Biblio5

the distance between nodes, repeater spacing, and the delays in the repeaters and in switches. Data rate (in bps) and repeater delays depend on the type of medium used to transmit messages.4 The velocity of propagation of the medium is a most important parameter. Table 13.2 provides information of delays for three types of transmission media and for various call distances.

Although propagation delay in most circumstances is the greatest contributor to overall delay, processing delays must also be considered. These are a function of the storage requirements and processing times in SPs, STPs, number of SPs/ STPs, signaling link loading and message length mix. (Ref. 11). Table 13.3 provides data on maximum overall signaling delays.

13.10 NUMBERING PLAN FOR INTERNATIONAL SIGNALING POINT CODES

The number plan described in ITU-T Rec. Q.708 (Ref. 12) has no direct relationship with telephone, data, or ISDN numbering. A 14-bit binary code is used for identification

Table 13.3 Maximum Overall Signaling Delays

a The values given in the table are mean values.

Source: ITU-T Rec. Q.709, Table 5/ Q.709, p. 5 (Ref. 11).

4Remember that the velocity of propagation is a function of the type of transmission medium involved.

404CCITT SIGNALING SYSTEM NO. 7

•Setup of a signaling connection;

•Establishment of the optimum size of NPDUs (network protocol data units);

•Mapping network address onto signaling relations;

•Selecting operational functions during data-transfer phase (e.g., layer service selection);

•Providing means to distinguish network connections; and

•Transporting user data (within the request).

13.11.4.2 Data-Transfer Phase. The data-transfer phase functions provide the means of a two-way simultaneous transport of messages between two end-points of a signaling connection. The principal data transport phase functions are listed as follows. These are used or not used in accordance with the result of the selection function performed in the connection-establishment phase.

•Segmenting/ reassembling;

•Flow control;

•Connection identification;

•NSDU delimiting (M-bit);

•Expedited data;

•Missequence detection;

•Reset;

•Receipt confirmation; and

•Others.

13.11.4.3 Connection Release Functions. Release functions disconnect the signaling connection regardless of the current phase of the connection. The release may be performed by an upper-layer stimulus or by maintenance of the SCCP itself. The release can start at each end of the connection (symmetric procedure). Of course, the principal function of this phase is disconnection.

13.11.5 Structure of the SCCP

The basic structure of the SCCP is illustrated in Figure 13.9. It consists of four functional blocks as follows:

1. SCCP Connection-Oriented Control. This controls the establishment and release of signaling connections for data transfer on signaling connections.

2. SCCP Connectionless Control. This provides the connectionless transfer of data units.

3. SCCP Management. This functional block provides the capability, in addition to the signal route management and lower control functions of the MTP, to handle the congestion or failure of either the SCCP user or signaling route to the SCCP user.

4. SCCP Routing. On receipt of the message from the MTP or from the functions listed previously, SCCP routing either forwards the message to the MTP for transfer or passes the message to the functions listed. A message whose called party address is a local user is passed to functions 1, 2, or 3, whereas one destined for a remote user is forwarded to the MTP for transfer to the distant SCCP. (Ref. 13)

Figure 13.9 General SCCP overview block diagram. Note the listing under user, left side; we find a listing of SCCP primitives. For a discussion of primitives consult Ref. 18. (From ITU-T Rec. Q.714, Figure 1/ Q.714, p. 307, [Ref. 15].)

13.12 USER PARTS

13.12.1 Introduction

SS No. 7 user parts, along with the routing label, carry out the basic signaling functions. Turn again to Figure 13.5. There are two fields in the figure we will now discuss: the SIO (service information octet) and the SIF (signaling information field). In the paragraphs that follow we briefly cover one of the user parts, the TUP (telephone user part). As shown in Figure 13.10, the user part, OSI layer 4, is contained in the signaling information field to the left of the routing label. ITU-T Rec. Q.723 (Ref. 14) deals with the sequence of three sectors (fields and subfields of the standard basic message signal unit shown in Figure 13.5).

Turning now to Figure 13.10, we have from right to left, the SIO, the routing label, and the user information subfields (after the routing label in the SIF). The SIO is an octet in length made up of two subfields: the service indicator (4 bits) and the subservice field (4 bits). The service indicator, being 4 bits long, has 16 bit combinations with the following meanings (read from right to left):

Figure 13.11 Initial address message format. (From CCITT Rec. Q.723, Figure 3/ Q.723, p. 23 [Ref. 14].)

tion code (CIC) indicates the one circuit (speech circuit in the TUP case) among those directly interconnecting the destination and originating points.

For the OPC and DPC, unambiguous identification of signaling points is carried out by means of an allocated code. Separate code plans are used for the international and national networks. The CIC, as shown in Figure 13.10, is applicable only to the TUP. CCITT Rec. Q.704 shows a signaling link selection (SLS) field following (to the left) the OPC. The SLS is 4 bits long and is used for load sharing. The ISDN user part address structure is capable of handling E.164 addresses in the calling and called number and is also capable of redirecting address information elements.

13.12.2 Telephone User Part (TUP)

The core of the signaling information is carried in the SIF (see Figure 13.10). The TUP label was described briefly in Section 13.7.2.1. Several signal message formats and codes are described in the following paragraphs. These follow the TUP label.

One typical message of the TUP is the initial address message (IAM). Its format is shown in Figure 13.11. A brief description is given of each subfield, providing further insight of how SS No. 7 operates.

Common to all signaling messages are the subfields H0 and H1. These are the heading codes, each consisting of 4 bits, giving 16 code possibilities in pure binary coding. H0 identifies the specific message group to follow. “Message group” means the type of message. Some samples of message groups are:

H1 contains a signal code or identifies the format of more complex messages. For

408 CCITT SIGNALING SYSTEM NO. 7

instance, there are four types of address message identified by H0 c 0001, and H1 identifies the type of message, such as:

Moving from right to left in Figure 13.11, after H1 we have the calling party subfield consisting of 6 bits. It identifies the language of the operator (Spanish, English, Russian, etc.). For example, an English-speaking operator is coded 000010. It also differentiates the calling subscriber from one with priority, a data call, or a test call. A data call is coded 001100 and a test call is coded 001101. Fifty of the 64 possible code groups are spare.

Continuing to the left in Figure 13.11, 2 bits are spare for international allocation. Then there is the message indicator, where the first 2 bits, B and A, give the nature of the address. This is information given in the forward direction indicating whether the associated address or line identity is an international, national (significant), or subscriber number. A subscriber number is coded 00, an international number is coded 11, and a national (significant) number is coded 10.

Bits D and C are the circuit indicator. The code 00 in this location indicates that there is no satellite circuit in the connection. Remember that the number of space satellite relays in a speech telephone connection is limited to one relay link through a satellite because of propagation delay.

Bits F and E are significant for common-channel signaling systems such as CCIS, CCS No. 6, and SS No. 7. The associated voice channel operates on a separate circuit. Does this selected circuit for the call have continuity? The bit sequence FE is coded:

10Continuity check performed on previous circuit

11Spare

Bit G gives echo suppressor information. When coded 0 it indicates that the outgoing half-echo suppressor is not included, and when coded 1 it indicates that the outgoing half-echo suppressor is included. Bit I is the redirected call indicator. Bit J is the alldigital path required indicator. Bit K tells whether any path may be used or whether only SS No. 7-controlled paths may be used. Bit L is spare.

The next subfield has 4 bits and gives the number of address signals contained in the initial address message. The last subfield contains address signals where each digit is coded by a 4-bit group as follows:

The most significant address signal is sent first. Subsequent address signals are sent in successive 4-bit fields. As shown in Figure 13.11, the subfield contains n octets. A filler code of 0000 is sent to fill out the last octet, if needed. Recall in Chapter 7 that the ST signal is the “end of pulsing” signal and is often used on semiautomatic circuits.

Besides the initial address message, there is the subsequent address message used when all address digits are not contained in the IAM. The subsequent address message is an abbreviated version of the IAM. There is a third type of address message, the initial address message with additional information. This is an extended IAM providing such additional information as network capability, user facility data, additional routing information, called and calling address, and closed user group (CUG). There is also the forward setup message, which is sent after the address messages and contains further information for call setup.

CCITT SS No. 7 is rich with backward information messages. In this group are backward setup request; successful backward setup information message group, which includes charging information; unsuccessful backward setup information message group, which contains information on unsuccessful call setup; call supervision message group; circuit supervision message group; and the node-to-node message group (CCITT Recs. Q.722 and Q.723 (Refs. 14, 16).

Label capacity for the telephone user part is given in CCITT Rec. Q.725 (Ref. 17) as 16,384 signaling points and up to 4096 speech circuits for each signaling point.

REVIEW EXERCISES

1. What is the principal rationale for developing and implementing Signaling System No. 7?

2. Describe SS No. 7 and its relationship with OSI. Why does it truncate at OSI layer 4?

3. Give the two primary “parts” of SS No. 7. Briefly describe each part.

4. OSI layer 4 is subdivided into two sublayers. What are they?

5. Layer 2 carries out the functions of the signaling link. Name four of the five functions of layer 2.

6. What are the two basic categories of functions of layer 3?

7. What is a signaling relation?

8. How are signaling points defined (identified)?

410 CCITT SIGNALING SYSTEM NO. 7

9. What does a routing label do?

10. What are the two methods of error correction in SS No. 7?

11. What are the three types of signal units used in SS No. 7?

12. Discuss forward and backward sequence numbers.

13. The routing label is analogous to what in our present telephone system? Name the three basic pieces of information that the routing label provides.

14. Define labeling potential.

15. What is the function of an STP? Differentiate STP with signaling point.

16. Why do we wish to limit the number of STPs in a specific relation?

17. What are the three measures of performance of SS No. 7?

18. Regarding user parts, what is the function of the SIO? Of the network indicator?

19. What is the purpose of circuit continuity?

20. With the TUP, address signals are sent digit-by-digit embedded in the last subfield of the SIF. How are they represented?

21. What are the two overall set of services of the SCCP?

22. What is the purpose of the SCCP?

REFERENCES

1. Specifications of Signaling System No. 7 (Q.700 Series), Fascicle VI.6, CCITT Yellow Books, VIIth Plenary Assembly, Geneva, 1980.

2. W. C. Roehr, Jr., “Signaling System No. 7,” in Tutorial: Integrated Services Digital Network (ISDN), W. Stallings, ed., IEEE Computer Society, Washington, DC, 1985.

3. Introduction to CCITT Signaling System No. 7, ITU-Rec. Q.700, ITU, Helsinki, 1993.

4. Functional Description of the Message Transfer Part (MTP) of Signaling System No. 7, ITU- T Rec. Q.701, ITU, Helsinki, 1993.

5. R. L. Freeman, Reference Manual for Telecommunication Engineering, 2nd ed., Wiley, New York, 1994.

6. BOC Notes on the LEC Networks—1994, Bellcore Special Report SR-TSV-002275, Issue 2, Piscataway, NJ, April 1994.

7. Signaling System No. 7—Signaling Link, ITU-T Rec. Q.703, ITU, Helsinki, 1993.

8. Signaling System No. 7—Signaling Network Functions and Messages, ITU-T Rec. Q.704, ITU, Geneva, 1996.

9. Signaling System No. 7—Signaling Network Structure, ITU-T Rec. Q.705, ITU, Geneva, 1993.

10. Signaling System No. 7—Message Transfer Part Signaling Performance, ITU-T Rec. Q.706, ITU, Geneva, 1993.

11. Signaling System No. 7—Hypothetical Signaling Reference Connection, ITU-T Rec. Q.709, Helsinki, 1993.

12. Signaling System No. 7—Numbering of International Signaling Point Codes, ITU-T Rec. Q.708, Helsinki, 1993.