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Ccs7

  1. 1. General Telecom Common Channel Signalling System #7 Handout 770 00438 0590 VHBE Ed. 07
  2. 2. Status Released Change Note Short Title CCS #7 All rights reserved. Passing on and copying of this document, use and communication of its contents not permitted without written authorization from Alcatel. 2 / 218 770 00438 0590 VHBE Ed. 07
  3. 3. Contents Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Situation of CCS #7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.1 1.2 Classification of Signalling Systems. . . . . . . . . . . . . . Brief History of NNI - Signalling Systems. . . . . . . . . 1.2.1 Analogue Signalling. . . . . . . . . . . . . . . . . . 1.2.2 CAS Signalling. . . . . . . . . . . . . . . . . . . . . . . 1.2.3 CCS #7 Signalling. . . . . . . . . . . . . . . . . . . Objectives and Fields of Application of CCS #7. . . 13 15 15 15 18 19 General Principles and Definitions . . . . . . . . . . . . . . . . . . . 23 2.1 2.2 Signalling Network Components . . . . . . . . . . . . . . . . Signalling Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 28 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.1 Architecture of CCS #7 . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 MTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 User Parts . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 SCCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CCS #7 and the OSI Model . . . . . . . . . . . . . . . . . . . . Selection of most relevant Items. . . . . . . . . . . . . . . . . 31 33 34 34 35 36 Message Transfer Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.1 MTP-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Digital Signalling Data Links . . . . . . . . . . 4.1.2 Analogue Signalling Data Links . . . . . . . . MTP-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 MTP-2 Functionality . . . . . . . . . . . . . . . . . 4.2.2 Signal Units . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 MTP-2 Procedures . . . . . . . . . . . . . . . . . . MTP-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Signalling Message Handling functions . 4.3.3 Signalling Network Management Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 39 40 42 42 43 47 68 68 68 Signalling Connection Control Part . . . . . . . . . . . . . . . . . . 103 5.1 5.2 1 11 103 104 106 106 106 1.3 2 3 3.2 3.3 4 4.2 4.3 5 770 00438 0590 VHBE Ed. 07 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Coding Principles of SCCP Messages . . . . . 5.2.1 Routing Label . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Message Type Code . . . . . . . . . . . . . . . . . . 5.2.3 Message Information . . . . . . . . . . . . . . . . . 77 3 / 218
  4. 4. Contents 5.3 Global Title Translation. . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Addressing information . . . . . . . . . . . . . . . 5.3.2 Protocol class . . . . . . . . . . . . . . . . . . . . . . . Provision of Connection-Oriented Services. . . . . . . 5.4.1 Phases in a Connection-Oriented Setup. 5.4.2 Interface to an OSI Transport Layer Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3 SCCP Message Layout for Connection-Oriented Message Delivery 109 112 114 114 114 ISDN User Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 6.1 6.2 126 127 128 129 133 134 137 137 147 149 153 154 155 157 161 5.4 6 6.3 6.4 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Coding Principles of ISUP messages . . . . . . 6.2.1 The Routing label . . . . . . . . . . . . . . . . . . . . 6.2.2 Circuit Identification Code (CIC) . . . . . . . 6.2.3 Message Type Code . . . . . . . . . . . . . . . . . . 6.2.4 Message Information . . . . . . . . . . . . . . . . . ISUP Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Call Setup Messages . . . . . . . . . . . . . . . . . 6.3.2 Call Release Messages . . . . . . . . . . . . . . . 6.3.3 Suspend and Resume Messages . . . . . . . 6.3.4 Continuity Messages . . . . . . . . . . . . . . . . . 6.3.5 Confusion Message . . . . . . . . . . . . . . . . . . 6.3.6 Facility Messages . . . . . . . . . . . . . . . . . . . . 6.3.7 Access Related Messages . . . . . . . . . . . . . ISUP Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Parameters, mainly used in Call Setup Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Parameters, mainly used in a Call Release Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3 Parameters, mainly used in Suspend / Resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.4 Parameters, mainly used in Continuity Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.5 Parameters, mainly used in Facility Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.6 Parameters, mainly used in Access Related Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 118 161 185 186 187 188 190 Transaction Capabilities Application Part . . . . . . . . . . . . . 195 7.1 7.2 195 199 200 201 201 7.3 4 / 218 Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 The Component Sub-layer . . . . . . . . . . . 7.2.2 The Transaction Sub-layer . . . . . . . . . . . . Identification of an Operation . . . . . . . . . . . . . . . . . . . 770 00438 0590 VHBE Ed. 07
  5. 5. Contents 7.4 201 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Appendix A References . . . . . . . . . . . . . . . . . . . . . . . . . 215 Request for Your Comments . . . . . . . . . . . . . . . . . . . . . . . . . 770 00438 0590 VHBE Ed. 07 Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 5 / 218
  6. 6. Contents Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 Figure 25 Figure 26 Figure 27 Figure 28 Figure 29 Figure 30 Figure 31 Figure 32 Figure 33 Figure 34 Figure 35 Figure 36 Figure 37 Figure 38 Figure 39 Figure 40 Figure 41 Figure 42 6 / 218 Transmission and Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 CCS #7 is an NNI Signalling System. . . . . . . . . . . . . . . . . . . . 14 CCS #7 performs both Line- and Register Signalling. . . . . 15 PCM Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 CAS Multi-frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Explicit speech channel identification in CCS #7. . . . . . . . . . 19 CCS in an exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 CCS #7 Network Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Signalling Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Hierarchy of a Signalling Link Set . . . . . . . . . . . . . . . . . . . . . . 27 Signalling Links, LinkSets, Routes and Route Sets. . . . . . . . . . 27 Associated Mode and Quasi-Associated Mode . . . . . . . . . . 29 Functional Diagram of CCS #7 . . . . . . . . . . . . . . . . . . . . . . . . 32 Concise CCS #7 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Comparison of CCS #7 and the OSI model. . . . . . . . . . . . . 36 MTP Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Signalling Data Links in a Digital Environment. . . . . . . . . . . . 40 Signalling Data Links in an analogue environment. . . . . . . . 41 Signal Unit Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Bit Stuffing and Flag Insertion . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Check Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Pull-Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Positive Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Negative Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 MTP-2 : Sender Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 MTP-2 : Receiver Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Example of Basic Error Recovery Method (1) . . . . . . . . . . . . . 57 Example of Basic Error Recovery Method (2) . . . . . . . . . . . . . 58 Example of Basic Error Recovery Method (3) . . . . . . . . . . . . . 59 Example of Basic Error Recovery Method (4) . . . . . . . . . . . . . 60 Example of the Preventive Cyclic Retransmission Method . . . 63 Leaky Bucket Mechanism for the SUERM. . . . . . . . . . . . . . . . . 66 Message Discrimination, Message Distribution, Message Routing and the Signalling Message flow . . . . . . . . . . . . . . . . . . . . . . . 69 Outlook of the Service Information Octet (SIO) . . . . . . . . . . . 70 Outlook of the Routing Label . . . . . . . . . . . . . . . . . . . . . . . . . . 70 MTP Routing Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Structure of an international Point Code . . . . . . . . . . . . . . . . . 72 National and International CCS #7 planes . . . . . . . . . . . . . . 73 National and International CCS #7 planes . . . . . . . . . . . . . . 74 Use of SLS for loadsharing purposes. . . . . . . . . . . . . . . . . . . . 76 Traffic Management, Link Management and Route Management functions, and their possible interactions. . . . . . . . . . . . . . . . . 78 Handshake Procedure for Changeover . . . . . . . . . . . . . . . . . . 79 770 00438 0590 VHBE Ed. 07
  7. 7. Contents Figure 43 Figure 44 Figure 45 Figure 46 Figure 47 Figure 48 Figure 49 Figure 50 Figure 51 Figure 52 Figure 53 Figure 54 Figure 55 Figure 56 Figure 57 Figure 58 Figure 59 Figure 60 Figure 61 Figure 62 Figure 63 Figure 64 Figure 65 Figure 66 Figure 67 Figure 68 Figure 69 Figure 70 Figure 71 Figure 72 Figure 73 Figure 74 Figure 75 Figure 76 Figure 77 Figure 78 Figure 79 Figure 80 Figure 81 Figure 82 Figure 83 Figure 84 Figure 85 Figure 86 Figure 87 Figure 88 770 00438 0590 VHBE Ed. 07 Changeover Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . COO and COA Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Handshake procedure for Changeback . . . . . . . . . . . . . . . . . Changeback Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CBD and CBA format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Forced Rerouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Controlled Rerouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MTP Restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TRA message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Management Inhibit messages . . . . . . . . . . . . . . . . . . . . . . . . . User Part Unavailable message . . . . . . . . . . . . . . . . . . . . . . . . Use of UPU message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DLC message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CSS, CNS and CNP messages . . . . . . . . . . . . . . . . . . . . . . . . . TFP TFA and TFR messages. . . . . . . . . . . . . . . . . . . . . . . . . . . . , TFP : Example 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TFP : Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transfer Prohibited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RST and RSR message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TFC message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RCT message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SCCP General Message Layout . . . . . . . . . . . . . . . . . . . . . . . . SCCP Structure of the message content . . . . . . . . . . . . . . . . . Routing, involving Global Title Translation. . . . . . . . . . . . . . . Example of Routing with a Global Title . . . . . . . . . . . . . . . . . . Outlook of the Unitdata message . . . . . . . . . . . . . . . . . . . . . . 3 phases of a Connection-Oriented message delivery . . . . Connection Oriented SCCP : Setup Phase. . . . . . . . . . . . . . . Connection Oriented SCCP : Data Transfer Phase . . . . . . . . Connection Oriented SCCP : Release Phase. . . . . . . . . . . . . . SCCP Primitives and messages . . . . . . . . . . . . . . . . . . . . . . . . . CR message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CC message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CREF message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DT1 message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DT2 message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RLSD message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RLC message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ISUP Message Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Example of CIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Circuit Identification Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meaning of the CIC for Multirate connections . . . . . . . . . . . . Meaning of the CIC for n * 64 Kbit/sec connections . . . . . . ISUP Structure of the message content . . . . . . . . . . . . . . . . . . ISUP Scenario for Connection Setup (En bloc) . . . . . . . . . . . . Overlap Sending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 81 83 83 84 85 86 87 87 89 91 92 94 94 95 96 96 96 99 100 101 105 108 109 112 113 115 116 117 117 118 120 121 121 122 123 124 124 128 129 130 131 133 136 144 146 7 / 218
  8. 8. Contents Figure 89 Figure 90 Figure 91 Figure 92 Figure 93 Figure 94 Figure 95 Figure 96 Figure 97 Figure 98 Figure 99 Figure 100 Figure 101 Figure 102 Figure 103 Figure 104 Figure 105 Figure 106 Figure 107 Figure 108 Figure 109 Figure 110 Figure 111 Figure 112 Figure 113 Figure 114 Figure 115 Figure 116 Figure 117 Figure 118 Figure 119 Figure 120 Figure 121 Figure 122 Figure 123 Figure 124 Figure 125 8 / 218 Quasi-Associated Mode Signalling . . . . . . . . . . . . . . . . . . . . ISUP Scenario for Call Release . . . . . . . . . . . . . . . . . . . . . . . . . Call Suspension for an analogue user . . . . . . . . . . . . . . . . . . Call Suspension for an ISDN user . . . . . . . . . . . . . . . . . . . . . . Call Suspend and Resume for an ISDN user . . . . . . . . . . . . . Continuity Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Successful UUS3 request and use . . . . . . . . . . . . . . . . . . . . . . Backward Call Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Called Party Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calling Party Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calling Party's Category . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CUG interlock code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Event Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Forward Call Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nature of Connection Indicators . . . . . . . . . . . . . . . . . . . . . . . Optional Backward Call Indicators . . . . . . . . . . . . . . . . . . . . . Call Rerouting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Call Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optional Backward Call Indicators . . . . . . . . . . . . . . . . . . . . . Call Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Redirection Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subsequent number. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transmission Medium Requirement . . . . . . . . . . . . . . . . . . . . . User-to-user Indicators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automatic Congestion Level . . . . . . . . . . . . . . . . . . . . . . . . . . . Suspend / Resume Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuity Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Facility Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . User-to-user Indicators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Circuit Group Supervision Message Type . . . . . . . . . . . . . . . . Range and Status parameter. . . . . . . . . . . . . . . . . . . . . . . . . . . Complete ISUP MSU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Example of TCAP communication . . . . . . . . . . . . . . . . . . . . . . TCAP Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TCAP Addressing Information . . . . . . . . . . . . . . . . . . . . . . . . . . Communication between TCAP Users . . . . . . . . . . . . . . . . . . . Example of a Transaction scenario . . . . . . . . . . . . . . . . . . . . . 147 148 150 151 152 154 156 164 167 170 172 173 174 175 176 177 177 178 179 180 180 182 183 183 185 186 187 188 189 190 191 193 196 197 198 199 203 770 00438 0590 VHBE Ed. 07
  9. 9. Contents Tables Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14 Table 15 Table 16 Table 17 Table 18 Table 19 Table 20 Table 21 Table 22 770 00438 0590 VHBE Ed. 07 Comparison of CCS #7 and CAS Signalling Systems. . . . . . 22 Status Indications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Recommended Parameters for T and D. . . . . . . . . . . . . . . . . . 66 Zone codings for the international Point Codes. . . . . . . . . . . 72 NI Codings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Service Indicator Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Subsystem Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 SCCP : Message Type Codings . . . . . . . . . . . . . . . . . . . . . . . . . 119 ISUP Message Types, functions and coding for a Call Setup. 138 ISUP Message Types, functions and coding for a Call Release. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 ISUP Message Types, functions and coding for a call suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 ISUP Message Types, functions and coding for continuity procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 ISUP Message Types, functions and coding in case of confusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 ISUP Message Types, functions and coding for call facilities. 155 ISUP Message Types, functions and coding for (un)blocking procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 ISUP Message Types, functions and coding for reset procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 ISUP Parameters names and codes, mainly used in Call Setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 ISUP Parameters names and codes, mainly used in Call Release. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 ISUP Parameters names and codes, mainly used in Call Suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 ISUP Parameters names and codes, mainly used in Continuity Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 ISUP Parameters names and codes, mainly used in Facility Messages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 ISUP Parameters names and codes, mainly used in Access Related Messages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 9 / 218
  10. 10. Contents 10 / 218 770 00438 0590 VHBE Ed. 07
  11. 11. Preface Preface Obviously, signalling has always played a very important part in the field of telecommunications, since it provides the means of information interchange between two or more nodes in the network. The way of performing signalling functions has evolved together with the evolution of the transmission equipment and of the used switching systems. Both of them were originally analogue, evolved into digital and will perhaps in the future be optical. The traditional analogue signalling methods are not a subject of this document, though a very brief overview will be given first, to be able to better situate Common Channel Signalling System #7 (CCS #7). CCS #7 is an advanced, digital signalling and control system, standardized by ITU-T in the 1980's. Most of the recommendations, upon which this document is based, were finalized on 03/93. Nowadays, CCS #7 is gaining a lot of importance. However, this document does not describe the complete CCS #7, since this would be too elaborate. It does describe the following parts of the CCS #7 environment : " Introduction (ITU-T Q.700) " Message Transfer Part (MTP) (ITU-T Q.701 - Q.704) " Signalling Connection Control Part (SCCP) (ITU-T Q.711 - Q.714) 770 00438 0590 VHBE Ed. 07 11 / 218
  12. 12. Preface " ISDN User Part (ISUP) (ITU-T Q.761 - Q.764) " Transaction Control Application Part (TCAP) (ITU-T Q.771 - Q.775) 12 / 218 770 00438 0590 VHBE Ed. 07
  13. 13. 1 Situation of CCS #7 1 Situation of CCS #7 In order to fully understand the use of CCS #7, a short overview of signalling principles will be given in this chapter. 1.1 Classification of Signalling Systems. Topological Classification In a telecommunication network, two core functions can be found: " The information has to be transported in a cost-efficient way from a source to a destination over a physical line = Transmission. " In a telephone exchange, an inlet has to be through-connected to the correct outlet = Switching. This is shown in figure 1. Exchange Exchange UNI UNI NNI Transmission Transmission Ç Ç Switching Figure 1 770 00438 0590 VHBE Ed. 07 User Ç User Transmission Switching Transmission and Switching 13 / 218
  14. 14. 1 Situation of CCS #7 In order to be able to perform the switching function, a communication will be required between the calling subscriber and his own switching unit. This is the User-to-Network Interface (UNI). A communication will also be required between each switching unit and the next one in the call sequence. This is the Network-to-Network Interface (NNI). Thus, topologically, 2 large families of Signalling Systems can immediately be introduced : " UNI Signalling Systems : D Analogue Subscriber Signalling System (ASSS). D Digital Subscriber Signalling System #1 (DSS1). Commonly called "ISDN-Signalling" or "D-channel protocol". It is described in ITU-T Q.921 for the Layer 2 functionality and in ITU-T Q.931 for the Layer 3 functionality. D Digital Subscriber Signalling System #2 (DSS2). The adaptation of DSS1 for Broadband purposes (ATM-switching). The Layer 3 functionality is described in ITU-T Q.2931. " NNI Signalling Systems : D Channel Associated Signalling System (CAS). D Common Channel Signalling System #. 7 (CCS #7). CCS #7 is an NNI Signalling System. Figure 2 Functional Classification CCS #7 is an NNI Signalling System. Another method of classifying Signalling Systems, is according to their functionality. At least, 2 types of information will always have to be "signalled" between adjacent points : " " 14 / 218 The intention to seize or to release a local line (in the case of UNI Signalling) or a trunk circuit (in the case of NNI Signalling). = Line Signalling. The call destination (under the form of the dialled digits) will have to be passed from the "register" of the previous step to the "register" of the next exchange. = Register Signalling. 770 00438 0590 VHBE Ed. 07
  15. 15. 1 Situation of CCS #7 CCS #7 performs both Line- and Register Signalling. Figure 3 1.2 1.2.1 CCS #7 performs both Line- and Register Signalling. Brief History of NNI - Signalling Systems. Analogue Signalling. What ? Analogue Signalling is the oldest kind of signalling. The line signalling relies often on a Direct Current (DC) to transport the information. In the adjacent exchange, a current detector will be used to determine this DC, i.e. if a connection is being requested. Further , the register signalling, i.e. the transport of the dialled digits can rely on different types of mechanisms. Common kinds are R1 or R2 signalling. They use Multi-frequency (MFC) tones to represent the digits. The peer exchange hears these tones and translates them back into the dialled digits. Limitations 1.2.2 Obviously, Analogue Signalling has a lot of limitations. The main disadvantage is that only a minimum number of states can be represented by voltages and currents. Thus, Analogue Signalling offers only the basic capabilities required for telephony, like seizure of circuits, a disconnect, etc. When more supplementary services and flexibility can be offered by a network, it goes without saying that, in parallel, also signalling becomes more complicated. Then, Analogue Signalling becomes insufficient. CAS Signalling. What ? 770 00438 0590 VHBE Ed. 07 A more recent kind of signalling is CAS Signalling. CAS is generally transported over a digital 2 Mbit/sec PCM-link. A Pulse Code Modulation (PCM) -link transports sequences of frames. Each frame contains 32 bytes, each belonging to 1 out of 32 channels. In general, channel 0 will be used for synchronization purposes (thus it will not transport any user information), while channel 16 will be reserved for the signalling. The frame structure, transported over a PCM link is shown in figure 4. 15 / 218
  16. 16. 1 Situation of CCS #7 0 15 16 17 1 Synchronization 31 0 Signalling Information 1 frame = 125 µsec Bitrate of 1 channel = Bitrate of the link Figure 4 = 8 bits = 64 Kbit/sec 125 µsec 32 * 64 Kbit/sec = 2,048 Mbit/sec PCM Structure Using CAS, the line signalling information is transported in channel 16, in the following way : " Channel 16 of frame 0 contains multi-frame synchronization. " Channel 16 of frame 1 contains 4 bits information related with channel 1 and 4 bits related with channel 17. " Channel 16 of frame 2 contains 4 bits information related with channel 2 and 4 bits related with channel 18. " Channel 16 of frame 15 contains 4 bits information related with channel 15 and 4 bits related with channel 31. This sequence of 16 frames, shown in figure 5, is repeated continuously and is called a Multi-frame. 16 / 218 770 00438 0590 VHBE Ed. 07
  17. 17. 1 Situation of CCS #7 0 1 15 Frame 0 16 17 31 Multiframe Synchronisation Frame 1 Ch 1 info Ch 17 info Frame 2 Ch 2 info Ch 18 info Ch 3 info Frame 3 Ch 19 info MULTI FRAME ... Ch 15 info Frame15 Figure 5 Ch 31 info CAS Multi-frame Examples of the 4 bit codes used are : " 1001 : IDLE " 0001 : SEIZURE " 1101 : ACK Note that only the line signalling is carried inside channel 16, by means of the CAS protocol. Register signalling, on the other hand, is carried in a digitalized format, in the user channel itself. Limitations A little calculation can show that signalling by means of CAS will be rather inefficient. " The period of a multiframe is 16 * 125 µsec = 2 msec. Thus, in a worst case situation, it can take up to 2 msec to transport information in 1 direction and again 2 msec for the other direction. I.e. the line signalling alone, can take up to 4 msec. The reason for this is that the sender is not free to choose in which frame he transmits his signalling information regarding a specific channel, in other words the time to send is "channel associated". Thus, in a worst case, the sender has to wait even if there is no other signalling information to be send. " 770 00438 0590 VHBE Ed. 07 Transportation of only 1 digit, can take up to 100 msec. 17 / 218
  18. 18. 1 Situation of CCS #7 Clearly, CAS line signalling, completed with register signalling over the user channels, is a too inefficient system for modern telecom requirements. 1.2.3 CCS #7 Signalling. With the introduction of computer control in the exchanges, a much more efficient signalling system came under consideration : Common Channel Signalling System #7. First, remember that both Analogue Signalling, as well as the register signalling of CAS are both in-band, meaning that the same channel will be used for the control data (signalling) as for the user data (e.g. voice). Out-Band On the other hand, one of the qualities of CCS #7 is the separation of the control information from the user information into logically separate (but not necessarily physically separate) paths. Thus CCS #7 Signalling is completely out-band. User data will always be conveyed over a separate channel then the control information. This separation offers several benefits, as there are : " " Hardware (HW) intended for user paths can be kept cheaper and faster. " Independent optimization of user paths and control paths can be done. " Message-Oriented Potential for integration of signalling for multimedia. Voice circuits remain idle, until the distant party answers the call. On the contrary, using in-band signalling systems, voice circuits remain busy, even if the distant party never answers the call until the calling party hangs up. Thus the utilization of voice circuits will be higher, using CCS #7. By consequence, the availability of voice circuits will be higher and the need for additional circuits decreases. Secondly, CCS #7 is message-oriented, i.e. the signalling function comes down to a transport of signalling information from the computer controlling one exchange to the computer controlling the next exchange, under the form of a message, having a clearly defined contents and functional definition. Thus, any call handling event (line signalling) or item of signalling information (register signalling) is converted by appropriate software into an information message. This information message is transmitted over a dedicated signalling channel to the 18 / 218 770 00438 0590 VHBE Ed. 07
  19. 19. 1 Situation of CCS #7 destination exchange. The processor in the destination exchange will receive the information message and execute the appropriate call handling action. Clearly, this approach offers a lot more possibilities and makes CCS #7 much more powerful than its predecessors. Common Channel Finally, the signalling channel used between the two exchanges is a common transfer path for signalling info between these exchanges. It can carry the signalling info for a multiplicity of trunk connections between these exchanges. This has lead to the name of "Common Channel Signalling". Typically, one signalling channel will carry the signalling information for about 1000 voice channels. On the other hand, common channel signalling requires additional measures, such as " Ordered Delivery of the signalling packets. " Adressing information, it has to be communicated quite explicitly which voice channel the signalling is about. This is demonstrated in figure 6. " Agreements concerning the kind of communication will have to be signalled, since not all voice channels will be 100% equivalent. n speech channels 1 2 Figure 6 1.3 n 3 1 Signalling channel Explicit speech channel identification in CCS #7. Objectives and Fields of Application of CCS #7. According to Ref.[2.], the overall objective of CCS #7 is to provide an internationally standardized, general purpose, Common Channel Signalling System, 770 00438 0590 VHBE Ed. 07 19 / 218
  20. 20. 1 Situation of CCS #7 " that is optimized for operation in digital telecommunications networks in conjunction with stored program controlled exchanges. CCS #7 is optimized for operation over 64 kbit/sec digital channels (e.g. channel 16 of a PCM link). However, it is also suitable for operation over analogue channels and at lower speeds. Further, it is suitable for point-to-point terrestrial an satellite links. " that can meet present and future requirements of information transfer for inter-processor transactions within telecommunication networks for call control, remote control, and management and maintenance signalling. Thus, CCS #7 is intended to be multi-purpose in multi-service networks. Indeed , modern telecom networks require more and more control facilities, which rely on an efficient information transport system. For example : D D " Taxation information and measurement results are collected in a taxation center. Maintenance of a small local exchange can be carried out remotely from a network service center. The information needed for these applications can again be transported by making use of the transfer functions of CCS #7. that provides a reliable means for transfer of information in correct sequence and without loss or duplication. Reliable transfer should even be ensured in the presence of transmission disturbances or network failures. The arrangements should ensure error detection and correction. Normally, a redundancy of signalling links should be provided, as well as functions for automatic diversion of signalling traffic to alternative paths. The use of a common channel signalling in an exchange requires the use of 3 components. 1. A transmission system capable of transmitting digital information between both exchanges without errors. 2. The information will be assembled from the different users into a "Post Office" which will route the information to the correct destination. After reception of messages the same "Post Office" function will distribute the messages to the correct users. 3. Different programs inside both exchanges (e.g. call handling), will generate and interpret the information messages. 20 / 218 770 00438 0590 VHBE Ed. 07
  21. 21. 1 Situation of CCS #7 An example of a possible configuration is shown in figure 7. Exchange A TC TC Post Office TC TC Exchange B PCM link (31 user channels) PCM link (31 user channels) PCM link (30 user channels) + 1 SIGNALLING CHANNEL PCM link (31 user channels) PCM link (31 user channels) Collection of signalling messages from the Trunk Circuits Figure 7 CCS in an exchange TC TC Post Office TC TC Dispatching of signalling messages to the Trunk Circuits To summarize, a comparison of CCS #7 and CAS systems can be found in table 1. 770 00438 0590 VHBE Ed. 07 21 / 218
  22. 22. 1 Situation of CCS #7 Table 1 Comparison of CCS #7 and CAS Signalling Systems. CCS #7 Special HW and Software (SW) is required for the message transport facility. CAS No special HW or SW is required for the message transport facility. No Multifrequency Analysers are required to Special Multifrequency Analysers are reĆ send and receive MFC tones. quired during the call setup. Only a few signalling channels are required Line signalling requires all channels 16 of between 2 exchanges. all PCM links between the 2 exchanges. Very fast signalling. Slow signalling. (1 message is sent in 2 - 4 msec) (Σ 100 msec / digit) CCS can be used for other types of inĆ formation, e.g. charging, maintenance, .. CAS will only be used for call handling inĆ formation. Failures will have a major impact upon the system. They will influence many circuits. Therefore, protection mechanisms have to be foreseen. Failures will have a limited impact upon the system. They will only affect the related PCM-link. Conclusion CCS #7 is an ideal solution for signalling in a modern computer controlled telecommunication network. It provides an information exchange capability, network wide for all interested users. CCS #7 is standardized by ITU-T in the beginning of the 1980's. It is to be used for signalling in international and national telephone applications and should facilitate the transition to a fully digital telephone network and later on to an ISDN. Since CCS #7 will be able to operate in a pure telephone network as well as in an Integrated Services Digital Network (ISDN), a high degree of flexibility will be required in the system definition. This flexibility is obtained by making use of a modular structure. 22 / 218 770 00438 0590 VHBE Ed. 07
  23. 23. 2 General Principles and Definitions 2 General Principles and Definitions 2.1 Signalling Network Components Signalling Points As previously explained, the CCS #7 network is logically separated from any other network and is used solely for the switching of data messages, pertaining to e.g. call handling for the telephone network. All nodes in the CCS #7 network are called "Signalling Points (SP)". Many kinds of Signalling Points may exist. " Exchanges " Service Control Points (SCP) for the Intelligent Network (IN) " Operation, Administration & Maintenance (OAM) Centers In the scope of this document, two specific kinds of Signalling Points are of interest. " Signalling End Point (SEP) In the scope of CCS #7, this is a local node in a telecommunications network, to which subscriber lines are attached, and which is served by CCS #7. It is a source or a sink of signalling traffic. 770 00438 0590 VHBE Ed. 07 23 / 218
  24. 24. 2 General Principles and Definitions " Signalling Transfer Point (STP) All CCS #7 messages travel from one SEP to another through the services of a STP The STP switches the messages as . received from the various SEP's through the network to their appropriate destinations, i.e. to the destination SEP or , perhaps first to another STP . In an STP there is no switching-oriented processing of the , signalling message, i.e. the contents of the signalling message is not examined. Three levels of STP's exist : National, International and Gateway. A typical CCS #7 Network structure is shown in figure 8. Signalling Points are deployed in pairs for redundancy and diversity. To make sure the CCS #7 network is always operational, alternate and multiple paths are provided. Thus, SEP's are connected to at least 2 STP's. Further, multiple paths exist between different STP's. STP STP SEP SEP SEP STP STP SEP Ç Ç Ç Ç Figure 8 SEP SEP SEP CCS #7 Network Structure Ç Ç Ç Ç SEP Note that a Signalling Point can act as an STP for one signalling message, while it can be an SEP for another message. Thus, this classification is on the level of the individual messages. At first, being an SEP or an STP is no property of an exchange, but how an exchange behaves for a particular signalling message. However, if it is certain that a Signalling Point will always act as an STP the classification can be shifted to the node level. Therefore, , Signalling Points can act in the following ways. " 24 / 218 SEP 770 00438 0590 VHBE Ed. 07
  25. 25. 2 General Principles and Definitions " " Note STP Mixed Functionality SEP / STP An STP will rarely be a stand-alone system built for the sole purpose of STP functionality. More typically, it will be integrated in an exchange. Obviously, a message will always be transmitted from one Signalling Point to another. This gives rise to the following terminology : " Originating Signalling Point : a Signalling Point at which a message is generated. " Destination Signalling Point : a Signalling Point to which a message is destined. Each Signalling Point is identified with a "point code". This point code identifies the Signalling Point in a unique way inside the (national or international) CCS #7 network. In a similar way, we will use the terms "originating point code" and "destination point code" in order to identify the source and destination of a CCS #7 message. Signalling Relations 770 00438 0590 VHBE Ed. 07 Any 2 Signalling Points, for which the possibility of compatible communication exists are said to have a "Signalling Relation". This is shown in figure 9. Although SP1 and SP 4 do not have any direct physical line, able to carry signalling information, they do have a Signalling Relation with each other, since they can communicate via SP2 or via SP3, which will act as STP's in that case. 25 / 218
  26. 26. 2 General Principles and Definitions Point Code 3 Point Code 2 Point Code 4 Point Code 1 line carrying user information line carrying signalling information signalling relation Figure 9 Signalling Links Signalling Relations CCS #7 uses "Signalling Links" to convey the signalling messages between two Signalling Points. A number of Signalling Links that directly interconnect two Signalling Points constitute a "Signalling Link Set". Typically, though not necessarily, a Signalling Link Set comprises all Signalling Links between 2 Signalling Points. A possible application of a Signalling Link Set is, for example, that switching equipment will alternate transmission across all the links in a linkset to ensure equal usage of all links. A group of links within a link set that have identical characteristics (e.g. the same bit rate) are called a "Signalling Link Group". All of this is represented in figure 10. 26 / 218 770 00438 0590 VHBE Ed. 07
  27. 27. 2 General Principles and Definitions SG1 SL SL Point Code A SG2 SG1 SL SG3 Point Code B Signalling Link Set SL = Signalling Link SG = Signalling Link Group Figure 10 Hierarchy of a Signalling Link Set Signalling Routes The pre-determined path, consisting of a succession of STP's and the interconnecting Signalling Links, that a message takes through the CCS #7 network between the originating and the destination point is called the "Signalling Route" for that Signalling Relation. All the Signalling Routes that may be used between an originating and a destination point by a message traversing the CCS #7 network is called the "Signalling Route Set" for that Signalling Relation. The complete terminology of Signalling Links, Signalling Link Sets, Signalling Routes and Signalling Route Sets is shown in figure 11. SL Point Code B SL Set SR Point Code A SR Set Point Code D SR Point Code C SL = Signalling Link SL Set =Signalling Link Set SR = Signalling Route SR Set= Signalling Route Set Figure 11 Signalling Links, LinkSets, Routes and Route Sets. 770 00438 0590 VHBE Ed. 07 27 / 218
  28. 28. 2 General Principles and Definitions 2.2 Signalling Modes The term "Signalling Mode" refers to the association existing between the path taken by a signalling message and the Signalling Relation to which the message belongs. Associated Mode In the "Associated Mode", Signalling Points are directly connected by means of signalling links. Information related to a particular Signalling Relation are as a result sent over the signalling link directly connecting the originating point with the destination point In other words, when using the Associated Mode of CCS #7, all signalling messages travelling between two Signalling Points are sent on a direct link interconnecting the two points. A different routing is not allowed in this mode. As a result, STP's will never be used. Non-Associated Mode In the "Non-Associated Mode", two Signalling Points do not have to be directly connected by a signalling link. The signalling information can be sent via multiple STP's, while the user information may follow a direct path to the destination. Since messages can be routed indirectly, multiple paths become available between two Signalling Points. As a result, consecutive messages for the same destination can follow different paths. This can lead to problems caused by mis-sequencing of messages. For example, a message informing the destination about a seizure could arrive at the destination after a message giving extra digits, if that seizure message follows a faster route. Quasi-Associated Mode This problem is solved in the "Quasi-Associated Mode". It is a limited case of the Non-Associated Mode where the path taken by the message through the CCS #7 network is the same for each message, pertaining to the same call. In this way, a correctly sequenced delivery of all the information is guaranteed. On one hand, the sequence of events is essential in telephone signalling. On the other hand, the message transport system of CCS #7 does not include features to avoid out-of-sequence arrival of messages or other problems that would typically arise in a fully Non-Associated Mode. Because of these 2 reasons, CCS #7 is specified for use in the Associated and in the Quasi-Associated Modes only. Again, these 2 modes are depicted in figure 12. 28 / 218 770 00438 0590 VHBE Ed. 07
  29. 29. 2 General Principles and Definitions Associated Mode Quasi-Associated Mode (in-sequence delivery is guaranteed) STP SP STP SP SP SP line carrying user information line carrying signalling information Figure 12 Associated Mode and Quasi-Associated Mode Since the Associated Mode of signalling would require direct links between all Signalling Points which have a Signalling Relation, a mesh network would be required between all Signalling Points. This is not a cost-effective solution since signalling networks only represent very small traffic values. As a result, normal CCS #7 networks will always make use of the Quasi-Associated Mode of signalling in which the use of Signalling Transfer Points will create a star network offering a cost-effective CCS #7 network with alternative routing possibilities. 770 00438 0590 VHBE Ed. 07 29 / 218
  30. 30. 2 General Principles and Definitions 30 / 218 770 00438 0590 VHBE Ed. 07
  31. 31. 3 Architecture 3 Architecture 3.1 Architecture of CCS #7 Telecommunication networks change enormously. The purely telephone network is evolving into a general purpose network capable of transporting all kinds of user information. In order to cope with this changing environment, a very flexible signalling system is required, which can perform signalling functions for all kinds of telecom applications, already existing, or even future applications yet to be defined. In order to obtain this flexibility in CCS #7, a modular, and layered structure is required. The fundamental principle of CCS #7 is the division of functions into a common "Message Transfer Part (MTP)" on one hand, and separate "User Parts" on the other hand. The same common MTP module is extracted and serves as a transport system, even for different User Parts. Note The term "user" in this context refers to any functional entity that utilizes the transport capability provided by MTP This is illustrated in figure 13. 770 00438 0590 VHBE Ed. 07 31 / 218
  32. 32. 3 Architecture Signalling Link Common Transfer Functions Link Control Functions Signalling Data Link Link Control Functions MTP3 User Message Processing MTP2 MTP1 MTP2 User Message Processing MTP3 MTP User Part Figure 13 Common Transfer Functions User Part Functional Diagram of CCS #7 If we represent this functionality in a vertical, layered manner, which is a more widely accepted kind of representation, we get the main structure of CCS #7, which is drawn in figure 14. ISUP TCAP (Q.77x) (Q.76x) SCCP MTP Figure 14 (Q.71x) (Q.70x) Concise CCS #7 Structure The ITU-T Recommendations, describing the different modules of CCS #7 can also be found in figure 14. It can be noted that they all start with Q.7xx, which is consistent with the name CCS #7. Besides that, CCS #7 is the successor of Signalling Systems #4, #5, and #6. 32 / 218 770 00438 0590 VHBE Ed. 07
  33. 33. 3 Architecture 3.1.1 MTP The lowest layer is the "Message Transfer Part (MTP)". The overall function of MTP is to serve as a transport system, providing reliable transfer of signalling messages between the locations of communicating user functions. MTP consists out of 3 sub-layers. " MTP-1 (or Signalling Data Link Functions) It takes care of the physical and electrical layer functions of the information transfer; in other words the pure transmission of bits from Signalling Point to Signalling Point. The functionality of this layer corresponds to OSI-layer 1. " MTP-2 (or Signalling Link Functions) It takes care of error-free transmission of messages over one individual signalling link. This layer performs error detection and error correction actions on these messages. The functionality of this layer corresponds to OSI-layer 2. " MTP-3 (or Signalling Network Functions) falls into two major categories. D First, it takes care of the "Signalling message handling functions". This functionality includes routing, message discrimination and distribution. In every signal point, MTP-3 will analyze the contained addressing information and message discrimination will decide whether the message has to be routed further, or whether the message has to be delivered to the correct higher layer (= message distribution). D Secondly, MTP-3 will also perform "Signalling network management functionality". These control the current message routing and configuration of the signalling network facilities and in the case of signalling network failures, control the reconfigurations and other actions to preserve or restore the normal message transfer capability. The functionality of MTP-3 corresponds partly to OSI-layer 3. Again, to summarize, the complete MTP is capable of sending messages through the network. In addition, error detection and correction, as well as flow control functionality is provided. 770 00438 0590 VHBE Ed. 07 33 / 218
  34. 34. 3 Architecture 3.1.2 User Parts On top of MTP several "User Parts" can run. Each User Part , supports a specific application, and can use its own set of signalling messages and related defined actions for a certain type of user. Note It should be clear that all User Parts make use of MTP which will , deliver the information to the requested destination, in the correct sequence and virtually free of errors. Indeed, this capability is common. The User Parts will create messages informing a similar user elsewhere in the network of their intention. The communication between these users, that is to say, the messages that are generated by each user are standardized in related Q.7XX protocols. If a new application is required, a new user part can independently be defined by ITU-T, containing the messages for that specific application. The most important, actual User Parts are the following. " " "ISDN User Part (ISUP)" : signalling in the ISDN network. " "Broadband-ISDN User Part (B-ISUP)" : signalling in the Broadband ISDN network. " 3.1.3 "Telephone User Part (TUP)": signalling support for normal telephony. "Data User Part (DUP)" : signalling for dedicated data networks. SCCP In the study period 1980 - 1984, it was decided to increase the functionality of the transport system by including a new layer. The "Signalling Connection Control Part (SCCP)" contains some extra functionality in comparison with MTP . For example, it will support a more efficient routing algorithm (connection oriented or connection-less). Further, SCCP will also support extended addressing facilities. Signalling messages can be routed to a Signalling Point based on, for example, dialled digits. SCCP is capable of translating the global title (e.g. dialled digits) into a Signalling Point Code and a sub-system number. This capability is called "global title translation". 34 / 218 770 00438 0590 VHBE Ed. 07
  35. 35. 3 Architecture In addition, SCCP can also be used as a platform to run an OSI transport layer. As a result, all Open System Interface (OSI) services can run on top of the CCS #7 network. A number of extra functions are supported running on top of SCCP The most important one is : . " " "Mobile Application Part (MAP)" The purpose of this protocol is to provide a mechanism by which cellular provider information may be passed from one cellular network to another. " 3.2 "Transaction Capabilities Application Part (TCAP)". This function can be used to connect to, and retrieve information out of network databases. It is also used to support remote control of other entities in a real-time environment. "Operation, Maintenance & Administration Part (OMAP)". It provides communication and control functions throughout the network. It is typically located in a remote maintenance center, where administration of system databases and maintenance access takes place. CCS #7 and the OSI Model The OSI model accepted by the International Telecommunication Union (ITU-T) (Ref [28.]) in 1980 offers a structured approach to the problem of datacommunication. This approach allows standardized procedures to be defined. It uses a 7 layers model. Since CCS #7 is in fact nothing more than a particular kind of datacommunication system ( one used for signalling purposes), a very similar structuring with respect to the OSI model can be found. This similarity is very high for the MTP part, though it becomes more difficult and inexact for the higher layers. The dissimilarity is due in part to the fact that CCS #7 was developed before the OSI model. Figure 15 shows the relationship between CCS #7 and the OSI model. It can be noted that neither the OSI Transport layer nor the OSI Session layer are strictly defined in CCS #7. Further, the functionality of the OSI Network layer is spread over more CCS #7 modules (i.e. MTP-3, SCCP User Parts). , 770 00438 0590 VHBE Ed. 07 35 / 218
  36. 36. 3 Architecture CCS #7 OSI TCAP L7 : Application I S U P SCCP L6 : Presentation L5 : Session L4 : Transport L3 : Network MTP-3 MTP-2 L2 : Data Link MTP-1 L1 : Physical Figure 15 3.3 Comparison of CCS #7 and the OSI model. Selection of most relevant Items. For reasons of conciseness, the complete CCS #7 system can not be discussed in this document. A selection is made to discuss the most relevant items. They are : " see chapter 4. " SCCP : see chapter 5. " ISUP : see chapter 6. " 36 / 218 MTP : TCAP : see chapter 7. 770 00438 0590 VHBE Ed. 07
  37. 37. 4 Message Transfer Part 4 Message Transfer Part The Message Transfer Part is described in quite a lot of detail since it is the only part of CCS #7 which will be used by all users. For this reason, the actions taken here are described in considerable detail. " Note MTP can send information messages to any destination inside the national CCS #7 network where the call originated or to any location in the international CCS #7 network. It is however unable to deliver messages between two exchanges in different countries. " MTP provides error free transportation of signalling information , not only on a link-by-link basis, but across the whole CCS #7 network. " MTP has the ability to react to system and network failures, and can take the necessary actions to . " MTP can provide information exchange in a digital but also in an analogue transmission environment. As described earlier, the MTP of CCS #7 will provide a transport system consisting of three sub-layers, which are shown in figure 16. They will be explained in detail, hereafter. 770 00438 0590 VHBE Ed. 07 37 / 218
  38. 38. 4 Message Transfer Part Layers 3-7 Layer 3 MTP-3 Layer 2 Layer 1 MTP-2 MTP-1 Signalling Link Functions Signalling Data Link Signalling Network Functions Signalling Message Handling TUP Signalling Network Management ISUP Figure 16 4.1 MTP MTP Structure MTP-1 MTP-1 is described completely in Ref.[4.]. It provides the access to a transmission system, capable of transmitting the information bits of a message. CCS #7 will make use of existing bit-transport equipment, provided in the telephone network on the inter-exchange level. They are called Signalling Data Links. A Signalling Data Link is a bidirectional transmission path for signalling, comprising 2 data channels operating together in opposite directions at the same data rate. The channels used to carry CCS #7 information shall be dedicated exclusively for this use. No other information should be carried together with the signalling information in the same channel. Equipment such as echo suppressors, or A / µ law convertors attached to the transmission link must be disabled for the channel carrying the CCS #7 information, in order to assure bit integrity of the transmitted data stream. A Signalling Data Link has the following characteristics : " A mechanical interface : A connector providing physical access to the outside world 38 / 218 770 00438 0590 VHBE Ed. 07
  39. 39. 4 Message Transfer Part " An electrical interface : The set of electrical signals required to send a digital 0 or a digital 1 " A functional interface : A set of extra functions provided on the link, supplementary to data transport itself. A Signalling Data Link can be analogue or digital. It can be a terrestrial or a satellite transmission link. 4.1.1 Digital Signalling Data Links In a digital environment, 64 kbit/sec digital paths will normally be used. In a "Time Division Multiplexing (TDM)" environment, making use of a 2.048 Mbit/sec PCM link, CCS #7 will make use of one of the 32 channels. The standard time slot that will be used, shall be time slot 16. However, when time slot 16 is not available, any other time slot may be used (except channel 0). It goes without saying that the signalling rate will indeed be 64 kbit/sec. In order to improve reliability, minimum two signalling links are to be provided between a pair of exchanges. In other words, if two exchanges are to be interconnected using CCS #7, two of all PCM links will support a signalling channel together with 30 channels for user information, while all the other PCM links systems will support 31 channels for user information. This is represented in figure 17. 770 00438 0590 VHBE Ed. 07 39 / 218
  40. 40. 4 Message Transfer Part Exchange A Exchange B PCM link (31 user channels) TB PCM link (31 user channels) TB CCS #7 Info PCM link (30 user channels 1 signalling channel) TB PCM link (30 user channels 1 signalling channel) TB PCM link (31 user channels) TB PCM link (31 user channels) TB TB Figure 17 TB TB TB TB CCS #7 Info TB TB Digital Trunk Board Signalling Data Links in a Digital Environment. Of course, besides a 2.048 Mbit/sec PCM link, also other kinds of links can be used. For instance, a 8.448 Mbit/sec link. In that case, time slots 67 to 70 will preferably be used to carry the signalling information. 4.1.2 Analogue Signalling Data Links In an analogue telephone environment (if no Time Division Multiplexed connection is provided between the two Signalling Points), data will be transmitted by making use of modems. Since the CCS #7 protocol relies on a full duplex connection, a full duplex modem must be utilized. ITU-T recommends using modems of 4.8 Kbit/sec or higher bit rates, to limit the transmission delay of the information in the network. Specifically, use of modem Recommendation V.27 or V.27bis is proposed. These specifications define the following parameters : " " signalling rate : 1600 baud " 40 / 218 bitrate : 4800 bit/sec modulation type : 8 DPSK 770 00438 0590 VHBE Ed. 07
  41. 41. 4 Message Transfer Part " carrier frequency : 1800 Hz " four wire " full duplex " synchronous transmission The modem signal can not be switched through the network of the exchange and the trunk circuit. It has to be sent to the Frequency Division Multiplexing (FDM) transmission equipment through separate cabling. This configuration is shown in figure 18. Exchange A Exchange B TB TB D/A TB TB CCS #7 Info D/A D/A TB D/A TB D/A D/A TB D/A D/A TB FDM System 4.8 kbit/sec 4.8 kbit/sec Modem Modem TB Digital Trunk Board D/A Figure 18 CCS #7 Info Speech PCM coder/decoder Signalling Data Links in an analogue environment. Obviously, the data speed is inferior to the speed of a digital connection (64 kbit/sec). However, it is also possible to use a data speed of 64 kbit/sec in an analogue environment. Special modems (groupband modems: V.36 recommendation) exist. They convert a signal of 64.000 bit/sec into an FDM group of 60 kHz to 108 kHz. This frequency band can be easily transmitted through an FDM system. As a result, 64 kbit/sec can be supported. 770 00438 0590 VHBE Ed. 07 41 / 218
  42. 42. 4 Message Transfer Part 4.2 4.2.1 MTP-2 MTP-2 Functionality Obviously, MTP-2 is the layer above MTP-1. MTP-2 is described completely in Ref.[5.]. While MTP-1 will transmit the messages from exchange to exchange. MTP-2 will provide reliable transfer of signalling messages between 2 directly connected Signalling Points. Amongst others, it will make sure that the message is free of errors and that no information will be lost during transmission. Both of these are well-known OSI-Layer 2 functions. Signalling messages, delivered by superior hierarchical levels (= the User Parts), are transferred over the signalling link in variable length "Signal Units". In addition to the signalling information, the Signal Units also include transfer control information for proper operation of the signalling link. To be more precise, the MTP-2 functionality is the following : " Signal Unit Delimitation The beginning and the end of a Signal Unit are indicated by a unique 8-bit pattern, called the "flag". Of course, measures are taken to ensure that the pattern of the flag can not be imitated elsewhere in the Signal Unit. This is called "Bit Stuffing" and it will be explained later. " Signal Unit Alignment Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (more than 6 consecutive 1s) is received. Another possibility for loss of alignment is when the maximum length of the Signal Unit is exceeded. In those cases, the Signal Unit received is considered to be in error. Loss of alignment does not immediately cause the signalling link to be taken out of service. This will only be done if an excessive amount of errors has occurred. " Error Detection The error detection function is performed by means of 16 checks bits provided at the end of each Signal Unit. " Error Correction Two forms of error correction are provided. Both methods are based upon retransmissions of errored or lost Signal Units. The 2 methods are : 42 / 218 770 00438 0590 VHBE Ed. 07
  43. 43. 4 Message Transfer Part D the "Basic Error Correction method (BER)" : This method applies for signalling links, using non-intercontinental terrestrial transmission, and for intercontinental links where the one-way propagation is less than 15 msec. D the "Preventive Cyclic Retransmission method (PCR)" : This method applies for signalling links where the propagation delay is greater than 15 msec and for all signalling links via satellite. " Initial Alignment The initial alignment procedure is appropriate to both first time initialization and alignment when restoring from a link failure. " Signalling Link Error Monitoring Two signalling link error rate monitoring functions are provided : D the "Signal Unit Error Rate Monitor (SUERM)" : This is used while the link is in service and is based upon a Signal Unit error counter, incremented an decremented using the leaky-bucket principle. D the "Alignment Error Rate Monitor (AERM)" : This is used during the alignment procedure and is based upon a linear count of errors. Too many errors cause the link to be taken out of service and the alignment procedure can restart. " Flow Control Flow control allows traffic to be throttled when level 2 becomes congested. The congested receiving end notifies the remote transmitting end by an appropriate Signal Unit and it also withholds acknowledgements of all incoming Signal Units. Before explaining all these functions in more detail, we will first take a closer look at the characteristics of the used Signal Units. 4.2.2 Signal Units A Signal Unit is nothing more than a packet, to be transmitted over the CCS #7 network, but because of many applications, 770 00438 0590 VHBE Ed. 07 43 / 218
  44. 44. 4 Message Transfer Part different packet structures and capabilities are foreseen. In fact, CCS #7 uses 3 different structures of Signal Units. " "Message Signal Units (MSU's)" : MSU's will transport information sent from a certain User Part of a Signalling Point to a User Part of another Signalling Point. " "Link Status Signal Units (LSSU's)" : LSSU's are sent between two Signalling Points to indicate the status of the signalling link on which it is carried. Therefore. the LSSU is only of significance between two Signalling Points and will not be transmitted further over the network. More precisely, the LSSU's will be used for the following reasons: D D " Flow control : In case of congestion, a busy status will be sent by means of LSSU's until the condition has disappeared. During the initial alignment phase of the link, LSSU's are used in order to properly start up the link. "Fill-in Signal Units (FISU's)" : The signalling link will continuously send Signal Units between the two Signalling Points even when there is no payload to be delivered and the CCS #7 network is idle. In this case empty Signal Units, called FISU's will be sent. They do not contain any information part. By means of these FISU's, the integrity of signalling links is constantly monitored in CCS #7. Otherwise, a link could degrade in the absence of traffic and this would only be noticed at a transmission, which would fail. Note that only MSU's will be transmitted in case of error, LSSU's and FISU's are not. The 3 types of Signal Units have different formats, which are shown in figure 19. They are easily distinguishable by means of their length and their length indicator. 44 / 218 770 00438 0590 VHBE Ed. 07
  45. 45. 4 Message Transfer Part 16 SIF SIO 8n, n w 3 8 2 ÇÇ ÇÇ ÇÇ ÇÇ ÇÇ ÇÇ CRC ÇÇ ÇÇ ÇÇ MSU CRC SF 16 LSSU 8 or 16 FISU CRC 16 BIB : BSN : CRC : F : FIB : FSN : LI : SF : SIF : SIO : Figure 19 2 LI F I B 6 1 B FSN I B BSN F 7 1 7 8 LI F I B FSN B I B BSN F 6 1 7 1 7 8 LI F I B FSN B I B BSN F 6 1 7 1 7 8 2 Backward Indicator Bit Backward Sequence Number Cyclic Redundancy Check Flag Forward Indicator Bit Forward Sequence Number Length Indicator Status Field Signalling Information Field Service Information Octet First bit transmitted Signal Unit Formats The meaning of the individual fields will now be explained. " Flag An 8 bit opening flag "01111110" is used as message separator. There is only one opening flag, and no closing flag. " Sequence Numbers Just as many other layer 2 protocols, CCS #7 uses the technique of sequence numbering to perform the error correction procedures. D 770 00438 0590 VHBE Ed. 07 The "Forward Sequence Number (FSN)" is the sequence number of the Signal Unit in which it is carried. 45 / 218
  46. 46. 4 Message Transfer Part D The "Backward Sequence Number (BSN)" is the sequence number of a Signal Unit being acknowledged. Both the FSN and the BSN are 7 bits in length and thus can span a cyclic sequence ranging from 0 to 127. " Indicator bits The "Forward Indicator Bit (FIB)" and the "Backward Indicator Bit (BIB)" , together with the FSN and the BSN are used to perform the error control. The indicator bits will be used to request a retransmission. During normal conditions, both indicator bits should be the same. When a retransmission is being requested, the Signal Unit being sent by the Signalling Point requesting the retransmission will have an inverted BIB. The BSN will identify the last received Signal Unit and thus indicates the point from where the retransmission has to begin. This procedure will be discussed later in more detail and is called the "Basic Error Correction method". " Length Indicator The "Length Indicator (LI)" of 6 bits is included in the Signal Unit. It indicates the number of octets following the LI and preceding the check bits. It represents a number in binary code ranging from 0 to 63. Figure 19 clearly shows that the LI differentiates between the 3 types of Signal Units : ⇒ FISU D D LI = 1 or 2 ⇒ LSSU D Note LI = 0 LI w 3 ⇒ MSU If the information part is smaller than 63 bytes, the LI will show the actual length. However, if the length of the information part exceeds 62 bytes, the length field is set to 63, indicating a length of the information field of 63 or higher. Apart from this, the length of the SIF can never be longer than 272 bytes. " Spare fields Spare fields are always coded as 0. " Information Part This part of the message contains the actual information to be sent to the destination. Depending on the type of Signal Unit, three different contents can be found in the information part. 46 / 218 770 00438 0590 VHBE Ed. 07
  47. 47. 4 Message Transfer Part D Service Information Octet (SIO) The SIO is only present in an MSU and is used by MTP-3 to identify the type of protocol, used at level 4 (i.e. TUP , ISUP TCAP ...) and the type of standard. A standard can , , be a national standard or an international standard. If the protocol at level 4 is based on the ITU-T standard, then the SIO field would indicate this as an international standard. If the protocol is any other type of protocol (e.g. ANSI), the SIO field would indicate this as a national standard. This information is used by MTP-3 to determine the type of Signal Unit, the protocol and how it should be decoded. It will be explained in more detail in chapter 4.3. D Signalling Information Field (SIF) The SIF is only present in an MSU and is used to transfer the actual control information, as well as the routing label used by MTP-3. Also this field will be explained in more detail in chapter 4.3. D Status Field (SF) The SF carries the link status information for the link on which it is carried. It is only present in an LSSU. The LSSU is not transmitted on parallel links; the SF will always apply on the status of the link on which it is carried. " Check Bits In order to detect errors in the information, the message is extended with a frame check sequence. This sequence of 16 check bits contains the result of a mathematical calculation carried out on the data. 4.2.3 MTP-2 Procedures We will now discuss all the MTP-2 procedures in more detail. Signal Unit Delimitation As already mentioned, an 8 bit opening flag "01111110" is used as message separator. There is only one opening flag, and no closing flag. Thus, the end of a Signal Unit coincides with the beginning of the next Signal Unit. By checking the incoming bit stream for the flag patterns, the receiver will be able to detect the start and the end of the Signal Units. 770 00438 0590 VHBE Ed. 07 47 / 218
  48. 48. 4 Message Transfer Part Naturally, it is possible that the bit pattern "01111110" would appear in the message itself. This would lead to errors, since this octet would be interpreted as a flag. Thus, it has to be ensured that the flag code is not imitated in any other part of the Signal Unit. Therefore, the transmitting signalling link terminal inserts a 0 after every sequence of five consecutive 1s, before the flags are attached. In the same way, at the receiving signalling link terminal, after flag detection and removal, each 0 which directly follows a sequence of five consecutive 1s is deleted. This technique is called bit stuffing. In this way, it is ensured that there is never an occurrence of six consecutive 1s except of course for the flag. The technique of bit stuffing is demonstrated in figure 20. Example Information. 0111111011111011 Bit Stuffing : Detection of 5 consecutive 1s ⇒ insertion of a 0. Extra Bits Information after Bit Stuffing. 011111 0 1011111 0 011 Flag Insertion : Precede message with a flag. Information ready for transport. Figure 20 Flag 01111110 011111 0 1011111 0 011 Bit Stuffing and Flag Insertion If a pattern of more than 6 consecutive 1s does occur, which normally should be impossible, it is called a "ones-density violation". Then, the Signal Unit is considered to be out of alignment. 48 / 218 770 00438 0590 VHBE Ed. 07
  49. 49. 4 Message Transfer Part Signal Unit Alignment A link is considered to be in alignment when Signal Units are received in sequence, without ones-density violations and with the proper number of octets (based on the message type). The total length of the Signal Unit must be a multiple of 8 bits. The total length of the SIF (of a MSU) may not exceed 272 octets. If one of these errors occurs, the "octet counting mode" is entered and the next valid flag is searched for. Until then all the received bits are discarded. The octet counting mode is left when the next correct Signal Unit is received. When there is an excessive number of alignment errors, the link is taken out of service. The typical cause of alignment errors is usually clock signals not being properly synchronized on both ends of a link. The network management procedure at level 3 is responsible for realigning the link. Error Detection The error detection function is performed by means of 16 check bits at the end of each Signal Unit. These check bits are generated by the transmitting signalling link terminal, by applying a certain algorithm on the preceding bits. The bits over which the algorithm runs begin at the first bit of the BSN until and including the last bit of the Information Part. Stuffed bits are excluded. These bits are indicated in figure 21. As a result, errors in the information part as well as in all the other fields of the Signal Unit can be detected. Information Part Ç Ç Ç CRC LI F I B FSN B I B BSN F Bits used for CRC calculation. (Stuffing bits excluded.) Figure 21 Check Bits The used algorithm is a "Cyclic Redundancy Check (CRC-16)" with x16 + x12 + x5 + 1 as the generator polynomial. If no consistency is found between the received check bits and the preceding bits of the Signal Unit, according to the algorithm, then 770 00438 0590 VHBE Ed. 07 49 / 218
  50. 50. 4 Message Transfer Part the presence of errors is indicated and the Signal Unit is discarded by the receiver. Error Correction As already mentioned, two forms of error correction are provided. Both methods are based upon retransmissions of errored or lost Signal Units. Basic Error Correction Method The first method is the Basic Error Correction method . This method relies on positive and negative acknowledgements and corrects errors by retransmissions. It ensures correct transfer of Signal Units over the link, in sequence and with no double delivery. As a consequence, no resequencing is required within the User Parts. A transmitting user will store each transmitted MSU in a retransmission buffer, where it will remain until a positive acknowledgement has been received. Multiple messages can stay in this buffer, all waiting for acknowledgement. Of course, sequence numbers will have to be used to refer to the different MSU's. This will be the FSN of the MSU. Note It is sufficient to talk about MSU's instead of Signal Units in general. Obviously, it makes no sense to request a retransmission of a FISU. Therefore, the FSN of a FISU will be the FSN of the previous MSU, in other words, FSN's are not increased for the transmission of FISU's. The same goes for an LSSU. The receiver, after having received a MSU, will use the CRC-16 calculation to decide on the correctness of the message. If errors are detected in the message, it is discarded. Further, the receiver will also check the FSN to find out if the current message is the next one. Only the next logical message will be accepted. Acknowledgments Positive acknowledgements are used to indicate correct transfer of Signal Units. A positive acknowledgement of a Signal Unit with sequence number n will automatically acknowledge all not yet acknowledged Signal Units with a lower sequence number. On the other hand, negative acknowledgements are used as explicit requests for retransmissions of lost or corrupted Signal Units. If a retransmission is requested, it will cause the Signal Unit to be retransmitted, but also all subsequently transmitted Signal Units will be retransmitted in the original order. Therefore method can be classified as a "Pull Back Method" (in contrast to a "Selective 50 / 218 770 00438 0590 VHBE Ed. 07
  51. 51. 4 Message Transfer Part Retransmission Method"). An example of such a Pull Back is shown in figure 22. Only after all retransmissions have been accomplished, the sender will continue to transmit new Signal Units, in this case M6. Sender M1 M2 M3 M4 M5 M3 M4 M5 M6 M3 NOK Receiver M3 NOK M1 Figure 22 M2 M3 M4 M5 M3 M4 M5 M6 Pull-Back The only thing that needs to be explained now is how positive acknowledgements and negative acknowledgements are communicated from the receiving party towards the sender. Positive Acknowledgements A positive acknowledgement is transmitted in the following way. The receiver can communicate the acceptance of one or more MSU's by assigning the FSN of the latest accepted MSU to the BSN of the next Signal Unit (which can be an MSU, LSSU or even a FISU) in the opposite direction. The BSN of subsequent MSU's sent back will remain the same until a further MSU is acknowledged, which will cause again a change of the BSN sent. This is demonstrated in figure 23. Note that the Signal Units sent from A to B are necessarily MSU's (otherwise the FSN would not increase), while the Signal Units sent from B to A can be any kind. The first Signal Unit from B to A acknowledges MSU 24, while the second one acknowledges correct reception of MSU 25 and MSU 26. 770 00438 0590 VHBE Ed. 07 51 / 218
  52. 52. 4 Message Transfer Part Sender A FIB FSN 1 FSN 1 FSN 1 BSN BIB BSN BIB BSN 25 FIB BIB 24 FIB Receiver B 26 BSN BIB 24 1 BSN BIB 26 Figure 23 Negative Acknowledgments FSN FIB FSN FIB 1 Positive Acknowledgement If a negative acknowledgement has to be sent, then the BIB of the Signal Unit (that will be sent in the opposite direction) will be inverted. This new BIB value will be maintained in subsequently sent Signal Units until a new negative acknowledgement is to be sent. The BSN will assume the value of the FSN of the last accepted MSU. There is no danger in loosing a negative acknowledgement (e.g. if the message is discarded because of an error). The BSN and the toggled BIB will also be transmitted in subsequent messages. It should be stressed that the actual value of a FIB (which is transmitted) or a BIB (which is received) has no own significance. Just their relative values matter. In figure 24, the receiving side requests a retransmission from MSU 23 onwards. Party A will detect this because the values the FIBtransmitted and the BIBreceived differ. 52 / 218 770 00438 0590 VHBE Ed. 07
  53. 53. 4 Message Transfer Part Sender A Receiver B FIB FSN 1 BIB BSN 24 BSN BIB 22 FSN FIB 0 BIBreceived 0 FIBtransmitted ⇓ RETRANSMISSION ! Figure 24 Negative Acknowledgement When the sender starts with the retransmission, he shall invert the FIB to match the value of the BIB of the received retransmission request. Both indicator bits should now match again, though they will have switched in value. They will retain this value until a new retransmission is requested again. This Basic Error Correction method operates independently in the 2 transmission directions. Indeed, until now, we have only spoken about the direction from A to B and we have only used the FSN and the FIB of Signal Units from A to B and the BSN and the BIB of Signal Units from B to A. In a complete analogue way, the method will operate for the other transmission direction. A method like this, in which the feedback to the sender is returned as a part of another information message, are also called "Piggy-Backing" methods. To summarize, the actions of a sender are given in figure 25, while the actions of a receiver are given in figure 26. 770 00438 0590 VHBE Ed. 07 53 / 218
  54. 54. 4 Message Transfer Part For each new Signal Unit : " Construct SIF and SIO for an MSU or SF for an LSSU " Add layer 2 information : D Add length indicator D Add FSN D Add current FIB D Add feedback information related to messages sent in the opposite direction = Add BSN = Add current BIB (positive ack) or toggle (negative ack) " Calculate and add CRC " Perform Bit Stuffing and add Flag If a retransmission is requested : " Take Signal Unit out of the Retransmission buffer " Modify the following layer 2 information : D Toggle FIB D Add new feedback information related to messages sent in the opposite direction . " " Figure 25 54 / 218 Calculate and add CRC Perform Bit Stuffing and add Flag MTP-2 : Sender Actions 770 00438 0590 VHBE Ed. 07
  55. 55. 4 Message Transfer Part For each received Signal Unit : " Flag Detection and Removal " Bit Destuffing " Check CRC D " If CRC = NOK ⇒ Discard message Check the Information ⇒ Discard Information D If LI = 0 D If LI = 1 or 2 ⇒ Process LSSU D If LI > 2 ⇒ Process MSU • If FIBreceived 0 BIBtransmitted ⇒ Discard message /* This happens when a retransmission was requested, but is not yet executed. */ • If FSNreceived 0 FSNexpected ⇒ Discard message and toggle BIBtransmitted /* This happens when a retransmission is requested. */ • If FSNreceived = FSNexpected ⇒ Accept message and increment FSNexpected " Check the Feedback D If BIBreceived = FIBtransmitted ⇒ Delete all messages up to the BSNreceived from the retransmission buffer. D If BIBreceived 0 FIBtransmitted ⇒ Invert FIBtransmitted and start retransmissions. Figure 26 MTP-2 : Receiver Actions 770 00438 0590 VHBE Ed. 07 55 / 218
  56. 56. 4 Message Transfer Part The test FIBreceived = BIBtransmitted carried out in the receiver is carried out for the following reason. When a retransmission has been requested, it will take some time before the retransmitted messages will start arriving in the receiver. In the meantime, all the messages that arrive will be ignored anyway because the sequence number will not have the expected value. The test will allow the receiver to reject these messages in a fast way. Example 56 / 218 To finish let us take a look at a concrete example. In the figures 27, 28, 29 and 30, a number of snapshots are taken, illustrating the use of the Basic Error Control method. 770 00438 0590 VHBE Ed. 07
  57. 57. 4 Message Transfer Part Step 1 In figure 27, the sender increases the FSN to 53. Then he stores the MSU in a retransmission buffer and sends it. The MSU will be kept in the retransmission buffer until a positive acknowledgment has been received. (1) When the receiving MTP finds no errors, he sends the number 53 back as a BSN. (2, 3 and 4). Meanwhile , the transmitting party gets a positive acknowledgement, regarding MSU 51, so he can delete this from the retransmission buffer (5). FSN =53 FSN = 52 FSN = 51 F I B 1 1 F S N 53 ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ retransmission buffer FLAG transmit buffer 2 check FSN 53 accepted 3 BSN = 53 4 5 ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ FLAG B S N B I B 51 1 ... ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ FSN =53 FLAG B S N B I B 53 1 If BSN= 51 is received, we know that we can delete all messages until 51 from the retransmission buffer Figure 27 Example of Basic Error Recovery Method (1) 770 00438 0590 VHBE Ed. 07 57 / 218
  58. 58. 4 Message Transfer Part Step 2 In figure 28, the receiver detects an error in MSU 54, i.e. the cyclic redundancy check yields a negative result. Therefore, the MSU will not be accepted, even more, it will simply be ignored. Thus the FSNexpected keeps the old value, i.e. 54 (1), i.e. the receiver will continue looking for MSU 54. Also the BSN will remain at value 53, since this was the last correctly received message. The consequence is that a second positive acknowledgement for MSU 53 will be transmitted. (2) Of course, this is no problem. In fact, until now receive control is not even aware that an error has occurred, and will still signal back that the previous message was ok. Meanwhile , the transmitting party gets a positive acknowledgement, regarding MSU 52, so he can delete this from the retransmission buffer (3). 1 FSN =55 FSN =54 FSN = 53 FSN = 52 F I B F S N 55 ... F S N 1 54 ÉÉÉÉ ÉÉÉÉ F I B retransmission buffer 1 FLAG FSN 54 not accepted BSN = 53 BIB =1 2 ÉÉÉÉ ÉÉÉÉ FLAG B I B 52 1 ... ÉÉÉÉ ÉÉÉÉ 3 B S N FLAG B S N B I B 53 1 If BSN= 52 is received, we know that we can delete all messages until 52 from the retransmit buffer Figure 28 58 / 218 Example of Basic Error Recovery Method (2) 770 00438 0590 VHBE Ed. 07
  59. 59. 4 Message Transfer Part Step 3 In figure 29, MSU 55 arrives at the receiver. This time the cyclic redundancy check is ok and he will compare this FSNreceived with the FSNexpected, but this was still number 54. (1) Therefore, receive control will detect that an MSU is missing and he will ask for a retransmission of MSU 54 and all subsequent units. Thus, he will invert the BIBtransmitted to 0. The BSN will still remain at value 53, indicating a transmission is requested from 54 onwards. (2) Meanwhile , the transmitting party gets a positive acknowledgement, regarding MSU 53, so he can delete this from the retransmission buffer (3). 1 FSN =56 FSN =55 FSN = 54 FSN = 53 F I B F S N 56 ... F S N 1 55 ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ F I B retransmission buffer 1 FLAG FSN 55 0 54 (= expected) BSN = 53 BIB = 0 2 ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ FLAG B I B 53 1 ... ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ 3 B S N FLAG B S N B I B 53 0 If BSN= 53 is received, we know that we can delete all messages until 53 from the retransmit buffer Figure 29 Example of Basic Error Recovery Method (3) 770 00438 0590 VHBE Ed. 07 59 / 218
  60. 60. 4 Message Transfer Part Step 4 In figure 30, all Signal Units arriving at the receiver will be ignored, since they carry a FIBreceived, different from the BIBtransmitted. Meanwhile ,the retransmission request arrives at the sender. Therefore, he will toggle his FIBtransmitted, and will start retransmitting from MSU 54 onwards (= BSNreceived + 1). The toggling of the FIBtransmitted is necessary to allow the receiver to identify when the retransmissions arrive at his side. Only from that moment on, MSU's will be accepted again. FSN =56 FSN = 55 FSN = 54 F I B 0 F I B F S N 54 F S N ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ retransmission buffer ... FLAG BIB = 0 0 FIB ³ retransmit starting from BSN +1 = 54 3 1 FIBreceived 0 BIBtransmitted BSN = 53 BIB =0 2 ÉÉÉÉ ÉÉÉÉ FLAG Figure 30 B I B 53 0 B S N ... ÉÉÉÉ ÉÉÉÉ B S N FLAG B I B 53 0 Example of Basic Error Recovery Method (4) In a more general case than the example, free time occurs on the line. During this free time FISU's are sent. They have the same FSN and FIB as the last sent MSU. Their BSN and their BIB can however be used as part of the error control procedure. Transmission errors that can not be detected by the CRC will not be detected. However, it has been shown that if we suppose a long term bit error rate of 10-6 on the signalling data link, then the overall probability of having undetectable transmission errors after the CRC calculation is 10-10 which is a sufficiently low value. Lines with a long delay 60 / 218 Let us consider a link with a total loop delay (= total time between sending of the signal unit and receipt of the positive acknowledgement) of 600 ms. 770 00438 0590 VHBE Ed. 07
  61. 61. 4 Message Transfer Part The maximum number of messages that can be sent and not yet acknowledged is 127, since the length of the FSN is only 7 bits. The maximum MSU loading of the link is : 127 * (duration of 1 message signal unit in ms) / 600 ms Taking a length of 120 bits for a message signal unit, this duration is 120 bits / ( 64 000 bits/sec ) = 1.8 ms Thus the maximum loading is : 127 * 1.8 / 600 = 0.4 So we find that the link can only be loaded up to 40%. This means that there will be at least 60% of free time, which, when the Basic Error Control Method would be used, would be spent in sending FISU's. The main cause of this low loading is the long total loop delay. To prevent this a second error correction method exists for these circumstances. This method is called the Preventive Cyclic Retransmission Method and it makes use of these periods of free time on the line to send retransmissions. Preventive Cyclic Retransmission Method A second procedure called Preventive Cyclic Retransmission Method will be used on lines on which the propagation delay is in excess of 15 msec. Round-trip delays on very long lines become excessive. In case of a failure of one MSU, use of the Basic Error Recovery Method would result in very long delays before the error condition could be signalled back to the transmitter. The Preventive Cyclic Retransmission Method will, of course, also use retransmissions to correct errors. However they will be sent automatically when time is available. This method is not the favored method since it will use a higher number of retransmissions than the Basic Error Correction Method. The following principles are applied : " " 770 00438 0590 VHBE Ed. 07 As in the previous method, the receiver will only accept the next message and ignore all incoming messages with a different sequence number. As in the previous method, positive acknowledgements are sent back to the transmitter by making use of the BSN. If an acknowledgement arrives, the related message and all previous messages are removed from the retransmission buffer. 61 / 218
  62. 62. 4 Message Transfer Part " In contrary to the previous method, only positive acknowledgements exist. In other words, retransmissions can not be requested by the receiver. Thus, the FIB and the BIB will not be used and will always be set to 1. " If no new messages are available for transmission, the sender will spontaneously retransmit the messages which are still present in the retransmission buffer in a cyclic way. Only unacknowledged MSU's will be in the retransmission buffer, so, in essence, the sender is retransmitting what he perceives to be not received yet. " If a sender is sending retransmissions and it receives additional MSU's to transmit, he stops the retransmission and will send the new MSU's first. After that he will resume the retransmissions. " If the retransmission buffer is full or if the limit of a certain number N of non-acknowledged messages has been reached, a retransmission will take place and will continue until enough MSU's have been acknowledged. This is called forced retransmission. It is done to maintain the efficiency of error correction in those cases where automatic correction by preventive cyclic retransmission alone is impossible (e.g. a high load on the signalling link, where constantly new messages are available). It complements the "spontaneous retransmissions". " If no messages are available for transmission and the retransmission buffer is also empty, FISU's will be sent. The theory behind this method is that eventually an MSU will reach the other Signalling Point, with or without retransmission. True, the retransmissions may flood the link. On the other hand, the messages are guaranteed a higher rate of success if continually transmitted in this fashion. Figure 31 shows an example of this method. Suppose that MSU with number 49 arrives with an error. The CRC yields a negative result. The receiver keeps on sending a confirmation for MSU 48. However, since message 49 is still in the retransmission buffer, it is highly likely that the MSU has already been sent back to the receiver. As a result, sooner or later, the message will arrive without errors, followed by all the following MSU's. The following initial situation is assumed. " " 62 / 218 Message 45,46 and 47 are present in the retransmission buffer. Message 48,49 are present in the transmission buffer. 770 00438 0590 VHBE Ed. 07
  63. 63. 4 Message Transfer Part " The receiver is expecting message 48. Receiver B Sender A Transmission Retransmission 49 48 FIB 47 46 45 FSN 1 45 1 46 1 47 1 45 1 46 1 48 accepted 49 1 BSN 48 1 BIB FSNexpected = 48 50 /* New MSU's are sent first */ 49 accepted /* No new MSU's are available. Therefore, all MSU's present in the retransmission buffer will be retransmitted cyclically .*/ /* A new MSU is available. The transmission of this MSU has a higher priority than the retransmissions. It is done first. */ 50 accepted POSITIVE ACKNOWLEDGEMENT for 45 Transmission Retransmission 50 49 48 47 46 1 47 /* No new MSU's are available.Retransmission is resumed 1 48 in a cyclic way, i.e. where the 1 49 previous retransmissions ended. */ Figure 31 Example of the Preventive Cyclic Retransmission Method 770 00438 0590 VHBE Ed. 07 63 / 218
  64. 64. 4 Message Transfer Part With this we can conclude the paragraph of the Error Recovery Capability of MTP-2. Initial Alignment The procedure is applicable to activation and restoration of a signalling link. Its purpose is to reestablish the timing and alignment of Signal Units so that the affected Signalling Point can determine where Signal Units begin and end, i.e. the flag is detected. The out-of-alignment condition occurs when the flag has been simulated within the data (ones-density violation) or the SIF is too long (i.e. longer than 272 octets), which would indicate that the flag was missed. The procedure resets both the transmitting and the receiving Signalling Points at level 2 and does not affect other links. Only the signalling link to be aligned is involved in the initial alignment procedure. The procedure provides a "normal" proving period for "normal" initial alignment and an "emergency" proving period for "emergency" initial alignment. The decision to apply either the normal or the emergency procedures is made by MTP-3. Normal alignment will be used when there are other links associated with the affected link (such as in a linkset). An emergency alignment is used when there are no other links to the adjacent Signalling Point within the linkset. The emergency alignment goes through the same procedure but within a shorter time period. Both alignment procedure employs four different alignment Status Indications, shown in table 2. These indications are carried within the Status Field (SF) of the LSSU's. They are transmitted over the signalling link between both Signalling Points which are "lining up". Table 2 Status Indications O Out of Alignment N Normal Alignment E Emergency Alignment OS 64 / 218 Out of Service 770 00438 0590 VHBE Ed. 07
  65. 65. 4 Message Transfer Part There are five states entered during alignment. Timers associated with each state ensure that the Signalling Point does not get stuck in any one state. The following explanation describes very briefly each state. " Idle : This state indicates that the procedure is suspended and it is the first state entered. It is also resumed whenever the alignment procedure is aborted. " Not Aligned : This state is entered when alignment is initiated by MTP-3. LSSU's are used to send a status of O (out-of-alignment) to the other side. " Aligned : This state indicates that the link is aligned and is capable of detecting flags and Signal Units without error. LSSU's with a status indication N or E will be transmitted. " Proving : The proving period is used to test the integrity of the link at a Signalling Point. During the proving period, FISU's are sent and errors are counted. There are 2 proving periods : normal and emergency. The normal proving period lasts for 2.3 seconds. During this time, no more than 4 errors may occur. This will be counted by the AERM (Alignment Error Rate Monitor) " The emergency proving period lasts for 0.6 seconds. During this time, only 1 error may occur. This is also monitored by the AERM. " When excessive errors have occurred according to the procedure, the link is returned to the idle state, and the process begins over and over again. " Aligned Ready : After a link has successfully passed the alignment procedure, the link is returned to an in-service state, where MSU's are transmitted and normal processing is allowed. Signalling Link Error Monitoring Two link error rate monitor functions are provided : 770 00438 0590 VHBE Ed. 07 65 / 218
  66. 66. 4 Message Transfer Part " the Signal Unit Error Rate Monitor (SUERM) : This is used while the link is in service. The SUERM has as its function the estimation of the error rate of the Signal Units, in order to decide about the quality of the signalling link. The Signal Units are those which are rejected by the acceptance procedure (faulty CRC, ones-density violation, SIF > 272). It will be implemented in the form of an up/down counter : D The counter will be decremented at a fixed rate (for every D received Signal Units, with or without errors). D The counter will be incremented for every errored Signal Unit. This kind of mechanism is called a "Leaky Bucket" mechanism and it is shown in figure 32. An increment of the counter corresponds with pouring some liquid into a fictitious "bucket", which is emptied on a slow rate because of a leak in its bottom. If a certain threshold T is reached (bucket overflow), an excessive error rate indication will be passed to MTP-3. When the link is brought into service, the monitor count should start from zero, which corresponds to an empty bucket. Bucket Contents = Threshold T Figure 32 Errored Signal Units. T One Unit flows out every D received SU. Leaky Bucket Mechanism for the SUERM. Recommended values for T and D can be found in Ref.[5.]. They are shown in table 3. Table 3 Recommended Parameters for T and D. Bitrate D 64 kbit/sec 64 Signal Units 256 Signal Units lower bitrates 66 / 218 T 32 Signal Units 256 Signal Units 770 00438 0590 VHBE Ed. 07

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