Chapter 4 Signaling For Analog Telephony 01022013 Bahman

Chapter 4 Signaling For Analog Telephony 01/02/2013 Bahman R. Alyaei 1

1 Introduction • Signaling conveys the intelligence needed for one subscriber to interconnect with any other in the PSTN. • Signaling tells the switch that a subscriber desires service. • It gives the local switch the data necessary to identify the required distant subscriber and hence to route the call properly. 01/02/2013 Bahman R. Alyaei 2

Continue… • It provides supervision of the call along its path. • It gives the subscriber certain status information, such as dial tone, busy tone, and ringing. • Signaling information can take place from subscriber to switch or between (and among) switches. 01/02/2013 Bahman R. Alyaei 3

Continue… • Signaling information can be transmitted by means such as 1. Duration of pulses 2. Combination of pulses 3. Frequency of signal 4. Combination of frequencies 5. Presence or absence of a signal 6. Binary code 7. For dc systems, the direction or level of transmitted current 01/02/2013 Bahman R. Alyaei 4

Continue… • There are several classifications of signaling: 1. General: a) Subscriber or Intra-exchange signaling. b) Inter-switch or Inter-exchange signaling. 2. Functional: a) Audible–visual (call progress and alerting). b) Supervisory signaling. c) Address signaling. d) Inter-register signaling. 01/02/2013 Bahman R. Alyaei 5

Continue… 3. Channel: a) Channel Associated Signaling (CAS). b) Channel Disassociated Signaling or Common Channel Signaling (CCS). 4. Direction: a) Forward signaling. b) Backward signaling. 01/02/2013 Bahman R. Alyaei 6

Continue… A functional breakdown of signaling 01/02/2013 Bahman R. Alyaei 7

2 Forward and Backward Signals • Call-control signals are categorized as of Forward and Backward signals. 1. Forward signals are sent in the direction in which the call is set up (from S 1 to S 2), such as the request-for-service signal and the digits of the called number. 2. Backward signals are sent in the opposite direction such as dial-tone, ringing-tone, and answer are backward signals. 01/02/2013 Bahman R. Alyaei 8

2 Supervisory Signaling • It is also called Line Signaling. • It provides information on line or circuit condition and indicates whether a circuit is in use (busy) or idle. • It informs the switch and interconnecting trunk circuits whether a calling party is “off hook” or “on hook” or whether a called party is “off hook” or “on hook”. 01/02/2013 Bahman R. Alyaei 9

Continue… • Supervisory information–status must be maintained end to end on every telephone call. • Supervisory signaling informs the switch when to start and stop the call duration metering to establish call charges. • As we know, dialing a subscriber line is merely interruption of the subscriber loop’s off-hook condition, often called “make and break”. 01/02/2013 Bahman R. Alyaei 10

Continue… • The “make” is a current flow condition (offhook), and the “break” is the no-current condition (on-hook). • Supervisory signaling can be divided into two groups: 1. The forward supervision signals: Request and disconnect by calling party, request the start or end of a connection, flash, clear forward, and seizure. 2. The backward supervision signals: Answer and disconnect by called party, busy tone, and clear backward. 01/02/2013 Bahman R. Alyaei 11

Continue… • But, how do we know the difference between supervisory and dialing? • Primarily by duration, and the amplitude and direction of the current flow. • By duration we mean, the on-hook interval of a dial pulse which is relatively short and is distinguishable from an on-hook disconnect signal (subscriber hangs up). 01/02/2013 Bahman R. Alyaei 12

Continue… 01/02/2013 Bahman R. Alyaei 13

3 Audible-Visual Signaling • Is a category of signaling functions inform the calling subscriber regarding call progress. It has two classes: 1. The alerting signals: are forward signals used to informs the called subscriber of a call waiting (ringing), or an extended “offhook” condition of his or her handset (offhook warning). 2. Progress Tones: These are audible backward signals (dial-tone, ringing-tone, busy-tone) sent by an exchange to the calling subscriber, indicating the progress of a call. 01/02/2013 Bahman R. Alyaei 14

4 Address Signaling • These signals are also known as digits or selection signals or register signals. • They are forward signals that are sent by the calling subscriber when dialing the called party number (DTMF). • Address signaling takes place while the telephone is off-hook. 01/02/2013 Bahman R. Alyaei 15

Continue… • If more than one switch is involved in the call setup, signaling is required between switches (both address and supervisory). • Address signaling between switches in conventional systems is called inter -register signaling. 01/02/2013 Bahman R. Alyaei 16

5 Inter-switch signaling • Is the signaling required to communicate switches with each other. • It is also called inter-exchange signaling or Link-By-Link Signaling. • It includes address and supervision signaling between switches. 01/02/2013 Bahman R. Alyaei 17

6 Subscriber Signaling • Is the signaling between a subscriber and the local exchange only. • It is also called intra-exchange signaling or End-to-End Signaling. • It includes forward and backward supervisory signaling, audible signaling, and address signaling. 01/02/2013 Bahman R. Alyaei 18

7. 1 Channel Associated Signaling (CAS) • In which the signaling information were carried by the same medium (subscriber line, trunk) on the same or different channel (time or frequency slot) that carried the speech during the call. • In other words, the signaling goes right along with the speech signal it is associated with, on the same medium. 01/02/2013 Bahman R. Alyaei 19

Continue… Channel Associated Signaling (CAS) 01/02/2013 Bahman R. Alyaei 20

Continue… Channel Associated Signaling (CAS) As shown, the signaling channel is separate, but associated. With conventional analog signaling, it would be a single solid line, where the signaling is embedded with its associated traffic. 01/02/2013 Bahman R. Alyaei 21

7. 2 Common Channel Signaling (CCS) • • • Signaling may or may not go on the same medium or path. Signaling information is transmitted on a separate or dedicated channel (line, frequency slot, time slot). If the signaling is performed through a signaling network on separate circuits or medium then it is called fully disassociated channel signaling. 01/02/2013 Bahman R. Alyaei 22

Continue… Common Channel Signaling (CCS) signaling is conveyed on a separate circuit (or time slot) 01/02/2013 Bahman R. Alyaei 23

Continue… Fully disassociated channel signaling 01/02/2013 Bahman R. Alyaei 24

8 AC Signaling • Traditionally, ac signaling systems are divided into three categories: 1. Low-frequency system 2. In-band system 3. Out-band (out-of-band) system. 01/02/2013 Bahman R. Alyaei 25

8. 1 Low-Frequency AC Signaling Systems • An ac signaling system operating below the limits of the conventional voice channel (i. e. , < 300 Hz) are termed low frequency. • Low-frequency signaling systems are onefrequency systems, typically 50 Hz, 80 Hz, 135 Hz, or 200 Hz. • low-frequency signaling is limited to metallic-pair transmission systems. 01/02/2013 Bahman R. Alyaei 26

8. 2 In-Band Signaling • The conventional voice channel occupies the band of frequencies from 300 Hz to 3400 Hz. • In-band signaling refers to signaling systems using an audio tone, or tones inside the conventional voice channel, to convey signaling information. • As the term implies, in-band signaling is where signaling is carried out directly in the voice channel. 01/02/2013 Bahman R. Alyaei 27

Continue… • In-band signaling is broken down into three categories: (1) One frequency (SF or single frequency). • Used for supervision such as idle tone. (2) Two frequency (2 VF or two voice frequency). • Is used for both supervision (line signaling) and address signaling. (3) Multi-frequency (MF). • Is used for inter-register signaling. 01/02/2013 Bahman R. Alyaei 28

8. 3 Out-of-Band Signaling • In which the supervisory information is transmitted out of band (i. e. , above 3400 Hz). • It is a single-frequency system. • Some out-of-band systems use “tone on when idle, ” indicating the on-hook condition, whereas others use “tone off”. • Out-of-band signaling is used exclusively on carrier systems, not on wire trunks. 01/02/2013 Bahman R. Alyaei 29

Continue… Single-frequency signaling: (A) in-band; (B) out-of-band. 01/02/2013 Bahman R. Alyaei 30

Signaling For Intra-exchange Call Example: • In this example, subscriber S 1 would like to communicate with subscriber S 2. • Both S 1 and S 2 are served by a local exchange X. • The directory number of called subscriber S 2 is 347 -9654. • The request will proceed as follow: 01/02/2013 Bahman R. Alyaei 31

Continue… An inter-exchange call signaling diagram 01/02/2013 Bahman R. Alyaei 32

Continue… • Calling subscriber S 1 starts by going offhook which is interpreted by the exchange as a request for service. • In response, the exchange returns dialtone, indicating that it is ready to receive digits. • S 1 then sends the digits of the called number, using the dial or the keypad of the telephone. 01/02/2013 Bahman R. Alyaei 33

Continue… • After receipt of 3 -4 -7, the exchange recognizes one of its exchange codes and thus knows that it is the destination exchange for the call. • The exchange can identify the called subscriber S 2 after receipt of the complete called number and checks whether S 2 is free. 01/02/2013 Bahman R. Alyaei 34

Continue… • Assuming that this is the case, it sends a ringing signal to alert S 2 and informs S 1 about the call progress with a ringing -tone. • When S 2 goes off-hook, an answer signal is generated. • The exchange then sets up a path in its switch block between the subscriber lines. 01/02/2013 Bahman R. Alyaei 35

Continue… • The conversation starts, and the exchange begins to charge S 1 for the call. • At the end of the call, the subscribers put the handsets back in the cradles of the telephones. • The signals generated by these actions from the calling and called subscribers are known as, respectively, the clear-forward and the clear-back signal. 01/02/2013 Bahman R. Alyaei 36

Signaling For Inter-exchange Call • Example: • S 1 belongs to exchange A and would like to communicate with S 2 which belongs to exchange C. • Assume that S 1 received the dial tone from Exchange A and in return, S 1 sent the telephone number of S 2 to the exchange A. 01/02/2013 Bahman R. Alyaei 37

An Inter-exchange call signaling diagram 01/02/2013 Bahman R. Alyaei 38

Continue… • When exchange A receive the called number from S 1, it will decides to route the call via intermediate exchange B. • Exchange A seizes an available trunk T 1 and sends a seizure signal on the trunk. • Exchange B responds with a proceed-tosend signal, indicating that it is ready to receive the digits of the called number. 01/02/2013 Bahman R. Alyaei 39

Continue… • Exchange A sends the digits and then sets up a path in its switch block between the subscriber line of S 1 and T 1. • When exchange B has received the complete called number, it seizes an available trunk T 2 to destination exchange C and sends a seizure signal on the trunk. 01/02/2013 Bahman R. Alyaei 40

Continue… • Exchange C responds with a wink signal, after which exchange B sends the digits of the called number and cuts through a path between trunks T 1 and T 2. • Exchange C then checks whether called subscriber S 2 is idle. 01/02/2013 Bahman R. Alyaei 41

Continue… Supervision signals for a call a) sent by outgoing exchange B and b) sent by incoming exchange C 01/02/2013 Bahman R. Alyaei 42

Continue… • If this is the case, C sends a ringing signal to S 2 and ringing-tone on trunk T 2. • Because exchanges A and B have cut through, there is a connection between the calling subscriber S 1 and exchange C, and subscriber S 1 hears a ringing-tone. • When S 2 answers, exchange C cuts through a path between trunk T 2 and subscriber S 2. 01/02/2013 Bahman R. Alyaei 43

Continue… • It also sends an answer signal on T 2, and exchange B repeats the signal on trunk T 1. • Assuming that originating exchange A is responsible for charging the call, it establishes a billing record that includes the calling and called numbers, the date, and the time of answer. • The conversation now begins. 01/02/2013 Bahman R. Alyaei 44

Continue… • In this example, called party S 2 hangs up first. • Exchange C sends a clear-back signal to exchange B, which repeats the signal to exchange A. • But, intra-exchange calls, and interexchange calls are usually controlled by the calling party, then 01/02/2013 Bahman R. Alyaei 45

Continue… • On receipt of the clear-back, exchange A stops charging and enters the time when it received the clear-back in the billing record of the call. • It also starts a 30– 60 second timer. It then awaits a clear-forward from calling party S 1, or the expiration of the timer, and initiates the release of the connection when one of these events occurs. 01/02/2013 Bahman R. Alyaei 46

Continue… • Exchange A releases its path between S 1 and trunk T 1 and sends a clear-forward signal to exchange B, which releases its path between T 1 and T 2 and repeats the clear-forward to exchange C. • This exchange then clears its path between T 2 and called subscriber S 2. 01/02/2013 Bahman R. Alyaei 47

Continue… • When exchanges B and C have completed the release of, respectively, T 1 and T 2, they send release-guard signals to, respectively, exchanges A and B. • When A and B receive the releaseguard, they know that they can again seize, respectively, T 1 and T 2 for new calls. 01/02/2013 Bahman R. Alyaei 48

Chapter 5 Introduction To Transmission For Telephony 01/02/2013 Bahman R. Alyaei 49

1 Purpose and Scope • Transmission: deals with the delivery of a quality electrical signal from point X to point Y. • The basic building block for transmission is the telephone channel or voice channel (VC). • VC implies spectral occupancy, whether the voice path is over wire, radio, or coaxial cable or over a fiber-optic system. • Theoretically, VC occupies the band of frequencies from 0 to 4 k. Hz. • Practically, VC occupies the band of frequencies between 300 and 3400 Hz. 01/02/2013 Bahman R. Alyaei 50

Continue… • 1. 2. 3. 4. 5. There are five basic transmission impairments we must deal with regarding the voice channel: Attenuation distortion. Phase distortion. Noise. Echo. Singing. 01/02/2013 Bahman R. Alyaei 51

2. 1 Attenuation Distortion • Is the result of imperfect amplitude-frequency response of the voice path (transmission channel). • It can be avoided if all frequencies within the passband are subjected to exactly the same loss (or gain). 01/02/2013 Bahman R. Alyaei 52

2. 2 Phase Distortion • A voice channel may be regarded as a bandpass filter (radio channel). • A signal takes a finite time to pass through a telecommunication network which is called delay time. • This time is a function of the velocity of propagation and the operating frequency, which varies with the media involved. • Absolute delay is the delay a signal experiences while passing through the channel end-to-end at a reference frequency. 01/02/2013 Bahman R. Alyaei 53

Continue… • But we see that the propagation time is different for different frequencies, with the wave-front of one frequency arriving before the wave-front of another in the passband. • A modulated signal will not be distorted on passing through the channel if the phase shift changes uniformly with frequency, whereas if the phase shift is nonlinear with respect to frequency, the output signal is distorted compared to the input. 01/02/2013 Bahman R. Alyaei 54

2. 3 Noise • • 1. 2. 3. 4. Is any undesired signal in a communication circuit. Noise is broken down into four categories: Thermal noise. Inter-modulation noise. Crosstalk. Impulse noise. 01/02/2013 Bahman R. Alyaei 55

Continue… • Thermal Noise: occurs in all transmission media and all communication equipment, including passive devices. • It arises from random electron motion and is characterized by a uniform distribution of energy over the frequency spectrum with a Gaussian distribution of levels. 01/02/2013 Bahman R. Alyaei 56

Continue… • Every equipment element and the transmission medium proper contribute thermal noise to a communication system if the temperature of that element or medium is above absolute zero. • Inter-modulation noise: is the result of the presence of inter-modulation products. If two signals with frequencies f 1 and f 2 are passed through a nonlinear device or medium, the result will contain IM products that are spurious frequency energy components. 01/02/2013 Bahman R. Alyaei 57

Continue… • Crosstalk: refers to unwanted coupling between signal paths. • There are essentially three causes of crosstalk: 1. Electrical coupling between transmission media, such as between wire pairs on a voice frequency (VF) cable system 2. Poor control of frequency response (defective filters or poor filter design). 3. Nonlinear performance in analog (FDM) multiplex systems. 01/02/2013 Bahman R. Alyaei 58

Continue… • There are two types of crosstalk: 1. Intelligible: where at least four words are intelligible to the listener from extraneous conversations in a 7 -sec period. 2. Unintelligible: crosstalk resulting from any other form of disturbing effects of one channel on another. 01/02/2013 Bahman R. Alyaei 59

Continue… • Impulse noise: is noncontinuous, consisting of irregular pulses or noise “spikes” of short duration, broad spectral density, and relatively high amplitude. • In the language of the trade, these spikes are often called “hits. ” A technician may say that the circuit is getting “hit up”. 01/02/2013 Bahman R. Alyaei 60

2. 4 Echo • Echo: in telephone systems is the return of a talker’s voice. • To be an impairment, the returned voice must suffer some noticeable delay. Thus we can say that echo is a reflection of the voice. • The cause of echo is impedance mismatches that might be present any place in the electrical telephone connection. • It affects the talker more than the listener. 01/02/2013 Bahman R. Alyaei 61

2. 5 Singing • Singing: is the result of sustained oscillations due to positive feedback in telephone amplifiers or amplifying circuits. • Singing may be regarded as echo that is completely out of control. This can occur at the frequency at which the circuit is resonant. • The cause of echo is impedance mismatches between the balancing network of the hybrid and its two-wire connection associated with the subscriber loop. 01/02/2013 Bahman R. Alyaei 62

3 Two-Wire and Four-Wire Transmission • A telephone conversation inherently requires transmission in both directions. • Two-wire transmission: when both directions are carried on the same pair of wires such as subscriber loop. • Two-wire operation: in which oppositely directed portions of a single telephone conversation occur over the same electrical transmission channel or path. 01/02/2013 Bahman R. Alyaei 63

Continue… Simplified schematic of two-wire/four-wire operation 01/02/2013 Bahman R. Alyaei 64

Continue… • Carrier and radio systems require that oppositely directed portions of a single conversation occur over separate transmission channels or paths. • Four-wire transmission: in which the transmit and receive signals are carrier on different lines or frequency slot, or time slot. 01/02/2013 Bahman R. Alyaei 65

Continue… • Thus we have two wires for the transmit path and two wires for the receive path, or a total of four wires for a full-duplex (twoway) telephone conversation. • Trunk circuit is an example of four-wire transmission. • Hybrid: is a transformer network that converts two-wire to four-wire transmission and vice versa. 01/02/2013 Bahman R. Alyaei 66

Continue… • Present practice with many telephone companies and administrations is to convert the two-wire subscriber loop, after it terminates in the mainframe, to either a T 1 or E 1 digital format, which, of course is four-wire. • Because each subscriber has his or her own Hybrid with an incoming Analog-to. Digital Conversion (ADC). 01/02/2013 Bahman R. Alyaei 67

4 Multiplexing • Is the process of combining of two or more signals (voice channels) into a single wave from which the signals can be individually recovered. • Of course the medium has to be able to accommodate the required bandwidth of the multiplexed signals. 01/02/2013 Bahman R. Alyaei 68

Continue… • On a wire pair we might combine 24, 30, or 48 channels. • On LOS microwave we can commonly carry 1800 analog channels or hundreds of digital channels. • On fiber optics we can carry thousands of digital channels. 01/02/2013 Bahman R. Alyaei 69

Continue… • There are essentially two generic ways we can multiplex voice channels: 1. In the frequency domain using frequency division multiplex (FDM). 2. In the time domain using time division multiplex (TDM). 01/02/2013 Bahman R. Alyaei 70

Continue… • FDM: is a method of allocating a unique band of frequencies in a comparatively wideband frequency spectrum of the transmission medium to each communication channel on a continuous time basis. • TDM: is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing. 01/02/2013 Bahman R. Alyaei 71

Chapter 6 Long Distance Networks 01/02/2013 Bahman R. Alyaei 72

1 General • The design of a long-distance network involves basically three considerations: 1. Routing scheme given inlet and outlet points and their traffic intensities. 2. switching scheme and associated signaling. 3. Transmission plan. 01/02/2013 Bahman R. Alyaei 73

Continue… • The system designer must specify type of traffic (how many people wish to talk, how often, and for how long), lost-call criteria or Go. S, forecast growth, and Qo. S. • The trade-off of all these factors with “economy” is probably the most vital part of initial planning and downstream system design. 01/02/2013 Bahman R. Alyaei 74

Continue… • We must point out that switching enhances the transmission facilities (carrier, radio, and cable systems). • From an economic point of view, it would be desirable to make transmission facilities adaptive to traffic load. • It is switching that makes transmission facilities adaptive by improving traffic concentration through discrete utilization of transmission trunks from different sources. 01/02/2013 Bahman R. Alyaei 75

Continue… • The transmission system must not excessively degrade the signal to be transported. • The transmission system must meet a reliability constraint and availability and must have an alternative route scheme in case of facility loss, whether switching node or trunk route. How? 01/02/2013 Bahman R. Alyaei 76

2 The Design Problem • In long-distance network design, the first step is Toll (transit) exchange placement. • Then, a country must be divided into toll areas. • The size of a toll area is impacted by a number of interacting disciplines, such as subscriber density at the end of a forecast period (say 10 years for the argument) for the candidate area; this will impact numbering and tariff areas. 01/02/2013 Bahman R. Alyaei 77

Interconnection of switching exchanges in North America 01/02/2013 Bahman R. Alyaei 78

Continue… • One report recommends, as a goal, that a numbering area be no greater than 70, 000 km 2 nor have less than 100, 000 subscribers at the end of a numberingplan validity period. • One rough rule of thumb is that a tariff area/toll area have no more than a 50 -km diameter. In rural regions, toll areas/tariff areas may be considerably larger. 01/02/2013 Bahman R. Alyaei 79

Continue… • Subscribers should be able to dial a shorter number when calling other subscribers inside their own numbering area (local area). • But when dialing toll area, a trunk prefix (zero digit) must be dialed with the area code (051). 01/02/2013 Bahman R. Alyaei 80

Continue… • Thus when a switch receives the initial digit as zero, it is prepared to receive the longer toll number for inter-area dialing, whereas if it receives any other digit, it is prepared for receipt of a subscriber number for intra-area dialing. • This will reduce the number of digits to be analyzed by a switch for proper routing and charging. 01/02/2013 Bahman R. Alyaei 81

Continue… • 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. • Our thinking has changed. We are moving away from the hierarchical concept (albeit slowly) to one using more direct routes. 01/02/2013 Bahman R. Alyaei 82

3 Link Limitation • Link: is the connectivity from one exchange to an adjacent exchange. • End-to-End: is the connectivity between the subscriber and a local exchange. • 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. 01/02/2013 Bahman R. Alyaei 83

Continue… • 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 call. 2. International portion. 3. National connection of country terminating the call. 01/02/2013 Bahman R. Alyaei 84

Continue… An international connection to illustrate the nomenclature adopted and the maximum number of links in tandem for an international connection. 01/02/2013 Bahman R. Alyaei 85

Continue… • 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. • For national network planning we assume that there are no more than four links in tandem. • we also assume that there are no more than four links in tandem on the international connectivity. 01/02/2013 Bahman R. Alyaei 86

4 International Network • International routing plan is known as free routing structure. Some of its highlights by ITU-T are: 1. It is not hierarchical. 2. Direct traffic should be routed over final (fully provided) or High Usage (HU) circuit groups. 3. No more than four international circuits in tandem should be involved between originating and terminating ISCs. 01/02/2013 Bahman R. Alyaei 87

Continue… 4. When a group consists of both terrestrial (earth) and satellite circuits, the choice of routing should be governed by: • 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. 01/02/2013 Bahman R. Alyaei 88

6 Network Design • At the top of a country’s hierarchy there is one or more international switching centers. • The next level down, as a minimum, would be the long-distance (Toll) network. • Thence down to a local network consisting of local serving exchanges and tandem exchanges. 01/02/2013 Bahman R. Alyaei 89

Continue… • Hence, a country is divided into toll areas. • And a Toll Area is made up of a grouping of Local Areas probably coinciding with a numbering (plan) area. • The link limit is immediately enforced starting at the international switching center (ISC) and heading downwards to the local serving exchange. 01/02/2013 Bahman R. Alyaei 90

Continue… • And we know that, no more than four links in tandem are permitted. • Using a High-Usage (HU) route, perhaps only two links would be required to reach the ISC — that is, from Local Serving Exchange to Toll Exchange, thence directly to the ISC. 01/02/2013 Bahman R. Alyaei 91

Continue… • HU route: is defined as any route that is not a final route which could be established between any pair of exchanges in the network if traffic intensities and distances involved proved this strategy economical. • Direct route: is a special type of HU route connecting exchanges of the lowest rank in the hierarchy. • Final route: is a route from which no traffic can overflow to an alternative route. 01/02/2013 Bahman R. Alyaei 92

Continue… • It is a route that connects an exchange immediately above or below it in the network hierarchy. • It makes up the backbone of the network. • When HU routes are established, traffic between the exchanges involved will first be offered to the HU route, and overflow would take place through the final route. 01/02/2013 Bahman R. Alyaei 93

A typical hierarchical network, Dashed lines show high-usage trunks 01/02/2013 An example of a hierarchical network with alternative routing Bahman R. Alyaei 94

Hierarchical structures of AT & T and ITU-T 01/02/2013 Bahman R. Alyaei 95

7 Traffic Routing In International Network • 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. 01/02/2013 Bahman R. Alyaei 96

7. 2 Network Topology • As we know that a network comprises a number of nodes interconnected by circuit groups directly or indirectly. • Theses circuits could be oneway or both-way circuits. 01/02/2013 Bahman R. Alyaei 97

Continue… A simplified network with circuit groups connecting pairs of nodes with one-way and both-way (two-way) working. 01/02/2013 Bahman R. Alyaei 98

Continue… • Direct route consists of one or more circuit groups connecting adjacent nodes. • Indirect route is a series of circuit groups connecting two nodes providing an end-to-end connection via other nodes. 01/02/2013 Bahman R. Alyaei 99

7. 2. 1 Network Architecture • As was mentioned earlier, there is no hierarchy for ISCs. • Hence, telecommunication companies and administrations are free to determine the most suitable utilization of their individual ISCs. 01/02/2013 Bahman R. Alyaei 100

7. 3 Routing Scheme • A routing scheme: defines how a set of routes is made available for calls between a pair of nodes. • There are two routing schemes: 1. Fixed routing schemes • The routing pattern is always the same. 2. Dynamic routing schemes • The set of routes in the routing pattern varies. 01/02/2013 Bahman R. Alyaei 101

Continue… • Dynamic routing schemes are classified to 1. Time-Dependent Routing: • Routing patterns are altered at fixed times during the day (or week) to allow changing traffic demands to be provided for. 01/02/2013 Bahman R. Alyaei 102

Continue… Example of time-dependent routing 01/02/2013 Bahman R. Alyaei 103

Continue… 2. State-Dependent Routing: • Routing patterns vary automatically according to the state of the network. This is adaptive 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 (e. g. , 10 s). 01/02/2013 Bahman R. Alyaei 104

Continue… Example of state-dependent routing 01/02/2013 Bahman R. Alyaei 105

Continue… 3. Event-Dependent Routing: • Routing scheme routes traffic locally away from congested links by retaining routing choices where calls are successful. 01/02/2013 Bahman R. Alyaei 106

Continue… An example of automatic rerouting 01/02/2013 Bahman R. Alyaei 107