ITU-R M 822-1-1994 Calling-Channel Loading for Digital Selective-Calling (DSC) for the Maritime Mobile Service《海上移动业务数字选择性呼叫的呼叫信道装载》.pdf
《ITU-R M 822-1-1994 Calling-Channel Loading for Digital Selective-Calling (DSC) for the Maritime Mobile Service《海上移动业务数字选择性呼叫的呼叫信道装载》.pdf》由会员分享,可在线阅读,更多相关《ITU-R M 822-1-1994 Calling-Channel Loading for Digital Selective-Calling (DSC) for the Maritime Mobile Service《海上移动业务数字选择性呼叫的呼叫信道装载》.pdf(13页珍藏版)》请在麦多课文档分享上搜索。
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
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