3 Shipborne equipment performance acceptance criteria and tolerances
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC.1/Circular.1307 – Guidance on the Survey and Certification of Compliance of Ships with the Requirement to Transmit LRIT Information – (9 June 2009) - Annex – Guidance on the Survey and Certification of Compliance of Ships with the Requirement to Transmit LRIT Information - Appendix 1 – Conformance Test - 3 Shipborne equipment performance acceptance criteria and tolerances

3 Shipborne equipment performance acceptance criteria and tolerances

  3.1 When considering the performance of the LRIT system and the implications of the performance shipborne equipment the following issues need to be taken into account:

  • .1 The overall LRIT system utilizes a sophisticated array of hardware components, software systems and satellite/terrestrial communications networks, which include without limitation:

    • .1 Shipborne equipment

    • .2 Communications satellites;

    • .3 Land Earth Stations;

    • .4 Terrestrial communication networks;

    • .5 CSP routing/switching systems; and

    • .6 ASP systems;

  • .2 In common with all real world systems the overall LRIT system may suffer from data losses and data latency, i.e. non-delivery/late-delivery of messages;

  • .3 Data losses in the system as a whole are the result of a complex interaction of each of its components. The cause of losses can be as technical as a packet collision in the space segment or as practical as an equipment blockage caused by a ship funnel or crane, in port, etc.;

  • .4 In order to specify the conditions of the conformance testing scheme it should be assumed that the average loss rate of messages sent to and from the existing potential equipment is 4% in each direction. Thus, the loss on a command and response pair such as Data Network Identifier (DNID) download and its acknowledgement is 8%, and the loss on an automatic LRIT information transmission is 4%;

  • .5 Data latency in the system arises from the store-and-forward nature of the communication networks used. Each part of the communication chain, first stores a message and then forwards it to the next link which produces a robust system but one which has intrinsically variable latency;

  • .6 In order to specify the conditions of conformance testing scheme it should be assumed that 1 h latency is within the normal operation of the system;

  • .7 The design of a test for the conformance testing scheme allows for the real world performance of the overall LRIT system. Hence, it should be a design objective of the test that terminals are not incorrectly failed and thus cause shipowners to needlessly replace equipment;

  • .8 Consider a poll request, it should be assumed that 8% of poll request/responses are lost, therefore, a fair test requires that the poll request is repeated in a sequence of attempts;

Table 3
Poll attempts and expectations
Attempt P(Success) P(Failure) Expectation Cumulative P(Success) Cumulative Expectation
1 0.92000 0.08000 0.92000 0.92000 0.92000
2 0.07360 0.00640 0.14720 0.99360 1.06720
3 0.00589 0.00051 0.01766 0.99949 1.08486
Notes:
Column heading Explanation
Attempt Number of attempts
P(Success) Probability of success on that attempt
P(Failure) Probability of failure on that attempt
Expectation Contribution to the expected number of attempts
Cumulative P Cumulative probability of success
Cumulative Expectation Cumulative expectation of number of attempts required for a successful download

  • .9 Table 3 demonstrates that if the system retries the poll three times there is a 1 in 1,000 chance of wrongly failing the equipment due to statistical variation. The cumulative expectation shows that the cost of testing apparent non-compliant terminals is only increased by 8% compared to a regime which had only a single attempt;

  • .10 Consequently, it is recommended that each command is retried a maximum of three times, unless statistical variation can be excluded because of a network response, e.g., if the ship is in port and the equipment confirmed by the communication system as logged-out;

  • .11 The ASP recognized by the Administration or approved to conduct conformance testing should properly account for different network regions. For example, the Inmarsat-C network has four ocean regions and the test provider must ensure that they have addressed the region containing the equipment;

  • .12 This calculation assumes uncorrelated failures within the network which it is assumed to be true where there is a reasonable interval between attempts. To increase the likelihood that this assumption is correct it is recommended that the system retries are separated by at least 15 min; and

  • .13 A similar analysis may be performed for receipt of automatic transmission of LRIT information. The more transmissions of LRIT information that are received the greater confidence there may be that the equipment is conformant while not wrongly failing units. To achieve a 1 in 1,000 confidence of wrongly failing units, it is recommended that 40 out of 48 transmissions of LRIT information are received with a transmitting interval of 15 min, and 10 out of 12 transmission of LRIT information with a transmission interval of 1 h.

  3.2 The table set out below specifies for each conformance test the related acceptance criteria and the tolerances or limits within which the functional performance of existing equipment should be before it is considered as being acceptable.

Table 4
Shipborne equipment performance acceptance criteria and tolerances
CTN Acceptance criteria Tolerances
EL1 CSP acknowledgement received Maximum of 3 attempts separated by a minimum of 15 min
1 Confirmed by shipowner declaration on testing registration form prior to testing including certificate type and reference number None
2 Validated in conjunction with CTN 9 None
3 Validated in conjunction with CTN 9 None
4a Validated in conjunction with CTN 9 None
4b Assumed compliant in accordance with standard IMO guidelines and regulations None
5a Validated in conjunction with CTN 9 None
5b Confirmed by ASP recognized by the Administration or approved to conduct conformance testing based upon the confirmed inclusion of MEM code 11 (in the case of Inmarsat-C) and in the case of alternate hardware the compliance of the received message structure with the equipment manufacturers published standard for a message containing the generated Date and Time stamp None
5c Confirmed by ASP recognized by the Administration or approved to conduct conformance testing based upon the confirmed inclusion of MEM code 11 (in the case of Inmarsat-C) and in the case of alternate hardware the compliance of the received message structure with the equipment manufacturers published standard for a message containing the generated Date and Time stamp None
6 Forms the subject of this test specification which will be if the results are satisfactory and a Statement of conformity is issued by the ASP conducting the test (and the subsequent issuance of a Certificate of compliance by the Administration) None
7 CSP acknowledgement received and nil position reports received within 15 min Maximum of 3 attempts separated by a minimum of 15 min
8 Confirmed by manufacturer or by validation of technical specification None
9a Confirmed subsequent to receipt of 48 consecutive transmissions of LRIT information transmitted at 15-min intervals 3 attempts separated by a minimum of 15 min Acceptable results: a minimum of 40 out of 48 transmissions are received (>82% success rate)
9b Confirmed subsequent to receipt of 12 consecutive transmissions of LRIT information transmitted at 60-min intervals 3 attempts separated by a minimum of 15 min Acceptable results: a minimum of 10 out of the 12 transmissions are received (>82% success rate)
9c Confirmed by ASP or manufacturer or by validation of technical specification None
9d Confirmed by ASP or manufacturer of by validation of technical specification None
9e Validated in conjunction with CTN 9a and 9b Satisfactory: a minimum of 50 out of 60 transmissions are received (>82% success rate)
10 Confirmed subsequent to receipt of 1 polled transmission of LRIT information within 30 min 3 attempts separated by a minimum of 15 min Satisfactory: a minimum 1 out of 1 transmissions are received (100% success rate)
11 Confirmed by ASP or manufacturer of by validation of technical specification if internal GPS, or if external GPS confirmed by shipowner declaration on testing registration form prior to testing None
12 Confirmed by shipowner declaration on testing registration form prior to testing None
13 Confirmed by the ASP recognized by the Administration or approved to conduct conformance testing based upon confirmation that all communication links from the equipment – satellite – CSP – ASP are direct and secure with no third party ASP involvement None
EL2 CSP acknowledgement or CSP declaration received 3 attempts separated by a minimum of 15 min
Parent topic: Appendix 1 – Conformance Test
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