Biblio5
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6.12 DIGITAL NETWORK |
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Table 6.6 CCITT Error Performance Objectives for International ISDN Connections |
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Performance Classification |
Objectivec |
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Fewer than 10% of 1-min intervals to have a bit error ratio worse |
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Degraded minutesa,b |
than 1 × 10−6d |
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Fewer than 0.2% of 1-s intervals to have a bit error ratio worse |
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Severely errored secondsa |
than 1 × 10−3 |
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Fewer than 8% of 1-s intervals to have any errors |
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Errored secondsa |
(equivalent to 92% error-free seconds) |
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aThe terms degraded minutes, severely errored seconds, and errored seconds are used as a convenient and concise performance objective “identifier.” Their usage is not intended to imply the acceptability, or otherwise, of this level of performance.
bThe 1-min intervals mentioned in the table and in the notes are derived by removing unavailable time and severely errored seconds from the total time and then consecutively grouping the remaining seconds into blocks of 60. The basic 1-s intervals are derived from a fixed time pattern.
cThe time interval T(L), over which the percentages are to be assessed, has not been specified since the period may depend on the application. A period of the order of any one month is suggested as a reference.
dFor practical reasons, at 64 kbps, a minute containing four errors (equivalent to an error ratio of 1.04 × 10−6) is not considered degraded. However, this does not imply relaxation of the error ratio objective of 1 × 10−6.
Source: CCITT Rec. G.821 (Ref. 12).
1. Time that the connection is available; and
2. Time that the connection is unavailable.
The following BERs and intervals are used in CCITT Rec. G.821 in the statement of objectives (Ref. 12):
•A BER of less than 1 × 10−6 for T(0) c 1 min;
•A BER of less than 1 × 10−3 for T(0) c 1 s; and
•Zero errors for T(0) c 1 s.
Table 6.6 provides CCITT error performance objectives.
6.12.3.4 Jitter Jitter was discussed in Section 6.9.2, where we stated that it was a major digital transmission impairment. We also stated that jitter magnitude is a function of the number of regenerative repeaters there are in tandem. Guidelines on jitter objectives may be found in Ref. 15.
6.12.3.5 Slips
6.12.3.5.1 From a Bellcore Perspective. Slips, as a major digital network impairment, are explained in Section 6.12.2.1. When stratum-3 slip conditions are troublefree, the nominal clock slip rate is 0. If there is trouble with the primary reference, a maximum of one slip on any trunk will result from a switched reference or any other rearrangement. If there is a loss of all references, the maximum slip rate is 255 slips the first day for any trunk. This occurs when the stratum-3 clocks drift a maximum of 0.37 parts per million from their reference frequency (Ref. 13).
6.12.3.5.2 From a CCITT Perspective. With plesiochronous operation, the number of slips on international links will be governed by the sizes of buffer stores and the
150 DIGITAL NETWORKS
Table 6.7 Controlled Slip Performance on a 64-kbps International
Connection Bearer Channel
Performance Category |
Mean Slip Rate |
Proportion of Timea |
(a)b |
≤ 5 slips in 24 h |
>98.9% |
(b) |
> 5 slips in 24 h |
<1.0% |
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and |
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≤ 30 slips in 1 h |
<0.1% |
(c) |
>30 slips in 1 h |
aTotal time ≥1 year.
bThe nominal slip performance due to plesiochronous operation alone is not expected to exceed 1 slip in 5.8 days.
Source: CCITT Rec. G.822 (Ref. 14).
accuracies and stabilities of the interconnecting national clocks.22 The end-to-end slip performance should satisfy the service requirements for telephone and nontelephone services on a 64-kbps digital connection in an ISDN.
The slip rate objectives for an international end-to-end connection are specified with reference to the standard hypothetical reference connection (HRX)-which is 27,500 km in length.
The theoretical slip rate is one slip in 70 days per plesiochronous interexchange link assuming clocks with specified accuracies (see Section 6.12.2.2) and provided that the performance of the transmission and switching requirements remain within their design limits.
In the case where the international connection includes all of the 13 nodes identified in the HRX and those nodes are all operating together in a plesiochronous mode, the nominal slip performance of a connection could be 1 in 70/ 12 days (12 links in tandem) or 1 in 5.8 days. In practice, however, some nodes in such a connection would be part of the same synchronized network. Therefore, a better nominal slip performance can be expected (e.g., where the national networks at each end are synchronized). The nominal slip performance of the connection would be 1 in 70/ 4 or 1 in 17.5 days. Note that these calculations assume a maximum of four international links.
The performance objectives for the rate of octet slips on an international connection of 27,500 km in length of a corresponding bearer channel are given in Table 6.7 CCITT (Ref. 14) adds that further study is required to confirm that these values are compatible with other objectives such as error performance given in Section 6.12.3.3.
REVIEW EXERCISES
1. What is the major overriding advantage of binary digital transmission? Give at least two secondary advantages.
2. Name the three steps to develop a PCM signal from an analog signal.
3. Based on the Nyquist sampling theorem, what is the sampling rate for a nominal 4-kHz voice channel? for a 56-kHz radar product signal?
4. If I’m transmitting 8000 frames per second, what is the duration of one frame?
22CCITT is looking at the problem from an international switching center gateway. It will connect via digital trunks to many national networks, each with their own primary reference source (PRS).
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REVIEW EXERCISES |
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5. |
Give a simple definition of quantization distortion. |
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6. |
Our system uses linear quantization. How can we reduce quantization distortion |
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(noise)? |
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7. |
What is the negative downside to increasing quantization steps? |
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8. |
Companding used in PCM systems follows one of two laws. Identify each law—its |
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name and where is it applied. |
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9. |
How many bits are there in a PCM word? How many different binary possibilities |
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can be derived from a PCM word or codeword? |
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10. |
If a PCM code received starts with a 0, what do we know about the derived voltage |
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sample? |
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11. |
Bits 2, 3, and 4 of the PCM code word identify the segment. How many total |
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segments are there? |
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12. |
In DS1, of what use is the framing bit? |
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13. |
How is the identification of frame beginning carried out in E1? |
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14. |
How is signaling carried out in DS1? in E1? |
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15. |
How do we arrive at 1.544 Mbps for DS1? |
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16. |
With the AMI line code, how is the zero coded? |
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17. |
If, with bipolar transmission, under “normal” circumstances, the |
first 1 |
is a |
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1.0 V, what would the second “1” be? |
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18. |
Where does a wire-pair cable or light-wave regenerative repeater derive its timing |
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from? |
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19. |
What is the purpose of stuffing on a higher-order PCM multiplex? |
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20. |
In the North American digital hierarchy, just from its nomenclature, what does an |
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M34 multiplex do? |
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21. |
On a digital link, how would excessive loss affect signal quality? |
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22. |
Define jitter from the perspective of a PCM link. |
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23. |
The principal cause of systematic phase jitter is a function of |
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24. |
What value BER can we expect as a mean for the North American digital net- |
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work? |
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25. |
What is the threshold BER for the digital network? Why is it set at that point? |
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Give at least three “economic” advantages of digital switching. |
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27. |
Define the function of a time-slot interchanger. |
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28. |
If we have a switch that is only a single time-slot interchanger for an E1 system, |
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how many different subscribers could I be connected to? |
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29. |
What are the three functional blocks of a conventional time-slot interchanger (i.e., |
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a time switch)? |
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30. |
How can the capacity of a time-slot interchanger be increased? Give two methods. |
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152 DIGITAL NETWORKS
31. How can a switch manufacturer sell just one switch to cover both E1 and DS1 regimes?
32. What is the function of a junctor in a digital switch?
33. What are the two primary factors that can change a national digital network structure?
34. What causes a slip on a digital network?
35. What is the difference between controlled slips and uncontrolled slips?
36. Argue the efficacy of using an external timing source to synchronize network elements.
37. What is a disciplined oscillator?
38. What is holdover stability?
39. According to CCITT, if we can maintain long-term frequency departure to better than 1 × 10 11, what kind of slip performance can we expect?
40. Define a burst errored second.
41. What kind of slip rate could we expect if all network timing references were lost?
REFERENCES
1. IEEE Standard Dictionary of Electrical and Electronic Terms, 6th ed., IEEE Std. 100-1996, IEEE, New York, 1996.
2. Reference Data for Radio Engineering, 5th ed., ITT, Howard W. Sams, Indianapolis, Indiana, 1968.
3. J. Bellamy, Digital Telephony, Wiley, New York, 1991.
4. D. R. Smith, Digital Transmission Systems, 2nd ed., Van Nostrand Reinhold, New York, 1993.
5. Transmission Systems for Communications, 5th ed., Bell Telephone Laboratories, Holmdel, NJ, 1982.
6. Second Order Digital Multiplex Equipment Operating at 8448 kbps Using Positive/ Zero/
Negative Justification, CCITT Rec. G.745, Fascicle II.4, IXth Plenary Assembly, Melbourne, 1988.
7. Functional Criteria for Digital Loop Carrier Systems, Bellcore Technical Reference TR- NWT-000057, Issue 2, Bellcore, Piscataway, NJ, 1993.
8. J. C. McDonald, ed., Fundamentals of Digital Switching, 2nd ed., Plenum Press, New York, 1990.
9. Digital Network Synchronization Plan, Bellcore Generic Requirements, GR-435-CORE, Bellcore, Piscataway, NJ, 1994.
10. Timing Requirements at the Outputs of Reference Clocks and Network Nodes Suitable for Plesiochronous Operation of International Digital Links, CCITT Rec. G.811, Fascicle III.5, IXth Plenary Assembly, Melbourne, 1988.
11. Transport Systems Generic Requirements (TSGR): Common Requirements, Bellcore GR-499- CORE, Issue 1, Bellcore, Piscataway, NJ, Dec. 1995.
12. Error Performance of an International Digital Connection Forming Part of an ISDN, CCITT Rec. G.821, Fascicle III.5, IXth Plenary Assembly, Melbourne, 1988.
REFERENCES 153
13. BOC Notes on the LEC Networks—1994, Special Report SR-TSV-002275, Issue 2, Bellcore, Piscataway, NJ, 1994.
14. Controlled Slip Rate Objectives on an International Digital Connection, CCITT Rec. G.822, Fascicle III.5, IXth Plenary Assembly, Melbourne, 1988.
15. R. L. Freeman, Telecommunication Transmission Handbook, 4th ed., Wiley, New York, 1988. 16. Physical/ Electrical Characteristics of Hierarchical Digital Interfaces, CCITT Rec. G.703,
ITU Geneva, 1991.
17. J. C. McDonald, ed., Fundamentals of Digital Switching, 2nd ed., Plenum Press, New York, 1990.
Fundamentals of Telecommunications. Roger L. Freeman
Copyright 1999 Roger L. Freeman
Published by John Wiley & Sons, Inc.
ISBNs: 0-471-29699-6 (Hardback); 0-471-22416-2 (Electronic)
7
SIGNALING
7.1 WHAT IS THE PURPOSE OF SIGNALING?
The IEEE (Ref. 1) defines signaling as the “exchange of information specifically concerned with the establishment and control of connections and the transfer of user-to-user and management information in a telecommunication network.”
Conventional signaling has evolved with the telephone network. Many of the techniques we deal with in this chapter are applicable to a telecommunication network which is principally involved with telephone calls. With telephony, signaling is broken down in three functional areas:
1. Supervisory;
2. Address; and
3. Call progress audible-visual.
Another signaling breakdown is:
•Subscriber signaling;
•Interswitch (interregister) signaling.
7.2 DEFINING THE FUNCTIONAL AREAS
7.2.1 Supervisory Signaling
Supervisory signaling provides information on line or circuit condition. It informs a switch whether a circuit (internal to the switch) or a trunk (external to the switch) is busy or idle; when a called party is off-hook or on-hook, and when a calling party is on-hook or off-hook.
Supervisory information (status) must be maintained end-to-end on a telephone call, whether voice, data, or facsimile is being transported. It is necessary to know when a calling subscriber lifts her/ his telephone off-hook, thereby requesting service. It is equally important that we know when the called subscriber answers (i.e., lifts the telephone off-hook) because that is when we may start metering the call to establish charges. It is also important to know when the calling and called subscribers return their telephones to the on-hook condition. That is when charges stop, and the intervening trunks comprising the talk path as well as the switching points are then rendered idle for use
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156 SIGNALING
by another pair of subscribers. During the period of occupancy of a speech path end- to-end, we must know that this particular path is busy (i.e., it is occupied) so no other call attempt can seize it.
7.2.2 Address Signaling
Address signaling directs and routes a telephone call to the called subscriber. It originates as dialed digits or activated push-buttons from a calling subscriber. The local switch accepts these digits and, by using the information contained in the digits, directs the call to the called subscriber. If more than one switch is involved in the call setup, signaling is required between switches (both address and supervisory). Address signaling between switches is called interregister signaling.
7.2.3 Call Progress—Audible-Visual
This type of signaling we categorize in the forward direction and in the backward direction. In the forward direction there is alerting. This provides some sort of audible-visual means of informing the called subscriber that there is a telephone call waiting. This is often done by ringing a telephone’s bell. A buzzer, chime, or light may also be used for alerting.
The remainder of the techniques we will discuss are used in the backward direction. Among these are audible tones or voice announcements that will inform the calling subscriber of the following:
1. Ringback. This tells the calling subscriber that the distant telephone is ringing. 2. Busyback. This tells the calling subscriber that the called line is busy.
3. ATB—All Trunks Busy. There is congestion on the routing. Sometimes a recorded voice announcement is used here.
4. Loud warble on telephone instrument—Timeout. This occurs when a telephone instrument has been left off-hook unintentionally.
7.3 SIGNALING TECHNIQUES
7.3.1 Conveying Signaling Information
Signaling information can be conveyed by a number of means from a subscriber to the serving switch and between (among) switches. Signaling information can be transmitted by means such as:
•Duration of pulses (pulse duration bears a specific meaning);
•Combination of pulses;
•Frequency of signal;
•Combination of frequencies;
•Presence or absence of a signal;
•Binary code; and
•Direction and/ or level of transmitted current (for dc systems).
7.3 SIGNALING TECHNIQUES |
157 |
7.3.2 Evolution of Signaling
Signaling and switching are inextricably tied together. Switching automated the network. But without signaling, switching systems could not function. Thus it would be better said that switching with signaling automated the network.
Conventional subscriber line signaling has not changed much over the years, with the exception of the push-button tones, which replaced the dial for address signaling. ISDN, being a full digital service to the subscriber, uses a unique digital signaling system called DSS-1 (Digital Subscriber Signaling No. 1).
In the 1930s and 1940s interregister and line signaling evolved into many types of signaling systems, which made international automatic working a virtual nightmare.1 Nearly every international circuit required special signaling interfaces. The same was true, to a lesser extent, on the national level.
In this section we will cover several of the more common signaling techniques used on the analog network which operated with frequency division multiplex equipment (Section 4.5.2). Although these signaling systems are obsolete in light of the digital network, the concepts covered here will help in understanding how signaling works.
7.3.2.1 Supervisory/ Line Signaling
7.3.2.1.1 Introduction. Line signaling on wire trunks was based essentially on the presence or absence of dc current. Such dc signals are incompatible with FDM equipment where the voice channel does not extend to 0 Hz. Remember, the analog voice channel occupies the band from 300 Hz to 3400 Hz. So the presence or absence of a dc current was converted to an ac tone for one of the states and no-tone for the other state. There were two ways to approach the problem. One was called in-band signaling and the other was called out-of-band signaling.2
7.3.2.1.2 In-Band Signaling. In-band signaling refers to signaling systems using an audio tone, or tones, inside the conventional voice channel to convey signaling information. There are two such systems we will discuss here: (1) one-frequency (SF or single frequency), and (2) two-frequency (2VF). These signaling systems used one or two tones in the 2000 Hz to 3000 Hz portion of the band, where less speech energy is concentrated.
Single-frequency (SF) signaling is used exclusively for supervision, often with its adjunct called E&M signaling, which we cover in Section 7.3.2.1.4. It is used with FDM equipment, and most commonly the tone frequency was 2600 Hz. Of course this would be in four-wire operation. Thus we would have a 2600-Hz tone in either/ both directions. The direction of the tone is important, especially when working with its E&M signaling adjunct. A diagram showing the application of SF signaling on a four-wire trunk is shown in Figure 7.1.
Two-frequency (2VF) signaling can be used for both supervision (line signaling) and address signaling. Its application is with FDM equipment. Of course when discussing such types of line signaling (supervision), we know that the term idle refers to the onhook condition, while busy refers to the off-hook condition. Thus, for such types of line signaling that are governed by audio tones of which SF and 2VF are typical, we have the conditions of “tone on when idle” and “tone on when busy.” The discussion
1Line signaling is the supervisory signaling used among switches. 2Called out-band by CCITT and in nations outside of North America.
158 SIGNALING
Figure 7.1 Functional block diagram of an SF signaling circuit. Note: Wire pairs “receive” and “transmit” derive from the FDM multiplex equipment. Note also the E-lead and M-lead.
holds equally well for in-band and out-of-band signaling methods. However, for in-band signaling, supervision is by necessity tone-on idle; otherwise subscribers would have an annoying 2600-Hz tone on throughout the call.
A major problem with in-band signaling is the possibility of “talk-down,” which refers to the premature activation or deactivation of supervisory equipment by an inadvertent sequence of voice tones through the normal use of the channel. Such tones could simulate the SF tone, forcing a channel dropout (i.e., the supervisory equipment would return the channel to the idle state). Chances of simulating a 2VF tone set are much less likely. To avoid the possibility of talk-down on SF circuits, a time-delay circuit or slot filters to by-pass signaling tones may be used. Such filters do offer some degradation to speech unless they are switched out during conversation. They must be switched out if the circuit is going to be used for data transmission (Ref. 2).
It becomes apparent why some administrations and telephone companies have turned to the use of 2VF supervision, or out-of-band signaling, for that matter. For example, a typical 2VF line signaling arrangement is the CCITT No. 5 code, where f 1 (one of the two VF frequencies) is 2400 Hz and f 2 is 2600 Hz. 2VF signaling is also used widely for address signaling (see Section 7.3.2.2 of this chapter; Ref. 3).
7.3.2.1.3 Out-of-Band Signaling. With out-of-band signaling, supervisory information is transmitted out of band (i.e., above 3400 Hz). In all cases 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.” The advantage of out-of-band signaling is that either system, tone on or tone off, may be used when idle. Talk-down cannot occur because all supervisory information is passed out of band, away from the speech-infor- mation portion of the channel.
The preferred CCITT out-of-band frequency is 3825 Hz, whereas 3700 Hz is commonly used in the United States. It also must be kept in mind that out-of-band signaling
7.3 SIGNALING TECHNIQUES |
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Figure 7.2 SF signaling (a) in-band and (b) out-of-band.
is used exclusively on carrier systems, not on wire trunks. On the wire side, inside an exchange, its application is E&M signaling. In other words, out-of-band signaling is one method of extending E&M signaling over a carrier system.
In the short run, out-of-band signaling is attractive in terms of both economy and design. One drawback is that when channel patching is required, signaling leads have to be patched as well. In the long run, the signaling equipment required may indeed make out-of-band signaling even more costly because of the extra supervisory signaling equipment and signaling lead extensions required at each end, and at each time that the carrier (FDM) equipment demodulates to voice. The major advantage of out-of-band signaling is that continuous supervision is provided, whether tone on or tone off, during the entire telephone conversation. In-band SF signaling and out-of-band signaling are illustrated in Figure 7.2. An example of out-of-band signaling is the regional signaling system R-2, prevalent in Europe and nations under European hegeonomy (see Table 7.1.)