[Multimedia: Beyond Video-on-Demand]
Hi Pat,
Don't remember this being posted . It's an interesting look beyond
The networks put in place for video-on-demand services could be extended to provide a host of other multimedia services.
Maurizio Naldi
June 1997 -- Special 30th Anniversary Issue: June 1967 - June 1997
Telecommunications Magazine
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A huge effort is being undertaken by telecom network operators all around the world to enrich the service portfolio and capture the emerging needs of the residential market. This is accompanied by a massive restructuring of both the access and the transport networks to accommodate for the bandwidth that these new services will require. Given the wide range of possibilities made real by the upcoming bandwidth availability, the choice of services, a basic element of any marketing strategy, is a rather complicated issue.
The first interactive multimedia (IMM) service that was thought of as revenue-promising was video-on-demand (VoD). It had the desirable features of aiming at a mass market and of being easily accepted by customers already used to cable television or video rental. The impression was generated that VoD could constitute the killer application for the broadband superhighways. It was soon realised, though, that the building of such a powerful infrastructure just for the on-demand delivery of movies was an inefficient choice, to say the least. A host of multimedia services can in fact be offered on the same transport platform as VoD and spur service demand.
IMM SERVICES
The basic issue all network operators are wondering about is: what multimedia services can be expected to have a broad appeal on the mass market and pay back the huge investments for the access network evolution? Though the whole issue may look like a guessing game, there have been serious attempts to understand users' expectations and formulate plans according to these forecasts. An insight into the problem of the definition of the services basket can be gained by looking at the surveys conducted among experts and at the field trials set up by network operators. An example of the first category is the survey conducted within the TITAN[1] project, undertaken in 1992 by 12 companies. Funded by the European Commission as part of the RACE II programme, the project aimed to develop a tool for the techno-economic evaluation and comparison of access network technologies and architectures. In the framework of this project, a Delphi survey (a survey by experts during which a set of questions is first submitted and then resubmitted at a later date together with a summary of the first round results) was conducted to forecast the demand for advanced services in the residential market. The prospective services were then ranked according to the preferences expressed by the experts, not all of which belonged to the telecoms arena.
The results, reported as percentages of the experts' indications, show that the video-on-demand was judged to be the most appealing service. Nevertheless other services, namely videotelephony and simple office-like applications, followed VoD closely. Overall, the list of services was made up of a balanced mix of entertainment and serious (education, information retrieval and exchange) applications. A common feature of most IMM services is the asymmetry of their bandwidth requirements. As the information required is generally contained in a server, and the interaction consists mainly of sending user commands, the bandwidth needed downstream is typically much lower than in the reverse direction.
These experts' opinions, collected some years ago, should, however, be compared against the actual choices made by operators in their field trials. A survey of 17 of these trials[2] reveals that network planners have confirmed the views expressed in the TITAN project, by investing much on VoD, but experimenting with other services as well (particularly entertainment services). In any case, the investment risk can be lowered by accommodating for a number of services rather than VoD alone. Parallel to the traditional operators approach, what has happened in recent times is that the growth of Internet has brought along, despite its inadequacy for real-time services, a host of interactive applications that can support most of the services appearing in both charts. A proof of the success of Internet is in fact the rapid development of the World Wide Web. Internet users can connect globally to a huge variety of information recipients and retrieve both texts and images (and even videos) in a very simple fashion by the use of the HTTP protocol.
The number of Web servers is continually growing: a rough estimate made by Webcrawler in April 1996 led to a figure of 1,500,000 - 2,000,000. In addition to the differences in the network platform, a striking novelty of the Internet is the vast fragmentation of service providers. While in the traditional operators' approach the service providers are a limited number, nearly anyone can set up, with limited expense, a Website and offer its services. Another example of an Internet application that has allowed video interactivity out of the traditional operator platforms is CU-SeeMe, a software package developed by Cornell University that allows up to eight users to communicate in a videoconference format, without the need of expensive and dedicated equipment.
ACCESS TECHNOLOGIES
Offering multimedia services requires bandwidth, and the basic copper infrastructure could not cope with the amount of information that would pass over it in the new scenario. Network operators are therefore enhancing, and even rebuilding, their access networks. As with the services, the number of options is quite large, and growing. A basic sub-division can be made between techniques based on a physical medium or on radio, the common feature of the first, larger class being the extension of optical fibre from the transport network to nearer the end-user. Starting from the lowest switching point, the access network can be sub-divided into the secondary distribution network (which connects the primary distribution point to the distribution cabinet) and what is commonly referred to as the last drop (connecting the distribution cabinet to the customer premises)[3].
The secondary distribution network, traditionally realised on coaxial cable, is being replaced by an optical infrastructure. Two aspects can be examined to characterise the different alternatives for this network: its extension and its topology. As to the extension, the options can be identified by indicating the termination point of the optical infrastructure. Ranking them in order of increasing fibre deployment (the optical/electrical conversion is placed closer and closer to the customer premises), we can therefore list fibre-to-the-cabinet (FTTCab), fibre-to-the-curb (FTTC), fibre-to-the-building (FTTB) and fibre-to-the-home (FTTH).
The last solution realises the provision of a direct optical access to each customer and, though being regarded as the ultimate goal, is far from being economically feasible for the residential market. ODN solutions can also differ as to their topology: either a point-to-point or a point-to-multipoint connectivity is possible. The point-to-multipoint solution, realised through a tree-and-branch passive optical network (PON) is becoming increasingly popular. Though not being thoroughly defined, the basic arrangement consists of a central optical line termination (OLT) serving a number of optical network units (ONU). Line rates should be asymmetric and corresponding to SDH rates (typically 622 Mbps downstream and 155 Mbps upstream). ATM is supported through a TDMA format, and traffic concentration is achieved with a dynamic bandwidth allocation scheme controlled by the OLT.
The more expensive alternative of point-to-point connectivity (leading to a star topology) can boast, on the contrary, of the advantage of getting rid of complex medium access control (MAC) protocols, relying instead on an already standardised physical layer such as SDH. What is more convenient depends on the services mix: in the current scenario of strongly asymmetric services, PONs are certainly better suited; a significant increase in the traffic generated by end users would instead lead to point-point solutions.
As to the last drop, the network operators can choose between the enhancement of their copper-based infrastructure and the creation of a coaxial one (cable television operator, which already possess just a coaxial distribution network, have no choice problem). The coaxial solution, HFC (hybrid fibre-coax) has been adopted in most trials, probably due to the absence of reliable broadband copper solutions at the time the trials were set up. At present, it can still provide a bandwidth advantage over some, but not all, the copper-based counterparts.
Two critical points are the support of upstream transmission (not available in all the HFC implementations) and the definition of a MAC protocol. On the contrary, much progress has been made on copper-based solutions, presenting a double advantage: they allow re-use of the installed copper, and they grant a full compatibility with the telephone service (a single distribution network is used for both telephone and multimedia services)[4]. Though many variants exist, and tend to proliferate, the discussion on copper access technologies can be limited to ADSL (asymmetric digital subscriber line) and VDSL (very high rate digital subscriber line), the latter allowing a higher speed but being plagued by a lower degree of definition. Both are asymmetric in nature. In the downstream direction the available bandwidth is a decreasing function of the distance, overcoming by far the present limitations of the telephone network. The upstream rates are instead a maximum of 640 kbps for ADSL and 2.3 Mbps for VDSL.
In both techniques, a four kHz region at the lower end of the spectrum is dedicated to POTS. In ADSL the remaining bandwidth is instead allocated between the upstream and the downstream data either by using frequency division multiplexing or by using echo cancellation (the two streams overlap and are separated by means of a technique already used in V.32 and V.34 modems). At present only a segregation scheme is instead adopted for the two streams in VDSL, the upstream data using the 300-700 kHz portion of the spectrum, and the downstream data being allocated the spectrum over one MHz. The ADSL solution has already been tested by more than 30 telephone companies, showing a notable resistance to impulse noise. It is claimed that ADSL modems can tolerate impulses lasting no longer than 0.5 milliseconds, thus being compatible with the quality required by video compression algorithms such as MPEG2. Much more critical is the evolution of VDSL, whose definition requires a thorough knowledge of the telephone line characteristics and RF emissions and susceptibility.
In the field of radio access, the most relevant distribution mechanism is represented by MMDS (multichannel multipoint distribution service) also known with the popular oxymoron 'wireless cable'. This is indeed a redistribution of satellite channels by a service centre through a two GHz link to the antennas installed on subscribers' roofs, from which the signal goes to the television sets through coaxial cables. Though MMDS is aimed at television rebroadcasting, a similar system, named LMDS (Local Multipoint Distribution Service) operating in the 28 GHz band, is being tested to provide a wider range of services, such as banking transactions, videoconferencing and home shopping. MMDS has a range of 40-50 kilometres, while a more restricted coverage applies to LMDS. In comparison with access systems based on physical media, MMDS looks like a niche technology whose natural target is represented by areas of scattered populations.
ITALY'S CASE
In Italy the national telecom operator, Telecom Italia, has undertaken two initiatives in the field of the multimedia services offering to the residential market[5]. The first one has been the realisation, started at the end of 1994, of a technical trial of interactive video services (particularly VoD, home shopping and gaming) through the existing copper infrastructure and the use of ADSL technology. About 500 households were connected to a central video-server. The project has now been extended to paying customers located in Rome and Milan. The second, more impressive one, has been the rollout of a massive deployment plan of fibre optics in the access network. The project, started in early 1995, is called SOCRATE (an Italian acronym for Telecom Optical-Coaxial Access Network Development). The first phase calls for building an HFC network covering the main urban areas. As to the services, their introduction has been scheduled in the following three phases: 1. Television broadcasts (pay per channel), pay per view, near video-on-demand; 2. VoD, video games, distance learning, home shopping; 3. POTS, ISDN, and data services.
References
1.K. Stordahl, E. Murphy: "Forecasting long-term demand for services in the residential market", IEEE Communications Magazine, vol. 33, no. 2, February 1995. 2.T.S. Perry: "The trials and travails of interactive the TV", IEEE Spectrum, vol. 33, no. 4, April 1996. 3.A. Gillespie, B. Orth, A. Profumo, S. Webster: "Evolving access networks: a European perspective", available at comsoc.org, March 1997. 4.K. Maxwell: "Asymmetric Digital Subscriber Line: interim technology for the next forty years", IEEE Communications Magazine, vol. 34, no. 10, October 1996. 5.U. Ferrero, M. De Bortoli, P. Passeri, S. Augusto, F. Parente: "Evolution of the access network for broadband services in Italy", CSELT Technical Reports, vol. XXIV, no. 4, August 1996.
Maurizio Naldi, works with the teletraffic engineering department of Telecom Italia. He is also a part-time lecturer at the University of Rome "Tor Vergata". He can be contacted on: m.naldi@ieee.org . |
