depending on the state of the network or
depending on whether calls succeed or fail. Irfan Khan
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Time-Dependent Routing
With this type of routing scheme, routing patterns are altered at fixed
times during the day (or week) to allow changing traffic demands to beprovided for.
State-Dependent Routing
This is a routing scheme where routing patterns vary automatically
according to the state of the network. This is adaptive routing.
Each exchange compiles records of successful calls or outgoing trunk
group occupancies. This information is then distributed through the
network to other exchanges or passed to a centralized database.
Concept of state-dependent routing Irfan Khan
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Event-Dependent Routing
Routing patterns are updated locally on the basis ofwhether calls
succeed or fail on a given route choice.
Each exchange has a list of choices, and the updating favors
those choices which succeed and discourage those which suffer
congestion.
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R t S l ti
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Route Selection
The action to select a definite route for a specific call.
is where the routes in a set are always
tested in sequence and the first available route is selected.
routing, the routes in a set are tested in nospecific order.
The selection can be sequential or nonsequential.
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The decision to select a route can be based on :
The state of the outgoing circuit group or the states ofthe series of circuit groups in the route.
The incoming path of entry
Class of service ( Voice, data)
Type of call (Operator, Ordinary subscriber, test call etc)
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Call Control Procedures
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Call Control Procedures
Call control procedures define the entire set of interactive signals
necessary to establish, maintain, and release connection betweenexchanges.
Two types of call control procedures are:
1. Progressive Call Control
Progressive call control uses link-by-link Signaling to pass
supervisory controls sequentially from one exchange to the next
In the irreversible case, call control is always passed
downstream toward the destination exchange.
Call control is reversible when it can be passed backwards
(maximum one node), toward the originating exchange. Irfan Khan
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2.Originating Call Control
Originating call control requires that the originating exchange
maintain control of the call setup until a connection betweenoriginating and terminating exchanges has been completed.
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Applications
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Applications
Automatic Alternative Routing
One type of progressive (irreversible) routing is automatic alternativerouting (AAR).
There are two principal types of this routing available:
1. When there is a choice of direct circuit groups between the twoexchanges.
2. When there is a choice of direct and indirect routes between the two
exchanges.
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Automatic Rerouting (Crankback)
Automatic rerouting is a routing facility enabling connection of call
attempts encountering congestion during the initial call setup phase.
If a signal indicating congestion is received from exchange B,
subsequent to the seizure of an outgoing trunk from exchange A,
the call can be rerouted at exchange A.
Blocking from B to D activates signal S1 to A. Blocking from D to F
activates signal S2 to A. Irfan Khan
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Load Sharing
Each outgoing routing pattern (A, B, C, D) may include alternative
routing options.
Routing schemes can be developed to ensure that call
attempts are offered to route choices according to apreplanned distribution.
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Dynamic Routing Examples
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Dynamic Routing Examples
Example State-Dependent Routing
A centralized routing processor is employed to select optimum routing
patterns on the basis of actual occupancy levels of the circuit groupsand exchanges in the network which are monitored on a periodic basis.
This routing technique inherently incorporates fundamental principles of
network management in determining routing patterns. These include: Avoiding occupied circuit groups.
Not using overloaded exchanges for transit.
In overload circumstances, restriction of routing direction connections.
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Example of Time-Dependent Routing.
For each originating and terminating exchange pair, a particular route
pattern is planned depending on the time of day and the day of the
week.
This type of routing takes advantage of idle circuit capacity in other
possible routes between originating and terminating exchanges which
may exist due to noncoincident busy hours.
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E l f E D d R i
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Example of Event-Dependent Routing
This type of routing scheme routes traffic away from congested
linksby retaining routing choices where calls are successful.
It is simple, adapts quickly to changing traffic patterns, and
requires only local information.
In a fully connected network, calls between each originating and
terminating exchange pair try the direct route with a two-link
alternative path selected dynamically.
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The long-distance network is entirely four-wire. As the network
is extended, delay becomes more of a problem.
Propagation
Processing time
Echo in telephone systems is the return of a talkers voice. Echo is
a reflection of voice.
The cause of echo is impedance mismatches that might be
present any place in the electrical telephone connection.
Two factors determine the degree of annoyance of echo: its
and its
Delay has two components:
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Singing is the result of sustained oscillations due to positivefeedback in telephone amplifiers or amplifying circuits
Circuits that sing are unusable and promptly overload multiplex
equipment, particularly FDM equipment.
Singing may be regarded as echo that is completely out of
control.
This can occur at the frequency at which the circuit is resonant.
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The mismatch is usually between the two-wire side and the hybrid,
where the balancing transformer provides the other side of the match.
Impedance match is described by a term called return loss. The higherthe return loss value, the better the match.
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We relate return loss, measured in dB, to the impedancesof the
two-wire line we callLand the balancing network N by:
If the balancing network (N) perfectly matches the impedance of
the two-wire line (L), then ZN = ZLand return loss would be infinite.
We use the term balance return loss ( ITU-T Rec. G.122 ) and classify
it as two types:
1. Balance return loss from the point of view of echo (Echo return
loss). This is the return loss measured between the frequencies 300
and 3400 Hz.
2. Balance return loss from the point of view of stability. This is the
return loss measured between 0 and 4000 Hz. Irfan Khan
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Improved return loss at the term set (hybrid).
Adding loss on the four-wire side (or on the two-wire side). Reducing the gain of the individual four-wire amplifiers.
Echo paths in a four-wire circuit Irfan Khan
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Delay is measured in one-way or round-trip propagation time
measured in milliseconds.
Practice in North America .where echo delay is less than that, then echo can be controlled
by adding loss.
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