Message Transfer Part (MTP)

MTP acts as the carrier for all SS7 message, providing reliable transfer of message from one signaling point to another.This function include levels one, two and three. In addition to providing signaling point to signaling point communication, the MTP also provide error detection and correction

The methodology of MTP are very similar to those used in other bit oriented protocol (BOPs), such as X.25.Sequence numbering and error checking mechanism are very similar
The signal unit structure used in all SS7 message provides all the information required by MTP level two and three.Flow control is provided through the use of a special signal unit called the Link Status Signal Unit (LSSU)

MTP provides all functions of layers one,two and three.Level two of MTP provides error detection/correction, as well as error checking through the cjeck bit field.In addition to these two fundamental functions, the following is provided:

1.Signal unit delimitation
2.Signal unit alignment
3.Signal unit error detection
4.Signal unit error correction
5.Signal unit initial alignment
6.Signal unit error monitoring
7.Flow control


Structure of MTP level two


The backward sequence number and the forward sequence number are used for sequencing packet and are used by level two to ensure tha all transmitted packets are received.They are also used for positive and negative acknowledgements.The indicator bits are used to request a retransmission.The lenght indicator allows level two to determine the type of signal unit being sent and the cyclic redundancy check (CRC) field is used to detect data errors in the signal unit


Flag

flag is used to indicates the beggining of a signal and the end of a signal unit.The flag in US network is used to indicate both beginning of the signal unit and the end another.In some other networks there can be both an opennung and closing flag


Sequence numbering

The SS7 protocol used sequence numbering like many other layer two bit oriented protocols.The SS7 technique is just a little different from, say X.25 but the principle is the same.Both a forwars sequence number (FSN and a backward sequence number (BSN) are provided


Indicator bits

The indicator bits are used to request a retransmission.There are two indicator bits; a forward indicator and a backward indicator bit.During normal conditions, both indicator should be of the same value (0 or 1).When a retransmission is being requested, the signal unit being sent by the signal point requesting the restransmission will have an intervet backward indicator bit.The forward indicator bit retains its original value.This indicates to the distant signaling point that an error occured and retransmission must take place.


Length indicator

it is used by level two to determine which type of signal unit is being sent.The value of the length indicator can be:
1. 0 (Fill in signal unit)
2. 1 or 2 ( link status signal unit)
3. 3 or more (message signal unit)


Cyclic redundancy check (CRC)

it is the last field in the signal unit.This field is calculated using the fields immediatly following the flag, up to the chek bit field itself