1、160 Rec. ITU-R M.822-1 RECOMMENDATION ITU-R M.822- 1 CALLING-CHANNEL LOADING FOR DIGITAL SELECTIVECALLING (DSC) FOR THE MARITIME MOBILE SERVICE (Question ITU-R 918) ( 1992- 1994) The ITU Radiocommunication Assembly, considering a) maritime mobile service bands (MF, HF and VHF); that the Radio Regula
2、tions contain designated and dedicated channels for the digital selective-calling in the b) communication systems in conjunction with the digital selective call; c) that the ITU-R studied the subject of calling-channel loading and developed a methodology on how to determine acceptable calling-channe
3、l loading and on how to decide how many calling channels would be required in each of the bands, that Question ITU-R 9/8 requests studies of the information necessary for the operation of future maritime recommends 1. O. 1 E on an MF or HF channel or 0.15 E on a VHF channel; that the maximum traffic
4、 offered to any calling channel used for digital selective calling should not exceed 2. that, as a consequence of 5 1, where a DSC channel is used for distress and safety calling the probability of a distress call attempt being mutilated will be less than O. 1 %, assuming that two distress call atte
5、mpts are transmitted per hour; 3. routine DSC calls which can be supported is: that, as a consequence of 4 1 and 2 and based on the calculations given in Annex 1, the maximum number of - - - 24 callsk on an MF or HF non-distress and safety coast station transmit channel; 28 callsk on an MF or HF non
6、-distress and safety ship station transmit channel; 500 callsk on the single DSC VHF channel (channel 70); 4. and safety DSC calls which can be supported on the dedicated MF and HF distress and safety channels is 20 callsh; 5. scanning be based on the calculations given in Annex 2. that, as a conseq
7、uence of 0 2 and based on the calculations given in Annex 1, the maximum number of urgency that the implications of using conventional scanning receivers in terms of the probability of calls lost due to ANNEX 1 Channel loading calculations 1. General ALOHA traffic theory 1.1 The characteristics of D
8、SC on common calling channels may be compared to a classical ALOHA channel which is a form of demand-assignment time-division multiple access suited to channels with random access where there is a high peak-to-average calling density. The number of calls originating per unit time then follows a Pois
9、son - diqtrihiitinn _ COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services_ ITU-R RECMN*M. B22-3 94 4855232 0523938 OT8 W Rec. ITU-R M.822-1 161 Z.2 According to classical ALOHA calculations, if R is the average number of DSC calls plus re
10、-transmissions per unit time and z is the call duration, the probability that a given call will have to be re-transmitted (due to call collisions) is given by: 1.3 R z, the channel traffic is therefore equivalent to the total traffic loading (including repetitions j in erlang. 1.4 It should be noted
11、 that the factor of 2 in the equation in 1.2 is due to the fact that, with purely random calling, there is a time period of duration 22 in which no other call must originate if a collision is to be avoided, i.e. IT from the start of transmission of any particular call. Any call originating during th
12、is period will overlap another call to a greater or lesser extent and thus collide. Z.5 If r is the average number of calls offered to the channel (excluding retransmissions), it follows that: R = r + R (1 - e-2Rr) and hence: rz = R where rz is the channel utilization, i.e. the amount of time that t
13、he channel is occupied by the offered DSC calls (excluding the time occupied by retransmissions). This value is also equivalent to the offered traffic in erlang. 1.6 If the channel utilization (rz) is plotted against the channel traffic (Rz) the graph in Fig. 1 is obtained. If, in the above equation
14、, the differential of rz with respect to Rz is equated to zero, it will be found, as illustrated on the graph, that rz reaches a maximum value of 1/2e = 0.184 when Rz = 0.5. This value of rz is therefore the maximum capacity of the channel since at higher values of Rz the channel becomes unstable an
15、d its utilization reduces due to an increasing number of repetitions. FIGURE i Channel utilization versus channel traffic 5 4 h Y 1 0.5 O - Maxat0.184 O 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 Channel utilization (TZ) W2 1.7 It can be seen that the relationship between the channel utilization (r
16、z) and the channel traffic (RT) is substantially linear up to around rz = 0.1 E and produces an acceptable margin to cope with peak concentrations of traffic. At this value Rz = 0.13 E and the probability of a re-transmission being required due to call collisions, from 1.2, is therefore: COPYRIGHT I
17、nternational Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services162 ITU-R RECMN*M- 822-1 94 4855212 0521939 T34 Rec. ITU-R M.822-1 2. MF/HF DSC calling 2.1 MFIHF routine calling 2.1.1 Routine (non-distress and safety) DSC calling at MF/HF uses paired frequencies
18、 in which the coast station transmit channel will contain DSC calls to ships which have a duration of 8.2 s (including a 200-bit dot pattern) and acknowledgements from coast stations to ships which have a duration of 6.4 s (having a 20-bit dot pattern). Although the ALOHA channel calculations given
19、in Q 1 assume that the call duration z is a constant, the calculations are stili considered to be valid for slight variation in the length of DSC calls bearing in mind that the O. 1 E figure (0 1.7) is a conservative figure compared to the maximum channel utilization of 0,184 E. 2.1.2 The ship stati
20、on transmit channel will contain DSC calls to coast stations which have a duration of 6.4 s (including a 20-bit dot pattern) and acknowledgements from ships to coast stations which also have a duration of 6.4 s (20-bit dot pattern). 2.1.3 For the coast station transmit channel of a paired frequency
21、the total number of DSC calls and acknowledgements allowable within the O. 1 E limit per channel is therefore: 0.1 x 3600 8.2 + 6.4 = 24 calls (and 24 acknowledgements) per hour 2.1.4 For the ship station transmit channel of a paired frequency the total number of DSC calls and acknowledgements allow
22、able within the 0.1 E limit per channel is therefore: 0.1 x 3600 6.4 + 6.4 = 28 calls (and 28 acknowledgements) per hour 2.1.5 It should be noted that, as a result of imperfect propagation and interfering signais, in some cases calls or acknowledgements will need to be repeated (over and above any r
23、epetitions as a consequence of call collisions) to establish each successful traffic communication. However, due to the conservative nature of the O. 1 E figure (see Q 1.7, 2.1.1 and Fig. I), the above figures for the offered calls per hour are considered to be acceptable. 2.2 MFIHF distress and saf
24、ety calling 2.2.1 At MF and HF, dedicated frequencies exist for DSC distress and safety (including urgency) calling. However, the use of such channels for safety and urgency calling is allowed on condition that the total channel loading is maintained below 0.1 E (see Recommendation ITU-R M.541, Anne
25、x 1, Q 4). For the purposes of the following calculations, it is assumed that this figure is intended to apply to the offered traffic. 2.2.2 Assuming that two single frequency DSC distress call attempts (each consisting of five consecutive distress calls) are transmitted on any particular DSC distre
26、ss and safety channel per hour and that each call attempt results in one DSC distress call acknowledgement, the distress traffic loading can be calculated as follows: Duration of single distress call (with 200-bit dot pattern) = 7.2 s Duration of distress acknowledgement (with 200-bit dot pattern) =
27、 8.6 s (2 x 5 x 7.2) + (2 x 8.6) 3 600 = 0.0248 E Therefore traffic loading 2.2.3 If the total allowable traffic = 0.1 E, safety and urgency traffic may occupy 0.1 - 0.0248 = 0.0752 E. 2.2.4 The duration of a safety or urgency call addressed to an individual ship (worst case, including an address an
28、d a 200-bit dot pattern) = 8.2 s. The duration of a safety or urgency call transmitted by a ship to a coast station (containing an address and a 20-bit dot pattern) = 6.4 s. The acknowledgement in each case (with a 20-bit dot pattern) also has a duration of 6.4 s. COPYRIGHT International Telecommuni
29、cations Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*M. 822-1 94 = 4855232 0521940 756 Rec. ITU-R M.822-1 163 2.2.5 number of such calls allowed per hour (where each call results in one acknowledgement) is therefore given by: Assuming that an equal number of safe
30、ty or urgency calls are transmitted to ships and coast stations, the total 0.0752 x 3600 - 19.8 (8.2 + 6.4) + (6.4 + 6.4)/2 - 2.2.6 In practice, the majority of safety or urgency calls transmitted by coast stations will be addressed to all ships (for which the duration of the DSC call is 7.2 s) and
31、no DSC acknowledgement will result. Hence it may be assumed that, on average, 20 safety or urgency culls per hour may be transmitted on MF/HF distress and safety frequencies. 2.2.7 with safety or urgency calls may be calculated as follows: The probability of a complete distress call attempt being mu
32、tilated by collisions with other distress calls or - the probability of a single distress call being mutilated (from 8 1.7): - but the five calls within a distress call attempt cannot interfere with each other and therefore the traffic loading due to calls capable of mutilating a distress call may b
33、e reduced by 4 x 7.2/3600 = 0.0080. Furthermore, a distress acknowledgement to a distress call attempt will not interfere since it follows the distress call attempt and therefore an additional channel loading of 8.6/3600 = 0.0024, i.e. a total of 0.0104, may be subtracted; - therefore, the probabili
34、ty of a single distress call being mutilated: - therefore, the probability of all five calls within the call attempt being mutilated: (0.2127)5 = 0.0004 (=0.04%) 3. VHF DSC calling 3.1 VHF routine calling 3.1.1 All DSC calling at VHF (Le. routine and distress and safety calling) uses a single freque
35、ncy channel (channel 70) for all calls and acknowledgements. Individual station calls and acknowledgements (including urgency and safety calls) have a duration of 0.533 s. 3.1.2 If the calling is random the channel capacity for O. 1 E of offered traffic is: 0.1 x 3600 0.533 + 0.533 = 337.7 calls (an
36、d 337.7 acknowledgements) per hour 3.1.3 However, all VHF DSC routine calling requires listening to the channel and transmitting only after detection of the absence of another call or the cessation of another call (see Recommendation ITU-R M.541, fi 3.6) and therefore it may be assumed that truly ra
37、ndom calling will only apply to stations which are out of range of the station transmitting an existing DSC call and to distress and safety calls. Examples of such “unheard call” cases are those of two ships, both within range of a particular coast station but out of range of each other, and two coa
38、st stations, both within range of a particular ship but out of range of each other. 3.1.4 The above “listen before transmission” calls may be compared to a slotted ALOHA system where transmissions are only permitted to start at the beginning of a time slot which have the same duration as the call du
39、ration. In this case, since calls, if they collide, will do so completely instead of merely overlapping to a certain extent, the factor of 2 in the equation given in 0 1.2 and explained in 0 1.4 is reduced to a factor of 1. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLice
40、nsed by Information Handling Services ITU-R RECMN+M. 822-3 94 = 4855232 052194L 692 W 164 Rec. ITU-R M.822-1 3.1.5 call collisions) is given by: Therefore, in this slotted ALOHA case the probability that a given call will have to be re-transmitted (due to 3.1.6 By carrying out similar calculations t
41、o those in 0 1 it will be found that the maximum value of channel utilization (rz) is doubled to 0.368 and, in comparison with 0 1.7, a channel utilization of = 0.2 E equates to a channel traffic loading (Rz) of 0.26 which still results (using the formula in 0 3.1.5) in a probability of a re-transmi
42、ssion being required due to call collisions of 0.229. 3.1.7 The equivalence of the “listen before transmission” VHF DSC calling to the slotted ALOHA situation is only strictly valid if any delay between the detection of no call and the transmission of a VHF DSC call is a minimum and constant value b
43、etween different equipments. In other words, there should be no random delay built into the equipment otherwise the doubling of the channel utilization will be reduced. 3.1.8 Applying a similar calculation to that in $ 3.1.2 it can be seen that the VHF DSC channel would have a capacity of 675 offere
44、d calls (and 675 acknowledgements) per hour if all calls were of the “listen before transmission” type. A more rigorous and complex mathematical analysis of the VHF situation may be conducted using calculations of “carrier sense multiple access” (CSMA) packet switching systems. 3.1.9 In practice, fo
45、r the reasons stated in 0 3.1.3, there will be a mix of random “unheard” calls and the “listen before transmission” calls and if a 50/50 mix is assumed, the channel should support approximately 500 calls per hour which is equivalent to 0.15 E. 3.2 VHF distress and safety calling 3.2.1 Assuming that
46、two VHF DSC distress call attempts (each consisting of five consecutive distress calls) are transmitted on channel 70 per hour and that each call attempt results in one DSC distress call acknowledgement, the distress traffic loading can be calculated as follows: Duration of single distress call (wit
47、h 20-bit dot pattern) = 0.45 s Duration of distress acknowledgement (with 20-bit dot pattern) = 0.567 s (2 x 5 x 0.45) + (2 x 0.567) 3 600 = 0.001565 E Le. traffic loading 3.2.2 It should be noted that VHF distress calls are transmitted without listening to the channel. The probability of a complete
48、 distress call attempt being mutilated by collisions with non-distress calls (routine, safety or urgency calls) may be calculated as follows: - The channel traffic loading on VHF, assuming a 50/50 mix of “listen before transmission” and “unheard” calls, based on the figures for Rz given in 0 1.7 and
49、 3.1.6 will be between 0.13 and 0.26 so an average value of 0.20 may be assumed. - Since a distress call may be transmitted at any random time, the classical ALOHA formula applies to the probability of a collision between a single distress call transmission with a non-distress call (0 1.2), .e.: = 1 - ,-2 x 0.20 = 0.330 - However, the distress call attempt consists of five consecutive distress calls and therefore this five-call transmission, once started, will prevent the transmission of any new non-distress calls within listening ra
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