1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Series J Supplement 2 (1 1/98) SERIES J: TRANSMISSION OF TELEVISION, SOUND PROGRAMME AND OTHER MULTIMEDIA SIGNALS Guidelines for the implementation of Annex A of Recommendation J.112, “Transmission systems fo
2、r interactive cable television services“ Example of Digital Video Broadcasting (DVB) interaction channel for cable television distribution ITU-T J-SERIES RECOMMENDATIONS TRANSMISSION OF TELEVISION, SOUND PROGRAMME AND OTHER MULTIMEDIA SIGNALS General Recommendations General specifications for analog
3、ue sound-programme transmission Performance characteristics of analogue sound-programme circuits Equipment and lines used for analogue sound-programme circuits Digital encoders for analogue sound-programme signals Digital transmission of sound-programme signals Circuits for analogue television trans
4、mission Analogue television transmission over metallic lines and interconnection with radio-relay links Digital transmission of television signals Ancillary digital services for television transmission Operational requirements and methods for television transmission Interactive systems for digital t
5、elevision distribution Transport of MPEG-2 signals on packetised networks Measurement of the quality of service Digital television distribution through local subscriber networks J. 1-J.9 5.10-J.19 5.20-5.29 J.30-J.39 J.40-J.49 J.50-J.59 J .6 all users of this Recommendation are therefore encouraged
6、to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. 111 IT-T Recommendation J. 1 10 (1 997), Basic principles for a worldwide common family of systems
7、for the provision of interactive television services. IT-T Recommendation J. 1 12 (1998), Transmission systemsfor interactive cable television services. ITU-T Recommendation J.83 (1997), Digital multi-programme systems for television, sound und data services for cable distribution. ITU-T Recommendat
8、ions 1.363 series, B-ISDN ATM Adaptation Layer specification. Pl 31 41 3 Abbreviations For the purposes of this Supplement, the following abbreviations apply: AAL 5 ATM Adaptation Layer 5 ATM Asynchronous Transfer Mode BC Broadcast Channel BIM Broadcast Interface Module BRA Basic Rate Access CATV Ca
9、bleTV J series - Supplement 2 (11/98) 1 CB radio DAVIC DVB EMC FIP HFC IB IC ID IEEE IIM INA IP IRD ISDN LAN LLC MAC MPEG NIU ONU OOB os1 PSTN RC RCC RIP RMS SDH SMATV SNR STB STU TCP TDMA TS Citizens Band radio Digital Audio Visual Council Digital Video Broadcasting Project1 ElectroMagnetic Compati
10、bility Forward Interaction Path Hybrid Fibre Coax In-Band Interaction Channel JDentifier Institute of Electrical and Electronics Engineers Interactive Interface Module Interactive Network Adapter Internet Protocol Integrated Receiver Decoder Integrated Services Digital Network Local Area Network Lin
11、k Layer Control Media Access Control Moving Picture Expert Group Network Interface Unit Optical Node Unit Out-of-Band Open Systems Interconnection Public Switched Telephone Network Return Channel Return Channel - Cable Return Interaction Path Root Mean Square Synchronous Digital Hierarchy Satellite
12、Master Antenna Television Signal-to-Noise power Ratio Set Top Box Set Top Unit Transmission Control Protocol Time Division Multiple Access Transport Stream The Digital Video Broadcasting Project (DVB) is a consortium of broadcasters, manufacturers, network operators and regulatory bodies created for
13、 designing sandards for the delivery of digital television. 2 J series - Supplement 2 (11/98 - uc Upstream Channel VCI Virtual Channel Identifier VPI Virtual Path Identifier 4 System model Figure 1 shows the system model which is to be used within DVB for interactive services i. Broadcast Broadcast
14、Channel Channel Set Top Box (STB) Interaction TO906770-99/dOl Interaction Channel network dependent Figure 1 - Generic system reference model or interactive systems In the system model, two channels are established between the service provider and the user: - Broadcast Channel (BC): A unidirectional
15、 broadband BC including video, audio and data. BC is established from the service provider to the users. It may include the Forward Interaction Path (FIP). - Interaction Channel (IC): A bidirectional interaction channel is established between the service provider and the user for interaction purpose
16、s. It is formed by: Return Interaction Path (RIP): From the user to the service provider. It is used to make requests to the service provider or to answer questions. Also commonly known as Return Channel (RC) or Upstream Channel (W. Forward Interaction Path (FIP): From the service provider to the us
17、er. It is used to provide some sort of information by the service provider to the user and any other required communication for the interactive service provision. It may be embedded into the BC. It is possible that this channel is not required in some simple implementations which make use of the BC
18、for the carriage of data to the user. J series - Supplement 2 (11198) 3 In this Supplement the word “channel“ denotes logical link and “path corresponds to a physical link. The user terminal is formed by the Network Interface Unit (NIU) consisting of the Broadcast Interface Module (BIM) and the Inte
19、ractive Interface Module (IIM) and the Set Top Unit (STU). The user terminal provides interface for both broadcast and interaction channels. The interface between the user terminal and the interaction network is via the IIM. The interactive system is composed of FIP (downstream) and RIP (upstream).
20、The general concept is to use FIP to act as a transmission medium for MAC control channel and to carry a part of the downstream data. This allows the NIUs to adapt to the network and send synchronized information upstream. RIP is divided into time slots which can be used by different users, using th
21、e technique of Time Division Multiple Access (TDMA). One MAC control channel is used to control up to 8 UCs, which are all divided into time slots. A time marker and an upstream counter at the INA is sent periodically to the NIUs, so that all NIUs work with synchronized clock and same upstream count
22、er value. This gives the opportunity to the INA to assign time slots to different users. Three major access modes are provided with this system. The first one is based on contention access, which lets users send information at any time with the risk of having a collision with other users transmissio
23、ns. The second and third modes are contention-less-based, where the INA either provides a finite amount of slots to a specific NIU, or a given bit rate requested by a NIU until the INA stops the connection on NIUs demand. These access modes are dynamically shared among time slots, which allows NIUs
24、to know when contention-based transmission is or is not allowed. This is to avoid a collision for the two contention-less-based access modes. Periodically, the INA will indicate to new users that they have the possibility to go through sign-on procedure, in order to give them the opportunity to sync
25、hronize their clock to the network clock, without risking collisions with already active users. This is done by leaving a larger time interval for new users to send their information, taking into account the propagation time required from the INA to the NIUs and back. 5 Protocol stack model For asym
26、metric interactive services supporting broadcast to the home with narrowband RC, a simple communication model consists of the following layers: - Network-dependent physical layer: where all the physical (electrical) transmission parameters are defined; - Network-dependent access mechanism layer: def
27、ines all the relevant data structures and communication protocols like data containers, etc.; - Network-independent application layer: the interactive application software and runtime environments (e.g. home shopping application, script interpreter, etc.). DVB-RCC (Annex MJ.112 2) addresses the lowe
28、r two layers (the physical and transport), leaving the application layer open to competitive market forces. A simplified model of the OS1 layers was adopted to facilitate the production of specifications for these nodes. Figure 2 points out the lower layers of the simplified model and identifies som
29、e of the key parameters for the lower two layers. Following the user requirements for interactive services, no attempt will be made in this Supplement to consider higher medium layers. This Supplement addresses the HFCICATV network specific aspects only. The network-independent protocols will be spe
30、cified separately. 4 J series - Supplement 2 (11/98) I I Access mechanism Packet structure Modulation Channel coding Freq. range Filtering Equalization Power Network-independent protocols (Network-dependent protocols) Figure 2 - Layer structure for generic system reference model 6 Specification outl
31、ine A multiple access scheme is defined in order to have different users share the same transmission media. Downstream information is broadcast to all users of the networks. Thus, an address assignment exists for each user which allows the INA to send information singlecast to one particular user. T
32、wo addresses are stored in Set Top Boxes (STB) in order to identify users on the network: - MAC address: It is a 48-bit value representing the unique MAC address of the NIU. This MAC address may be hard coded in the NU or be provided by external source. - NSAP address: It is a 160-bit value represen
33、ting a network address. This address is provided by higher layers during communication. Upstream information may come from any user in the network and shall therefore also be differentiated at the INA using the set of addresses defined above. This interactive system is based either on Out-of-Band (O
34、OB) or In-Band (IB) downstream signalling. However, STBs do not need to support both systems. In the case of OOB signalling, a Forward Interaction Path (FIP) is added. The presence of this added FIP is in that case mandatory. However, it is also possible to send higher bit rate downstream informatio
35、n through a DVB-C channel whose frequency is indicated in the FIP. In the case of IB signalling, the FIP is embedded into the MPEG-2-TS of a DVB-C channel. NOTE - it is not mandatory to include the FIP in ail DVB -C channels. Both systems can provide the same quality of service. However, the overall
36、 system architecture will differ between networks using IB STBs and OOB STBs. Both types of systems may exist on the same networks providing that different frequencies are used for each system. Upstream and OOB downstream channels are divided into separate channels of 1 MHz or 2MHz bandwidth for dow
37、nstream and i MHz, 2 MHz or 200 kHz for upstream. Each downstream channel contains a synchronization frame used by up to 8 different UCs, whose frequencies are indicated by the Media Access Control (MAC) protocol. J series - Supplement 2 (11/98) 5 Within UCs, users send packets with TDMA type access
38、. This means that each channel is shared by many different users, who can either send packets with a possibility of collisions when this is allowed by the INA, or request transmission and use the packets assigned by the INA to each user specifically. Assuming each upstream path can therefore accommo
39、date a large number of users at the same time, the upstream bandwidth can easily be used by all users present on the network at the same time. The TDMA technique utilizes a slotting methodology which allows the transmit start times to be synchronized to a common clock source. Synchronizing the start
40、 times increases message throughput of this signalling channel since the message packets do not overlap during transmission. The period between sequential start times are identified as slots. Each slot is a point in time when a message packet can be transmitted over the signalling link. The time ref
41、erence for slot location is received via the downstream channels generated at the delivery system and received simultaneously by all STUs. This time reference is not sent in the same way for OOB and iB signalling. Since all NIus reference the same time base, the slot times are aligned for all NIUS.
42、However, since there is propagation delay in any transmission network, a time base ranging method accommodates deviation of transmission due to propagation delay. Since the TDMA signalling link is used by NIUS that are engaged in interactive sessions, the number of available message slots on this ch
43、annel is dependent on the number of simultaneous users. When messaging slots are not in use, an NIU may be assigned multiple message slots for increased messaging throughput. Additional slot assignments are provided to the NlU from the downstream signalling information flow. There are different acce
44、ss modes for the upstream slots: - reserved slots with fixed rate reservation (Fixed rate access: the user has a reservation of one or several time slots in each frame enabling, e.g. for voice, audio.); - reserved slots with dynamic reservation (Reservation access: the user sends control information
45、 announcing his demand for transmission capacity. He gets grants for the use of slots.); - contention-based slots (These slots are accessible for every user. Collision is possible and solved by a contention resolution protocol.); - ranging slots (These slots are used upstream to measure and adjust t
46、he time delay and the power.). These slots may be mixed on a single carrier to enable different services on one carrier only. If one carrier is assigned to one specific service, only those slot types will be used which are needed for this service. Therefore, a terminal can be simplified to respond t
47、o only those slot types assigned to the service. 6.1 Bit rates and framing For the interactive downstream OOB channel, a rate of 1.544 Mbit/s or 3.088 Mbids may be used. For downstream IB channels, no other constraints than those specified in DVB-C (5.83 3) exist, but a guideline would be to use rat
48、es multiples of 8 kbit/s. Downstream OOB channels continuously transmit a frame based on TI -type framing, in which some information is provided for synchronization of upstream slots. Downstream IB channels transmit some MPEG-2-TS packets with a specific PID for synchronization of upstream slots (at
49、 least one packet containing synchronization information shall be sent in every period of 3 ms). For upstream transmission, the INA can indicate three types of transmission rates to users - specifically 3.088 Mbit/s, 1.544 Mbit/s or 256 kbit/s. The INA is responsible for indicating which rate may be used by NIUs. It would imply that all NIUs are able to either transmit with 256 kbit/s, 1.544 Mbit/s, or 3.088 Mbit/s. Only the implementation of one of these bit rates would be mandatory. 6 J series - Supplement 2 01/98) Upstream framing consists of packets of 5 12 bits (256 s
