1、 Rec. ITU-R BO.1724-1 1 RECOMMENDATION ITU-R BO.1724-1 Interactive satellite broadcasting systems (television, sound and data) (Question ITU-R 26/6) (2005-2007) Scope This Recommendation intends to specify the interactive satellite broadcasting systems (television, sound and data) under Question ITU
2、-R 26/6 when a satellite return channel using geostationary-satellite systems is operated using the System A in Recommendation ITU-R BO.1516 in the broadcasting-satellite service (BSS). Two systems are recommended for this subject. The first system is the European Telecommunication Standards Institu
3、te (ETSI) Standard ETSI EN 301 790V1.3.1, known as DVB-RCS. The second one is the Telecommunications Industry Association (TIA) Standard TIA 1008-A. Normative texts for these specifications are given in URLs and descriptive summaries are provided in its Annexes. The ITU Radiocommunication Assembly,
4、considering a) that substantial progress in digital broadcasting technologies has resulted in the implementation of operating digital systems in the broadcasting satellite service (BSS); b) that ITU-R has developed Recommendation ITU-R BO.1516 for digital multiprogramme television systems operating
5、in the 11/12 GHz frequency range; c) that it is important to ensure the maximum commonality and compatibility with other return channel solutions for the different broadcasting media; d) that interactivity is recognized to be a desirable feature for broadcast services, the need for suitable systems
6、being addressed by Question ITU-R 16/6, and that Question ITU-R 26/6 addresses the provision for anonymous reception for broadcasts; e) that the interaction channel systems for interactive services should ensure the access to all the terminals within the downlink service area; f) that the interactio
7、n channel solutions based on satellite should be compatible for individual and community reception systems; g) that the existence of a return channel may offer to the broadcasting entities the opportunity to provide interactive broadcasting services; h) that there is a need to identify interfaces fo
8、r the interconnection of the satellite-broadcasting systems with other broadcasting media; j) that ITU-R has approved Recommendation ITU-R BT.1369 Basic principles for a worldwide common family of systems for the provision of interactive television services; k) that ITU-R has approved Recommendation
9、 ITU-R BT.1434 Network independent protocols for interactive systems; l) that ITU has developed Recommendations on interaction channel using public switched telephone network/integrated services digital network (PSTN/ISDN), mobile phone systems, cable systems, microwaves, etc., 2 Rec. ITU-R BO.1724-
10、1 recommends 1 that, when a satellite return channel using geostationary-satellite systems is operated using the System A in Recommendation ITU-R BO.1516 in the BSS, the following two standards may be used: European Telecommunications Standards Institute (ETSI) Standard ETSI EN 301 7901V1.3.1 (2003-
11、03): http:/www.itu.int/ITU-R/study-groups/rsg6/etsi/index.html, or Telecommunication Industry Agency (TIA) Standard TIA-1008-A: http:/www.itu.int/md/R03-WP6S-C-0191/en; 2 that the descriptive summaries of the standards provided in Annexes 1 and 2, as well as the comparison Table 1, may be used to as
12、sist administrations in selecting an appropriate standard to meet their needs. TABLE 1 Comparison Table for ETSI EN 301 790 V.1.3.1 and TIA-1008-A Item ETSI EN 301 790 TIA-1008-A Broadcast channel Recommendation ITU-R BO.1516 Recommendation ITU-R BO.1516Return channel modulation QPSK CE-OQPSK Return
13、 channel coding Viterbi/Reed Solomon rates 1/2, 2/3, 3/4 or turbocode rates 1/3, 2/5, 1/2, 2/3, 3/4, 4/5, 6/7 Viterbi rate 1/2 or turbocode rate 1/2, 2/3, 4/5 Return channel data rate No restriction 64, 128, 256, 512, 1 024, 2 048 ksymbol/s Return channel spacing (minimum) 1.35 symbol rate 1.25 symb
14、ol rate Return channel burst size Variable length, defined as 1, 2 or 4 ATM cells, or 1, 2 N: N = 1 to 12 MPEG2 cell(s) Slotted Aloha fixed length bursts (definable), dynamic stream variable length bursts from specified minimum to full frame length Return channel Media access control method TDMA/FDM
15、A bandwidth, transmission rate, coding rate, and duration of traffic slots are fixed or dynamic, slot by slot. Frequency hopping can be performed on a slot-by-slot basis TDMA/FDMA (frequency assigned when inroute request made) Can frequency-hop on a frame-by-frame basis Return channel burst ARQ prot
16、ocol? Yes, as per IP on MPEG Yes, with selective retransmit Return channel bandwidth management Continuous rate dynamic capacity, rate-based dynamic capacity, volume-based dynamic capacity, free capacity assignment Slotted Aloha, dynamic stream with definable hangover period, inroute quality of serv
17、ice, constant bit rate 1The word “shall” in this ETSI standard should be considered as “should” in this ITU-R Recommendation. Rec. ITU-R BO.1724-1 3 Annex 1 Descriptive summary of ETSI Standard EN 301 790 V1.3.1 TABLE OF CONTENTS Page 1 Introduction 6 2 Reference model of the satellite interaction n
18、etwork . 6 2.1 Protocol stack model. 6 2.2 System model 6 2.3 Reference model of the satellite interactive network . 7 3 Forward link . 8 4 Return link baseband physical layer specification and multiple access definition. 9 4.1 RCST synchronization 9 4.1.1 Timing control 9 4.1.2 Carrier synchronizat
19、ion 9 4.1.3 Burst synchronization 10 4.1.4 Symbol clock synchronization. 10 4.2 Burst format 10 4.2.1 TRF burst formats 10 4.2.2 Synchronization (SYNC) and acquisition (ACQ) burst formats . 11 4.3 Modulation 12 4.4 MAC messages . 12 5 Protocol stack . 13 6 Capacity request categories 15 6.1 Continuo
20、us rate assignment (CRA) 15 6.2 Rate-based dynamic capacity (RBDC). 15 6.3 Volume-based dynamic capacity (VBDC) . 15 6.4 Absolute volume-based dynamic capacity (AVBDC) 15 6.5 Free capacity assignment (FCA) 15 7 Multiple access . 16 7.1 MF-TDMA . 16 8 Security, identity, encryption . 16 4 Rec. ITU-R
21、BO.1724-1 1 Introduction This Annex provides a specification for the provision of an interaction channel for GSO satellite interactive networks with fixed return channel satellite terminals (RCSTs). This specification facilitates the use of RCST for individual or collective installation (e.g. SMATV)
22、 in a domestic environment. It also supports the connection of such terminals with in-house data networks. This specification may be applied to all frequency bands allocated to GSO satellite services. 2 Reference model of the satellite interaction network 2.1 Protocol stack model For interactive ser
23、vices supporting broadcast to the end user with return channel, a simple communications model consists of the following layers: Physical layer: where all the physical (electrical) transmission parameters are defined. Transport layer: defines all the relevant data structures and communication protoco
24、ls like data containers, etc. Application layer: the interactive application software and runtime environment (e.g. home shopping application, script interpreter, etc.). A simplified model of the OSI layers was adopted to facilitate the production of specifications for these layers. Figure 1 shows t
25、he lower layers of the simplified model and identifies some key parameters. FIGURE 1 Layer structure for generic system reference model The present text addresses the satellite interactive network dependent aspects only. 2.2 System model Figure 2 shows the system model which is to be used within dig
26、ital video broadcasting (DVB) for interactive services. In the system model, two channels are established between the service provider and the user: Broadcast channel: A unidirectional broadband broadcast channel including video, audio and data is established from the service provider to the users.
27、It may include the forward interaction path. Rec. ITU-R BO.1724-1 5 Interaction channel: A bidirectional interaction channel is established between the service provider and the user for interaction purposes. It is formed by: Return interaction path (return channel): from the user to the service prov
28、ider. It is used to make requests to the service provider, to answer questions or to transfer data. Forward interaction path: from the service provider to the user. It is used to provide information from the service provider to the user and any other required communication for the interactive servic
29、e provision. It may be embedded into the broadcast channel. It is possible that this channel is not required in some simple implementations which make use of the broadcast channel for the carriage of data to the user. The RCST is formed by the network interface unit (consisting of the broadcast inte
30、rface module and the interactive interface module) and the set top unit. The RCST provides interface for both broadcast and interaction channels. The interface between the RCST and the interaction network is via the interactive interface module. FIGURE 2 A generic system reference model for interact
31、ive systems 2.3 Reference model of the satellite interactive network An overall satellite interactive network, within which a large number of RCSTs will operate, will comprise the following functional blocks, as shown in Fig. 3: Network control centre (NCC): An NCC provides monitoring and control fu
32、nctions. It generates control and timing signals for the operation of the satellite interactive network to be transmitted by one or several feeder stations. 6 Rec. ITU-R BO.1724-1 Traffic gateway (TG): A TG receives the RCST return signals, provides accounting functions, interactive services and/or
33、connections to external public, proprietary and private service providers (e.g. databases, pay-per-view TV or video sources, software download, teleshopping, telebanking, financial services, stock market access, interactive games, etc.) and networks (e.g. internet, ISDN, PSTN, etc.). Feeder: A feede
34、r transmits the forward link signal, which is a standard satellite digital video broadcast (DVB-S) uplink, onto which are multiplexed the user data and/or the control and timing signals needed for the operation of the satellite interactive network. FIGURE 3 Reference model for the satellite interact
35、ive network The forward link carries signalling from the NCC and user traffic to RCST. The signalling from the NCC to RCST that is necessary to operate the return link system is called “forward link signalling” in the following. Both the user traffic and forward link signalling can be carried over d
36、ifferent forward link signals. Several RCST configurations are possible depending on the number of forward link receivers present on the RCST. 3 Forward link The RCST should be able to receive digital signals conforming to ETSI Standards EN 300 421, TR 101 202, ETS 300 802, EN 300 468, EN 301 192 an
37、d ETR 154. Rec. ITU-R BO.1724-1 7 4 Return link baseband physical layer specification and multiple access definition Specifications for the baseband physical layer are given in this section. Figure 4 represents the generic digital signal processing to be performed at the RCST transmitter side, from
38、the burst formatting of the serial information bit-stream, to the modulation representing the digital to analogue conversion. The signal processing to be performed by each subset is described in the following sections. FIGURE 4 Block diagram of the RCST return link baseband signal processing 4.1 RCS
39、T synchronization 4.1.1 Timing control The synchronization of the RCST is an important feature of the satellite interactive network. Constraints are imposed on the RCST to obtain an efficient TDMA system with minimum interference between users and maximum throughput, although they can be minimized i
40、f the NCC performs tasks such as satellite frequency translation error and common-mode Doppler compensation for RCST carrier frequency. For this reason, the synchronization scheme is based on information contained within the forward link signalling as follows: network clock reference (NCR); signalli
41、ng in DVB/MPEG-2 transport stream (TS) private sections. The NCR is distributed with a specific PID within the MPEG-2 TS that carries the forward link signalling. The NCR distribution follows the PCR distribution mechanism as defined in ISO/IEC 13818-1, which is usually derived from an MPEG video en
42、coder, whereas here the NCR is derived from the NCC reference clock. The NCC reference clock will have an accuracy of 5 ppm or better. 4.1.2 Carrier synchronization The MPEG-2TS that carries the forward link signalling contains an NCR information which provides a 27 MHz reference of the NCC referenc
43、e clock to the RCST. The RCST reconstructs the reference clock from the received NCR information as implemented in MPEG decoders for MPEG-2 transport streams (MPEG-2 TS). The RCST then performs a comparison to determine the offset between the local reference clock which controls the RCST up-converte
44、r local oscillator and the reference clock recovered from the received NCR. It then compensates the carrier frequency according to this offset. This local carrier synchronization provides a way of adjusting the transmit frequency of all RCST on the network to almost the same frequency. Normalized ca
45、rrier frequency accuracy should be better than 108(root mean square). 8 Rec. ITU-R BO.1724-1 4.1.3 Burst synchronization The RCST retrieves the centre frequency, the start time and the duration of their transmit bursts by examining the forward link signalling. The contention between RCST on the retu
46、rn link is resolved as described in this specification. The bursts are sent according to the burst time plan (BTP) received in the forward link signalling. The BTP is expressed in terms of centre frequency and absolute start time (given in NCR-counter value) of superframes and associated frequency a
47、nd time offsets of burst allocations along with a description of the time-slot properties. A superframe always starts at a given value of the RCST local NCR counter, which serves as a reference for all burst allocations within the superframe. For the purpose of synchronizing to the network the RCST
48、reconstructs, in addition to the reference clock, the absolute value of the NCC reference clock. The RCST compares the reconstructed value with the NCR value given by the BTP. The time reference for counting time-slots occurs when the values are equal. Burst synchronization accuracy is expected to b
49、e within 50% of a symbol period. The resolution is 1 NCR count interval. The burst synchronization accuracy is the worst-case deviation of the scheduled start of burst time and the actual start of burst time at the transmitter output. The scheduled start of burst time is the point in time when the ideal reconstructed NCR equals the value written in the TBTP for that burst. The ideal reconstructed NCR is defined as observed at the output of an ideal delay-less DVB-S receiver. Compensation for the receiver delay, if required to achieve the specified accuracy,
