11.1 Introduction


A TFS, Transport File System (TFS) or the ATSC A95 TSFS, provides a mechanism for delivering various data types simultaneously from a small number of data servers to a large number of clients , also referred to as receivers [A95].

A TFS is typically part of a large system and data transmission food chain that collectively enables receivers to retrieve files (see Figure 11.1). Essentially, it is similar to the commonly used network file system, except that it is broadcasted. Files could often be transferred from the data source to the receiver through a terrestrial broadcast or an ISP. Both the broadcaster and the ISP may pull these files from a Web-server containing the data source.

Figure 11.1. The positioning of the DCM-CC file system

A single broadcast can deliver the data simultaneously to an arbitrary large number of receivers at a fixed cost and without presenting the hot-spot problem . Therefore, the file broadcasting system should deliver high traffic files (i.e., the initial working set of all iTV applications), and the interactive ISP channel is used for custom portions of the data.

11.1.1 Digital Storage Media Command and Control (DSM-CC) History

Work on the MPEG DSM-CC was initially chartered to provide VCR like functions as an annex to MPEG-2 ISO/IEC 13818-1. However, it subsequently expanded in MPEG-2 ISO/IEC 13818-6 to provide a Client-Network-Server paradigm for selection, access and control of distributed video sources. The ISO/IEC 13818-6 includes the following set of capabilities:

  • User -Network Client Configuration (DSM-CC U-N)

  • User-Network Session protocol

  • User-User Directory, Stream control, File access (DSM-CC U-U)

  • Interactive and broadcast download

  • Broadcast object carousel

  • Switched digital broadcast channel change protocol

The DSM-CC work was further extended as part of the MPEG-4 ISO/IEC 14496-6, with the DSM-CC Multimedia Integration Framework (DMIF). DMIF extends the DSM-CC to enable the convergence of interactive, broadcast and conversational multimedia into one specification, including the following:

  • An end-to-end session across multiple network provider implementations

  • Integration with multiple network technologies, such as ATM and the Internet

  • Integration with specific object domains, such as CORBA and JAVA

  • Multiple devices participating as peers within the same session

  • Fully symmetric consumer and producer operations within a single device

  • The scheduling and real-time switching/multiplexing of bit streams

11.1.2 Relationship to IP-based standards

As opposed to IP-based protocols, the MPEG-based approach of DSM-CC is well suited for simultaneous broadcasting to millions of receivers. As an example, a common problem occurring when one attempts to process data requests over IP from millions of receivers simultaneously, is the hot-spot problem; this phenomenon is utilized to stage a denial of service attach. Nevertheless, some argue that combining IP-multicast with the RTSP, RTP, the Resource ReSerVation Protocol (RSVP) RFC 2205, and the Service Control Protocol (SCP) achieves the desired broadcasting capability. For simplicity, a comparison can be made between DSM-CC U-N and U-U protocols and the RSVP and RTSP protocols; the differences are as follows :

  • RSVP is not able to account for the stacks above which IP packets are carried. DSM-CC U-N and that are carried over IP networks. In addition DSM-CC can account for network processing resources that are used, such as multicast servers and translators. DSM-CC U-N can be implemented independently from DSM-CC U-U, and therefore it can operate with RTSP, RTP and SCP.

  • RTSP provides the equivalent of the DSM-CC U-U directory resolve and stream control whereas RTP and SCP provide the equivalent of MPEG-2 transport and allows for the selection of a transport within the confines of connections between the sender and the receiver (or receivers for multicast). DSM-CC U-U provides a far more capable stream control through the use of NPT state diagrams. DSM-CC also allows requesting for direct connections between the receiver and any sender. This enables taking advantage of geographical proximity and load balancing.

11.1.3 DSM-CC Functional Model

DSM-CC is defined in terms of a simple functional reference model, shown in Figure 11.1. The model is one of Client and Server entities (jointly called Users) that use a Network to communicate with each other.

Clients are, in general, devices such as set top boxes that consume multi-media content. Servers are entities that provide multi-media content and services. A server may be a distributed system and contain multiple computing platforms, some functionally specialized e.g., video pumps to provide VOD service. A server thus may have multiple transport connection points to the network.

The intent is that DSM-CC be applicable to a wide range of physical networks with different realizations of connections, including one-way point to multipoint (broadcast) connections. Therefore, the definition of a network includes any collection of communicating elements that provides connections for users. The definition of a connection is equally broad: a transport capability to transfer information between two or more endpoints.

Popular residential broadband networks that support DSM-CC include HFC and Fiber to the Curb (FTTC). In many HFC networks connections are implemented using an MPEG-2 TS in the downstream (Server to Client) direction and a packet contention scheme in the other direction. Many FTTC networks implement connections as bi-directional ATM Virtual Channel Connections (VCC). DSM-CC allows a network to be non-homogeneous. servers may connect to a standard ATM backbone network whereas clients connect to an access network (such as HFC or FTTC).

The U-U and U-N models could be viewed as providing various service levels:

  • DSM-CC U-N allows the use and logging of heterogeneous resources made available to a session. This allows the proper collection of revenue and its disbursement to the network providers. Although the choice of a specific resource is not visible to a user, the user implicitly participates in its choice through service selection.

  • DSM-CC U-N enables ISPs to provide broadband access to their clients irrespective of their chosen access technology The ISP gateway router can be connected to ATM or frame relay and the clients can use over-the-air broadcast, Public Switched Telephone Network (PSTN), Public Land Mobile Network (PLMN), Narrow- band Integration Services Digital Network (NISDN), BISDN, Asymmetric Digital Subscriber Line (ADSL), HFC ,FTTC, and so on.

  • To broadcasters, DSM-CC IS provides unique features for broadcast download and broadcast object carousel data operation.

Figure 11.2 depicts connections for carriage of DSM-CC U-N and U-U information. The U-U information flow is used between client and server. The U-N information flows between the network and the client or the server. U-N messages are exchanged over U-N connections, their purpose is to control sessions and network resources (sessions and resources are discussed in the next section). Figure 11.2 shows the network containing a session and Resource Manager (SRM) entity. The SRM is the entity that terminates the U-N connection from a user. An SRM could be distributed over a geographical area in order to cover the spread of a global network spanning many network providers. The SRM can police connections based on policies set at service subscription, it can be the point in the network that provides configuration information to users, and it can authenticate the clients.

Figure 11.2. DSM-CC Functional Reference Model

11.1.4 DSM-CC Usage

The DSM-CC Carousel technology is often used by data broadcasters, especially by cable headends, to implement a file broadcasting system. In addition to the unidirectional broadcast mode, DSM-CC enables operating in interactive mode as well. In unidirectional mode, the server chooses which files to deliver, and the client receivers discover which files are available through the use of directories and file names broadcast to all receivers. In interactive mode, some receivers may request that a server place a specific file in the transport for a period of time. In most cases, receivers do not have enough resources to store all of the data that is being broadcast. Even if the receiver could store all of the data it receives, there is no guarantee that the receiver receives an error-free copy of the data in a single transmission of the data. This is because in a broadcast environment the receiver has no way to request that a server resend any data that was missed or received in error.

11.1.4.1 Random Tuning

To cope with random tuning, for example during channel surfing, and ensure that all clients are able to receive an error free copy of the data, the DSM-CC carousel server, or emitter, repeatedly sends the data over a period of time so that every interested receiver receive the data it needs as it needs it (see Figure 11.3). If a receiver misses some of the data, or the data has an error, that receiver must wait for the next occurrence of the data in the transport stream, at which point it can discover and acquire the data needed. This is achieved using either a data or object carousels, cyclically repeating the transmission of their content.

Figure 11.3. Repeated retransmission of Carousels to cope with channel surfing.

11.1.5 Transport Structure

The MPEG-2 DSM-CC transport consists of multiple programs, each comprising audio, video, and data components, where the latter is used to carry the file system (see Figure 11.4). The data component is carried over the same physical media as the audio and video components , and therefore it shares bandwidth (i.e., bits per second) with them. As a result, as the resolution of the video is increased, the number of packets available for the data is decreased, reducing the data download rate and increasing the carousel repeat period. The longer the carousel period, the longer it takes for receivers to launch applications.

Figure 11.4. The Structure of an MPEG-2 Transport.

11.1.6 DSM-CC Directory Structure

Each DSM-CC file system has a single service gateway, which is the root directory of that file system. Each entry in that directory, as well as in any other directory, may point to other directories or to files. Although not required by the DSM-CC specification, the ATSC A95 TSFS extends DSM-CC with the capability to use URIs and define a complex Web of file systems [A95]. The TSFS requires that each entry in the service gateway directory specifies a base-URI, which serves as the base for all the files and directories in the sub-tree rooted at that entry.

Figure 11.5 illustrates a simplified directory structure involving two file systems integrated as a single unified DCM-CC file system. Each component of a broadcasted file system comprises a DSM-CC service gateway, each listing a number of directory links each associated with a base-URI. For example, service gateway A in Figure 11.4 specifies two directories associated with the base-URIs of lid://cbs.org (LIDs [LID]) and http://pbs.org , as well as a file whose full absolute URI is http://abc.com/main.html . Each of the entries in the service gateway specifies one or more path segments. The directory whose base-URI is lid:cbs.com has a subdirectory labeled news/events ; indeed, it is possible to associate a number of path segments with a directory entry. The news/events subdirectory contains two files: local.html and world.html, with the full absolute URIs lid://cbs.com/news/events/local.html and lid://cbs.com/news/events/world.html , respectively.

Figure 11.5. Sample DSM-CC directory and URI structure.

A file may be associated with multiple URIs using a multiparent directory structure. For example, the file kids .html is accessible through lid://cbs.com/kids.html and http://pbs.org.educational/kids.html . Further, remote links can be used to mount additional file systems. For example, the file games .html is accessible from Gateway A through http://pbs.org/olympic/history/games.html as well as from Gateway B through http://fox.com/sports/games.html .

11.1.7 Carousel Protocol Stack

The DSM-CC Object carousel used for broadcasting files relates to other components of the MPEG-2 stack by means of encapsulation, namely data is encoded in several DSM-CC layers (see Figure 11.6). The base layer are the transport packets. Packets are assembled to form DSM-CC sections. The payloads of private (i.e., non-PSI) sections are used to carry DSM-CC sections whose payloads, in turn , carry DSM-CC data download sections carrying modules. DSM-CC data download carries DSM-CC object carousel structures. Each object within the DSM-CC object carousel is either a directory or a file (or a stream in some cases); these objects are used to assemble the broadcasted file system. Finally, the application operates on top of the file system extracted from the DSM-CC object carousel structure.

Figure 11.6. The DCM-CC Stack

11.1.8 Adoption

DSM-CC is a tool-kit, and therefore organizations can choose to implement any subset of DSM-CC (see Table 11.1). Companies involved with DSM-CC implementations include: Bellcore, CCETT, CSELT, DiviCom, Graphics Communication Laboratories, Hitachi, Lucent, Matsushta, Media Transfer, Microware, Motorola, nCube, Nippon Telephone and Telegraph, Nokia, Nortel, NYNEX, Oracle, Philips, PowerTV Inc., and Scientific Atlanta.

Table 11.1. Some of the Subsets Adopted by Various Standardization Organizations

DVB has adopted the following:

  • User-Network Session protocol

  • User-User Directory, Stream control, File access

  • Interactive and broadcast download

  • Broadcast object carousel

ATSC as adopted the following:

  • User-User Directory, Stream control, File access

  • Interactive and broadcast download

  • Broadcast object carousel

SCTE has adopted the following:

  • User-Network Session protocol

  • User-User Directory, Stream control, File access

  • Interactive and broadcast download

  • Broadcast object carousel

ATM Forum has adopted the following:

  • Audiovisual Multimedia Services (AMS)

  • User-Network Session



ITV Handbook. Technologies and Standards
ITV Handbook: Technologies and Standards
ISBN: 0131003127
EAN: 2147483647
Year: 2003
Pages: 170

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