Systemy sygnalizacji i Zarządzania Sieciami System sygnalizacji nr 7-wprowadzenie Krzysztof Wajda Katedra Telekomunikacji AGH Październik, 2016
Goals of the course introduction of main problems and definitions related to signalling description of functions accomplished by signalling detailed desciption of Signalling System No 7 signalling in intelligent networks and advanced services B-ISDN environment and signalling for contemporary networks
Outline Common channel signalling SS7 structure and architecture Signalling points Overview of common transfer functions Overview of higher parts of signalling protocol stack
Common Channel Signalling This concept was possible with significant advances in network software engineering SS6 as first attempt to CCS This is out-of-band signalling method in which common data channel conveys signalling messages related to a number of trunks
CCS6 -CCIS CCIS - Common Channel Interoffice Signalling CCS6 - Signalling System No. 6 introduced in 1976 by AT&T Features of CCS6: not a layered system only for trunk signalling routing based on the basis of permanent virtual signalling circuits
CCS6 - drawbacks large efforts to manage banded routing limited lengths low speed links non-modular structure
SS7 -intro Work on SS7 began in mid 1970s CCITT (ITU-T currently) as leading force 800 service and calling card service as first attempts to new concept used layered approach to system construction
SS7 supports signalling for: Public Switched Telephone Networks ISDN Public mobile networks Operations, administrations and maintenance as a basis for interactions with network data bases and service control points for advanced services
SS7 network structure Sieć sygnalizacyjna SS7 Sieć telefoniczna - centrala telefoniczna STP Signalling Transfer Point - SP signalling point
Signalling network elements SP - Signalling Point STP - Signal Transfer Point signalling links SCP - Service Control Points OAM - Operation, Administration and Maintenance centres
Signalling network structure Reflects the structure of telecommunication network to be served by signalling and administration aspects Signalling system can be planned: purely on a per signalling relation basis - associated signalling for large traffic volumes and some quasi-associated signalling for low volume signalling relations as an independent network with regard to common channel signalling needs - this leads to mainly quasi-associated signalling with scarce associated signalling for high volume traffic patterns
Worldwide signalling network Structured into two functionally independent levels: international national such structure helps to split responsibility and separates numbering plans
Signalling points Originating point of the message is a signalling point at which message is generated, i.e. there is location of the source User Part function Destination point of the message is a signalling point to which message is destined, i.e. there is location of the receiving User Part function Signalling point at which a message is received on one signalling link and transferred to another link is a Signal Transfer Point (STP)
Routing concepts In classical networks, based on different criteria: static alternative time-based state-based
Signalling relation Signalling relation exists between two nodes when they are capable of exchanging signalling messages through signalling network
Signalling modes Mode refers to the type of association between path taken by the signalling message and its corresponding signalling relation: Associated Non-associated Quasi-associated
Associated mode Messages corresponding to a signalling relation between two points are transported over a link set directly inteconnecting those signalling points exchange A exchange B exchange SP signalling link set signalling relation
Non-associated mode Messages corresponding to a particular signalling relation between two points are transported over two or more linksets in tandem passing through one or more signalling points other than the origin and destination of messages exchange A exchange C exchange SP signalling link set signalling relation exchange B
Quasi-associated mode It is limited case of the non-associated mode - route (path) taken by message through signalling network is predetermined and fixed (at a given point in time) exchange A exchange C exchange SP signalling link set signalling relation exchange B
SS7 - general description Model OSI application l. presentation l. UP session l. transport l. services network l. data link l. physical l. NSP transport UP = User Part NSP = Network Service Part
SS7 - main building blocks Message Transfer Part (MTP): block which ensures reliable transmission of functional signalling messages with high availability User Parts (UP): block which provides functions to manage number of services
SS7 protocol model Model OSI Model SS7 OMAP ASEs application l. TCAP ISDN-UP presentation l. session l. Puste transport l. SCCP network l. MTP Level 3 data link l. physical l. MTP Level 2 MTP Level 1 OMAP = Operations Maintenence and Administration Part ASE = Application Service Element TCAP = Transaction Capabilities Application Part ISDN-UP = ISDN User Part SCCP = Signaling Connection Part MTP = Message Transfer Part
SS7 - building blocks MTP SCCP TUP ISDN-UP DUP TC MAP INAP
Standardization process of SS7 ITU-T (former CCITT) Bellcore ANSI all national standard institutes
ITU-T recommendations Key recommendations General description Q.700 MTP Q.701-Q.704, Q.706, Q.707 TUP Supplementary services Q.730 DUP Q.741 Q.721-Q.725 ISDN-UP Q.761-Q.764, Q.766 SCCP Q.711-Q.714, Q.716 Transaction Capabilities Operations, maintenance and administration part (OMAP) Q.771-Q.775 Q.795
ITU-T recommendations Additional recommendations Signalling network structure Q.705 Numbering of international Q.708 signalling point codes Hypothetical signalling reference Q.709 connection PBX application Q.710 Test specifications Q.780-Q783 Monitoring and measurements Q.791
Basic architectural and functional components of SS7
Outline Signalling link Network layer issues The Signalling Connection Control Part (SCCP) Signalling Network structures
Signalling link MTP - structure and functions Physical layer requirements and standards Signalling unit formats Link protocol operation Error detection and recovery Flow control issues
MTP Provides a reliable transfer and delivery of signalling messages across the signalling network Consists of levels 1-3 of SS7 protocol: Signalling Data Link Functions (Level 1) Signalling Link functions (Level 2) Signalling Network Functions (Level 3) Uses datagram mode Signalling link przęsło sygnalizacyjne
MTP functional diagram Signalling Link User Message Processing Common Transfer Functions link control functions Signalling Data Link link control functions Common Transfer Functions User Message Processing MTP Users MTP users MTP
Signalling Data Link Functions Signalling Data Link is a bidirectional transmission path for transfer of signalling messages; two data channels operating together in opposite directions with the same data rate analog signalling link (4 khz) accessible by modems digital data links: 56 kbit/s (ANSI standard) 64 kbit/s (ITU-T standard) minimum data rate allowed is 4.8 kbit/s (delay requirements)
Signalling link functions Correspond to the OSI s data link layer (layer 2), provide reliable transfer of signalling messages between two adjacent signalling points, signalling units have variable length and are called signal units.
Summary SS7 is first widely accepted and used signalling concept for more than 30 years Introduced with relevant advancements in engineering concepts and computer science services Flexible and extendable standard
References ITU Recommendation, Q.700, Introduction to CCITT Signalling System No. 7, March 1993 G. Danilewicz, W. Kabaciński, System Sygnalizacji nr 7, WKŁ, 2005 D. Minoli, G. Dobrowski, Principles of signaling for Cell Relay and Frame Relay, Artech House, 1995 Lectures available at: http://www.kt.agh.edu.pl/~wajda/students/systemysygnalizacji-i-zarzadzania-4r-eit/
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