The distribution network is the set of switches and lines between the source and destination. As we saw in Fig. 7-22, it consists of a backbone, connected to a local distribution network. Usually, the backbone is switched and the local distribution network is not.
The main requirement imposed on the backbone is high bandwidth. It used to be that low jitter was also a requirement, but with even the smallest PC now able to buffer 10 sec of high-quality MPEG-2 video, low jitter is not a requirement anymore.
Local distribution is highly chaotic, with different companies trying out different networks in different regions. Telephone companies, cable TV companies, and new entrants, such as power companies, are all convinced that whoever gets there first will be the big winner. Consequently, we are now seeing a proliferation of technologies being installed. In Japan, some sewer companies are in the Internet business, arguing that they have the biggest pipe of all into everyone’s house (they run an optical fiber through it, but have to be very careful about precisely where it emerges). The four main local distribution schemes for video on demand go by the acronyms ADSL, FTTC, FTTH, and HFC. We will now explain each of these in turn. ADSL is the first telephone industry’s entrant in the local distribution sweepstakes. We studied ADSL in Chap. 2 and will not repeat that material here. The idea is that virtually every house in the United States, Europe, and Japan already has a copper twisted pair going into it (for analog telephone service). If these wires could be used for video on demand, the telephone companies could clean up.
The problem, of course, is that these wires cannot support even MPEG-1 over their typical 10-km length, let alone MPEG-2. High-resolution, full-color, full motion video needs 4–8 Mbps, depending on the quality desired. ADSL is not really fast enough except for very short local loops.
The second telephone company design is FTTC (Fiber To The Curb). In FTTC, the telephone company runs optical fiber from the end office into each residential neighborhood, terminating in a device called an ONU (Optical Network Unit). On the order of 16 copper local loops can terminat e in an ONU.
These loops are now so short that it is possible to run full-duplex T1 or T2 over them, allowing MPEG-1 and MPEG-2 movies, respectively. In addition, videoconferencing for home workers and small businesses is now possible because FTTC is symmetric.
The third telephone company solution is to run fiber into everyone’s house. It is called FTTH (Fiber To The Home). In this scheme, everyone can have an OC-1, OC-3, or even higher carrier if that is required. FTTH is very expensive and will not happen for years but clearly will open a vast range of new possibilities when it finally happens. In Fig. 7-7 we saw how everybody could operate his or her own radio station. What do you think about each member of the family operating his or her own personal television station? ADSL, FTTC, and FTTH are all point-to-point local distribution networks, which is not surprising given how the current telephone system is organized.
A completely different approach is HFC (Hybrid Fiber Coax), which is the preferred solution currently being installed by cable TV providers. It is illustrated in Fig. 2-47(a). The story goes something like this. The current 300- to 450-MHz coax cables are being replaced by 750-MHz coax cables, upgrading the capacity from 50 to 75 6-MHz channels to 125 6-MHz channels. Seventy-five of the 125 channels will be used for transmitting analog television.
The 50 new channels will each be modulated using QAM-256, which provides about 40 Mbps per channel, giving a total of 2 Gbps of new bandwidth. The headends will be moved deeper into the neighborhoods so that each cable runs past only 500 houses. Simple division shows that each house can then be allocated a dedicated 4-Mbps channel, which can handle an MPEG-2 movie.
While this sounds wonderful, it does require the cable providers to replace all the existing cables with 750-MHz coax, install new headends, and remove all the one-way amplifiers—in short, replace the entire cable TV system. Consequently, the amount of new infrastructure here is comparable to what the telephone companies need for FTTC. In both cases the local network provider has to run fiber into residential neighborhoods. Again, in both cases, the fiber terminates at an optoelectrical converter. In FTTC, the final segment is a point-to-point local loop using twisted pairs. In HFC, the final segment is a shared coaxial cable. Technically, these two systems are not really as different as their respective proponents often make out.
