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Network Design Procedures & Traffic Routing

•Transmission Factors in Long Distance Telephony Networks •Control of Echo & Singing

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THE DESIGN PROBLEM

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

The economic decision arises to balance switching against transmission

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THE DESIGN PROBLEM

The economic decision arises to balance switching against transmission

Comparison of local versus long-distance networks

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THE DESIGN PROBLEM

In the past, for the long-distance network we could nearly always assume a hierarchical structure with three, four, or even five levels in the hierarchy.

Ideally, the highest levels would be connected in mesh for survivability.

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 no more than 12 links in tandem on any international connection, except for very large countries where 14 links may be acceptable

On an international connection, the 12 links in tandem are broken down into three groups, each 4 links in tandem as follows:

1. National connection of country originating call2. International portion3. 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

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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

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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”).

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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 (<400 ms) including both processing delay and propagation delay

The number of satellite circuits in the overall connection. No more than one GEO-link (consists of one up and one down link).

Select the circuit that provides the overall better transmission quality

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EXCHANGE LOCATION (TOLL/LONG-DISTANCE NETWORK)

Toll Areas

A country is divided into toll areas. Tariff areas and toll areas may be the same.

One rough rule of thumb is that a tariff area and thus a toll area have no more than a 50-km diameter.(Optional)

In rural regions, toll areas/tariff areas may be considerably larger.

In densely populated urban areas with heavy business concentration, these areas may be smaller

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 0.003 erlangs (see next slide) per subscriber line; thus a 4000-line toll exchange could serve just under a million subscribers maximumThe 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

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The erlang is a dimensionless unit.

One erlang represents a circuit occupied for one hour

Erlang =

Considering a group of circuits, traffic intensity in erlangs is the number of call-seconds per second or the number of call-hours per 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 work from:

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 any connection 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

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The next step is to examine traffic flows to and from each T.

Design Process

This information is organized and tabulated on a traffic matrix

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 country’s hierarchy is the international switching center.

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 areconnected in mesh.

The earlier AT&T network in the United States was a five-level hierarchy.

Four-link rule is fulfilled in this hierarchy

Design Process

© Irfan KhanAn 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 be tested in the same sequence although some routes may not be available for certain call types

The last choice route is the final route in the sense that no traffic streams using this route may overflow further.

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Suppose a country had four major population centers and could be divided into four areas around each center.

In this case, one of the tertiary exchanges would be an ISC

A sample network design

We define a FINAL ROUTE as aroute from which no traffic can overflow to an alternative route

“FINAL ROUTES” ARE SAID TO MAKE UP THE “BACKBONE” OF A NETWORK

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HIERARCHICAL REPRESENTATION SHOWING FINAL ROUTES

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A HIGH-USAGE (HU) ROUTE IS DEFINED AS ANY ROUTE THAT IS NOT A FINAL

ROUTE; IT MAY CONNECT EXCHANGES AT A LEVEL OF THE NETWORK

HIERARCHY OTHER THAN THE TOP LEVEL.

A DIRECT ROUTE IS A SPECIAL TYPE OF HU ROUTE CONNECTING EXCHANGES

OF THE LOWEST RANK IN THE HIERARCHY.

But in PTCL routing Hierarchy, we have direct routes

and indirect routes / overflow routes.

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TRAFFIC ROUTING IN THE NATIONAL NETWORKObjective of Routing

The objective of routing is to establish a successful connection between any two exchanges in the network.

The function of traffic routing is the selection of a particular circuit 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 on the 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 connectingadjacent nodes

Indirect route as a series of circuit groups connecting two nodes providing an end-to-end connection via other nodes

Network Architecture

Hierarchy of switching centers (e.g., local area, regional trunk, and international) with each level of the hierarchy performing differentfunctions.

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A simplified network with circuit groups connecting pairs of nodes with one-wayand both-way (two-way) working.

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Routing SchemeA routing scheme defines how a set of routes is made available for calls between a pair of nodes.There are fixed routing schemes and dynamic routing schemes

For a fixed routing scheme, the routing pattern is always the same

For a dynamic scheme the set of routes in the routing pattern varies.

Fixed Routing Scheme.

Dynamic Routing Schemes.

Routing patterns in a network may be fixed, in that changes in route choices 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.

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Time-Dependent Routing

Dynamic Routing Schemes.

With this type of routing scheme, routing patterns are altered at fixed times during the day (or week) to allow changing traffic demands to be provided 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

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Event-Dependent Routing

Routing patterns are updated locally on the basis of whether 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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Route Selection

The action to select a definite route for a specific call.

Sequential route selection is where the routes in a set are always tested in sequence and the first available route is selected.

For nonsequential routing, the routes in a set are tested in no specific 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 of the 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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Basic Origination Call Processing

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Call Control Procedures

Call control procedures define the entire set of interactive signals necessary to establish, maintain, and release connection between exchanges.

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

This type of call control can be reversible or irreversibleIn 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.

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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

Automatic Alternative Routing

One type of progressive (irreversible) routing is automatic alternative routing (AAR).

There are two principal types of this routing available:

1. When there is a choice of direct circuit groups between the two exchanges.

2. When there is a choice of direct and indirect routes between the twoexchanges.

Alternative routing takes place when all appropriate circuits in a group are busy.

Several circuit groups may be tested sequentially. The circuit order is fixed or is time-dependent.

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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.

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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 a preplanned distribution.

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Dynamic Routing ExamplesExample State-Dependent Routing

A centralized routing processor is employed to select optimum routing patterns on the basis of actual occupancy levels of the circuit groups and 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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Example of Event-Dependent Routing

This type of routing scheme routes traffic away from congested links by 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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TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONYThe long-distance network is entirely four-wire. As the network is extended, delay becomes more of a problem.

• Propagation• Processing time

Total one-way transmission time on a connection is governed by ITU-T Rec. G.114

Definition of Echo and SingingEcho Echo in telephone systems is the return of a talker’s 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 loudness and its length of delay

Delay has two components:

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Definition of Echo and Singing

Singing

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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Causes of Echo and Singing

Echo and singing can generally be attributed to the mismatch between the balancing network of the hybrid and its two-wire connection associated with the subscriber loop

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 higher the return loss value, the better the match.

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We relate return loss, measured in dB, to the impedances of the two-wire line we call L and the balancing network N by:

Return lossdB = 20 log10(ZN + ZL)/(ZN − ZL)

If the balancing network (N) perfectly matches the impedance of the two-wire line (L), then ZN = ZL and return loss would be infinite.

We use the term balance return loss ( ITU-T Rec. G.122 ) and classifyit 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.

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Echo and singing may be controlled by:

• 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

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Delay is measured in one-way or round-trip propagation time measured in milliseconds.

The ITU-T Organization recommends that if the mean round-trip propagation time exceeds 50 ms for a particular circuit, an echo suppressor or echo canceler should be used.

Practice in North America uses 45 msec as a dividing line.where echo delay is less than that, then echo can be controlled by adding loss.

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