IPTV-Beginning of Triple Play

Abstract:IPTV is directly related to the current 3 operational networks (the cable TV network, Internet and telecom network) and the services they offer. However, different network operators have different expectations of IPTV. For telecom operators, IPTV organizes and transports TV programs over IP networks; while for cable TV operators, IPTV provides digital TV programs and other information services through cable TV networks. IPTV is broadly categorized into broadcast services and interactive services, which are carried respectively by the unidirectional broadcast network and the bidirectional interactive network. Therefore, convergence of the bearer networks is hard to implement. Instead, the convergence will occur on the service layer, so as to make full use of the two existing bearer networks to develop services cost-effectively. IPTV is capable of delivering broadcast, multicast as well as interactive services. It is expected to be the beginning of triple play.

      IPTV has become one of the prevailing trends at home and abroad. IPTV is not only related to IP networks and services, but also associated with TV networks and services. It is obvious that IPTV is directly related to the current three operational networks (the cable TV network, Internet and telecom network) and the services they offer. Therefore, IPTV is expected to be the beginning of triple play.
Different operators have different definitions for IPTV. From the point of view of telecom and Internet operators, IPTV is a kind of telecom and Internet service that uses TV programs as information sources and transports them over IP networks. However, from the perspective of cable TV operators, IPTV delivers TV programs and IP information services by cable TV networks. The different definitions not only reflect the different thinking of the operators, but also show their desire to enter each other?s service areas. In this consideration, IPTV may also become the beginning of triple play.
This article analyzes IPTV in terms of its service capability, network capability as well as the basis for triple play. Generally, if service capability is well adapted to network capability, IPTV services may develop rapidly, and in turn, drive the network development. Therefore, it is important to make a scientific analysis of IPTV in the above three aspects.

1 IPTV Service Capability
From the perspective of telecom operators, IPTV organizes and transports TV programs over IP networks. The IPTV services include Live TV, Time-shifted TV, Pay TV and Near Video on Demand (NVOD).

      Live TV uses interactive bidirectional communication networks (the IP network) to transfer available TV programs from the broadcast TV network.

      Time-shifted TV allows a viewer to choose freely a program of a certain channel in a certain period of time (such as within 48 hours). In this case, TV programs appear in the program list instead of being broadcast continuously through the TV channels. Viewers choose and watch TV programs they are interested in. The only relation between TV channels and TV programs is the program list where TV programs are arranged in a channel sequence to facilitate viewers? selection. Technically, Time-shifted TV has little connection with TV broadcast. It is closer to the Video on Demand (VOD) service.

      NVOD is a type of service intermediate between Live TV and Time-shifted TV. Generally, it repeatedly plays a program or a channel with staggered start times (such as with an interval of 5 minutes). The advantage of this service is that a viewer can watch the program he likes by waiting for just a short time. The more the viewers use this service, the higher the system efficiency.

      From the perspective of cable TV operators, IPTV uses cable TV networks to provide digital TV programs and other information services. Currently, as the information services are completely based on IP technologies, IPTV makes use of cable TV networks just to deliver IP information services. This is a natural requirement for cable TV operators who are always seeking to explore new service opportunities. With the explosive growth of digital technologies, the data transfer capability of digital cable TV networks is significantly enhanced. Although digital TV is the major service delivered by cable TV networks, its commercial application is restricted by the defective business model and the actual economic endurance of users, which further causes the failure to make full use of network resources. Therefore, IP information services undoubtedly become the preferred developable services for cable TV operators.

      Although different network operators think about IPTV differently, they all see it as an opportunity to tap into each other?s service areas. In short, IPTV provides an opportunity to both the telecom and cable TV network operators. This is the basis for network convergence.

2 IPTV Network Capability
With regard to the IPTV network, it is the first problem to be solved to fulfill access to telephone, TV and data information services (or the telephone network, Internet and TV) through one subscriber line. With the characteristics of broadband, the subscriber line of cable TV may support transmission of services of all three networks simultaneously after a two-way reconstruction and digitization. The main reason is that broadcasting and TV require a higher concurrent bandwidth, which is difficult to fulfill by the access technologies of other networks. Therefore, triple play starts with the Hybrid Fiber Coaxial (HFC) cable. However, it is not an easy thing to conduct the two-way reconstruction of HFC. Aggregation of reverse noise restricts the number of users connected to a Point of Presence (POP) to 500. Besides, it only solves the last-mile access problem for users. As the HFC system operates in a bus mode, the downlink data between users are not separated. In order to ensure privacy of user data and set up an optimal business model, it is necessary to fulfill separation of downlink data, which is technically a very difficult issue.
With the advance of time and technology, there have been many solutions to last-mile broadband access. For example, both the Gigabit Ethernet Digital Subscriber Line (DSL) and the single-fiber dual-wavelength (one optical wave of a Passive Optical Network (PON) and one broadcast wave) have the ability to solve the last-mile access problem for all telephone, TV and data information services through one physical line. Although there are many available technologies, we find through careful analysis that fully-digitalized services can be divided into broadcast services and interactive information services.

      Broadcast services are characterized by the downloading of a great amount of concurrent media streams at the same instant. Live TV is a typical broadcast service. This type of service requires very broad and consistent bandwidth throughout the channel from an information source to the destination. To facilitate channel switching and simplify the complexity of service networks, channel switching is implemented at the user terminal. Therefore, each user receives the same information (unreadable information).  The broadcast network is the best choice for this type of service because it adopts the multi-source tree architecture and operates in a one-way broadcasting mode.

      With regard to interactive information services, information is obtained only by interaction and there is generally one media stream downloaded at one moment. Data sent from a source is often accessed exclusively by one user, so each user receives different data. Apparently, a two-way interactive network is applicable for this type of services.
These two different types of services should be carried by the above networks suitable for them respectively.

      Certainly, it is not impossible to use other bearer networks, but it seems to "take a musket to kill a butterfly". For example, a two-way interactive network can undoubtedly be used as a broadcast network, but it would cost more than a broadcast network and result in lots of idle network capabilities, so this is not a practical choice. Conversely, a more serious problem may occur when a one-way broadcast network acts as a two-way interactive network. To enable a two-way communication, the first thing is to make a two-way reconstruction of the subscriber line to solve the "last-mile" problem. Besides, it is necessary to build an overlap interactive network over the original one-way broadcast network for two-way information communication.

      According to the above analysis, the services are essentially divided into broadcast services and interactive information services, and there should accordingly be two types of bearer networks to support the two types of services: the one-way broadcast network and the two-way interactive network. As the cable TV network and interactive network are completely different bearer networks, it is quite difficult to converge them and impossible to converge the backbone networks. The only possibility is the convergence of subscriber access lines at the "last mile".

      Currently there are only two out of many DSL access technologies available for the convergence. One is the fiber DSL that employs the single-fiber dual-wavelength technology, i.e., the Wavelength Division Multiplexing (WDM) technology, which uses two wavelengths to carry the two types of access services respectively. The other is the two-way reconstructed and digitized HFC. Essentially employing the Frequency Division Multiplexing (FDM) technology, HFC uses one part of the frequency for access of two-way interactive services and the other part for access of broadcast services. By further analysis, it is found that these two access technologies have one thing in common, that is logically, they are completely independent of each other and can be carried by a single physical bearer. The logic path of the subscriber access line is shown in Figure 1, and the IPTV bearer network for broadcast services and interactive services is shown in Figure 2.

      If combining the networks in Figure 1 and Figure 2, as shown in Figure 3, we will interestingly find that the two networks are not converged. The user access parts seem to be converged, but in essence the multiplexing technology is used to merge superficially these two access technologies separate in logic and different in performance. Therefore, it is a fantasy to converge the broadcast network and the interactive network. In practice, it is impossible and unnecessary to merge the two networks into one. It will be a best choice for the two networks to coexist, deliver their own applicable services, and develop collaboratively.

3 Basis for Triple Play
Triple play, especially convergence of the cable TV network and telecom network, is truly an issue worthy of discussion. The convergence is hard to fulfill at the bearer network level. Although it is possible to merge the two networks separate in logic and different in performance, that is to place them in a physical device as by the HFC or
single-fiber dual-wavelength technology, the two networks are essentially independent of each other in logic with superficial combination by using the WDM or FDM technology. No real convergence is fulfilled.

      One might ask whether or not the networks should be converged and at which layer to converge them. The answer is that the networks are to be converged and the convergence will occur at the service layer. Objectively, the conditions for network convergence at the service layer have been ready, as services are all digitized and most of them are characterized by both broadcasting and interaction, which lays a sound foundation for the convergence. Service convergence means to make full use of the two existing bearer networks to develop services cost-effectively and efficiently and achieve win-win coexistence of the two networks.

      It is clear that broadband digital broadcast services including digital video broadcast services, digital audio broadcast services and data broadcast services belong to cable TV network services, for the service and network features are well matched. However, a problem will rise if a digital cable TV network is used to carry interactive communication services. Although the "last mile" may obtain an asymmetrical two-way communication capability by two-way DSL reconstruction,   there are no alternative means to implement the two-way communication capability of the entire network except adding an overlap interactive communication network over the backbone network. Interactive communication services include session communication services, retrieval communication service and message services, and it is most suitable and reasonable to use an interactive communication network to carry them. However, if the interactive communication network is used to carry broadcast services, many functions of the network will not be applicable. Besides, without the same bandwidth in the entire network, the interactive communication network, especially its user access network, must be reconstructed. Therefore, the most adaptive network will be selected in principle if it is capable of delivering satisfactory services independently. Only for the services that a type of network is unable to support is a combination of the two networks considered.

      IPTV is just such a service that a type of network is unable to support. Therefore, IPTV services can be seen as pioneer services that trigger the triple play. Although consensus has not yet been reached on IPTV, the existing IPTV systems have offered Live TV and Time-shifted TV services. Therefore, we can start with the practical IPTV systems to get into the issue of how IPTV initiates service convergence in the networks.

      Let?s have an analysis of Live TV first. It uses IP packets to encapsulate a TV program as media streams that are then transported from a TV station to users? homes. Live TV can deliver real-time and unchanged TV programs to users. Technically, the live TV service of IPTV is in essence the same as the present digital TV service in reconstruction. The present digital TV service uses MPEG2 for coding/decoding and Transport Stream (TS) packets for encapsulation, while the live digital TV service of IPTV uses IP packets for encapsulation and may not use MPEG2 for coding/decoding. It seems that encapsulation is a major difference between the two services, but there is actually no significant difference between them in encapsulation. With regard to service features and necessary resources, Live TV transports TV programs from a TV station to users without any alteration. It needs a great amount of channels and occupies large bandwidth. As the simplest and most effective way of switching TV channels is switching at the user terminal, a data transmission channel from the TV station to the user is required to provide the basically same bandwidth. Obviously, the broadcast network, or cable TV network can meet the requirement provided that a digital channel encoder is added to the present analog cable TV network. Therefore, Live TV has no direct connection with the interactive network, and the broadcast network is the best choice for Live TV.
Now let?s have an analysis of Time-shifted TV. Technically,

      Time-shifted TV is completely different from Live TV. The most significant difference is that each user of
Time-shifted TV occupies a stream exclusively and each stream can only serve one user instead of being shared, while each stream of Live TV can be shared by many users provided that they are watching the same TV program on the same channel. For Time-shifted TV, channels have no other practical sense except appearing in the program list. When a program list is generated, the channels will become a part of the catalog index, so that users can easily look up TV programs. The individual program is a basic unit for storage, transport and charging when it is on demand.

      From the perspective of the bearer network and users, each of two users of Time-shifted TV will occupy a stream exclusively and cannot share the same stream, even though they watch the same program at an interval of 1s. As the cable TV network adopts a bus structure, all users on the network share the network resources. One cable TV network provides a maximum broadcast bandwidth of 3 GHz-5 GHz and a maximum of 1 000 Standard-Definition Television (SDTV) streams. If used for Time-shifted TV, a cable TV network can only provide service for 1 000 users. Apparently, the average operation cost per user is still too high for even a cable TV network in a small city. Therefore, even a small city is very difficult to use its backbone network to deliver the Time-shifted TV service.

      Moreover, the present cable TV network adopts the HFC structure. It is a broadcast network based on a bus structure. All the users connected to an optical node share the network transmission resources. For example, if one optical node is connected with 500 to 1 000 users, each user can be allocated only a transmission resource of 1 to 2 SDTV streams. Therefore, the user access part of the cable TV network has too-limited a capacity to be used for Time-shifted TV. It is even less powerful than the access line of a communication network, for a passive optical network can provide each user with a transmission capacity of 30 Mb/s. On all accounts, it is very difficult for the digital cable TV network to be used to deliver the time-shifted TV service.

      It should be mentioned that Time-shifted TV generally provides nearly a hundred channels and 48-hour TV programs for each channel. Due to the very limited number of programs, Time-shifted TV is quite different from VOD. Also, as the Time-shifted TV system is less complicated, it is regarded as a simple form of VOD.
Additionally, there are another two IPTV services between the two basic IPTV services mentioned above. One is Pay TV, and the other is NVOD.

      Pay TV is another form of Live TV. Users watching the pay TV programs are charged by channels or by programs. To run the service cost-effectively, Pay TV generally adopts the broadcast mode to deliver TV programs. All users may receive data of the programs, but only the paid users can decode the programs by using a decryption key. Obtaining a decryption key requires the support of a two-way communication network. Users who are charged by programs must get the keys by interaction through the two-way communication network, although those who are charged monthly can pay and get the keys either in an out-band mode or by interaction through the two-way communication network. Therefore, it is impossible for a single broadcast network to complete the whole communication process of Pay TV. A two-way communication channel is necessary for Pay TV although it only transports a very limited capacity of information. Usually it is impossible for cable TV operators to build new communication networks just for Pay TV. Instead, they deliver the interactive communication service through the existing communication networks and have the two networks work together to fulfill the pay TV service. From the above analysis, we can see Pay TV enables win-win coexistence of the two networks.

      NVOD is a special Pay TV service that broadcasts the same TV program through multiple channels to ensure users can view the program in a specified time (generally in 5 minutes). This operation mode is often used for playing hot movies so that a great number of users can watch the hot movies within the shortest time at the lowest possible cost. Take a 2-hour movie as an example. Once a user selects the movie, the maximum waiting time for him is 5 minutes. Then the equation 2×60/5=24 shows that 24 channels are required to play the movie in the broadcast network. One digitized cable TV network can support about 1 000 SDTV channels. If half of the TV channels are used to support NVOD, the network can support the play of about 20 movies, and more movies are hardly supported. In addition to the broadcast channels, the interactive channels are required to support interaction between users and the system. Although the amount of interactive information between users and the program source in NVOD is much larger than that in Pay TV, it is still small. Hence, NVOD can still use the existing communication network to deliver interactive information (especially uplink information) and the existing broadcast channels to transport downlink information.

      Based on the above analysis of IPTV, it is hard to carry IPTV basic services by a single network (either a broadcast network or an interactive network). However, if the broadcast and interactive networks are combined favorably, the total cost of system construction and operation will be greatly reduced. Of course, this is from the point view of telecom operators.

      From the point of view of cable TV operators, the cable TV network can be used to carry IP services. At first glance, there seems no question about it. The 5 GHz bandwidth from a TV station to users seems able to carry whatever IP services, so the digital cable TV network would be fully used to carrying all IP services to minimize the cost. Through careful analysis, however, we find this is not the case. With regard to the overall network, a cable TV network is actually a transmission path with a total bandwidth of 5 GHz from the TV station to users. Also, it is a network that operates in the bus mode, and all users in the network share the 5 GHz bandwidth. If the network is used to transport interactive services, the total bandwidth shared by the users shall not be larger than 5 GHz so as to guarantee the QoS. For a network with 5 000 users, each user can obtain an average bandwidth of 1 Mb/s for one-way transmission; while for a network with 50 000 users, each user can only obtain an average bandwidth of
0.1 Mb/s for one-way transmission. Hence it is very difficult for a city to use such a small network with capacity of
50 000 users to deliver interactive data services (even only downlink services). But things will be completely different if such a network is used to deliver broadcast data services. The broadcast network originally features that all its users are allowed to receive all data broadcast by the network. As the cable TV network is a kind of broadcast network, it can be used to provide broadcast data services to all users. All users in the network can receive all the data but can only decode the authorized data according to different authorities. There are a great amount of broadcast data services such as network newspaper, network magazines, network novels, network video broadcasting and network audio broadcasting. The most suitable network for these broadcast services is the broadcast network. Whether in terms of system complexity, or system investment and maintenance, it is not cost-effective to use an interactive network, as with the multicast technology, to deliver these services. Obviously, the broadcast network is suitable for transporting broadcast data. It is applicable when there are many users who share one data stream. The greater the number of shared users, the more cost-effective the broadcast network. Yet it is not applicable when each user occupies exclusively one stream. Even if there are no high-speed data, it is difficult to support one-user-one-stream applications. Thus, for a cable TV network operator, it is unfeasible to use the cable TV network to carry IP services. It is appropriate, however, to use the network to carry broadcast IP services, which will be the service trend for cable TV operators.

      Now it is clear that there are interactive services and broadcast data services existing in the real world. The former services feature one user one stream. They cannot support the sharing of data streams. Therefore, they should be supported by an interactive switching network. On the other hand, the characteristics of broadcast data services are such that all users share the same data stream. Therefore, they should be carried by the data broadcast network, that is, by the cable TV data network. In practice, it is difficult for broadcast data services to establish a good business model. For an unprofitable service, it is difficult to sustain development. Therefore, data multicast is often used to substitute data broadcast in real application to set up a good business model. The best way to implement multicast technology in the broadcast network is terminal blocking, i.e. adopting suitable network technology to enable that only a part of users in the network can decode data although all the users can receive the data. This is a trend of data services of the broadcast network.

      As IPTV involves a great amount of broadcast, multicast and interactive services, it is to be a beginning of triple play. Upon IPTV, the cable TV network and the interactive network will be more and more closely related and ultimately tend toward convergence.

4 Conclusions
The cable TV network and the interactive network have long been operating independently, with each delivering its own suitable services. The cable TV network offers broadcast services while the interactive network delivers interactive communication services. Therefore, the two networks have developed independently for quite a long time. Since the emergence of MPEG2 that provides a better solution to the problem of digitizing video TV signals, the cable TV network has started to be digitized. Due to the overestimate of their own network capabilities and the eagerness of tapping into each other?s service areas, operators of the two networks are driven to compete for the services of the rival, regardless of their own abilities and possible expenses. It deserves the operators? further consideration whether this competition is proper or not.

Manuscript received: 2006-03-27

Author Introduce:

  Jiang Lintao received his Bachelor?s degree from the Radio Department of Tsinghua University in 1970, and Master?s degree in data communication from the Electronic Engineering Department of Tsinghua University in 1982. He is the chief engineer of the China Academy of Telecommunications Research of Ministry of Information Industry (MII) of China. He is the chairman of the Standardization Technical Committee for IP and Multimedia, the vice chairman of ITU-T SG13, and also a member of the first, second and third Expert Group of Multimedia for the national "863" Program. He has long been engaged in the R&D and standard setting of multimedia, data communication and IP technologies. He was awarded the special government allowance issued by the State Council of China in 1992 and the title of "Chinese Young and Middle-aged Expert with Outstanding Contributions" in 1996.