Recommendation R.116 (R.11m)

      MAINTENANCE TESTS TO BE CARRIED OUT ON INTERNATIONAL
                           TDM SYSTEMS


       The CCITT,

considering

       a)   the savings to be made by reducing interruption time  on  TDM
links;

       b)   the importance of being able  to  determine  responsibilities
between the several parties who, of necessity are involved in maintenance 
questions for the networks;

       c)   the advantages of standardization regarding maintenance;

       d)   maintenance loops are standardized in Recommendation R.115,

unanimously declares the following:

       that when the quality of the TDM-link has deteriorated beyond  the  alarm
limit or if the local muldex gives an alarm, supplementary measurement should be 
performed. The following test and supervision methods can be used.

1.     Testing and supervision of TDM systems

1.1    Bit error rate

       The synchronization bits are supervised and an error rate alarm is issued 
when the error rate exceeds a preset limit, 10-3, 10-4 or 10-5.

1.2    Bit error counter

       All synchronization bit errors shall be registered in a cyclic counter and 
it shall be possible to read the value of this counter by command.

1.3    Routine supervision

       The operation  of  the  TDM  equipment  and  maintenance  channel  should
continuously be supervised by a repetitive test signal. An alarm is issued when a 
correct acknowledgement is not received for a specified number of test signals.  The 
alarm is reset automatically when the fault situation ceases.

1.4    Alarm reset

       It shall be possible to reset all alarms from the local side. The command 
shall also be able to reset the error rate value.

1.5    System alarms

       Failures that affect operation of the whole or a major part  of  the  TDM
equipment are classified as one category. The supervised functions are:

       -    Carrier:         Loss of carrier is detected by the data modem
                            through CCITT circuit 109 or  the  corresponding
                            circuit.












       -    Synchronization: Loss of synchronization is detected by the TDM
                            multiplexer in  accordance  with  the  respective
                            CCITT Recommendation.

       -    Multiplexing     Failure of the TDM central logic is  detected  by
         logic:             internal   supervision   facilties   within    the
                  multiplexer.

       -    Power:           Failure of the power supply is detected  when  the
                              telegraph   power   supply   exceeds    tolerance
                             limits.

1.6    Changing of active side

       When the TDM equipment is duplicated the active side can be  changed  by
command or manually.

       When the remote or local side  is  changed  automatically  or  manually,
information about what side is executive must be sent when the change has  been
executed.

1.7    Looptest on standby side

       When the TDM equipment is duplicated the standby modem can be tested by the 
setting of loop b by command. The test result is sent over the active maintenance 
channel.

1.8    Automatic restart

       When the remote TDM is automatically restarted, information shall be sent 
informing about the restart and alarm status.

1.9    Acknowledgement

       The acknowledgement consists of one character and should have the following 
values:

       5 - Acknowledgement;

       0 - Not acknowledgement.

2.     Format of the messages

       The messages which will be sent over the 50 baud  maintenance  channel
shall have the following structure:

       C1C2M1M2....Mn;

       C1C2: Message category (two characters);

       M1-Mn: Information (number of characters unlimited).

       After the reception of a message at the receiving end, the receiving end 
shall send one character to the originating end as an acknowledgement of  the
reception.

2.1    Message categories

       The purpose of the message categories (called MC) is to give a  direct
command or to inform the control equipment in an exchange, a maintenance centre 
or a TDM about what type of information the following message contains.







       The MC consist of two characters, and each character is a decimal number 
from 0 to 9. The numbers are coded according to alternative A (CSC) in R.115.

2.2    Information

       The information characters are a part of an order to  the  remote  TDM
equipment or information from the remote TDM-equipment, depending on the Message 
Category Signal.

       The number of information characters in a message is not limited.

       The characters are decimal numbers from 0 to  9,  coded  according  to
alternative A(CSC) in R.115.

3.     Maintenance messages

       Using the format described in  2 the maintenance messages shall  have  a
message category and information as given in the table below:


                                    TABLE 1
                                        
                              Maintenance messages


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