MTP2 (Message Transfer Part 2)

Questionnaire on MTP2: 

1.)   What is the difference between the MTP2 and MTP3

2.)   What are functions of MTP2.

3.)   What is the overhead of MTP2 in Signaling units (SUs).

4.)   How MTP2 identifies the type of SU.

5.)   What signifies the LI. What did it mean by length.

6.)   What is maximum size of SU.

7.)   What is bit stuffing.

8.)   How MTP2 identifies the SU not aligned.

9.)   What is error detection mechanism.

10.) What are error correction methods.

11.) Difference between the BER and PCR.

12.) Define states in initial Alignment.

13.) Define the procedure for Signaling link alignment.

14.) How MTP2 monitors the link failure.

15.) Difference between the SIE and SIN and when they are invoked.

16.) What is processor outage and Link Flow control.

17.) Describe the AERM.

Answers:

1.) MTP2 provides a reliable means of transfer for signaling information between two directly connected signaling points (SPs), ensuring that the signaling information is delivered in sequence and error-free while MTP3 is a protocol used in End to End signaling.

2.) MTP2 performs the following functions:
• Delimitation of signal units. - Insert the Flag in SU ( start and End of SU in 01111110 form)
• Alignment of signal units. - Check the Alignment of SU
• Signaling link error detection - 16 Bit CRC method used for detection.
• Signaling link error correction by retransmission - uses BER and PCR
• Signaling link initial alignment - to align the Link in service.
• Error monitoring and reporting - Uses SUERM and AERM methods
• Link flow control - use the SIB if congestion

3.) Following field in FISU/LSSU/MSU is the MTP2 overhead:

- Flag: 8 bit flag with the pattern 0111 1110 to indicate the start and end of SU.
 - BSN: 7bit BSN, indicates the last correctly received SU.
- BIB: 1bit, to toggle in case of error with the received SU.
- FSN: 7 bit FSN, identifies the transmitted SU.
- FIB:Toggle indicates the retransmission of SU.
- LI:  indicates how many octets resides between LI and CRC field.
- CK: 16 bit CRC check to detect the error in SU.

4 and 5.) MTP2 identifies the SU by length Indicator (LI): LI is to indicates how many octets resides between LI and CRC field.

LI =0 for FISU

LI= 1 or 2 for LSSU

LI>= 2 for MSU, because LI is 8 bit in size so takes upto 64 length so if the MSU size if more than 63 octets, it set it to 63 for more than 63 octets in MSU.

6.) Maximum size of MSU is 279 octats. FISU is 7 and LSSU is 9.

MSU=>SIF =272 + CK=2 + 2*Flag =2 + FSN, FIB=1 + BIB, BSN=1 + LI, spare=1
FISU=> CK=2 + 2*Flag =2 + FSN, FIB=1 + BIB, BSN=1 + LI, spare=1
LSSU=>  CK=2 + 2*Flag =2 + FSN, FIB=1 + BIB, BSN=1 + LI, spare=1 + SF=2

7.) Bit Stuffing: Flag is indicated as 0111 1110 to identifies the start and End of SU. This string can be found in Message as well, so to avoid the false flag in message, originating node add 0 after five consecutive 1's in message, is called bit stuffing.

8.) Out of alignment occurs if:

- SU size is not multiple of 8.
- more than six 1's detected in bit pattern.
- SU received greater than maximum size.

9.) Error detection is done by 16 bit CRC check.

10 .) MTP2 uses two method for error correction both are based on Retransmission.

- BER ( Basic Error Retransmission)
- PCR (Preventive Cyclic Retransmission)

11.) BER uses FSN, BSN, FIB and BIB to ack and Nack the SU but the PCR only uses the BSN and FSN, FIB and BIB are permanently set to 1.

12.) initial alignment states are:

- Idle
- Not Aligned
- Aligned
- Proving
- Ready
- In service

13.)Procedure of Signaling Link Initial Alignment:
     
                                            SPa                                             SPb
                                       
                                              SIOS  ----------------------->                            Idle State
                                              SIOS  ------------------------>
                             start T2  SIO        ----------------------- >                            Not Aligned State
                                stopT2              <-----------------------  SIO
                          start T3  SIN/SIE    ---------------------->                             Aligned
                           stop T3                 <-----------------------    SIN/SIE
                          start T4    SIN/SIE ---------------------->                                Proving
       Expires T4, start T1                  <--------------------- SIN/SIE
                                         FISU      ---------------------->                            Aligned/Ready
                                          FISU      --------------------->
                        stop T1                   <----------------------     FISU/MSU           In service  

14.) Link Failure is monitored by SUERM (Signaling Unit Error Rate Monitor);

It is leaky bucket mechanism, in which MTP2 monitor the error and increment the counter by 1, and if D number of consecutive correct SU received decrement by 1, if it is not 0. after the T value MTP2 informs to MTP3 about the link failure.

         Error --------------> +1
                                                              6 bit Counter               T -------------> Link Failure
          Good ------D-------> -1

     For 64 kbps links, the value of D is 256 and value of T is 64.

15.) If there is only one link (or none) is active between the two adjacent nodes and other node also start emergency alignment even if more than one  in service link is available, then SIE is invoked otherwise SIN.

16.) Processor Outage is indicates by SIPO in LSSU when SU can not transferred to MTP3.

Flow control indicates by SIB in LSSU when the MTP2 receive buffer become congested. It control by timer T6, remote congestion.

17.) AERM ( alignment Error rate Monitor) : AERM is active when link is in proving period of initial Alignment. counter start with 0, increased for every error-ed LSSU . If threshold value Tin, Tie (timer T4) reached for normal and emergency alignment, proving period expires. If proving period aborted M times,  link enters in Idle state.

The value of parameters for 64kbps links are:

• Tin = 4
• Tie = 1
• M = 5

• N = 16