15/01/2013Bahman R. Alyaei1 Chapter 2 Local Networks.

15/01/2013 Bahman R. Alyaei 1

Chapter 2Chapter 2

Local NetworksLocal Networks


1 Introduction1 Introduction

• The Local Area includes:1. The subscriber plant.

2. The local exchanges.

3. The trunk plant interconnecting these exchanges.

4. Trunks connecting a local area to the next level of network hierarchy, or the point of presence (POP).


A sample local area (arrows represent trunk pull; dashed lines delineate serving areas).

Geographical area of five exchanges


Basic Telecommunication Network


Continue…Continue…• To build the most economical local

network assuming an established Quality of Service (QoS), the following should be known:

1. Geographic extension of the local area of interest.

2. Number of inhabitants and existing telephone density.

3. Calling habits.


Continue…Continue…

4. Percentage of business telephones.

5. Location of existing telephone exchanges and extension of their serving areas.

6. Trunking scheme.

7. Present signaling and transmission characteristics.



We will assume that • Each exchange in the sample will be capable of

serving up to 10,000 subscribers.• All telephones in the area have seven-digit

numbers, the last four of which are the subscriber number of the respective serving area of each exchange.

• All subscribers are connected to their respective serving exchanges by wire pairs, resulting in some limiting subscriber loop length.


2 Subscriber Loop Design2 Subscriber Loop Design

• It is a dc loop in that it is a wire pair supplying a metallic path for the following:

1. Talk battery for the telephone transmitter.2. An AC ringing voltage for the bell.3. Current to flow through the loop when the

telephone instrument is taken out of its cradle (“off hook”), telling the serving switch that it requires “access,” thus causing a line seizure at that switch.

4. Dialing tones indicating the desired party.


2.3 Local Loop Design Techniques 2.3 Local Loop Design Techniques

• The loop length D is a critical parameter.• The greater the value of D, the greater the

attenuation that the loop suffers.• Likewise, there is a limit to D due to dc

resistance.• Also, the greater the wire diameter of the loop

pair, the less resistance there is per unit length.



• When we lift the telephone off hook, there must be enough current flow in the loop to actuate the local switch where the loop terminates.

• The high impedance voltage of the line is -48 V dc.

• The question is, when designing a subscriber loop, what its maximum length D would be?


Continue…Continue…• There are two variables that must be

established:1. The maximum loop resistance: this value is

a function of the switch circuit where the loop terminates and its value is 1000 Ω (old) or 2400 Ω (new)

2. The maximum loss or attenuation on the loop: In Europe it is 6 dB at 800 Hz, and in North America it is 9 dB at 1000 Hz.

• When budgeting a value for the loop, we should budget some value to the telephone set, 300 Ω has been assigned.


2.3.2 Calculating the Resistance2.3.2 Calculating the Resistance

• For copper conductor, the DC loop resistance is calculated as follow:

Where Rdc is the loop resistance in ohms per mile and d is the diameter of the conductor in inches.

2

1095.0

dRdc


Continue… Continue…

• Example: we wish a 10-mile loop and allow 100 Ω per mile of loop, what diameter of copper wire would be needed?

Hence, for 100 Ω per mile loop resistance, the diameter of the loop should be 0.80 mm



Using Table 2.4, we can compute maximum loop lengths for 1000 Ω signaling resistance. Use a 26-gauge loop. We then have



Let’s use the 2400 Ω switch as another example. Subtract 300 Ω for the telephone subset, leaving us with a net of 2100 Ω. We will use a 26-gauge wire pair on the loop, then from Table 2.4 we have


If we are limited to 6 dB loss on a subscriber loop, and we know the loss per 1000 ft for different AWG, then we can calculate the length of the subscriber loop for different AWG as follow


2.3.4 Loading2.3.4 Loading

• In some cases it is desirable to extend subscriber loop lengths beyond the limits described previously.

• Common methods to attain longer loops without exceeding loss limits are

1. Increase conductor diameter (AWG).

2. Use amplifiers or range extender.

3. Inductive loading.



• Loading a particular voice-pair loop consists of inserting inductances in series (loading coils) into the loop at fixed intervals.

• Inductive loading tends to:

1. Reduce transmission loss on the subscriber (i.e. better frequency response)

2. Decrease the velocity of propagation and increase the impedance.


Loading Coils

Loop Loop Loop Loop






• Loaded cables are coded according to the spacing of the load coils.

• Loaded cables are typically designated 19-H-44, 24-B-88, and so forth.

• Example: 19-H-44 means

1. 19 = AWG

2. H = 6000 ft spacing.

3. 88 = Inductance in mH (88 mH)


4.1 Exchange Area4.1 Exchange Area

• The size of an Exchange Area is also called a Serving Area.

• It obviously will depend largely on

1) Subscriber density.

2) Subscriber distribution.

3) Subscriber traffic.


4.2 Exchange Size4.2 Exchange Size

• Exchange sizes are often in units of 10,000 lines.

• 10,000 is the number of subscribers that may be connected when an exchange reaches “exhaust,” where it is filled and no more subscribers can be connected.

• The number of subscribers initially connected should be considerably smaller than when an exchange is installed.


5 Shape of Serving Area 5 Shape of Serving Area

• It has a considerable effect on optimum exchange size.

• If a serving area has sharply angular contours, the exchange size may have to be reduced to avoid excessively long loops.

• In other words, more exchanges must be installed in a given local geographical area of coverage.



• Fully circular exchange serving areas are impractical because either the circles will overlap or uncovered spaces will result.

Circular coverage area

Local Exchange AreaLocal

Geographical Area



• There are then two possibilities: square or hexagonal serving areas.

• Of the two, a hexagon more nearly approaches a circle.

• The size of the hexagon can vary with density with a goal of 10,000 lines per exchange as the ultimate capacity.

• Full coverage of local areas may only be accomplished using serving areas of equal triangles or squares.



Hexagonal serving area and interconnection of exchanges


Chapter 3Chapter 3

Switching in Analog Switching in Analog EnvironmentEnvironment


1.1 Background 1.1 Background

• The idea of automatic switching goes back to 1889.

• Strowger gave the problem a lot of thought on how to automate switching, removing the young lady from the scene.

• He invented the step-by-step (SXS) switch which the British call the Strowger switch.

• Today, switching in PSTN is entirely digital.



• We will study analog switching for two reasons:

1. Most of the concepts covered in the analog description hold for digital switching as well.

2. The chapter development provides an excellent historical perspective of the evolution of the telecommunications network.


1.2 Switching in The Telephone 1.2 Switching in The Telephone NetworkNetwork

• Switching establishes a path between two specified terminals (subscribers).

• A switch sets up a communication path on demand and takes it down when the path is no longer needed.

• It performs logical operations to establish the path and automatically charges the subscriber for usage.



• A commercial switching system satisfies, the following user requirements:

1. Each user has need for the capability of communicating with any other user.

2. The speed of connection is not critical, but the connection time should be relatively small compared to holding time or conversation time.



3. The GoS (probability of call blockage) is also not critical but should be low (p = 0.01).

4. The user expects and assumes conversation privacy but usually does not specifically request it.



4. The primary mode of communication for most users will be voice.

5. The system must be available to the user at any time the user may wish to use it.


2 Numbering, One Basis of 2 Numbering, One Basis of SwitchingSwitching

• A telephone subscriber looking into a telecommunication network sees a repeatedly branching tree of links.

• At each branch point there are multiple choices.



Branching tree of links (Telephone Network)



• If (A) would like to communicate with (N), a connection is built up utilizing one choice at each branch point.

• A telephone number is required to activate the switch at the branch points in the network to direct the call to the destination.



• A telephone number performs two important functions:

1. It routes the call. (ROUTING) 2. It activates the necessary equipment for

proper call charging. (CHARGING)• Hence, lifting the hand set and dialing a

telephone number will activate the switches to rout the call and setup the charging equipment.


Continue…Continue…• The switch capacity is defined as the total

number of subscribers that it can serve.• Hence, Total Number of Subscriber IDs =

Switch Capacity. • If we had a switch with a capacity of

(10,000 lines), then• It could serve up to (10,000 subscribers)

and we could assign telephone numbers (0000) through (9999).

• The critical points occur where the number of subscribers reaches 10,000.



• A given switch unit is usually most economical when operating with the number of subscribers near the maximum of its design.

• It is necessary for practical purposes to hold some spare capacity in reserve.

• Example: an exchanges with seven-digit subscriber numbers, such as



• The Local Area has (999)Local Exchanges, where (911) is blocked number.

• Each exchange can serve 10,000 subscribers allowing for no blocked numbers, such as 746 and 0000


16 Numbering Concepts For 16 Numbering Concepts For TelephonyTelephony

• There are four elements to an international telephone number.

1. International prefix.2. Country code. 3. National significant number.4. Subscriber number.• CCITT recommends that not more than

12 digits make up an international number.


Continue…Continue…• These 12 digits exclude the international prefix.• Example: International number 32-2-456-1234,

that is 10 digits which is less than 12 digits.


16.2 Definitions16.2 Definitions• Local Numbering Area: is the area in

which any two subscribers use the same dialing procedure to reach another subscriber in the telephone network.

• Subscribers belonging to the same numbering area may call one another simply by dialing the subscriber number.

• If they belong to different numbering areas, they must dial the trunk prefix plus the trunk code in front of the subscriber number.


Continue…Continue…• Subscriber Number: This is the number to be

dialed or called to reach a subscriber in the same local network or numbering area.

• Trunk Prefix (Toll-Access Code): This is a digit or combination of digits to be dialed by a calling subscriber making a call to a subscriber in his own country but outside his own numbering area. The trunk prefix provides access to the automatic outgoing trunk equipment.



• Trunk Code (Area Code): This is a digit or combination of digits (not including the trunk prefix) characterizing the called numbering area within a country.

• Country Code: This is the combination of one, two, or three digits characterizing the called country.


Continue…Continue…• Local Code: This is a digit or combination of

digits for obtaining access to an adjacent numbering area or to an individual exchange (or exchanges) in that area. The national significant number is not used in this situation.

• International prefix: combination of digits used by a calling subscriber to a subscriber in another country to obtain access to the automatic outgoing international equipment.



• Numbering system is classified according to its length in the serving area into:

1. Uniform Numbering: is a numbering scheme in which the length of the subscriber numbers is uniform inside a given numbering area.

2. Nonuniform Numbering: is a scheme in which the subscriber numbers vary in length within a given numbering area.


16.4 In-Dialing16.4 In-Dialing• Private Automatic Branch Exchange

(PABX): is a telephone exchange that serves a particular business or office.

• PBXs make connections among the internal telephones of a private organization, usually a business and also connect them to the PSTN via trunk lines.

• A PABX has a main number assigned by CO, for example the Air University telephone number.



• Example: suppose that a PABX main number is 543-7000, with an extension to that PABX of 678. To dial the PABX extension directly from the outside, we would dial

1. Main PABX number: 543-70002. PABX extension number: 6783. Extension dialed directly from outside:

543-7000678


3 Concentration and Expansion3 Concentration and Expansion

• Switch is defined as a device that connects inlets to outlets.

• The inlet may be a calling subscriber line, and the outlet may be the line of a called subscriber.

• In PSTN, subscribers share common transmission facilities; switches permit this sharing by concentration.

• Concentration: is a line-to-trunk ratio.



Inlet to outlets selector



• A local switching exchange concentrates traffic.

• Concentration reduces the number of switching paths or links within the exchange and the number of trunks connecting the local exchange to other exchanges.



• Tandem exchanges concentrate trunks in the local area for traffic relations (links) from sources of low traffic intensity, particularly below 20 erlangs, improving trunk efficiency.

• A switch also performs the function of expansion to provide all subscribers served by the exchange with access to incoming trunks and local switching paths.


4 Basic Switching Functions4 Basic Switching Functions

• There are eight basic functions of a conventional switch or exchange:

1. Interconnection2. Control3. Alerting4. Attending5. Information receiving6. Information transmitting7. Busy testing8. Supervisory



• Interconnection is made by double-ended connecting cords, connecting subscriber to subscriber or subscriber to trunk or trunk to trunk.

• The cords available are always less than half the number of jacks appearing on the board.


Continue…

• Concentration takes place at this point on a manual exchange.

• Distribution is also carried out because any cord may be used to complete a connection to any of the terminating jacks.

• Attending–Alerting function: The operator is alerted by a lamp when there is an incoming call requiring connection.



• Control functions: The operator performs numerous control functions to set up a call, such as

1. Selecting a cord,

2. Plugging it into the originating jack of the calling line,

3. Connecting her headset to determine calling information,



4. Selecting and busy testing the called subscriber jack, and then

5. Plugging the other end of the cord into the proper terminating jack and

6. Alerting the called subscriber by ring-down.



• Information receiving by the operator from the caller. (destination information)

• Information transmission: routing a call to another exchange (X) through a trunk circuit by orally asking the operator at (X).

• Supervision function: is performed by lamps to show when a call is completed and the cord taken down. (charging)


Continue…

• Concentration: is the ratio of the field of incoming jacks to cord positions.

• Expansion: is the number of cord positions to outgoing (terminating) jacks.

• The called subscriber at another moment in time may become a calling subscriber.



• Incoming and outgoing trunks may be separated.

• In this case they would be one- way circuits.

• If not separated, they would be both-way circuits, accepting both incoming and outgoing traffic


5 Introductory Switching Concepts5 Introductory Switching Concepts

• A switch in simple way, we can say that it has originating line appearances and terminating line appearances.

• All telephone switches have, as a minimum, three functional elements:

1. Concentration.

2. Distribution.

3. Expansion.



Originating and terminating line appearances.

Exchange or Switch X


Continue…Continue…• The figure shows the three different call

possibilities of a typical local exchange:1. A call originated by a subscriber who is served by

the exchange and bound for a subscriber who is served by the same exchange.

(route A–B–C–D–E).2. A call originated by a subscriber who is served by

the exchange and bound for a subscriber who is served by another exchange. (route A–B–F).

3. A call originated by a subscriber who is served by another exchange and bound for a subscriber served by the exchange in question.

(route G–D–E).


Continue…Continue…• Call concentration takes place in B.

• Call expansion takes place in D.

• The distribution stage in switching serves to connect by switching the concentration stage to the expansion stage.

The concept of concentration, distribution, and expansion



Symbols used in switching diagrams, A) concentration, B) Expansion



• The number of inputs to a concentration stage is determined by the number of subscribers connected to the exchange.

• The number of outputs of the expansion stage is equal to the number of connected subscribers whom the exchange serves.

• The outputs of the concentration stage are less than the inputs.

• These outputs are called trunks and are formed in groups.



• The sizing or dimensioning of the number of trunks per group is a major task of the systems engineer.

• Dimensioning: is to determine the number of trunks required on a route or connection between exchanges.

• The number is determined by the erlangs of traffic originated by the subscribers and the calling rate.

• One erlang represents a circuit occupied for 1-h.



• A group selector is used in distribution switching to switch one trunk (between concentration and expansion stages) to another.

• It is often found not only in switches that switch subscriber lines but also where trunk switching is necessary.

• A group selector alone is a tandem switch.



The group selector concept where both lines and trunks can be switched by the same matrix.


6 Crossbar Switch6 Crossbar Switch

• It is also called coordinate switch or a matrix switch.

• This concept was carried over to digital switch for the space switching function.

Typical diagram of a cross-point matrix.



• It is actually a matrix switch used to establish a speech path.

• An electrical contact is made by actuating a horizontal and a vertical relay (old) or through digital logic gates (new).

• A control circuit is used to establish the connection and latch it until the telephone set goes on-hook.



Main distribution frame (MDF)



Crossbar switch



Crossbar Exchange



Crossbar switch



Hungary large crossbar exchange


7 Multiples and Links7 Multiples and Links

• A multiple multiplies: It is a method of obtaining several outputs from one input and reverse is true. Thus access is extended.

Examples of ‘‘multiple’’: (A) single inputs to multiple outputs; (B) multiple inputs to a single output.



• Links: provide connection for a multiple of switch inputs from one stage to a multiple of switch outputs in another stage. Examples of links.


8.2 Availability8.2 Availability

• As we know that, a Switch is defined as a device that connects inlets to outlets.

• A switch has full availability: when each inlet has access to any outlet or every free inlet is at all times able to test every outlet.

• A switch has limited availability: when not all the free outlets in a switching system can be reached by inlets.


Continue…

An example of a switch with limited availability.


Continue…

An example of a switch with full availability.



Link and availability



• Full availability switching is more desirable than limited availability but is more expensive for larger switches.

• Full availability switching is generally found only in small switching configurations and in many new digital switches.



• At a switching array, availability describes the number of outlets that a free inlet is able to reach and test for a busy condition.

• The availability at a switching array can be assigned a value, namely, the number of outlets available to each inlet.



Two stage matrix (array), there are now four paths between input and output


8.3 Grading8.3 Grading• At a switching array with limited

availability the inlets are arranged into groups, called grading groups.

• All the inlets in a grading group always have access to the same outlets.

• Grading is a method of assigning outlets to grading groups in such a way that they assist each other in handling the traffic.



• Grading is one method of improving the traffic-handling capacities of switching configurations with limited availability.

• Grading is a scheme for interconnecting switching subgroups to make the switching load more uniform.



Three stage switching array



Two stage link network (grading).



• The figure shows twenty switches of size 10 × 10 used to connect 100 incoming trunks to 100 outgoing trunks.

• The links between primary and secondary are arranged systematically.

• The link 29 connects the outlet of 9 of primary switch 2 and inlet of 2 of secondary switch 9.


Full and limited availabilityi = number of inlets per switching matrix (grading group). m = number of switching matrices (grading groups) (4, in A and B).N = number of outlets (three in A; seven in B). k = availability [number of outlets per switching matrix (grading group)]. Note: This is a simplified illustration. In a typical switching array, k would equal 10 or even 20.


10 Switch’s System Control10 Switch’s System Control

• The basic function of a switch’s control system is to establish an appropriate speech path through the switch matrix.

• To carry out this function, the control system must know the calling and called ports on the matrix and be able to find a free path between them.

• The telephone number is the basic element to perform the control function.


10.2 Inter-exchange Control 10.2 Inter-exchange Control RegisterRegister

• Right after the switch inlet, there was a register translator (control register) that carried out three consecutive steps:

1. Information reception.

2. Internal control signaling.

3. Information transmission.



Interexchange control register concept.


Continue…

• The translator in this case determines:

1. The exchange code (first three dialed digits).

2. The outgoing trunk group.

3. The type of signals required by the group and the amount of information to be transmitted.

• The sender generates and transmits signaling information to distant exchanges.


Continue…Continue…• Outgoing trunk-control registers facilitate the

use of tandem exchange operation.

• Inter-exchange control registers enabled alternative routing. How?

• Answer: When first-attempt routing is blocked, this same technique can be used for second-attempt routing through an exchange by setting up a speech path with switching components different from those of the first setup that failed.


10.3 Common Control10.3 Common Control

• Is any control circuitry in a switch that is used for more than one switching device.

• Is a circuit that provides a means of control of the interconnecting switch network.

• Note: Network here means an arrangement of components, nodes, and interconnecting branches inside a switch.


Continue…Continue…• It may cover the entire switch or separate

control of the originating and terminating halves.• It first identify the input and output of the

terminals of the network that are free and then establish a path between them.

• This implies a busy-test of the path before setting the path up.

• Markers are one of the basic elements of common control of a crossbar switch.


Continue…

Functional block diagram of a marker

Input Grid

Output Grid



• On grid circuits the characteristics are such that with specified input and output terminals, various sets of linkage paths exist that can provide connection between the terminals. (North American Hierarchy).

• It is the marker, with terminal points identified, that locates a path, busy-tests it, and finally sets up a channel through the switch grid network.



• A marker always works with one or more registers.

• It is a rapidly operating device serving many calls per minute.

• It cannot wait for the comparatively slow input information (i.e. touch tones, or inter-register signaling information) supplied by an incoming line or trunk.



• Such information is stored in the register and released to the marker on demand.

• The register may receive the entire dialed number and store it before dumping to the marker.

• Or may take only the exchange code or area code plus the exchange code which is sufficient to identify the trunk group.



• The register will also identify the location of the call input or setup a control path to the input for the marker.

• The translator provides the marker information on access to the proper trunk group and type of signaling required on that trunk group.


11 Stored Program Control11 Stored Program Control

• In SPC, common control is carried out to a greater extent or entirely by computer-ware such as full scale computer, mini-microcomputer, microprocessors.

• Software may be hard-wired or programmable.

• 5ESS is an example of exchanges operating according to SPC.



• Control functions are entirely carried out by a central computer.

• The switch is entirely digital (i.e. processing) but the information received, and forwarded by it is analog.

• Compared to the ordinary Crossbar Switch, it is faster, smaller in size, and have moderate capacity.


11.2 Basic Functions of SPC11.2 Basic Functions of SPC

• There are four basic functional elements of an SPC switching system:

1) Switching matrix.

2) Call store (memory).

3) Program store (memory).

4) Central processor.



A simplified functional block diagram of an SPC exchange.



• The switching matrix can be made up of electromechanical cross-points or digital gates.

• The call store is often referred to as the scratch pad memory.

• This is a temporary storage of incoming call information ready for use, on command from the central processor.



• It also contains availability and status information of lines, trunks and service circuits, and internal switch circuit conditions.

• Circuit status information is brought to the memory by a method of scanning. All speech circuits are scanned for a busy/idle condition.



• The program store provides the basic instructions to the controller (central processor).

• In many installations, translation information is held in this store, such as DN to EN translation and trunk signaling information.



• DN: The directory number which is the dialed code.

• EN: The equipment number. Equipments such as trunk groups.


Continue…

5ESS exchange



5ESS exchange



5ESS switch



5ESS control room5ESS control room

15/01/2013Bahman R. Alyaei1 Chapter 2 Local Networks.

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