• ITU-T Common Channel Signalling System No. 7 (CCSS7, CCS7, CCS, CCS#7, C7, SS7 … ) • At present the dominant inter-exchange signalling system in digital networks (PSTN, ISDN, PLMN) • SS7 is in effect a robust, high-performance packet switched network, intended for secure transmission of signalling messages • ITU-T Q.700-series Recommendations
Channel-associated signalling (CAS)
Oldform of signalling (has mostly been replaced by SS7) Signalling occurs in-band on voice channels
signalling possible Exchange signalling not possible Exchange Exchange
circuit switched connection
Before a circuit switched connection exists, end-to-end signalling between originating and terminating local exchanges (or to/from databases) is not possible
Common channel signalling (CCS)Modern form of signalling (SS7 is based on this method) Signalling occurs out-of-band on dedicated channels
signalling possible anywhere anytime Exchange Exchange Exchange
Uses a separate packet-switched signalling network which is not related to circuit switched connections End-to-end signalling between originating and terminating local exchanges (or to/from databases) is possible anytimeCommon channel signalling (CCS)
Faster call setup times - compared to in-band signalling using multi-frequency (MF) signalling tones More efficient use of voice circuits Support for Intelligent Network (IN) services which require signalling to network elements (e.g., database systems) without the use of circuit switched connections Support for ISDN-type supplementary services which requireend-to-end signalling between terminals (or local exchanges) Improved control over fraudulent network usage
Signalling points (SP) in SS7
Every SP is identified by a signalling point code (SPC) STP STP STP
ISUP INAP MAP CAP
Signalling Transfer Point (only related to SS7 network, not related to network nodes in supported networks) Service Control Point (e.g. a database, such as HLR in GSM) ServiceSwitching Point (signalling termination in an exchange)
Application protocols used in SS7
Significance of SSP and SCP
During the processing of a circuit switched call, an SSP (Service Switching Point) in an exchange may be triggered to retrieve various switching related information (number analysis, time, location, security, charging...) from an SCP Thus, the SCP(Service Control Point) provides information necessary for advanced call-processing capabilities The usage of SSP and SCP depends on which IN (Intelligent Network) features a network operator has implemented (and which IN features the user has subscribed to).
Protocol layers (”levels”) of SS7
MAP TUP ISUP SCCP MTP level 3 CAP TCAP INAP
MTP level 2 (HDLC-type protocol) MTP level 1 (64kbit/s PCM time slot)
MTP - Message Transfer Part SCCP - Signalling Connection Control Part UP - User Part AP - Application Part
Application protocols in SS7
TUP (Telephone User Part) – is being replaced by ISUP ISUP (ISDN User Part) – for all signalling related to management of circuit switched connections MAP (Mobile User Part) – for transactions between exchanges (MSC, GMSC) anddatabases CAMEL = (HLR, EIR, AuC...) in mobile networks Customised INAP (Intelligent Network Application Part) – for IN applications in fixed networks CAP (CAMEL Application Part) – for extended IN functionality in mobile networks
Applications for Mobile networks Enhanced Logic
MTP level 1 (signalling data link level): Physical transmission in a 64 kbit/s PCM time slot. MTP level 2(signalling link level): HDLC-type frame-based protocol for flow control, error control (using ARQ), and signalling network supervision and maintenance functions. MTP level 3 (signalling network level): Used for routing in the signalling network (OPC ó DPC) between SPs with level 4 users (see SIO at level 2).
MTP level 2 frame formats
MSU (Message Signal Unit)
F CK SIF Level 3 signalling...
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