3. Message Transfer Part
The Message Transfer Part (MTP) is divided into three levels:
3.a MTP Level 1
The lowest level, MTP Level 1, is equivalent to the OSI Physical Layer. MTP Level 1 defines the physical, electrical and functional characteristics of the digital signaling link. Physical interfaces defined include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kp/s channels), V.35 (64 kb/s), DS- 0 (64 kb/s) and DS-0A (56 kb/s).
3.b MTP Level 2
MTP Level 2 ensures accurate end-to-end transmission of a message cross a signaling link. Level 2 implements flow control, message sequence validation and error checking. MTP Level 2 is equivalent to the OSI Data Link Layer. An SS7 message is called a signal unit (SU). There are three kinds of signal units: Fill-In Signal Units (FISUs), Link Status Signal Units (LSSUs) and Message Signal Units (MSUs)
FISUs are transmitted continuously on a signaling link in both directions unless other signal units (MSUs or LSSUs) are present. FISUs carry basic Level 2 information only (i.e., acknowledgment of signal unit receipt by a remote signaling point). Because a CRC checksum is calculated for each FISU, signaling link quality is checked continuously by both signaling points at either end of the link. FISUs are sent only at predefined timer intervals
LSSUs carry one or two octets (8-bit bytes) of link status information between signaling points at either end of a link. The link status is used to control link alignment and to indicate the status of a signaling point to the remote signaling point.
MSUs carry all call control, database query and response, network management and network maintenance data in the signaling information field (SIF). MSUs have a routing label, which allows an originating signaling point to send information to a destination signaling point across the network.
The flag indicates the beginning of a new signal unit and implies the end of the previous signal unit (if any). The binary value of the flag is 0111 1110. Before transmitting a signal unit, MTP Level 2 removes “false flags” by adding a zero-bit after any sequence of five one-bits. Upon receiving a signal unit and stripping the flag, MTP Level 2 removes any zero-bit following a sequence of five one-bits to restore the original contents of the message. Duplicate flags are removed between signal units.
2. BSN (Backward Sequence Number)
The BSN is used to acknowledge the receipt of signal units by the remote signaling point. The BSN contains the sequence number of the signal unit being acknowledged.
3. BIB (Backward Indicator Bit)
The BIB indicates a negative acknowledgment by the remote signaling point when toggled.