Signalling • The interchange of information • between the different functional parts • of a telecomm network
There are three distinct types of signalling: • 1.Between customer and exchange • 2. Within an exchange • 3.Inter-exchange
There are three distinct types of signalling: Local Loop Trunk Local Loop LX LX Calling Party Off Hook Digits Ring back Tone Call Progress Called Party Ring Off Hook Seize Start Digits Call Progress Supervision
Functions of Inter-Exchange Signalling • Supervisory functions • Selection functions • Operational functions
Speech Channels 17-31 Speech Channels 1-15 0 16 For Signalling For Frame Synchronization Channel Associated Signalling • Signalling is carried, associated with each channel. • For each channel Bits are reserved in channel 16 for signalling status.
Channel Associated Signalling Limitations of Channel-Associated Signalling • Slow • Inflexibility • Limited Capability • Closed Growth path
C7 Signalling A worldwide standard for inter-exchange signalling. The average call setup time for a toll call is 3 to 7 seconds, compared to 11 to 15 seconds before. Optimum use of trunk lines. It uses labeled messages which cab be sent on a signalling path separate from that used by the speech and data. Each signalling path conveys signalling information about many speech/data channels. A C7 Signalling link can handle several thousand calls. Long distance bandwidth is conserved since signalling is out of band and signalling for several trunks can be multiplexed on a single signalling channel.
C7 Signalling Additional user services can be offered with CCS, such as closed user groups (CUGs), credit card verification, and calling party identification. C7 is modular in structure Hence it is flexible and may serve different user sources i.e. it is a multi-user system. For example, C7 can be used to handle signalling for Telephony or ISDN users. The modular structure of C7 allows it to meet not only present, But future needs too. As C7 is an open system, it offers administrations the opportunity to adapt and expand their network according to their needs.
Characteristics of C7 • Fast 3 to 7 seconds, compared to 11 to 15 seconds • High Capacity Can handle several thousand calls • Modular Canserve different user sources • Flexible Labeled messages for speech and data • Economical Optimum use of trunk lines less EQP
Network Structure Signalling Point Nodes in the C7 Signalling network are referred to as Signalling Points (SPs). Signalling Link (SL). A signalling path between two adjacent Signalling Points (SPs) is called a Signalling Link. Link Set (LS). A number of Signalling Links between two SPs are together referred to as a Link Set.
Network Structure Originating Point (Op) The SP from where the signalling message is generated is called Originating Point. Destination Point (DP) The SP for which the signalling message is generated is called Destination Point. Signalling Transfer Point A Signalling Transfer Point (STP) is an SP through which messages are routed without being processed.
Signalling Point Codes • Every Signalling Point is identified by a unique Point Code to enable the system to route a message efficiently. This is done by specifying the: • Originating Point Code (OPC) • Destination Point Code (DPC) • The OPC and DPC are contained in a part of the C7 labeled message calledthe Routing Label.
Structure of a C7 Node • The C7 node was originally designed: • as a 4-level structure, • and to support Circuit-Related applications • The modern C7 node is based on the 7-layer OSI model • Can be used for non circuit-related applications
Messages Application Part User Part User Part Application Part Signalling Link Message Transfer Part Message Transfer Part A C7 nodes consists of three parts
Message Transfer Part It acts as a common message transfer system for the different applications and user parts in the node. Basically, the MTP is responsible for the reliable transfer of signalling messages between users. Messages from different users are packed into a standard format by the MTP and are transported over the network in this format.
User Parts • User Parts arelevel 4 parts of the node and are concerned with circuit-related applications e.g. • The Telephone User Part (TUP). • The ISDN User Part (ISUP)
Application Parts Later additions to the node which were developed in line with the OSI model. Some of these parts are concerned with non circuit-related applications such as network operation and maintenance.
Some More Additions to C7 SCCP ( Signaling Connection Control Part ) ISP ( Intermidate services Part ) TCAP ( Transaction Capability Part ) With the addition of these C7 becomes OSI compatible. ISP & TCAP provide layers 4-7.
MAP OMAP ISUP Level 4 TUP Level 4 TCAP ISP SCCP Message Transfer Part ( Level 1-3 ) Structure of C7 Node
Message Transfer Part • MTP Serves as a transport system for the reliable transfer of messages. • The MTP ensure that: • No errors are introduced into the signalling units • The SUs are sent in correct sequence • A SU is sent only once • SUs are sent in the correct direction • All SPs within the signalling network are informed of any changes in the signalling network e.g. link failure.
Types of SUs • 1) Message Signal Units (MSUs) • 2) Link Status Signal Units (LSSUs) • 3) Fill In Signal Units (FISUs)
Message Signal Units (MSUs) These are received from the User Parts and the Network management functions. They contain information relating to the control of calls or the signalling network.
MSU F CK SIF SIO LI F FSN B BSN F I I B B 8 16 8n, n>2 8 2 6 1 7 1 7 8 F = Flag. Structure of MSU BSN = Backward Sequence No. BIB = Backward Indicator Bit FSN = Forward Sequence No FIB = Forward Indicator Bit L1 = Length Indicator SIO = Service Info. Octet. SIF = Signalling Info.Field CK = Check bits
Structure of MSU Flag = 01111110 BIB and FIB bits are toggled to Indicate: A faulty SU Retransmission of SU Length Indicator LI is used: To distinguish between SU types and In MSU to indicate the size of SIF If L = 0 SU is a FISU L = 1 or 2 SU is a LSSU L > 2 SU is a MSU and the size of Data filed
Service Information Octet (SIO) SIO SSF SI • SI = Service Indicator • TUP , ISUP • SSF = Sub-Service Field • The SSF contains the Network Indicator (N1) bit. • This is used to discriminate between, • national and international signalling networks, • two national signalling networks • different level 4 parts
LSSU F B F CK SF LI I FSN I BSN F B B 8 16 8 OR 16 2 6 1 7 1 7 Link Status Signal Units (LSSUs) These are sent from level 2 of the MTP. They indicate a change in the status of the signalling link e.g. change from the status ‘Available” to the status “Blocked’ There are 6 different status indications: Out of Service (OS) Processor Outage (PO) Busy(B) Normal Alignment (N) Emergency alignment (E) Out of Alignment (O)
F B F CK LI I FSN I BSN F B B 8 16 2 6 1 7 1 7 8 Fill In Signal Units (FISUs) These are from level2 of the MTP. Sent when no other message is being sent on the signalling link. They are also sent to acknowledge MSUs when no other MSUs are being sent.
MTP Functional Levels Level 1: Signalling Data Link functions (SDL) Level 2: Signalling Link functions(SL) Level 3: Signalling Network functions
Level 1: Signalling Data Link functions (SDL) • Level 1 defines the characteristics of : • The Signalling Data Link and • the means to access it, • i.e. the transmission path. An example of a digital Signalling Data Link would be a 64 kbit/s transmission channel and semi-permanent connection through an electronic switch. • Level 1 consists of: • .Transmission Channels on the SL • .Switching Functions to Access SL
Level 2: Signalling Link functions(SL) • Level 2 Functions are: • Signal Unit Delimitation • Link Alignment • Error Detection • Error Correction
Level 2: Signalling Link functions(SL) • Signal Unit Delimitation: • The Flag is used for Signal Unit Delimitation. • The bit pattern of the flag is 01111110. • Flags are inserted at the beginning and end of a Signal Unit. • Bit Stuffing (to prevent the flag sequence being imitated ) • A ‘0’ is inserted after every five consecutive ‘1S’
Level 2: Signalling Link functions(SL) Link Alignment Alignmentis a level 2 function which occurs when A link : -Is first brought into service -Which has been out of service is brought back into service. The link may be aligned under ‘Normal’ or ‘Emergency’ procedures. The Signalling Link Management function of level 3 decides whether Normal or Emergency alignment is required, depending on whether the Link is required urgently or not.
Level 2: Signalling Link functions(SL) Link Alignment The following LSSU types are sent during Initial Alignment: -The Status Indicator Out of Service (SIOS) This is sent on a Link after initial power up of the link The Status Indicator Out of alignment (SIO) This is sent at the beginning of alignment to start the alignment procedure. Status Indicators Normal (SIN) This is sent for a period of 8 seconds during normal alignment. The Status Indicator Emergency alignment (SIE) This is sent during emergency alignments for a 0.5 second proving period The error rate of the SINs and SIEs is checked by: - Examining the CK bits - Incrementing the alignment error rate monitor if an error is detected
Level 2: Signalling Link functions(SL) Error Detection The Error Detection function of Level 2 is used to detect errors in SUs. The CK field is used in the Error Detection function. It contains 16 checksum bits. These CK bits are generated in Level 2. The CK bits are assigned a value on the basis of the preceding bits of an SU. The SU is then sent on the SL. At the receiving Signalling Point (SP), the CK bits are compared with the preceding bits of the SU. If they correspond, the SU is considered to be error free. If SU is not error free the SU will be discarded and the SUERM Unit Error Rate Monitor incremented.
Level 2: Signalling Link functions(SL) Error Correction The Signal Units are retransmitted. One method used is the Basic Error Correction method. All SUs that are transmitted are stored in a buffer, in the sending SP, until they are acknowledged. The Backward Sequence Number (BSN) is used for error correction. For non-satellite links
Level 3 Functions The functions of Level 3 are divided into two main categories: 1. Message Handling functions 1. Message Routing 2.Message Discrimination 3.Message Distribution 2. Network Management functions Signalling Traffic Management (STM) Signalling Route Management (SRM) Signalling Link Management (SLM)
Message Handling functions • The Message Handling functions manage the messages, • which come from or are sent on the Signalling Link. • These messages may have been generated by the: • User Parts (UP) Level 4 • SCCP and Application Parts or • Network Management functions (also in Level 3 ) • The Message Handling functions involve: • 1. Message Discrimination • 2. Message Routing • 3.Message Distribution
Message Handling functions • 1. Message Discrimination • Discriminates between incoming SUs which are for this Signalling Point and those destined for another Signalling Point, • The parts used in Discrimination are: • The Network Indicator (NI) in SIO • The Destination Point Code (DPC) in the Routing Label. • The Network Indicator (NI) is a 2/bit code found in the SIO. • The Network Indicator (NI) is used to distinguish between: • - National and International networks • - Two different National Signalling networks
Message Handling functions • 2. Message Routing • It involves directing the messages to the correct SL. • In User Message the DPC and SLS contain the information required for the Routing function. • In Management Messages the DPC and SLC contain the information required for the Routing.
Message Handling functions 3. Message Distribution Directs incoming Signal Units (SUs) to the correct destination, i.e. User Parts, SCCP or The Network Management functions (Level 3). The Service Indicator (SI) in the SIO field indicates to which destination the message will be sent.
User Parts Signalling Link Distribution Discrimination Routing Message Handling functions
The Network Management functions • Signalling Traffic Management (STM) • Changeover • Change back • Forced rerouting • Controlled rerouting. • Signalling Route Management (SRM) • Signalling Link Management (SLM) • Activation • Restoration • Deactivation
The Network Management functions Signalling Traffic Management (STM) Signalling Traffic Management (STM) is responsible for Diversion and Reduction of Traffic. There are four procedures which enable the MTP to carry out the (STM) functions: - Changeover - Changeback - Foced rerouting - Controlled rerouting.
The Network Management functions • Signalling Traffic Management (STM) • 1. Changeover • Traffic diversion to an alternative link is implemented by the Changeover procedure. • Stopping the transmission on the faulty link. • Retransmission of buffers in both sides. • Changeover is a handshaking procedure which ensures that: • No SUs are lost • No SUs are duplicated • The Sus sequence is maintained • This is done by exchanging Changeover Order (COO) and Changeover Acknowledgement (COA) SUs.
The Network Management functions • Signalling Traffic Management (STM) • 2. Change back Procedure • When the normal link becomes available again, • the Change-back procedure is initiated. • The change-back Procedure is as follows: • (SP) A prevents the traffic from being transmitted to via the alternative link. • Signalling Point (SP) A send a Change-back Declaration (CBD). • Signalling Point (SP) B replies with a Change-back Acknowledge (CBA). • Traffic then resumes on the normal link.
The Network Management functions • Signalling Traffic Management (STM) • Forced Rerouting • When a signalling route to a particular Signalling Point (S) is indicated as unavailable, Forced Rerouting is carried out by the Signalling Traffic Management (STM) functions. • Controlled Rerouting • Controlled Rerouting is used when: • Traffic is to be directed from an alternative Signalling Route back to the normal Signalling Route, which has become available. • A Transfer Restricted (TFR) SU is received, indicating congestion on a Signalling Route.
The Network Management functions Signalling Traffic Management (STM) Rerouting Procedure. A similar procedure is used in both cases. In a route restriction case (where traffic is restricted on a route due to congestion), traffic is transferred from the restricted route to an alternative route using a Controlled rerouting Buffer, e.g. - Traffic is stopped on the restricted route - MSUs are stored in the Rerouting Buffer - An alternative route is selected - Traffic resumes on the alternative route Flow Control ( If No alternative is available )
Signalling Link Management The Signalling Link Management (SLM) function is used to control the Signalling Link. There are three basic Signalling Link Management procedures: - Activation - Restoration - Deactivation
Signalling Link Management • Activation • -The process of bringing a SL into service. • Link Restoration • In case of SL failure, a Link Restoration procedure is defined to restore the faulty SL. • During restoration the following takes place: • (1) Initial Alignment • (2) Signalling Link Test
Signalling Link Management Link Deactivation A link in service can be taken out of service. For example a link will be taken out of service when it is required for another signalling route. This procedure is called Deactivation. Link Deactivation will only take place if there is no traffic on the Signalling Link (SL). The Signalling Terminal of the deactivated Signalling Link will also be taken out of service.