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Signaling in Telecommunication Networks. John G. van Bosse Copyright  1998 John Wiley & Sons, Inc. ISBNs: 0-471-57377-9 (Hardback); 0-471-22415-4 (Electronic) 6 SIGNALING SYSTEM NO. 6 This chapter describes signaling system No.6, the first-generation commonchannel signaling system. There are two versions of this system, both of which were first deployed in the mid-1970s. CCITT No.6 [l, 23 is still in use in the international network, on a number of transatlantic and transpacific trunk interofice signaling (CCIS), defined by the Bell groups. Common-channel System, has been used in the U.S. toll network [3,4], but has now been replaced by the North American version of signaling system No.7. There are many similarities between the CCITT No.6 and CCIS. In this chapter, we use the acronym SS6 when discussing matters that are common to both versions. was The definition of SS6 took place at the time that telecommunications still synonymous with telephony, and long-distance trunks were carried on analog voiceband transmission channels of FDM multiplexed transmission systems. In SS6, a signaling data link usually consists of a pair of these analog voiceband channels. However, SS6 signaling data links can also be implemented as pairs of digital (PCM) transmission channels. SS6 signaling can be used for FDM (analog) and PCM (digital) trunks, which equipment (CME-see Section may be equipped with circuit-multiplication 1.6.4). However, the signaling data links have to be carried on transmission channels without CME. SS6 was designed originally for call control applications only. Around 1980, the Bell System expanded CCIS to include query-response transactions between exchanges and centralized network databases. 126 SIGNALING Signaling 1 PA 1 Point A Signaling MA ST, MB 09 B (a) w (a) Figure 6.1 .l PB ' @I Voiceband Analog Channels I Signaling 6.1 Point B ST, 09 127 LINKS 6.1-1 SIGNALING Signaling SS6 Link signaling link. (a): analog signal. (b): digital bit stream. LINKS Link Components Figure 6.1-1 shows a SS6 signaling link in which the data link consists of a pair of analog voiceband (300-3400 Hz) transmission channels [5]. At signaling points A and B, modems M, and M, provide the interface between the signaling data link and the signaling terminals ST, and ST,. For transmission from A to B, modem MA converts the digital bit stream (b) from ST, into an analog signal (a) that is suitable for transport on the voiceband channel, and modem M, converts signal (a) back into a bit stream (b). A similar conversion sequence takes place in the other direction. SS6 originally used V.26.bis modems, defined by CCITT [6]. The relation between analog signal (a) and bit stream (b), at respectively the input and output of a modem, is shown inFig. 6.1-2. Signal (a) is an 1800 Hz sine wave that changes its phase at intervals T, of l/1200 s. The power spectrum of this signal is essentially contained within a 600-3000 Hz frequency band. There are four phase changes (with respect to the previous interval TB), each of which represents a specific combination of two consecutive data bits in the bit stream: Phase Change (degrees) + + + + 45 135 225 315 Bits 00 01 11 10 The bit transfer rate of the V.26.bis modem is therefore 2400 bits/s. In the U.S., the V.26-bis modems were replaced later on by modems that operate at 4800 bits/s. Signaling links with 2400 bits/s modems can carry Signaling Link Capadty. call-control messages for up to 3000 trunks. Under normal conditions, the signaling links carry messages for 1500 trunks. The 3000 trunk load occurs only in failure conditions, when the link carries its normal signaling traffic, and the traffic 128 SIGNALING SYSTEM NO. 6 0a I I I I -t I 1 I CCITT-43605 I Figure 6.1-2 Relation between analog signal and digital bit stream. (a): analog signal (shown as modem input). (b): bit stream (shown as modem output). (From Rec. V.26.bis. Courtesy of ITU-T.) of a failed companion link. Operation at this load increases the queuing delays (the time spent in output buffer OB) of the messages. On signaling links with 4800 bits/s modems, the capacities are 3000 trunks and 6000 trunks, respectively. 6.1.2 Signal Units and Blocks Working signaling links transmit a continuous stream of adjacent signal units (SU) in each direction. Each SU has a 20-bit information (INF) field, followed by an eight-bit check bit (CB) field for error detection (Fig. 6.1-3). Most SUs start with a heading field (H) and a signal information field (SI). Usually, H identifies a group of SU types, and SI defines a SU type within this group. Some SU types are defined completely by their H field, and do not have a SI field. The lengths of H fields are different in CCITT No.6 and CCIS. We distinguish “message” signal units (MSU) that carry information between the processors P, and P, of Fig. 6.1-1, and “link” signal units that convey information originated by signal terminal ST, and intended for ST,, and vice versa (52.2). Twelve consecutive signal units form a block. The last SU in a block is an “acknowledgment” signal unit (ACU)-see Fig. 6.1-4. The ACUs are “link” SUs that contain acknowledgments of the signal units in a received block. SUs Signal units SU1 through SUll are either MSUs or “synchronization” (SYU). A SYU is a “link” SU that is sent by a signaling terminal when no MSU CB Bits -----+ 1 ---------------------------e-D-H 20 21 ------v--m- a 28 SI Figure 6.1-3 Signal unit. (From Rec. 0257. Courtesy of ITU-T.) SIGNALING 4 Bits1121 CB *4 _-------------------- ---- LINKS 129 * -- 120 211---------- 128 lSU su su ACU su 1 su 2 su 3 su 4 su 5 su 6 su 7 su 8 su 9 su 10 su 11 ACU 12 Block su su Figure is waiting to be transmitted. synchronized (6.1.4). 6.1.3 6.1-4 Block structure of signal units. SYUs are also sent when a signaling link has to be Error Control SS6 error control consists of error detection, acknowledgments, and retransmission of MSUs received with errors, and takes place in the signaling terminals at both ends of the links [7]. Error Detection. Cyclical redundancy checking (see Section 5.2.3) is used. The check bits for each SU are calculated by mod 2 division of the number in INF by the divisor = 100010111. The eight bits representing the result (remainder), are inverted, and placed in the CB field of the SU. Signal units received error-free are accepted by the signaling terminal, and positively acknowledged. Signal units received with errors are discarded, and negatively acknowledged. Acknowledgments. The 11 SUs in a block sent by terminal ST, are acknowledged by the acknowledgment unit (ACU) in a block sent by STB, and vice versa. 130 SIGNALING SYSTEM NO. 6 Bits ll----- c Figure 6.1-5 14------7------~14~15-17~18-20~21 H = 011 Acknowledgment ___-_--_-_-. BA BC 281 CB signal unit (ACU). (From Rec. Q.259. Courtesy of ITU-T.) The layout of the ACU is shown in Fig. 6.1-5. The ACU heading is 011. Bits 4-14 indicate positive- or negative acknowledgments (0 or 1) of SU,-SU,, in a previously received block. A signaling terminal identifies each outgoing block by a block completed number BC (bits 18-20). This number is incremented cyclically, from 0 through 7, for consecutive transmitted blocks. When terminal STB has received a block with say BC = 3 from terminal ST,, it acknowledges the SUs in this block in an ACU in which it sets the block acknowledged number (bits 15-17) to BA = 3. Retransmission. Each ST retains all sent messages in its retransmission buffer until they have been positively acknowledged. Most SS6 messages are one-unit messages (fitting in one message SU). However, there are also multi-unit messages that are transferred in a number of consecutive message SUs. When a terminal receives a negative acknowledgment of a message SU that has been sent, it retransmits the entire message of which the negatively acknowledged SU is a part. In SS6, a MSU transmission error results in out-of-sequence message delivery (5.2.5). Out-of-sequence delivery of call-control messages pertaining to the same call causes call-processing problems. 6.1.4 Link Synchronization SS6 signaling terminals have to besynchronized (or aligned) with their incoming bit streams, so that they can determine the start points of SUs and blocks. Synchronization units are used to acquire synchronization when the link is turned on, and re-synchronization after a disturbance on the link [8]. The format of a SYU in CCITT No.6 is shown in Fig. 6.1-6. The combination H = 11101, SI = 1101 identifies the signal unit as a SYU. Bits 6-16 are coded 1100011. Bits 17. 20 contain a number (N) that indicates the position of the SYU within a block. SYUs in CCIS have a slightly different format. When a signaling link is turned on, both terminals start by sending blocks with Bits -----w (l____ 516----9llO------16(17-H= SI= 1100011 11101 1101 Figure 6.1-6 ITU-T.) CCITT No. 6 synchronization ____ 20121 N ______ -- ___. 281 CB signal unit (SYU). (From Rec. Cb.259. Courtesy of MESSAGES, LABELS, AND ROUTING 131 11 SYUs and one ACU. The “receive” part of a ST has a counter that steps up one unit with each bit in the incoming bit stream, and recycles when it has counted 12 x 28 = 336 bits (the number of bits in a block). When a terminal is aligned, the counter value should be 1 on receipt of the first bit of the blocks. In the initial part of the synchronization procedure, the signaling terminal (ST) looks for the SYU bit pattern 1110141014100011. When it recognizes this pattern, it knows that it has received bits 1 through 16 of a SYU. Moreover, from the value ofN, it knows the place of the SYU in the block, and thus can initialize the counter. In the second step of the procedure, both terminals inform each other about their progress by sending acknowledgments of the received SYUs. When the procedure completes successfully, the signaling link is put in service. On a working signaling link, the STs keep verifying their alignment by looking for the fixed pattern 011 (the ACU heading) during steps 309-311 of the counter. 6.2 6.2.1 MESSAGES, Message LABELS, AND ROUTING Structure As shown in Fig. 6.24, there are two SS6 message sizes. A one-unit message occupies a single signal unit, called a lone signal unit (LSU). Multi-unit messages (MUM) require several consecutive SUs, and consist of an initial signal unit (ISU) followed by one or more subsequent signal units (SSU). A call-control message pertains to a particular trunk, which is identified by Label 11 -----__---_LSU H 1m _____-SI 16117-----w BN 20121 -----------. TN 281 CB 0a Label 11--------w--ISU H ssu, Im- ______ SI BN 16117 _____ -20121 TN __---------. CB CB .\> ssu, 281 , .\q CB Figure 6.24 SS6 call-control messages. (a): one-unit message. (b): multi-unit message. Note: In CCITT No. 6, m = IO; in CCIS, m = 8. (From Rec. Q.257. Courtesy of ITU-T.) 132 SIGNALING SYSTEM NO. 6 TG (BN = 102) Figure Region 1 6.2-2 Band Region number assignments for trunk 2 group (TG). the label in the message. The label consists of two parts: a band number (BN) identifies a “band” of trunks, and a trunk number (TN) identifies a trunk within a band. The band and trunk numbers in CCITT No.6 occupy seven bits and four bits respectively. A CCITT No.6 label can thus identify up to 128 bands with up to 16 trunks in each band, for a theoretical label capacity of 2048 trunks. CCIS has nine-bit band numbers, and can identify up to 512 bands of up to 16 trunks (theoretical label capacity: 8192 trunks). The BN in a message is also used by signal transfer-points (STP) to route the message to its destination (the exchange at the distant end of the trunk). A BN thus cannot be shared by trunks in different trunk groups. As a consequence, a group of for example 20 trunks requires two band numbers, but uses only 20 of the 32 possible (BN,TN) combinations. This reduces the number of trunks that can be identified on a signaling link in actual networks. Since the number of available BNs is limited, they have to be “reused” in large signaling networks, such as the CCIS network in the U.S. Therefore, a particular BN value usually identifies different bands of trunks in different parts of the signaling network, and a particular band of trunks is usually identified by different band numbers on the various links in its signaling route. When a callcontrol message arrives at a signaling point, the affected band of trunks is determined from two data items: the BN in the message, and the identity of the signaling link on which the message came in. Figure 6.2-2 shows an example for a trunk group TG with at most 16 trunks (one band). On the “A” signaling between EX, and the STPs of region 1, the group is identified by BN = 34. On the “B” signaling links between the STPs in regions 1 and 2, the BN = 102. On the “A” signaling links between the STPs of region 2 and EX, the BN = 65. This is because these BNs were available at the time that the trunk group was installed. 6.2.2 Band Number Translation and Outgoing Link Selection We now explore the BN translations and outgoing link selections at the signaling MESSAGES, LABELS, AND ROUTING 133 points for messages from Ex, to Ex,, which relate to a specific trunk T of group TG, in Fig. 6.2-2 [4]. The trunk has TN = 5 (this number does not change when the messages traverse a STP). We assume that all signaling links are operational, and that the messages for TG are to be load-shared evenly by the four normal signaling routes for the group: R,: R,: R,: R,: AI-B,-Af A1 -B2-A4 AZ-B3-A3 AZ-B4-A4 All messages relating to a specific trunk have to use the same route. Otherwise, two consecutive messages for that trunk, say M, (sent first, on route R,) and M, (sent later, on route R2) could arrive out-of-sequence at EX, because, at the time that the messages are sent, the queuing delays at the signaling links in R, are large, and small on R,. Out-of-sequence delivery of messages relating to a trunk can cause problems in call processing. The selection of outgoing signaling links that accomplishes load sharing of signaling links, and also associates a signaling route with a particular trunk, can be done in several ways, for example: At exchanges, the primary outgoing A link for messages with odd or even TN go to respectively the odd- and even-numbered STPs in the region of the exchange. If the primary link fails, the messages are diverted to the other (alternative) A link. In this example, the messages for trunk T normally go out on A,, and reach STP,,. Every STP has a table for each attached signaling link, with entries for all incoming band numbers. Each entry contains the outgoing band number (BN,), and the identity of a primary and alternative outgoing link. For messages received by an STP on its A links, the value of BN, indicates the destination region for the message. In addition, when BN, is odd, the primary and alternative outgoing B links go to respectively the odd- and even-numbered STP of that region. If BN, is even, the primary and alternative B links go to respectively the even- and odd-numbered STP. In this example, the entry at STP, 1for messages received on link Al with BN = 34 thus indicates: Outgoing band number: BNo = 102, Primary outgoing SL: B, (to STP,,), Alternative outgoing SL: B, (to STP,,). For messages received by an STP on its B links, the BN identifies an exchange in the region of the STP. The primary outgoing link is the A link to the exchange, and the alternative link is the C link to the other STP in the region. At STP,,, the table entry for messages received on B, with BN = 102 thus indicates: 134 SIGNALING SYSTEM NO. 6 Outgoing band number: BN, = 65, Primary outgoing SL: A4, Alternative outgoing SL: C,. The normal route for messages from Ex, to Ex, for trunk T is thus A,-B,-&. At destination exchange Ex,, the label (BN = 65, TN = 5) indicates that the message concerns trunk T. Applying the same rules to messages relating to trunk T, and sent by exchange Ex,, the signaling route is AJ-B3-AZ. Messages relating to a trunk, and sent in opposite directions thus may traverse different signaling routes. When a primary signaling link fails, messages normally sent out on that link are diverted to the alternative link. Consider again the messages sent by Ex,, relating to trunk T. On failure of link A,, Ex, diverts the messages to link A,. On failure of link B,, STP,, diverts the messages to link B,, and on failure of link A4, STP,, diverts the messages to link C2 (and STP,, sends the messages on A& 6.3 CCITT NO.6 CALL CONTROL The CCITT No.6 signaling system is used for international transatlantic and transpacific trunk groups. The call-control features of CCITT No.6 are comparable to those of CCITT No.5 and R2 international signaling (Chapter 4). All signaling is link-by-link. The most important call-control messages [9] are described in Sections 6.3.1 through 6.3.4. In CCITT No.6 documents, these messages are usually denoted by three-character acronyms. The coding of the heading and signal information fields of the messages are listed in Table 6.3-l. Signaling procedures are outlined in Sections 6.3.5 and 6.3.6. 6.3.1 Initial Address Message address message (IAM) is the first forward message in a call. It indicates the seizure of a trunk, and contains the initial digits (in overlap address signaling), or all digits (in en-bloc address signaling), of the called number, and parameters that affect the routing and processing of the call. The IAM layout is shown in Fig. 6.3-l. Label L in the initial signal unit (ISU) identifies the trunk for which the message is intended. The subsequent signal units (SSU) have a heading code H = 00, and a length indicator (LI) that represents the number of SSUs beyond SSU,. SSU, contains a number of international routing indicators, which we have already encountered in No.5 and international R2 signaling. The initial Country Code indicator (c). This indicates whether the called number is an international number (C = 1; country code included), or a national number(C = 0; no country code included). CCITT NO. 6 CALL CONTROL 135 Bits - 1516 I1 H ISU 121 I10 SI CNE L ssu, H LI - ssu, H LI D, ssu, H LI D, D6 ssu, H LI D, DIO D2 281 CB CPC - CB D3 D4 CB D7 D8 CB Dll Dl2 CB Figure 6.3-l CCITT No. 6 Initial address message (IAM). Note: CCIT No. 6 one-unit messages have the format of ISU. (From Rec. Q.258. Courtesy of ITU-T.) Table 6.34 Coding of heading and signal information messages and signals. fields in CClTT No.6 call-control Acronym Name IIeading ADC AD1 AFC AFN ANC ANN BLA BLO CFL CGC CLB CLF COF COT FOT IAM LOS RAN RLG SAM SEC SSB UBA UBL UNN Address complete, charge Address incomplete Address complete, subscriber free, charge Address complete, subscriber free, no charge Answer, charge Answer, no charge Blocking acknowledgment Blocking Call failure Circuit group congestion Clear-back Clear-forward Confusion Continuity Forward-transfer Initial address message Line out of service Reanswer (after CLB) Release-guard Subsequent address message Switching equipment congestion Subscriber busy Unblocking acknowledgment Unblocking Unallocated national number 11011 11011 11011 11011 11000 11000 11010 11010 11001 11001 11000 11010 11001 11010 11010 10000 11011 11000 11000 10001 11001 11011 11010 11010 11011 Source: Signal Information 1010 1101 0001 0010 0010 0011 1101 1011 1000 0100 0100 0010 1110 0001 0011 0000 0110 0101 0001 0000 0011 0100 1110 1100 0101 Rec. Q.257. Courtesy of ITU-T. Nature of Circuit hdicator (N). This indicates whether the connection built up so far includes, or does not include, a satellite trunk (N = 1, or N = 0). This indicator is used by incoming exchanges. When a call is received with N = 1, the exchange avoids routing the call on another satellite trunk.