network protocols handbook.pdf

312Protocols GuideSS7 / C7 ProtocolProtocol NameMTP2 and MTP3: MessageTransfer Part level 2 and level3Protocol DescriptionMessage Transfer Part (MTP), a protocol in the SS7/C7 protocolsuite, transfers signal messages and performs associatedfunctions, such as error control and signaling link security. MessageTransfer Part (MTP) also provides reliable routing withina network. MTP has two parts, MTP level 2 (MTP2) and level3 (MTP3), that performs functions at the layers 2 and 3 respectivelyof the OSI 7 layers model.Message Transfer Part Level 2 (MTP2) resides at Layer 2 in theSS7 protocol stack. It is responsible for the reliable transmissionof signalling units over an individual Signalling Link. MTP2 reliabilityis achieved through retransmission techniques.Message Transfer Part level 3 (MTP3) is the network layer inthe SS7 protocol stack. It routes SS7 signalling messages topublic network nodes by means of Destination Point Codes, andto the appropriate signalling entity within a node by means of aService Info Octet. MTP3 is specified as part of the SS7 protocoland is also referred to as part of the B-ICI interface for ATM.MTP3 sits between MTP2 and the user parts (ISUP, TUP, SCCPand TCAP) of the SS7 protocol stack. B-ISUP is an ApplicationLayer protocol run over MTP3.MTP3 is split into two distinct parts, SMH (Signalling MessageHandling) and SNM(Signalling Network Management). The SNMpart looks after the general management of MTP, the SHM partdeals with the discrimination, distribution and routing of signallingmessages. MTP3 defines the functions and procedures ofthe signalling system for signalling message handling and signallingnetwork management. Signalling message handling consistsof the actual transfer of a signalling message and directingthe message to the proper signalling link or user part. Signallingnetwork management consists of controlling the signalling messagerouting and configuration of the signalling network facilitiesbased on predetermined information and the status of the signallingnetwork facilities.MTP3 provides a connectionless message transfer system forpassing information across a network. MTP3 includes a numberof link-protection features, to allow automatic rerouting ofsignalling messages around broken signalling transfer points.It includes certain management functions for congestion controlon signalling links.MTP2 User Adaptation Layer (M2UA) is used to access MTP2functions using SCTP (Streaming Control Transmission Protocol).MTP3 User Adaptation Layer (M3UA) is a protocol forsupporting the transport of any SS7 MTP3-User signaling (e.g.,ISUP, SCCP and TUP messages) over the IP Network.Protocol StructureThe format of the header of MTP2 is shown as follows:FlagBSN (7 bits)FSN (7 bits)LI (6 + 2 bits)SIOSIFChecksum (16 bits)Flag7 8bitBIBFIB• BSN - Backward sequence number. Used to acknowledgemessage signal units which have beenreceived from the remote end of the signalling link.• BIB - Backward indicator bit. The forward and backwardindicator bit together with the forward and backwardsequence numbers are used in the basic errorcontrol method to perform the signal unit sequencecontrol and acknowledgment functions.• FSN - Forward sequence number.• FIB - Forward indicator bit.• LI - Length indicator. This indicates the number of octetsfollowing the length indicator octet.• SIO - Service information octet.• SIF - Signalling information field.• Checksum - Every signal unit has 16 check bits forerror detection.The structure of the MTP-3 header is shown as follows:Service indicator4 8bitSubservice field• Service indicator - Used to perform message distributionand in some cases to perform message routing.The service indicator codes are used in internationalsignalling networks for the following purposes:• Signalling network management messages• Signalling network testing and maintenancemessages• SCCP• Telephone user part• ISDN user part• Data user part• Reserved for MTP testing user part.• Sub-service field - The sub-service field contains thenetwork indicator and two spare bits to discriminatebetween national and international messages.