18
SIGNAL PROCESSING: ELSEVIER Signal Processing: Image Communication 8 (1996) 173-190 COMMUNICATION A review of fully interactive video on demand Gordon Kerr* Room MLB4/32A, BT Laboratories, Martlesham Heath, Ipswich IPS 7RE, UK Abstract Is Video on Demand coming of age? From the early 1980s there have been various experiments on fully interactive video on demand services, and since the early 199Os,there has been a renewed and increased interest in the area as various technologies have emerged enabling such services to be offered in a technically feasible manner. This paper reviews the major technical aspects of early fully-digital Video on Demand systems, with an emphasis on the trial that British Telecommunications ran in the UK in 1994. Service, system, transmission, server and set-top aspects are covered, and there are some comments made on the customer experience of such services. 1. Introduction For well over a decade, there have been numer- ous developments and experiments to investigate the potential of providing on demand video re- trieval services (or VoD) to residential customers. In the UK a video library for the Westminster Switched-Star Network Cable-TV system (SSN) [IS, 131 was developed in the mid-1980s, followed by a small trial on the Bishops Stortford LLOFT (Local Loop Optical Fibre Trial) system (1992-93) [8]. This video library provided films on demand to customers on a fully one-to-one basis, each user having full control over the programme they were watching. World-wide, various experiments took place in the early 198Os, a summary of which was made in 1986 in [12]. *Tel.: + 44 1473 644553; fax: + 44 1473 643791; e-mail: [email protected] (main), [email protected] (alternate). Since the early 199Os,two major technology dri- vers for video on demand have emerged: first, video compression technology now permits good quality retrieval of moving video at a total of 1.5 Mbit/s or above using technology such as that in ISO/MPEG-1 and ISO/MPEG-2 standards 13, lo]; secondly, it is now possible to transmit a minimum of 1.5 Mbit/s downstream over the existing copper pair from a local exchange (Central Office in the USA) to customers in addition to the existing telephony [S, 181, along with a low-speed (say 16 kbit/s) bi-direc- tional channel for control. Such transmission is possible for customers within 5 km loop length of their exchange, and using unloaded pairs (probably in excess of 90% of UK customers), again with standardisation emerging in ANSI Cl]. This paper reviews the major technical aspects of early Video on Demand systems, with special men- tion of the early trials that BT has run in the UK. The service provided for the first trial was assumed to be linear films or equivalent, with full user con- trol on a one-to-one basis. 0923-5965/96/$15.00 @.J 1996 Elsevier Science B.V. All rights reserved SSDI 0923-5965(95)00049-6

A review of fully interactive video on demand

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SIGNAL PROCESSING:

ELSEVIER Signal Processing: Image Communication 8 (1996) 173-190 COMMUNICATION

A review of fully interactive video on demand

Gordon Kerr*

Room MLB4/32A, BT Laboratories, Martlesham Heath, Ipswich IPS 7RE, UK

Abstract

Is Video on Demand coming of age? From the early 1980s there have been various experiments on fully interactive video on demand services, and since the early 199Os, there has been a renewed and increased interest in the area as various technologies have emerged enabling such services to be offered in a technically feasible manner. This paper reviews the major technical aspects of early fully-digital Video on Demand systems, with an emphasis on the trial that British Telecommunications ran in the UK in 1994. Service, system, transmission, server and set-top aspects are covered, and there are some comments made on the customer experience of such services.

1. Introduction

For well over a decade, there have been numer-

ous developments and experiments to investigate

the potential of providing on demand video re-

trieval services (or VoD) to residential customers. In the UK a video library for the Westminster Switched-Star Network Cable-TV system (SSN) [IS, 131 was developed in the mid-1980s, followed by a small trial on the Bishops Stortford LLOFT (Local Loop Optical Fibre Trial) system (1992-93) [8]. This video library provided films on demand to customers on a fully one-to-one basis, each user having full control over the programme they were watching.

World-wide, various experiments took place in the early 198Os, a summary of which was made in 1986 in [12].

*Tel.: + 44 1473 644553; fax: + 44 1473 643791; e-mail: [email protected] (main), [email protected] (alternate).

Since the early 199Os, two major technology dri- vers for video on demand have emerged: first, video compression technology now permits good quality retrieval of moving video at a total of 1.5 Mbit/s or above using technology such as that in ISO/MPEG-1 and ISO/MPEG-2 standards 13, lo]; secondly, it is now possible to transmit a minimum of 1.5 Mbit/s downstream over the existing copper pair from a local exchange (Central Office in the USA) to customers in addition to the existing telephony [S, 181, along with a low-speed (say 16 kbit/s) bi-direc- tional channel for control. Such transmission is possible for customers within 5 km loop length of their exchange, and using unloaded pairs (probably in excess of 90% of UK customers), again with standardisation emerging in ANSI Cl].

This paper reviews the major technical aspects of early Video on Demand systems, with special men- tion of the early trials that BT has run in the UK. The service provided for the first trial was assumed to be linear films or equivalent, with full user con- trol on a one-to-one basis.

0923-5965/96/$15.00 @.J 1996 Elsevier Science B.V. All rights reserved SSDI 0923-5965(95)00049-6

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G. Kerr / Signal Processing: Image Communication 8 (1996) 173- 190 175

2. Outline service requirements 4.1. Fibre access

Before the major technical elements can be de- scribed, it is important to state the general service assumptions which are currently being made: - Full one-to-one on-demand retrieval video (no

simulcast or broadcast, so as to meet the UK regulatory requirements).

4.1. I. Fibre to the home (FTTH)

- At minimum, the basic VCR controls available to users (e.g. stop, back/forwards scan, fast forward/reverse, pause).

- Inclusion of additional controls to permit interactive services to be offered (such as home shopping, banking).

- Access to at least 1000 titles (initially films and similar material) available on-demand 24 hours per day, within a second or so of re- quest, of average length perhaps 1.5 hours.

- Minimum 2 Mbit/s total bit rate for video plus audio plus control, initially to ISO/MPEG-1 standards, with higher rates in the future.

- Potential roll-out to most of the UK. - Design must cater for significant penetration of

potential customer base in any one area, but must also be cost-effective at much lower pen- etration and also variable take-up of service.

- Design must cater for service over the system provided from multiple third party providers in addition to any in-built services (open access).

Experiments on various completely fibre systems and fibrelcopper pair hybrids, have been made in Bishops Stortford by BT [S], not for VoD services, but for telephony and broadband services, mostly using Passive Optical Network (PON) technolo- gies. These technologies rely on exploiting the massive potential bandwidth of optical fibre to permit a tree and branch physical cable structure to provide star-type (i.e. independent) services to cus- tomers. The PON technology used in these trials was not designed to provide efficient digital VoD services, but it is clear that with the use of either a modified PON technology, or other optical tech- nology, the bit-rate requirements for VoD services could easily be provided.

4.1.2. Fibre to the kerb (FTTK)

In the USA, trials using fibre to the kerb are being discussed, and will probably have high-speed (6 + Mbit/s) drops over standard copper pair to the customer’s home. This involves active (powered) cabinets in the street, which raises issues of main- tenance and future upgradeability, but does save the civil engineering costs associated with relaying the final part of the cable to the customer’s home.

4.1.3. Fibrelcoax 3. Overall system

Fig. 1 shows a potential overall system architec- ture. As with all system diagrams, assumptions are implicit; a number of these will be discussed in the sections that follow.

4. Access transmission

For a general coverage of local network telecom- munications systems, the reader is referred to [16]. Although there has been a major focus to date on transmission either over fibre, or copper, or some combination of the two, other options exist, includ- ing fibre plus radio final drop. Some options are dis- cussed below, but for more detailed information, the reader is referred to the references, where available.

The Cable TV companies in the US are currently upgrading their trunk subsystems (which equates to the first part of the link from local exchange to customers for a telecoms provider) to fibre, leaving the tree and branch coaxial system to serve a small number of homes passed. The fibre therefore can provide high-speed digital services in addition to any existing analogue TV channels, and these digital services can be broadcast to all the homes served by that fibre, the home equipment picking off the rel- evant digital information for the service requested.

4. I. 4. Conclusions on jibre

Various options exist, from Cable TV upgrade equipment, with hybrid fibrejcopper systems, through to FTTH solutions, none of which are currently optimal to provide full VoD services to

176 G. Kerr / Signal Processing: Image Communication 8 (I 996) 173-I 90

a large number of homes, but all of which have that potential. The precise solution chosen will depend heavily on what additional capacity and what other communications to each home the fibre owner will wish to have, in addition to the VoD capacity he will wish to provide.

All optical fibre solutions will involve the civil engineering costs associated with laying new cables in the access network, costs which are heavily based on labour charges and not readily susceptible to reduction even in volume.

4.2. Copper pair access

The great advantage of using the existing copper pair for VoD services is that the copper pairs are already installed for telephony, and the provision of VoD can therefore be considered as a marginal technical cost. Since with Cable TV systems a large proportion of the costs of installing are to do with the civil engineering costs of laying new cable, the use of the existing copper pair can potentially lead to some significant potential savings above other access transmission technologies.

The technique used for the transmission of in- formation between 1.5 and 6 Mbit/s over the cop- per pair is known as Asymmetric Digital Sub- scribers Loop (ADSL) [5,18], and is currently the subject of standardisation in the American Nation- al Standards Institute (ANSI) [l]. This paper does not attempt to do justice to the detailed technology

1 Splitter.

required, and the reader is referred to the above references for full information. The ADSL system is summarized in Fig. 2.

The ‘splitter and combiner’ at each end of the link is a highly complex filter, as we are dealing with a bidirectional system for both telephony and modulated data; in addition, all filtering must be of sufficient standard not to impair the telephony transmission beyond the stringent limits given in CCITT telephony standards or their equivalent ETSI standards and handle both the speech signals and also ringing current (75~ RMS), on/off hook impedance changes and loop disconnect or MF4 dialling. In addition, the telephony system must function when there is no mains power available at the customer’s end so emergency telephone access is still available. In the US, the telephone network appears resistive, and solutions to the above filter- ing have been found using wholly passive imple- mentations; in the UK, the network appears as a complex impedance, with additional side-tone requirements, so the filtering problem has been very hard to solve: lab prototypes of mixed pass- ive/active technologies now exist which will be able to be implemented in quantity in due course, and the design has been patented.

Current ADSL technology can reach 5 km or more over standard copper pairs at 2 Mbit/s, tak- ing into account worse-case cross-talk effects found within a multi-pair cable. For shorter reaches the technology can provide higher bit-rates, typically as given in Table 1.

1 I Customer’s Home

Telephony QP Digital Video

Streams

Local Exchange or RCU

Fig. 2. Copper pair access transmission system.

G. Kerr / Signal Processing: Image Communication 8 (1996) I73- 190 171

Table 1

Reach, km Downstream, Control, kbit/s Mbit/s (net) (duplex)

5.9 1.5 16 5.5 2 16 4.8 3 64

-3 6.3 384

These rates have been actively considered within the ANSI deliberations. Shorter reaches will dir- ectly cater for most customers within urban areas in the UK, or for all customers if the actual cop- per-pair drop is within the range required, and fibre is used from the exchange building to the start of the copper drop.

Using ADSL technology over the complete ac- cess link is seen as a short-term solution to provide advanced broadband services to residential cus- tomers; once major broadband services have be- come established (such as VoD), then the technical business case for fibre roll-out in the access network can be more readily derived.

5. Customers premises network (CPN)

Early configurations for the network for VoD in the customer’s home have tended to be as shown in Figs. 3 and 4 in recent UK trials.

It is probable that the above Network Termina- ting Equipment (NTE) will replace the customer’s existing master socket and the customer’s internal telephone system will plug into the new telephone socket provided on the new NTE.

6. Set top decoders

The function of the set top decoder is to decode the 2 Mbit/s (or above) data stream according to ISO/MPEG-1 [3] (later ISO/MPEG-2 [lo]) stan- dards and present the resulting video and stereo audio signals to the customer’s TV (and possibly his audio hi-fi system). In addition, the decoder accepts signals from its remote controller and sends those signals back to the NTE. The set top decoder often contains a graphics plane so that interaction with the customer can be more easily carried out by the network’s servers, especially when guiding the customer to his choice of item to view. The graphics plane can also enable more advanced items to be provided to the customers, such as education, train- ing and transactions.

The remote control provides keys for enabling the customer to make his choice of viewing, and keys to control his viewing. In addition, the customer may wish to use more complex or service- specific keypads or keyboards, and the signals from these will also need to be handled appropriately.

FIBRE, COAX, - _ INFORMATION

RADIO OR STREAM

SEPARATE NETWORK SET-TOP

PAIR TERMINATTNG

4 ) EQUIPMENT DECODER

4 CONTROL

VIDEO AND AUDIO

Fig. 3. Separate VoD transmission system.

G. Kerr 1 Signal Processing: Image Communication 8 (1996) 173-190

INFORMATION

TERMINATING

VIDEO AND AlJDIO

I I 1

Customer’s own internal telephone extensions

Fig. 4. ADSL system: integrated telephony and VoD.

In a more mature environment, the set-top/TV combination may optionally be replaced by a Per- sonal Computer (PC) with appropriate plug-in cards. This option will be particularly suitable for some to the more interactive services such as home education, shopping and banking, where the key- board and mouse provide a more appropriate user interface device.

7. Video server

In a fully one-to-one video on demand system, all programmes are likely to be transmitted from a video server to the customer. The video server will therefore need to:

Store all the programmes that customers may request. Provide independent access to requested pro- grammes on demand. Spool out programmes at precisely the required bit-rate to customers, that bit-rate being main-

tained whatever other processes the server needs to carry out.

_ Provide VCR-like controls in conjunction with the set top box.

_ Provide a facility, in conjunction with the set top box, to enable the customers to choose the item he wants to watch.

- Provide raw information for Service Manage- ment systems, including billing.

In early trials, various options exist for the imple- mentations of a server as the overall traffic require- ments are low. However, there are technical chal- lenges in servers that might be based in roll-out scenarios. In a system running at 2 Mbit/s, with 2000 hours of audiovisual material, the server needs to store 1.8 Tbytes of programme audiovisual material: for a server with a capability of handling 1000 simultaneous sessions, it needs to be able to maintain, at peak, 1000 channels at 2 Mbit/s, cor- responding to a steady-state spooling rate of 2 Gbit/s. On the other hand, the server will not be required to perform much information processing,

G.

unless that approach is used to provide the fast forward and fast reverse functions. It is therefore unlikely that the optimum technical solution for a video server will be a standard mini or mainframe computer system, although clearly a totally custom design will be too expensive to consider. The fol- lowing subsections discuss the various aspects of a video server.

7.1. Main processor system

Few current computer systems can maintain 2 Gbit/s across their main system bus; in most classic CPU/Bus systems, spooling from a disc to an output port would probably require multiple bus transfers for every byte transmitted. In addi- tion, without advanced direct spooling facilities from disk direct to I/O port, few CPUs would be able to handle the data fast enough to maintain the traffic levels required. Therefore, in all but the

Service management

smallest systems, some form of specialist processing system is required.

One logical approach which combines the ad- vantages of a standard computer system with a need for a more specialised spooling system is shown in Fig. 5. In Fig. 5 the standard data- base processor handles the control interface to the user (including keystrokes) and maintains a database of information on the programme, including pointers on where the programmes are actually stored on the streaming system. The streaming system is instructed by the database pro- cessor to spool an audiovisual programme from one location to a specific transmission path, and to continue to perform that function until it reaches an end-of-file or is interrupted again by the database processor.

The streaming system may be bespoke hardware, but is also potentially a modification of emerging parallel processing systems or even massively par- allel processing systems [4, 21.

rl- 1:::: - Standard processor database storage

B Audiovisual data storage

Specialised

streaming

system

Control

to/from customer

Downstream encoded digital audiovisual streams

Fig. 5. Main processor system for video server

180 G. Kerr J Signal Processing: Image Communication 8 (1996) 173% I90

The standard database processor is likely to be a standard Unix-based system. However, as some of the emerging parallel processing and massively parallel processing systems are being designed to handle the major database software engines (for example, Oracle, Sybase, etc.), it may eventually be the case that the two logical entities (standard database processor and the streaming system) be- come implemented on the one parallel system.

The structure of the streaming system needs to be able to cope with large amounts of data flow and ideally able to be increased in size to handle in- creased traffic and/or programme content, without too large an initial investment.

7.2. Programme storage

Though the scope of this paper does not permit a detailed discussion on storage technology, some indications can be given. It should be noted that the architecture of the spooling system will heavily impact the final choice of storage systems used. The storage of a programme of average 90 min length at 2 Mbit/s involves - 1.3 Gbyte; some programmes will be very popular at a given time, others will be barely used. It has been suggested that, at peak, 20% of customers might be watching the most popular title at one time, and since no simulcasting is permitted in a fully one-to-one video on demand system, this implies that 200 sessions may be ac- cessing the 1.3 Gbyte logical block of storage on a lOOO-session server. This is a major technical requirement, and various methods and system architectures have been considered to solve it

(e.g. C91).

7.2.1. Semiconductor storage

Semiconductor storage offers potentially the highest access speeds, so the capability of handling the most number of independent customer sessions. However, cost per byte is also high. In some archi- tectures, semiconductor storage can be used to ad- vantage to minimise the cost per byte per simulta- neous access at least for popular programmes. In other architectures, semiconductor storage is used as buffer or cache memory between disc storage and the transmission system.

7.2.2. Magnetic disc storage

The efficiency of magnetic disc storage has risen considerably over the years, and raw costs of well under $500 per Gbyte are being quoted as a bench- mark, with 3-9 Gbyte of storage on a single spindle within a 5.25 in or even 3.5 in disc drive bay. Read speeds are reasonably high, but major issues with magnetic discs when considering peak loading in- clude the disc access time (basically the time needed for the head to move to the new location on the disc and lock in) and self-calibration times (higher density discs often include a regular process to recalibrate the head positioning to take account of variations caused by temperature changes). An- other major issue is the finite lifetime of magnetic discs, especially when a system uses a large number of them.

Since some programmes on the video on demand server will be much more popular than others there has to be some method of ensuring popular pro- grammes are still available to customers; this can simplistically be achieved by making multiple co- pies of the popular programmes, but clearly this costs in disc space. Various techniques exist for balancing the load on multiple disc storage, includ- ing the use of disc striping, whereby the popular item is broken down into small sections, and those sections are placed on different physical disc drives, enabling concurrent sessions on the programme to be handled by different drives.

RAID Technology (Redundant Array of Inex- pensive Discs) can provide various levels of protec- tion against disc failure, enabling faulty discs to be removed and replaced and the contents rebuilt on- line without heavily effecting overall system perfor- mance. In simple terms, RAID technology ensures a sufficient level of redundancy across the disc drives to enable stored data to be rebuilt from that which remains intact; clearly, there is a penalty on overall disc capacity required, and also in access speeds.

7.2.3. Tape storage Tape storage provides one of the lowest costs of

storage media per byte, and is extensively used for back up and archive storage systems. It is unlikely to be of value in a video server with the popular programmes, but could be of use for the least

G. Kerr / Signal Processing: Image Communication 8 (I 996) 173- 190 181

popular programmes (maybe even the lower 50% of programmes on offer). The main considerations are: - Bit-rate is relatively slow, at 6 Mbit/s or so. - Access time, when used in a juke box or tape

handling system, is of the order of 10 s or more. - Tape handling systems, being mechanical, have

a finite reliability. - Random access is very slow (order of seconds). If the customer is connected directly to a tape drive for a programme, then the level of service he will receive is likely to be unacceptable; but a way round that is to have some spare disc space onto which the chosen programme can be cached for the duration of the customer session, and also to have already stored on some disc the first few tens of seconds of each programme archived on tape, so that the customer gets a response to his request and can start viewing immediately.

7.2.4. Optical storage

Optical storage tends to be robust, efficient in terms of byte densities (20-30 Gbyte on a 12 inch platter) and not so liable to corruption. However, access times available, whilst far better than those of tape, usually exceed those from magnetic disc. Optical drives are still relatively expensive, and any disc handling that might be considered again intro- duces another mechanical system which is liable to failure. Much development work continues on op- tical disc technology, and it may well prove to be an important storage technology for video server systems.

7.3. Implementation of VCR controls

The handling of the VCR controls (pause, play, fast forward, fast reverse, scan forward, scan re- verse) is an issue of the specific design of the server and to a lesser extent the set top box.

Pause will often be implemented simply by stop- ping the audiovisual stream from the server: the audiovisual decoder in the set top box will freeze on the current video frame and mute the audio. The set top box may choose to instruct the audiovisual decoder to freeze as soon as the pause key is detec-

ted, and ignore further data from the server, so giving the user a faster response time.

Play will cause the server to continue spooling out to the set top box audiovisual information from the current logical position in the main programme file.

Scan ForwardJReverse (visible) since in general there is limited transmission capacity between video server and set top box, the server cannot simply spool the audiovisual data at a faster rate and/or in reverse. Various techniques can be em- ployed to provide this facility: ~ Process the compressed (play) file in real time

and transmit that information required to scan forward (or reverse) to the user.

- At the time of loading the programme onto the server process the compressed (play) file, and create additional files containing scan forward and scan reverse information.

In the latter case the video server will need to have some form of cross-referencing between the scan forward, scan reverse and play files so the system spools from those files to the user at the relevant logical positions when moving between the various modes.

Fast Forward/Reverse corresponds to fast winding a tape on a VCR, so no video is expected by the user, but he will need some indication on how far he has ‘wound’ through the programme. possibly by a graphic or a numeric counter on- screen.

7.4. Implementation of the user interface

A separate section of this paper touches on as- pects of the user interface. The actual implementa- tion of the user interface is a function be shared between video server and set top box, the actual amount handled by the set top box being depen- dent on its processing, graphical and software capa- bilities. Various vendors are basing the control aspects of the set top box on personal computer technology. The user dialogue, when he identi- fies himself and selects a programme to watch, can be considered to be very similar to a standard database security and access application, albeit from a remote control unit.

182 G. Kerr J Signal Processing: Image Communication 8 (1996) 173% 190

8. Core network 8. I. Local video server

The core network is that part of the transmission system between any centralised parts of the net- work and the local exchange buildings (Fig. 6). This section introduces some possible options for the core network in an early roll-out scenario and discusses their relative merits.

In general, it is possible that a number of distinct video servers would provide service to a number of local exchange areas, the access transmission equipment, be it ADSL/copper or optical, being sited in these local exchanges. The servers may be- long to different providers, and may offer distinctly different services to the end customer, it is therefore important that the capacity of each server is open to all customers who want to use that service. In general, the video servers will not therefore be co- sited with each local exchange that they serve, so some form of transmission and switching system is required between the video servers and the local exchange areas they serve. However, a number of architectures could be used for the deployment of video servers, and these are discussed below.

In this model it is assumed that a commercial arrangement exists whereby a local video server is able to accommodate all of the customer’s service requirements, the Service Providers loading their information as required onto the local video server. Essentially no core network is required, and the need for third party access is covered by space being available on the local video server for third party information. If customers need to have access to a separate third party server, then some form of switching is required so the customer’s link is either served from the local video server or the third-party server.

This general architecture of a video server per local exchange requires little investment in a core network, but has drawbacks (see Fig. 7): _ expensive where the local exchange serves only

a few customers; ~ all updates to programme materials done man-

ually; _ all local exchanges require 24-hour maintenance

cover for the video server technology;

Customers

Fig. 6. Overall core network system.

G. Kerr/Signal Processing: Image Communication 8 (1996) 173-190 183

VIDEO

DOWNSTREAM

ANDAUDIO

TRANSMISSION CONTROL

END

:4---W CUSTOMERS

Local Exchange Building

Fig. 7. Basic local video server and transmission system.

LOCAL VIDEO SERVERS

OPERATIONS LOCAL VIDEO SERVERS

LOCAL VIDEO SERVERS

Fig. 8. Remotely updated local video servers.

- the local video server requires considerable data storage capacity to offer a wide service to end customer (e.g. over 1 Tbyte for 1000 titles).

8.2. Remote updating to local video server

The obvious extension to the above architecture is to permit updating of the local video server by telecommunications links from some form of cen-

tral operational unit to the required local ex- changes (Fig. 8). The links required would need capacity to meet the updating times derived by the service provider. For example, a single El 2 Mbit/s link would update one film in roughly 1.5-2 hours, so would be adequate for a maximum of, say, 10 film updates per day; a basic rate ISDN connection would update a film in roughly 24-32 hours, so would be adequate for a maximum of 5 film up- dates per week, assuming the service provider is

184 G. Kerr 1 Signal Processing: Image Communication 8 (1996) I73- 190

willing for the release of the film on the local video a communications link between the major video server to be at least a day after he loaded it onto the server and the local video server served. Two mod- central operations station. els can exist.

The advantages of this architecture are: _ all local video servers easily operated remotely; _ all local video servers updated easily from central

location. However, most of the disadvantages of the local video server architecture still remain.

1. The local video server requests a faster than real-time download of a copy of the programme available on the major video server which then handles the customer session thereon. There is a potential penalty in the response time pro- vided to the customer’s request unless very care- fully handled.

8.3. Hierarchical video services

In this architecture a new layer of video server is introduced, probably remotely, which handles the less popular programmer,, leaving the local video server to handle the most popular programmes (see Fig. 9). In this manner a considerable amount of data storage capability can be removed from each local video server, leaving the major video server to have one set for a number of local video servers served. As the major video server only handles the less popular material, it can take traffic demand from a number of local exchange areas without undue problem. The trade-off is the requirement for

MAJOR VIDEO SERVER

2. The local video server requests that the major video server directly handles the customer ses- sion for the programmes available only on the major video server.

In the latter case, a switch of some form (logical or literal) will be required in the local video server to cater for the traffic handled by the major video server. Again in this model, there is still a require- ment for the local video server to have full 24-hour maintenance cover in each local exchange, but there will be far less data storage to consider. The major video server/local video server links can also be used, at off-peak times, for updating the contents of each local video server data/pro- gramme store,

Fig. 9. Hierarchial video servers.

LOCAL VIDEO SERVER

LOCAL VIDEO SERVER

LOCAL VIDEO SERVER

G. Kerr/Signal Processing: Image Communication 8 (1996) 173-190 185

8.4. Remote video server

The models for a remote video server architec- ture are many, only a few are suggested here (Fig. 10).

SDH (Synchronous Digital Hierarchy) is the emerging European version of the ITU standard for transmission of digital information. In the USA, the complementary standards are referred to as ‘Sonet’ (Synchronous Optical Network). SDH per- mits transmission in a hierarchy of bit-rates to provide for differing traffic requirements. The reader is referred to [7, 171 for further information on this topic.

In the first model, a video server serves a cluster of local exchanges (say a medium-sized town and its satellite exchange areas) on a series of SDH rings, an Add/Drop Multiplexer (ADM) being used in each local exchange to gain access to the streams from the video server. The ADM is, in this model, essentially a switch and demultiplexer unit at the local exchange, and a multiplexer unit at the video server, so enabling full use of the SDH ring for video server traffic. A double ring is used for reliability, and is a standard feature in the SDH

VIDEO SERVER

hierarchy. A 622 Mbit/s ring could handle 250 end customer sessions (perhaps 750-1000 customers connected), a 2.4 Gbit/s ring perhaps 1000 end cus- tomer sessions (perhaps 3-4000 customers connec- ted), so for a typical town and a typical take-up of service, a number of these rings will be required. Note that the suggested capacity of these rings far exceeds that planned for telephony customers for local exchanges to main switching units in a town. However, for early or smaller installations, the architecture is robust, and available from manufac- turers.

Remote updating of the video server can be achieved via communication links as in the pre- vious architecture, or a hierarchy of video servers established as in the section above, with the lowest end being at main exchange level instead of the local exchange level.

8.5. A jlexible core network model

Many of the preceding models rely on either a static, or a well-defined service environment for VoD: in reality this is not likely to be the case, as

ADM = ADD/DROP MUX

TRANSMISSION ! r”‘“’ 1-1 T” * “‘““‘“SION

LOCAL EXCHANGE

LOCAL EXCHANGE

Fig. 10. Remote video server.

186 G. Kerr / Signal Processing: Image Communication 8 (1996) 173- 190

new third-party service providers may constantly - the regulatory environment; appear wanting to offer service to large areas of _ the relative cost of the transmission/switching served customers, new sets of customer coming options; onto the system, and the usual customer ‘churn’. In _ the cost of maintenance of video servers at the addition the demand for services will vary, depend- different locations in the various models; ing on the activities of providers of similar services, - the availability of suitable exchanges or nearby the regulatory environment and the general confi- accommodation for the video servers; dence of customers in using the services available. It _ the take-up and use of the VoD service; is therefore important that whatever core network - the geographical spread of VoD service in the architecture is used, it can cope with these fluctu- steady state; ations, many of which are hard to project, with the _ the impact of other interactive multimedia servi- best efficiency possible. The most favourable tech- ces, such as home shopping. nology for this would appear to be a specific use of Some of the above are not yet known to any cer- the emerging ATM (Asynchronous Transfer Mode) tainty, so there is currently much debate over what solutions [14]. ATM has some advantages for will be the optimum core network architecture for VoD: VoD. - Distributed switching is inherent in the packet

structure. - New video server feeds can readily be incorpor-

ated at any location in the ATM network. - Most carriers are considering working towards

an ATM-based main network. The current problems with ATM mostly centre on the relative immaturity of the technology. - Not much real-field experience. - Very little installed base. - Immature centralised management capabilities,

and standards.

9. The customer’s experience

The customer’s experience is a combination of many things, which include: ~ His experience of finding out about VoD, from

marketing, information from friends, hearsay, in- formation from media.

~ Equipment from suppliers still expensive, but projections indicate attractive potential pricing in the future.

In addition, the ATM traffic has to be carried on some transport system, potentially SDH in the UK, which is a relatively new technology, and it is only currently beginning to be rolled out in the BT network. Nonetheless, for a mature VoD core net- work architecture, the use of ATM at least in the core network would seem appropriate assuming video servers are located away from the majority of local exchanges.

- His experience of ordering VoD. ~ His experience of having the necessary equip-

ment installed in his home. ~ His experience of initially using the system (ease

of use, finding way around, controllability, breadth of choice, technical quality of video and audio, quality of programmes, attractiveness of presentation).

_ His experience of using the system after some months.

- His experience of how many faults he perceives he has had, how any call to a fault reporting number was handled, and his experience of how quickly and easily the fault was cleared.

- His perception of the correctness and appro- priateness of the bill.

8.6. Trade-o& ~ His perception of the handling of bill payments and enquiries.

The choice of core network architecture will, in the end, depend on: _ the levels of service that service providers are

prepared to pay for:

Most of these are major items in their own right, and are outside the scope of this paper. However, some comments will be made on some relevant items.

G. Kerr/Signal Processing: Image Communication 8 (1996) 173- 190

ACCESS

TRANSMISSION

* DIGITAL SIGNAL VOD

SYSTEM SOCKET SET

NTE ON ON

TOP

(COPPER OR WALL

IN-HOUSE CABLE BOX

FIBRE)

Fig. 11. Basic equipment in the home.

187

9.1. Equipment in the home

The minimum offering for a customer is one VoD channel to a house through one STB (usually located on the main TV) and this has been the equipment provided to customers on most early VoD trials. For multiple simultaneous VoD chan- nels in one house, a number of options exist, and these are the subject of ongoing discussions with potential suppliers and standards-making bodies (see Fig. 11).

9.2. Technical quality of signals

Current VoD systems rely heavily on digital technology, especially in the transmission over the local copper pair: the video signal needs to be compressed in order that the average local copper pair can carry the signal. Much has been written on the compression of video and audio signals, both for conversational and retrieval services, and the reader is referred to relevant references (e.g. [ 111). Most VoD systems globally are expected to use digital compression of video and audio to ISO/MPEG standards initially to MPEG-1 [3], but as soon as suitable integrated devices exist, to ISO/MPEG-2 [lo]. The impairments which a viewer may notice from such a compression sys- tem differ considerably from those he is used to from broadcast, satellite or video cassette recorder (VCR), the latter all being essentially analogue im- pairments.

Analogue impairments include noise, ghosting, loss of fine resolution on the picture and colour imbalance. On a standard VHS machine, consider- able bandwidth is lost compared with broadcast TV; when a VHS machine is run at half-speed (‘long

play’) even more bandwidth is lost, resulting in more reduction in picture resolution and loss of colour fidelity as well as an increase in noise. But many viewers seem to be very willing to accept these type of impairments, as witnessed by the huge number of people who do timeshift recording of broadcast TV programmes on ‘long play’ on their VHS machines.

Impairments on the VoD system will come from two sources: 1. Impairments due to bit-errors (including slip

and jitter). 2. Impairments due to the compression/decom-

pression process.

9.2. I. Impairments due to bit-errors

The compressed digital information, by its very nature, has very little redundancy remaining in the bit-stream, so any bit-error can cause a significant detectable impairment to the viewer. In general, at low error rates, these errors will appear as very small squares on the picture, disappearing very quickly. Higher error rates can cause the whole picture to disappear for a short time, or for large areas of ‘mosaics’ to appear on screen. Clearly, all of these effects are unacceptable to viewers, so the system is being designed for very low overall error rate. To that end, the access transmission systems will employ some form of forward error correction (FEC) to guarantee an error rate of perhaps less than 1 in 10i2. The video server and core network system will need to guarantee at least this level of performance as well. Interestingly, it may be that in the home equipment and wiring it may be the most difficult to achieve a very low error rate unless great attention is given to electromagnetic interference (EMI) and resilience to mains-born interference (e.g. from switching of a large electrical load, or

188 G. Kerr / Signal Processing: Image Communication 8 (1996) I73- 190

faulty fluorescent tubes, etc.). In all, the target is Although it is dangerous to attempt to pin down that at most the viewer might notice one very brief acceptable qualities and bit-rates, Table 2 is con- visual impairment during the viewing of one film. sidered an indication by the author.

It must be stated here that unlike the case of analogue broadcast or cable TV systems, the qual- ity to the end customer is consistent, whether he is close, or distant from his local exchange.

Live broadcasts/sports will probably benefit most from the extra bit-rate as the encoding will have to be done on-line, with no chance of any rebalancing or intervention as is possible for off- line encoding.

9.2.2. impairments due to compression/

decompression

On the video side, expert viewers tend to notice slight blockiness in detailed moving images, slight blurring of details in moving objects, small amounts of judder in fast-moving high-contrast edges and a drop in overall resolution compared with broadcast TV. These impairments relate heav- ily to overall picture content and available bit-rate. Ordinary viewers do not tend to notice the impair- ments, but the author is not aware of any large- scale controlled experiments on the acceptability of various levels of these types of impairments to ordinary viewers over a long period of time.

10. User interface

This paper does not intend to delve deeply into this highly specialised area, but intends simply to provide an overview and some pointers.

Not surprisingly, this state of affairs leads to a lot of debate in the relevant industries, with some ap- parently unsubstantiated statements being made by some people. In addition, the extra complication is that ISO/MPEG is a decoding standard, i.e. it defines how to decode a given bit-stream into a video/audio signal, but does not define the opti- mum method of encoding. Thus, very poor en- coders can exist, which meet ISO/MPEG, and the ‘industry best’ encoders can get better. So it is possible that some commentators have seen video and audio at a given data rate looking essentially unacceptable because of the poor encoder used.

Most VoD users will expect the VoD system to be an enhancement of their existing TV systems, with none of the problems of operating many cur- rent VCRs. They will not expect the VoD service to make the TV into some form of computer interface (highly inappropriate for the non-computer liter- ate), but instead may expect to be attracted and entertained all the time, even though the VoD sys- tem, by its very nature, needs to be interactive at the start of any session, and thereby elicit important information for the user, such as any passwords, identifying what he wants to watch. The one TV screen needs to convey to the user a very wide choice of services and programmes without con- fusion. This is the demanding problem to be solved by user interface designers.

Table 2

Total bit- Material

rate Mbits

Video quality Audio quality

1.5

2.0

4.0

12.0

CD-I/FMV Acceptable, FM Radio

VHS or Better

Films Near S-VHS FM Radio

Live Very good Nearly CD

broadcasts/sports

HDTV Distribution CD

HDTV

Early VoD systems, e.g. the video library at West- minster [13], used text and block graphics to elicit relevant information from the user. A coffee-table paper catalogue was provided to the user so they could decide from it the title they wanted to watch, and an associated four-digit code was entered at the appropriate prompt on the system. Whilst that may have been acceptable ten years ago, when it was probably the only technically feasible option for that early system, it is felt that for the majority of users, such a selection system will now be far from acceptable, and they will expect a far more attract- ive ‘log in/choice’ sequence. In truth, if the user has just switched from a mainstream broadcast TV channel, with all the potential links and continu- ities, being presented with a static graphic request- ing some input could appear very dull and boring.

G. Kerr / Signal Processing: Image Communication 8 (I 996) 173-I 90

VoD systems, by using digitally compressed video, can send random sequences of video clips to the user: user interface designers may choose to use such moving video sequences to elicit the required information for users.

11. BT trials

In 1994, BT installed and ran a small-scale tech- nology trial of video on demand. Sixty employee customers received a service of on-demand films and old TV programmes in their homes over a peri- od of six months. Users were able to control their viewing much like they would their own VCRs. The server was located in the local exchange (Kesgrave, near Ipswich), and the majority of customers re- ceived service over ASDL transmission systems (see Section 4.2) with others receiving over FTTH (see Section 4.1.1) and one or two over a mixture of fibre to the final distribution point with radio for the final drop. As this was a technology trial, no charges were made for the service. An enormous amount was learnt from the trial, much being tech- nical aspects at a systems level, and important pointers to operational and installation issues were raised as well.

At the time of writing, BT is working towards a major market trial of VoD services to 2500 pay- ing customers in the Ipswich/Colchester areas in East Anglia. The services will be marketed under the umbrella title BT Interactive TV and are plan- ned to include: - Movies on Demand, TV programmes on De-

mand. - Home Shopping on Demand. - Home Banking on Demand. ~ Education on Demand. ~ Local Information and What’s on. Customers will be connected over ADSL (Section 4.2) or over passive optical networks (see Section 4.1.1). One server will provide service to all cus- tomers, and transmission from this server to the local exchange buildings will be ATM over SDH (Section 8.4).

The services will demand approximately 2000 h of audiovisual information stored on the server, and a significant amount of this information will be

updated (‘churned’) during Agreements have been put companies to work together services.

189

the life of the trial. in place with major to provide the above

BT is also working towards a smaller-scale mar- ket trial of a more limited set of on-demand services over its existing Westminster Cable TV system. The technology of the delivery system has been de- signed to work over the existing legacy switched- star network [lS] and exploit the hooks left in the system from the Video Library developments of the 1980s [13].

12. Conclusions

The paper has given an outline of some of the network, transmissjon and storage aspects required for a potential fully interactive video on demand system in the UK. Video on demand is currently a fast-moving field, and numerous technical chal- lenges will have to be met before service can be offered in a cost-effective manner in volume. But early trials, and detailed discussions with potential suppliers, would indicate that the service is now technically feasible; market trials and further nego- tiations with suppliers will eventually help service providers to ascertain whether there actually is profitable business from this opportunity provided by the technology drivers.

Acknowledgements

Grateful acknowledgement is made to Geoff Morrison, Nigel Cole, Norman Kenyon and Richard Nicol, all of BT Labs, for their detailed comments on the various drafts of this paper.

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