1 Frequency/channels
1.1 As a rule, paired frequencies should be used at HF and MF, in which case an acknowledgement is transmitted on the frequency paired with the frequency or the received call. In exceptional cases for national purposes a single frequency may be used. If the same call is received on several calling channels, the most appropriate shall be chosen to transmit the acknowledgement. A single frequency channel should be used at VHF.
1.2 International calling
The paired frequencies listed in RR Appendix 31 (Appendix S17, Part A) and in Annex 5 of this Recommendation should be used for International DSC calling.
1.2.1 At HF and MF international DSC frequencies should only be used for shore-to-ship calls and for the associated call acknowledgements from ships fitted for automatic DSC operation where it is known that the ships concerned are not listening to the coast station's national frequencies.
1.2.2 All ship-to-shore DSC calling at HF and MF should preferably be done on the coast station's national frequencies.
1.3 National calling
Coast stations should avoid using the international DSC frequencies for calls that may be placed using national frequencies.
1.3.1 Ship stations should keep watch on appropriate national and international channels. (Appropriate measures should be taken for an even loading or national and international channels.)
1.3.2 Administrations are urged to find methods and negotiate terms to improve the utilization of the DSC channels available, e.g.:
- coordinated and/or joint use of coast station transmitters:
- optimizing the probability of successful calls by providing information to ships on suitable frequencies (channels) to be watched and by information from ships to a selected number of coast stations on the channels watched on-board.
1.4 Method or calling
1.4.1 The procedures set out in this section an applicable to the use of DSC techniques, except in cases of distress, urgency or safety, to which the provisions of RR Chapter NIX (SVII) are applicable.
1.4.2 The call shall contain information indicating the station or stations to which the call is directed, and the identification of the calling station.
1.4.3 The call should also contain information indicating the type of communication to be set up and may include supplementary information such as a proposed working frequency or channel; this information shall always be included in calls from coast stations, which shall have priority for that purpose.
1.4.4 An appropriate digital selective calling channel chosen in accordance with the provisions of RR Nos. 4323S to 4323AB (S52.128 to S52.137) or Nos. 4323AJ to 4323AR (S52.145 to S52.153), as appropriate, shall be used for the call.
2 Operating procedures
The technical format of the call sequence shall be in conformity with the relevant ITU-R Recommendations.
The reply to a DSC requesting an acknowledgement shall be made by transmitting an appropriate acknowledgement using DSC techniques.
Acknowledgements may be initiated either manually or automatically. When an acknowledgement can be transmitted automatically, it shall be in conformity with the relevant ITU-R Recommendations.
The technical format of the acknowledgement sequence shall be in conformity with the relevant ITU-R Recommendations.
For communication between a coast station and a ship station, the coast station shall finally decide the working frequency or channel to be used.
The forwarding traffic and the control for working for radiotelephony shall be carried out in accordance with Recommendation ITU-R M.1171.
A typical DSC calling and acknowledgement sequence contains the following signals (see Recommendation ITU-R M.493).
Composition of a typical DSC calling and acknowledgement sequence |
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NOTE 1 - if the calling sequence EOS signal incorporates a request for acknowledgement "RQ"(117) an acknowledgement is mandatory and shall incorporate the EOS signal "BQ"(122).
2.1 Coast station initiates call to ship
Figures 1 and 2 illustrate the procedures below in flow chart and by time sequence diagram respectively.
2.1.1 There are two categories of calls for commercial communications:
- routine call;
- ship's business call (see Recommendation ITU-R M.493. Annex 1, з6.4.1).
2.1.2 If a direct connection exists between the calling subscriber and the coast station, the coast station asks the calling subscriber for the approximate position of the ship.
2.1.3 If the ship's position cannot be indicated by the caller, the coast station operator tries to find the location in the information available at the coast station.
2.1.4 The coast station checks to see whether the call would be more appropriate through another coast station (see з1.3.2).
2.1.5 The coast station checks to see whether the transmission of a DSC is inappropriate or restricted (e.g. ship not fitted with DSC or barred).
2.1.6 Assuming a DSC is appropriate the coast station composes the calling sequence as follows:
- selects format specifier.
- enters address of the ship,
- selects category,
- selects telecommand information,
- inserts working frequency information in the message part of the sequence, if appropriate,
- usually selects "end of sequence" signal "RQ". However, if the coast station knows that the ship station cannot respond or the call is to a group of ships the frequency is omitted and the end of sequence signal should be 127, in which case the following procedures (з2.1.13 to 2.1.15) relating to an acknowledgement are not applicable.
2.1.7 The coast station verifies the calling sequence.
The call shall be transmitted once on a single appropriate calling channel or frequency only. Only in exceptional circumstances may a call be transmitted simultaneously on more than one frequency.
2.1.8 The coast station operator chooses the calling frequencies which are most suitable for the ship's location.
2.1.8.1 After checking as far as possible that there are no calls in progress, the coast station operator initiates the transmission of the sequence on one of the frequencies chosen. Transmission on any one frequency should be limited to no more than 2 call sequences separated by intervals of at least 45 s to allow for reception of an acknowledgement from the ship, or exceptionally (see Recommendation ITU-R M.493) to one "call attempt" consisting of up to five transmissions.
2.1.8.2 If appropriate, a "call attempt" may be transmitted, which may include the transmission of the same call sequence on other frequencies (if necessary with a change of working frequency information to correspond to the same band as the calling frequency) made in turn at intervals of not less than 5 min, following the same pattern as in з2.1.8.1.
2.1.9 If an acknowledgement is received further transmission of the call sequence should not take place. The coast station shall then prepare to transmit traffic on the working channel or frequency it has proposed.
2.1.10 The acknowledgement of the received call should only be transmitted upon receipt of a calling sequence which terminates with an acknowledgement request.
2.1.11 When a station called does not reply, the call attempt should not normally be repeated until after an interval of at least 15 min. The same call attempt should not be repeated more than five times every 24 h. The aggregate of the times for which frequencies are occupied in one call attempt, should normally not exceed 1 min. The following procedures apply at the ship:
2.1.12 Upon receipt of a calling sequence at the ship station, the received message is recorded and an appropriate indication is activated as to whether the call category is "routine" or "ship's business". The category does not affect the DSC procedures at the ship.
2.1.13 When a received call sequence contains an end of sequence signal RQ, an acknowledgement sequence should be composed and transmitted in accordance with з 2.
The format specifier and category information should be identical to that in the received calling sequence.
2.1.13.1 If the ship station is not equipped for automatic DSC operation, the ship's operator initiates an acknowledgement to the coast station after a delay of at least 5 s but no later than 41/2 min of receiving the calling sequence, using the ship-to-shore calling procedures detailed in з 2.2. However the transmitted sequence should contain a "BQ" end of sequence signal in place of the "RQ" signal.
If such an acknowledgement cannot be transmitted within 5 min of receiving the calling sequence then the ship station should instead transmit a calling sequence to the coast station using the ship-to-shore calling procedure detailed in з2.2.
2.1.13.2 If the ship is equipped for automatic DSC operation, the ship station automatically transmits an acknowledgement with an end of sequence signal "BQ". The start of the transmission of this acknowledgement sequence should be within 30 s for HF and MF or within 3 s for VHF after the reception of the complete call sequence.
2.1.13.3 If the ship is able to comply immediately the acknowledgement sequence should include a telecommand signal which is identical to that received in the calling sequence indicating that it is able to comply. If no working frequency was proposed in the call, the ship station should include a proposal for a working frequency in its acknowledgement.
2.1.13.4 If the ship is not able to comply immediately the acknowledgement sequence should include the telecommand signal 104 (unable to comply), with a second telecommand signal giving additional information (see Recommendation ITU-R M.493).
At some later time when the ship is able to accept the traffic being offered, the ship's operator initiates a call to the coast station using the ship-to-shore calling procedures detailed in з 2.2.
2.1.14 If a call is acknowledged indicating ability to comply immediately and communication between coast station and ship station on the working channel agreed is established, the DSC call procedure is considered to be completed.
2.1.15 If the ship station transmits an acknowledgement which is not received by the coast station then this will result in the coast station repeating the call (in accordance with з2.1.11). In this event the ship station should transmit a new acknowledgement. If no repeated call is received the ship station should transmit an acknowledgement or calling sequence in accordance with з 2.1.13.1.
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