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Signaling For Analog Telephony

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

2 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

3 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

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

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

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

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

8 2 Forward and Backward Signals
Call-control signals are categorized as of Forward and Backward signals. Forward signals are sent in the direction in which the call is set up (from S1 to S2), such as the request-for-service signal and the digits of the called number. 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

9 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

10 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

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

12 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

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

14 3 Audible-Visual Signaling
Is a category of signaling functions inform the calling subscriber regarding call progress. It has two classes: The alerting signals: are forward signals used to informs the called subscriber of a call waiting (ringing), or an extended “off-hook” condition of his or her handset (off-hook warning). 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

15 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

16 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

17 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

18 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

19 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

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

21 Channel Associated Signaling (CAS)
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

22 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

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

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

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

26 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 one-frequency 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

27 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

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

29 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

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

31 Signaling For Intra-exchange Call
Example: In this example, subscriber S1 would like to communicate with subscriber S2. Both S1 and S2 are served by a local exchange X. The directory number of called subscriber S2 is The request will proceed as follow: 01/02/2013 Bahman R. Alyaei

32 Continue… An inter-exchange call signaling diagram 01/02/2013
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33 Continue… Calling subscriber S1 starts by going off-hook which is interpreted by the exchange as a request for service. In response, the exchange returns dial-tone, indicating that it is ready to receive digits. S1 then sends the digits of the called number, using the dial or the keypad of the telephone. 01/02/2013 Bahman R. Alyaei

34 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 S2 after receipt of the complete called number and checks whether S2 is free. 01/02/2013 Bahman R. Alyaei

35 Continue… Assuming that this is the case, it sends a ringing signal to alert S2 and informs S1 about the call progress with a ringing-tone. When S2 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

36 Continue… The conversation starts, and the exchange begins to charge S1 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

37 Signaling For Inter-exchange Call
Example: S1 belongs to exchange A and would like to communicate with S2 which belongs to exchange C. Assume that S1 received the dial tone from Exchange A and in return, S1 sent the telephone number of S2 to the exchange A. 01/02/2013 Bahman R. Alyaei

38 An Inter-exchange call signaling diagram
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39 Continue… When exchange A receive the called number from S1, it will decides to route the call via intermediate exchange B. Exchange A seizes an available trunk T1 and sends a seizure signal on the trunk. Exchange B responds with a proceed-to-send signal, indicating that it is ready to receive the digits of the called number. 01/02/2013 Bahman R. Alyaei

40 Continue… Exchange A sends the digits and then sets up a path in its switch block between the subscriber line of S1 and T1. When exchange B has received the complete called number, it seizes an available trunk T2 to destination exchange C and sends a seizure signal on the trunk. 01/02/2013 Bahman R. Alyaei

41 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 T1 and T2. Exchange C then checks whether called subscriber S2 is idle. 01/02/2013 Bahman R. Alyaei

42 Supervision signals for a call
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

43 Continue… If this is the case, C sends a ringing signal to S2 and ringing-tone on trunk T2. Because exchanges A and B have cut through, there is a connection between the calling subscriber S1 and exchange C, and subscriber S1 hears a ringing-tone. When S2 answers, exchange C cuts through a path between trunk T2 and subscriber S2. 01/02/2013 Bahman R. Alyaei

44 Continue… It also sends an answer signal on T2, and exchange B repeats the signal on trunk T1. 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

45 Continue… In this example, called party S2 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 inter-exchange calls are usually controlled by the calling party, then 01/02/2013 Bahman R. Alyaei

46 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 S1, 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

47 Continue… Exchange A releases its path between S1 and trunk T1 and sends a clear-forward signal to exchange B, which releases its path between T1 and T2 and repeats the clear-forward to exchange C. This exchange then clears its path between T2 and called subscriber S2. 01/02/2013 Bahman R. Alyaei

48 Continue… When exchanges B and C have completed the release of, respectively, T1 and T2, they send release-guard signals to, respectively, exchanges A and B. When A and B receive the release-guard, they know that they can again seize, respectively, T1 and T2 for new calls. 01/02/2013 Bahman R. Alyaei

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

50 1 Purpose and Scope 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 kHz. Practically, VC occupies the band of frequencies between 300 and 3400 Hz. 01/02/2013 Bahman R. Alyaei

51 Continue… There are five basic transmission mpairments we must deal with regarding the voice channel: Attenuation distortion. Phase distortion. Noise. Echo. Singing. 01/02/2013 Bahman R. Alyaei

52 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

53 2.2 Phase Distortion A voice channel may be regarded as a band-pass 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

54 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

55 2.3 Noise 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

56 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

57 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 f1 and f2 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

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

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

60 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

61 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

62 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

63 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

64 Continue… Simplified schematic of two-wire/four-wire operation
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65 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

66 Continue… Trunk circuit is an example of four-wire transmission.
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 (two-way) 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

67 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 T1 or E1 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

68 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

69 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

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

71 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

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

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

74 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 GoS, forecast growth, and QoS. 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

75 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

76 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

77 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

78 Interconnection of switching exchanges in North America
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79 Continue… One report recommends, as a goal, that a numbering area be no greater than 70,000 km2 nor have less than 100,000 subscribers at the end of a numbering-plan 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

80 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

81 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

82 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

83 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

84 Continue… On an international connection, the 12 links in tandem are broken down into three groups, each 4 links in tandem as follows: National connection of country originating call. International portion. National connection of country terminating the call. 01/02/2013 Bahman R. Alyaei

85 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

86 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

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

88 Continue… 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

89 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

90 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

91 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

92 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

93 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

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

95 Hierarchical structures of AT & T and ITU-T
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96 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

97 Theses circuits could be one-way or both-way circuits.
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 one-way or both-way circuits. 01/02/2013 Bahman R. Alyaei

98 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

99 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

100 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

101 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: Fixed routing schemes The routing pattern is always the same. Dynamic routing schemes The set of routes in the routing pattern varies. 01/02/2013 Bahman R. Alyaei

102 Continue… Dynamic routing schemes are classified to
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

103 Continue… Example of time-dependent routing 01/02/2013
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104 Continue… 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

105 Continue… Example of state-dependent routing 01/02/2013
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106 Event-Dependent Routing:
Continue… 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

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

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