Abstract:
A file transfer method is disclosed in which in which a client ( 100 ) requests a file from a server ( 110 ) and the server ( 110 ) sends to the client ( 100 ) one or more data segments which data segments together constitute content of the requested file and additional content provided by a service provider. A benefit that may be provided by at least preferred embodiments is that a provider of download files, and in particular audio files, may add additional material such as advertising material to the download content. Targeting of the material to a particular user might be achieved by asking the user to provide information about their interests, and selecting for inclusion those advertisements that most closely accord with the user&#39;s a preferences. To reduce storage requirements, the server may store a virtual file reference for each user and construct a customised file dynamically.

Description:
COPYRIGHT NOTICE  
         [0001]    A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.  
         FIELD OF THE INVENTION  
         [0002]    This invention relates to a file transfer method and system and in preferred arrangements provides a personalized or individualized file transfer method and system.  
         BACKGROUND  
         [0003]    Many files are routinely transferred over a network (including the Internet) from a store of files on a server to a client. Conventionally, each file is of use to the client only when the entire file has been received. Moreover, it is normal that the same file will be requested many times, which means that the total file storage requirement of the server is not excessive in relation to the amount of network traffic.  
           [0004]    In the case where a user is given the opportunity to download a large audio file, such as an audio book or a music album from a server in MP3 format, the user may wish to download the file in portions (typically, corresponding chapters of a book or to music tracks in an album). This is particularly so because the entire file may take considerable time to download over a low-bandwidth Internet connection that might typically be used by a home user.  
         SUMMARY OF THE INVENTION  
         [0005]    An aim of this invention is to provide a method and a system that allows a client to download from a server a virtual file that has been tailored to the client&#39;s (or, more precisely, a user thereof&#39;s) specific requirements, preferably in more than one download session If the user so chooses, while reducing as far as possible the storage requirements of the server.  
           [0006]    Accordingly, from a first aspect, the invention provides a file transfer method in which a client requests a file from a server and the server sends to the client one or more data segments, which data segments together constitute content of the requested file and additional content provided by a service provider.  
           [0007]    A benefit that may be provided by at least preferred embodiments of the invention arises where a provider of download files, and in particular audio files, may add additional material such as advertising material to the download content. Advertising is considered to be more effective if it is targeted towards a particular user. This might be achieved by asking the user to provide information about the user&#39;s interests, and selecting for inclusion those advertisements that most closely accord with a user&#39;s previously provided preferences. The issue is similar with other content such as entertainment materials (for example, music programs and video films), computer programs (such as computer games and software applications), text-files (such as books or other documents), and so forth. A consequence of this is that the aggregate content that a user downloads, including the requested content and selected advertisements, may be tailored to each particular user. Even if the number of users who are delivered each download configuration of requested content plus advertisements is relatively small, the number of possible combinations of content may be very large. If each possible download combination had to be stored on the server as an individual file, the storage requirements for the server would become large. The system can be thought of as being a multi-user individualised file sender.  
           [0008]    At first sight, it might be thought that the storage requirement of the server could be reduced to a manageable level by creating a temporary individualised file on the server for each user. The service might then wait while this user downloaded this file and then immediately delete this file from the server. However, analysis pursuant to the invention reveals that this is not a viable solution as many of existing downloading clients (download managers) allow a user to pause a downloading process for an undetermined period, which might be as long as days or weeks, and then resume it at a later date. Therefore such temporary files would have to be maintained on the server for a sufficiently long length of time. For large numbers of users, this method would require very considerable storage space, which in many cases would be impracticable.  
           [0009]    It might also be thought that the storage requirement of the server could be reduced to a manageable level by simply sending individualised additional content (such as advertisements) as a separate files without first concatenating them to requested content (such as audio book chapters). However, analysis pursuant to the invention reveals that this is not a viable solution as the majority of users would not necessarily download the additional content such as advertisements or would not play them back even if the downloading process were initiated automatically.  
           [0010]    In embodiments of this invention, the server need not maintain a copy of each of the aggregate file of requested content plus additional material. Instead, the server need only maintain the one copy of each of the segments, typically as real server files. The server may return a virtual file reference and construct the data to be supplied in response to a download request “on the fly”.  
           [0011]    The server can select those segments that, when taken together, constitute the entire content most appropriate for the client user. The content may then be sent to the user as a single virtual file or as several files. It will be noted that the entire downloaded file need not exist on the server; it need only store the segments (for example, as actual files on the server) from which the download is made up. However, the invention does not exclude the possibility that in certain embodiments it may be advantageous to create a copy of the entire file on the server (at least temporarily or which may be retained for some predetermined time) to reduce processing requirements of the server.  
           [0012]    Another alternative to reduce the storage requirement of the server to a manageable level would be to provide users with a modified Downloading Manager. In such a case, when a user decides to download a file, such as an audio book, the modified Downloading Manager would be arranged to receive from information indicating what additional content (such as advertisements) should be provided to the user with the requested content. The Downloading Manager would then initiate downloading of the requested content (for example chapter-files of the book) and additional content (for example relevant advert-files) transparently. The Downloading Manager would also transparently concatenate requested and additional content on the user PC and present the user with content (for example chapter-files) that have additional content (for example advertisements) embedded therein, for example as indivisible part of chapter-files. This solution is a possible alternative which may be provided in an alternative aspect of the invention but has some drawbacks:  
           [0013]    1. It requires users to install a new Downloading Manger just for such a site (this can be alleviated by having the download manager install online).  
           [0014]    2. Some user devices might not have sufficient computing capabilities to perform the required tasks (e.g. WAP mobile phone, MP3 hi-fis with direct downloading facilities, etc.)  
           [0015]    3. Sending chapters and advertisements as a separate files would significantly increase possibilities for an unauthorised person to create a filter program that would bypass the Downloading Manager and filter out additional content.  
           [0016]    4. It is likely that in some formats, the requested content and additional content could not be easily concatenated together without complicated, processor-intensive and storage-intensive re-conversions. Some user personal computers or devices might not have sufficient computer resources to perform such a task.  
           [0017]    5. Such a Downloading Manager has to be continuously supported, developed and modified in conjunction with constant and accelerating development of different operating systems used by various user devices.  
           [0018]    In the case where additional content comprises advertisements which may change periodically, the case may arise where a user has paused a download containing an advertisement which has been replaced. Problems may arise if the real file corresponding to the old advertisement is simply replaced and the user attempts to resume the download. These can be tolerated or minimised simply by retaining old files on the server for a set length of time. However, advantageously, the server may maintain a register of real files required by existing download requests which have not been completed and to retain real files at least while so required.  
           [0019]    In a typical embodiment, the content requested by the client includes audio encoded speech or music. For example, the file may be encoded in MPEG Layer 3 (MP3) format. However, the method can be applied to any file format that can encode the required content. Advantageously, the additional content is provided in a format identical to or compatible with the content requested by the client. In this way, all of the content can be brought together by the client or the server to constitute, or to simulate, a unified file.  
           [0020]    In a method embodying the invention, segments may be subject to data compression prior to their being sent by the server to the client. The segments are typically then uncompressed by the client to restore them to their original format. Some files, in their native form, compress their data. MP3 files are a particular example. These files would not normally be further compressed in embodiments of the invention. Such compression may combine several segments (for example, several files) into one compressed archive. These files are then recovered by the user when the archive is uncompressed.  
           [0021]    The additional content may be selected by the server by identifying the client and providing additional content that is appropriate for that user. If the user cannot be identified, default and/or random additional content may be supplied. More specifically, in preferred embodiments, prior to making a file transfer request, a user will be asked provide personal information (for example, in an enrolment procedure to gain initial access to the server), and that personal information is consulted in order to determine the additional content to be sent to the user. The personal information may be obtained by requesting the user complete a questionnaire. Such personal information may be stored in a user database for future retrieval.  
           [0022]    A method embodying the invention may receive requests from a client and return data to the client using a network protocol. For example, the protocol may be hypertext transfer protocol (HTTP), wireless application protocol (WAP) or file transfer protocol (FTP). A particular advantage of this arrangement is that the client may be a conventional computer with Internet access or, in the case of the wireless application protocol, a WAP-enabled mobile communication device. In the case of hypertext transfer protocol, the method can typically operate over a proxy server or a firewall, and can also be accessed by a public access Internet terminal.  
           [0023]    In a file transfer method embodying the invention a list of segments to be downloaded to fulfil a request may be constructed and stored in a download database (for example, as a list of real files on a server).  
           [0024]    Advantageously, in a file transfer method embodying the invention, the server receives a request from the client, and the server assigns the request a unique request identifier. In many embodiments, a record of the request is then constructed in a database, the record being identified by the request identifier. In a typical method, the unique identifier can be used to construct a virtual URL from which the virtual file can be downloaded by the client; the virtual URL can then be returned by the server to the client. In such embodiments, part of the download reference may be used by software on the client as a local file store name for the downloaded file. For example, the reference may include a name of an audio file. This can help the user to identify the file that has been downloaded. The URL may include data to enable the client to perform error detection on the downloaded file, for example, by including a cyclic redundancy check code for the file.  
           [0025]    In alternative embodiments, the server may returns to the client a list of identifiers that identify segments to be downloaded to provide the content requested by the user. For example, the identifiers may identify real files on the server. In such embodiments, the client subsequently requests the segments specified in the list.  
           [0026]    Of particular advantage, in a file transfer method embodying the invention when a transfer is interrupted and subsequently resumed, on resumption, only those segments not previously sent to the client are then sent to the client. This allows a user to interrupt and resume a download with a minimum of duplicated data transfer. For example, a record may be kept (for instance, in the virtual file database) of the position in the virtual file to which a download has progressed, and, upon resumption, the download is re-started from that position.  
           [0027]    It should also be noted that in a method embodying the invention the segments need not be sent to the client in a contiguous stream. There may be pauses in the data stream within or between segments.  
           [0028]    In another arrangement, the client may store the received data on a data carrier, such as a tape, a memory card, or an optical or magnetic disc. This data carrier can then be conveyed to the user. Alternatively, the data may be downloaded directly to a user&#39;s computing device, such as a self-contained audio file player. In such embodiments, the client and the server may both be operated by a service provider. The client and the server may operate on a single computer. Typically, data is transferred to the data carrier upon receipt of a request from a user. Such a request may be conveyed by many alternative methods, including computer-based communication methods, telephone, fax or post.  
           [0029]    From a second aspect, the invention provides a file transfer server system comprising receiving means to receive a request for a file from a server, storage means for storing content requested by a user and additional content, and sending means to send to the client a plurality of data segments in turn that together constitute content of the requested file and additional content provided by the server.  
           [0030]    The receiving means of such a file transfer server system most typically communicates with a client over a network link using a network protocol, for example, hypertext transfer protocol (HTTP), wireless application protocol (WAP) or file transfer protocol (FTP). Naturally, many other protocols might also be used. Conveniently, the receiving means may be embodied within a web server.  
           [0031]    A server embodying this aspect of the invention may further comprise data storage means for writing segments that constitute content of the requested file and additional content provided by the server to a data carrier. This allows the requested file to be written to a data carrier (such as a tape, a memory card, or an optical or magnetic disc), which can subsequently be conveyed to a user. Alternatively or additionally, the server may further include interface means for transferring segments that constitute content of the requested file and additional content provided by the server to a user&#39;s computing device, such as a portable music file player.  
           [0032]    A file transfer server system embodying this aspect of the invention may include selection means for selecting those segments that, when taken together, constitute the entire content intended for the client. Typically, the selection means includes a dynamic file database. The dynamic file database may contain a record for each request, the record identifying the data segments to be sent to the client in order to meet the request. Moreover, the dynamic file database record may indicate the position within the virtual file to which a client download has progressed.  
           [0033]    Advantageously, a file transfer server system embodying this aspect of the invention may include a user database. Such a user database might include a list of users and personal information that relates to each user.  
           [0034]    In preferred embodiments, a file transfer server system includes a database server that manages the dynamic file database and/or the user database. The selection means may be embodied within the database server.  
           [0035]    In preferred embodiments, the file transfer server system returns to the client a virtual URL from which the virtual file can be downloaded by the client. Subsequently, a request for data from the virtual URL may be handled by the sending means, which might conveniently be embodied within a file transfer server. In such embodiments, the file transfer server may communicate with the client using one of hypertext transfer protocol (HTTP), wireless application protocol (WAP) or file transfer protocol (FTP).  
           [0036]    In alternative embodiments, the server returns to the client a list of identifiers that identify segments to be downloaded to provide the content requested by the user. For example, the identifiers may identify real files on the server.  
           [0037]    Most typically, a file transfer server embodying this aspect of the invention is embodied by a computer system executing server software. Moreover, a file transfer server embodying this aspect of the invention typically operates in accordance with a method embodying the first aspect of the invention.  
           [0038]    From a third aspect, the invention provides server software executable on a computer system to perform a method embodying the first aspect of the invention.  
           [0039]    From a fourth aspect, the invention provides a download client comprising a computer system programmed to send a request to a file transfer server that embodies the second aspect of the invention. However, it should be noted that such a server, in many embodiments, can communicate with conventional network clients such as a web browser or a file transfer client that operates using FTP.  
           [0040]    From a fifth aspect, the invention provides a method of operating a file transfer server optionally in accordance with the second aspect of the invention to respond to a request for a file comprising compiling a list of data segments to be sent in response to the request, which data segments together constitute content of the requested file and additional content provided by a service provider.  
           [0041]    Such a method may further comprising a step of identifying a user making the request and selecting those segments that, when taken together, constitute the entire content most appropriate for the user. In such embodiments, prior to responding to a file transfer request from a user, the server may obtain personal information from the user, and that personal information is consulted in order to determine the additional content to be sent to the user. For example, the server may obtain personal information by requesting the user complete a questionnaire. Thereafter, the personal information may be stored in a user database for future retrieval.  
           [0042]    Typically in a method embodying this aspect of the invention, the server receives requests from a client and returns data to the client using a network protocol, such as hypertext transfer protocol (HTTP), wireless application protocol (WAP) or file transfer protocol (FTP).  
           [0043]    In a method of operating a file transfer server, when the server receives a request, the server may assign the request a unique request identifier. A record of the request may then be constructed in a database, the record being identified by the request identifier.  
           [0044]    In embodiments according to the last-preceding paragraph, the unique identifier may then be used to construct a virtual URL from which a virtual file can be downloaded. The URL may then be returned by the server to a client. The URL may advantageously include a name of an audio file. Moreover, the URL may include data to enable the client to perform error detection on the downloaded file. (The URL effectively provides a reference to a virtual file. An application may refer to the virtual file by way of a hyperlink.)  
           [0045]    A method according to this aspect of the invention may comprise a further step of subject to processing prior to there being sent to a client to bring them into accordance with a requested file specification.  
           [0046]    Advantageously, in a method of operating a file transfer server, when a transfer is interrupted and subsequently resumed, on resumption, only those segments not previously sent to the client are then sent to the client. This may, for example, be implemented by a method in which a record is kept of the position in the virtual file to which a download has progressed, and, upon resumption, the download is re-started from that position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]    An embodiment of the invention will now be described in detail, by way of example and with reference to the accompanying drawings, in which:  
         [0048]    [0048]FIG. 1 is a diagram of a client and server system exchanging files by a method embodying the invention;  
         [0049]    [0049]FIG. 2 is a flowchart of a first stage in a method in which a user requests a file for download by way of a method embodying the invention;  
         [0050]    [0050]FIG. 3 is a flowchart of a method by which the size of a dynamic file can be calculated;  
         [0051]    [0051]FIG. 4 is a flowchart of a method by which a download interrupted by the user can be resumed;  
         [0052]    [0052]FIG. 5 is a flowchart of a method by which a virtual file can be retrieved from the system database;  
         [0053]    [0053]FIG. 6 is a flowchart of a method by which a virtual file can be opened by a client;  
         [0054]    [0054]FIG. 7 is a flowchart of a method by which a seek operation can be performed within a virtual file; and  
         [0055]    [0055]FIG. 8 is a flowchart of a method by which a virtual file can be transferred to a client; 
     
    
     DETAILED DESCRIPTION  
       [0056]    In FIG. 1, a client system is represented at  100 , and the server system at  110 . The server system  110  includes a web server  112 , a database server  114  and a file transfer server  116 . The client system  100  and the server system  110  are interconnected by a network link  120 , which typically includes the Internet.  
         [0057]    In this embodiment, the actions (preferably, but not necessarily in this order) that result in a file being delivered to a user are as follows:  
         [0058]    1. A user registers with a service that provides audio files (for example, audio books) for download. As part of the registration process, the user is asked to provide the service provider with information about their demographics, preferences and interests. (Note that this will typically happen only once, before the first time that the user accesses the service.)  
         [0059]    2. The user selects content, for example an audio book, to download on the basis of a selection presented in the web site (Step  210 ). Through a web browser, a user&#39;s client computer  100  sends a request for the selected file to a web server  112  over the network link  120 .  
         [0060]    3. The web server  112  returns a unique download reference to the client computer  100  at Step  212 . The web server  112  also sends details of the request to the database server  114 . This reference is used as an index to a unique database entry (Step  222 ).  
         [0061]    4. The database server  114  generates database entry (Step  222 ) in a dynamic file database  230  and creates in that entry a list of content segments, each of which is an actual file on the file transfer server  116 , that are required to fulfil the user&#39;s request (step  224 ), and sends the list to the file transfer server  116  (Step  226 ). The content segments include the content specifically requested by the user, together with additional content selected by the database server  114 . Such additional content includes audio advertisements to include within the audio book download, targeted at the user&#39;s specific interests, demographics etc.  
         [0062]    5. The web server  112  returns a URL to the client that includes the unique download reference (Step  232 ). The URL serves as a reference to a virtual file that the client can download.  
         [0063]    6. The file transfer server  116  sends the segments to the client over the network link  120  or the transfer server writes data to a data carrier by means of a device (such as a disc drive or a tape drive) connected to the server  116  or connected to another computer, or downloads the file to a portable computing device.  
         [0064]    7. The client may stop the download of the file at any point, and resume the download at a future date when it is more appropriate for the user.  
         [0065]    It should be understood that the web server  112 , the database server  114  and the file transfer server  116  need not be implemented as separate server computers. They could be implemented as separate processes executing on one computer or as a single software system executing on one computer. Equally, they could be distributed over separate computer systems, possibly at remote locations.  
         [0066]    Aspects of the above method will now be discussed in further detail with reference to the flowcharts of FIGS.  2  to  8 .  
         [0067]    First, with reference to the section marked  220  in FIG. 2, when an entry in the database  230  is created, it is indexed by a unique key. This key contains characters compatible with the URL specification. The key may also have encoded into it a checksum or CRC (cyclic-redundancy-check) and will be used to verify the key is genuine. The relationship between the URL and the key should be calculated in a way that is not obvious to an outside observer in order that it should not be a straightforward matter for a person to guess a valid URL.  
         [0068]    Database accesses are CPU and disk intensive. If the database were consulted every time a download request was received by the system, a malicious person could make a large number of invalid download attempts to mount a “denial of service” attack on the system. If the URL contains a CRC code, a checksum or other error-checking code, the URL can first be checked for internal validity before the database is inspected.  
         [0069]    An error-checking code (such as a CRC code) for the entire virtual file can also be calculated and incorporated into the URL. Prior to downloading data to the client, the code can be re-calculated from data within the server files. If this does not agree with the code in the URL, the content of the files on the server has changed. Appropriate action can then be taken to ensure that incorrect data is not sent to the client.  
         [0070]    This key will then be used to generate the URL defining the access to the file, and will be in this format—this ‘Unique URL’; it will never be repeated for different files.  
                                   &lt;Unique URL&gt; = &lt;FTP or HTTP&gt;://&lt;DNS or IP address&gt;/&lt;optional fixed       path/&gt;&lt;unique path&gt;                  
 
         [0071]    The &lt;unique path&gt; can be either of two formats:  
         [0072]    1) Form 1.  
                                                                       &lt;unique path&gt; := &lt;unique key&gt;/&lt;filename&gt;                e.g.                FTP://somewhere.net/fixedpath/a8283kjh2y7/someaudiobook.mp3                      
 
         [0073]    This results in the downloaded file being named ‘someaudiobook.mp3’.  
         [0074]    1) Form 2.  
                                   &lt;unique path&gt; := &lt;unique key&gt; &lt;filename extension e.g. “.MP3”&gt;       e.g.       HTTP://somewhere.net/a8283kjh2y7.mp3                  
 
         [0075]    This results in the downloaded file being named ‘a8283kjh2y7.mp3’.  
         [0076]    Form 1 may be easier for a human user to understand, but the Form 2 may be more appropriate for automatic storage by another computer.  
         [0077]    Form 1 of the unique URL may be extended to support virtual storage directories. Form 1 also has an advantage in that this ‘unique path’ up to the ‘unique key’ (e.g. FTP://somewhere.net/fixedpath/a8283kjh2y7/) describes a directory store, and hence can be used to present a virtual directory containing a list of dynamic files. This virtual dynamic file storage directory can then be presented by the FTP, WAP or HTTP server as a directory and such can be listed and separate files downloaded.  
         [0078]    A standard FTP client could result in this example output: (Server in normal text, client in bold)  
                                                                                                   OPEN somewhere.net           Username: anonymous           Password: guest           Okay           DIR                fixedpath   &lt;Dir&gt;                Okay.           CD fixedpath           Okay.           DIR           Permission denied.           CD a8283kjh2y7           Okay.           DIR                someaudiobook.mp3   9,272,394 bytes           someaudiobook2.mp3   10,134,292 bytes           someaudiobook3.mp3   5,230,028 bytes.                Okay.           GET someaudiobook2.mp3           File ‘someaudiobook2.mp3’ downloaded successfully.           QUIT                      
 
         [0079]    Notice the italic section towards the middle of this listing: until the unique path is specified, the directory cannot be listed. This is because the unique paths could be too numerous to list. It can also enhance security of the system.  
         [0080]    In the final DIR section, the contents of the audio book, split into sections is listed, and the file ‘audiobook2.mp3’ file is downloaded, but at no point does the file or directory itself need to exist on the file store.  
         [0081]    In this case the ‘unique key’ indexes a database record that contains a lists of references to multiple content descriptions of each virtual files.  
         [0082]    The process by which the size of a dynamic file can be calculated is illustrated in the flowchart of FIG. 3.  
         [0083]    The size of the dynamic file is obtained by requesting details of all segments of the dynamic file (Step  310 ) from the dynamic file database  230  using the download reference as a database key. The system then calculates the size of the file (Step  312 ). This is achieved in a loop  314  and summing the sizes of all the constituent parts including alignment adjustments for the destined output format  316 . For example, if the data was bit-aligned rather than byte-aligned, or adding additional formatting data and rounding up the size if necessary, and so forth. In the case of MP3 frame data, no alignment is typically necessary. Finally, any overall adjustment to the file size is made (Step  318 ) that might, for example, be inherent to the requested file format before the calculated size is returned to the requestor (Step  320 ).  
         [0084]    Once the virtual file size has been calculated, the value may be stored in the dynamic file database  230 . This can avoid the need to re-calculate the size when it is next required, and can also be used to verify that the file size has not been changed once it has first been calculated.  
         [0085]    With reference to FIG. 4, during download of a virtual file by a client, a value that specifies the current point in the file that the download has reached (for example, a byte offset within the dynamic file) is stored in the database record of the associated request. This enables resumption of an interrupted download.  
         [0086]    If the client has previously downloaded part of the virtual file, it may specify a resume point at which subsequent downloading is to be recommenced. If the dynamic file is known, the resume point can be checked whether it is inside the range of the file. This function is necessary for the file download to be resumed from an existing download point and the download continued.  
         [0087]    In the case of an FTP download, the resume point is specified before the file to download, and as such cannot be checked until the file is requested, hence the check the resume point ‘Retrieve Dynamic File’.  
         [0088]    In alternative embodiments, the resume point may be specified with the file and checked at this point.  
         [0089]    Otherwise, the size of the dynamic file is calculated (Step  420 ) as described above, and this value is compared with the resume point value (Step  422 ). If the resume point value indicates a position beyond the end of the dynamic file, the client is informed that a download can successfully be resumed (Step  424 ). Otherwise, the request is terminated with a suitable error message (Step  426 ).  
         [0090]    [0090]FIG. 5 shows the steps carried out when the dynamic file is requested, and opened by means of its unique reference. First, an open request is made to the dynamic file database  230 . This operation is checked for success (Step  512 ). If it has failed, then an error is generated (Step  514 ).  
         [0091]    Next, the dynamic file size is calculated (Step  516 ) and the system determines whether a resume point has been set for the file (Step  518 ). If the resume point has not been set the file is transmitted (Step  520 ) from the start. If a resume point is specified then the resume point is checked to be valid, and if so then the dynamic file seeks to a specified offset within the set of files (otherwise, an error condition is raised  530 ). The data is then delivered to the client until either the transfer of data is finished, or the operation is cancelled (Step  520 ). The dynamic file is then closed (Step  528 ).  
         [0092]    With reference to FIG. 6, to download a file, the client sends a request that includes the unique URL, which is received by the transfer server  116  (Step  610 ). The corresponding unique key can be determined from the open request. This key is used by the transfer server  116  to request the contents of the dynamic file from the database as a list  614  of its constituent parts (Step  612 ). This information is then stored with a file and is used later to recreate the data had the file actually existed (Step  616 ). If successful a dynamic file handle identifier is returned (Step  618 ). If unsuccessful a error is returned (Step  620 )—this would be reported to back to the file transfer, for example, by sending a “File not found.” Message.  
         [0093]    A procedure which may be followed to perform a seek in a dynamic file is shown in FIG. 7. The opened file handle to the dynamic file is adjusted to a new byte offset within the dynamic file, so reading data from the dynamic file handle would match the data at the supplied offset had the file existed as a real file. As noted above the offset must take into account any adjustments for alignment of data, and additional formatting data, or if near the end of the file any padding data. The offset within the virtual file is converted to an offset within a segment (referred to also as a “sub-file”) at step  710 , having first skipped those sub-files that are positioned in the virtual file entirely before the offset within the dynamic file (Step  712 ).  
         [0094]    The data within the ‘dynamic file’ may be transferred to the client by a method illustrated in FIG. 8. An outer loop  810  selects each constituent sub-file in turn in order as per the database information. An inner loop  812  then sends the data to the client. Within the inner loop  812 , the data may be realigned, reformatted or padded to suit the file format As each constituent file is completed, the next file is opened and to continue with the data transfer. Once the transfer is complete, an indicative signal is sent to the client (Step  814 ).  
         [0095]    The dynamic file database performs additional functions within the system. The database keeps track of how much of the file has been downloaded for security, accounting and billing purposes, and also the creation date.  
         [0096]    Accounting: The database keeps track of the number of full downloads and partial downloads of a virtual file. For example a user might download the first chapter of a book, listen to it and then decide not to download the rest of the book. The statistics for this could be gathered from the download database. They could be used, for example, to determine the most popular types of audio books and so that more of those could be recorded in the future.  
         [0097]    Billing: When a download of a virtual file has been completed, then the additional content such as advertisements may be billed to the advertisers. (It may be that a partially-downloaded virtual file will not be billed in this way because it is unlikely that a partially-downloaded virtual file will be of much value to the user).  
         [0098]    Changing contents: If a virtual file has never been downloaded (download count=0 and download_point=−1), its additional contents may still be changed. For example an advertisement may be replaced with revised text or it may have been removed (e.g. banned for non-payment by the advertiser). The additional contents of the virtual file could then be changed so that a user would download the new additional contents instead. This might be done without changing the length of the file by selecting new contents of the same or smaller length and appending blank data at the end of the file.  
         [0099]    Security: Each time a virtual file is downloaded, the point at which the download is resumed and how many concurrent downloads from different network addresses may be used to check if the file have been made public. This is important if no additional contents such as advertisements are included. For example, a user may choose to subscribe with payment for advertisement-free content. These extra resume points and download counts can be use to detect illegal use of the unique references, such as those made public and accessed by many different network addresses and/or downloaded from the start or different points. This can be then used to deny access to these virtual file or files automatically or by issuing notices to the administrators that the files have been accessed illegally (and then can be removed to deny further access).  
         [0100]    Creation Date: This is kept so that the file reference is only valid for a specific limited time (e.g. 2 months). Content that is time-related can be removed before it becomes irrelevant.  
         [0101]    The final format of the download delivered to the client can adopt various formats. For example:  
         [0102]    1. A very large MP3 file:  
         [0103]    (Advertisements+Chapter1+Advertisements+Chapter2 . . . ID3Tag). This may not be preferred by some users where the requested content is an audio book because not that useful as most books are read/listened to by the chapter. Such a large file is also difficult to navigate, so a user is likely to “loose their place” in the book.  
         [0104]    2. A set of smaller files consisting of chapters with advertisements: File1 :  
         [0105]    (Advertisements+Chapter1+ID3Tag); File2: (Advertisements+SmallChapter2+Advertisements+SmallChapter3+ID3Tag); . . . FileN:  
         [0106]    (Advertisements+LastChapter+ID3Tag).  
         [0107]    3. A single archive (e.g. ZIP format as it is well known and used) FILE.ZIP that contains File1: (Advertisements+Chapter1+ID3Tag); File2: (Advertisements+SmallChapter2+Advertisements+SmallChapter3+ID3Tag); . . . FileN:  
         [0108]    (Advertisements+LastChapter+ID3Tag).  
         [0109]    If a user wants the whole book as a single file (to listen to in one session), s/he would typically choose method 1. If the user wants to download and listen to the first chapter before s/he downloads the rest of the book s/he might typically select method 2. If the user wants to download the book in an archive which will extract the files as smaller, easier-to-use chapters s/he will typically use method 3.  
         [0110]    In order to make clear the advantage of this embodiment in a system that provides a service that provides users with audio books, consider the following table. This compares the storage requirements of a server being a system embodying the invention as compared with a server that maintains all required files as conventional files.  
                                                   Number of Users:   1,000,000   1,000,000   1,000,000   1,000,000       Number of downloads per year per user:   20   20   20   20       Number of downloads per year:   20,000,000   20,000,000   20,000,000   20,000,000       Average number of users downloading   54,795   54,795   54,795   54.795       files per day       Average size of 1 download   64 MB   128 MB   256 MB   512 MB       Total size of files selected (all unique)   3,506,849 MB   7,013,699 MB   14,027,397 MB   28,054,795 MB       for download in 1 day       min. period we have to maintain the   30 days   30 days   30 days   30 days       temporary file so user can “PAUSE” and       “RESUME” his Downloading Manager       Conventional Server       Download storage size of current and   105,205,479 MB   210,410,959 MB   420,821,918 MB   841,643,836 MB       existing files (including those not       completely downloaded yet (1))       (1) in Gigabytes   105,205 GB   210,411 GB   420,822 GB   841,644 GB       (1) in Terabytes   105 TB   210 TB   421 TB   842 TB       Large High performance hard disk drive size   36 GB   36 GB   36 GB   36 GB       Number of above size hard disk drives   2,923   5,845   11,690   23,379       needed to store this       Cost per drive   $800   $800   $800   $800           $2,338,400   $4,676,000   $9,352,000   $18,703,200       Number of disk controllers needed   488   975   1,949   3,897       Cost per controller   $600   $600   $600   $600           $292,800   $585,000   $1,169,400   $2,338,200       Total cost of storage   $2,631,200   $5,261,000   $10,521,400   $21,041,400       NOTE: The above cost excludes server       memory needed to cache file access       tables; cost of hosting; networking;       maintenance; IP addresses etc.       Server embodying the invention       Average database record per file   100 KB   100 KB   100 KB   100 KB       in MB   0.10 MB   0.10 MB   0.10MB   0.10MB       Average database record size for files per   164,384 MB   164,384 MB   164,384 MB   164,384 MB       days needed to download       in GB   164 GB   164 GB   164 GB   164 GB       Number of drives to store custom       download files   5   5   5   5       Number of disk controllers needed   1   1   1   1       Total cost of storage   $4,600   $4,600   $4,600   $4,600       Savings:   $2,626,600   $5,256,400   $10,516,800   $21,036,800           99.83%   99.91%   99.96%   99.98%           572 times   1,144 times   2,287 times   4,574 times                  
 
         [0111]    It will be noted in the above table that, although the size of the content may increase (for example if higher quality content is provided, for example at a higher bit rate, or with video or other content) the database requirements do not change correspondingly. The example of 100 KB per record is in practice more than ample to manage files of the sizes indicated and larger and in fact could be condensed if storage space were critical.  
         [0112]    It is noted that the additional content may comprise pre-existing files or files or other resources (for example, an advertisement dynamically created for the user using partially pre-recorded content and speech synthesis).  
         [0113]    It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.  
         [0114]    Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.  
         [0115]    Reference numerals appearing in the claims are by way of illustration only and are intended to have no limiting effect on the scope of the claims.