One of the most important challenges for next generation all-IP networks is the convergence and interaction of wireless and wired networks in a smooth and efficient manner. This challenge will need to be faced if broadcast transmission networks are to converge with IP infrastructure. The 2nd generation of DVB standards supports the Generic Stream, allowing the direct transmission of IP-based content using the Generic Stream Encapsulation (GSE), in addition to the native Transport Stream (TS). However, the current signalling framework is based on MPEG-2 Tables that rely upon the TS. This paper examines the feasibility of providing a GSE signalling framework, eliminating the need for the TS. The requirements and potential benefits of this new approach are described. It reviews prospective methods that may be suitable for network discovery and selection and analyses different options for the transport and syntax of this signalling metadata. It is anticipated that the design of a GSE-only signalling system will enable DVB networks to function as a part of the Internet.
The first generation of DVB standards [
Current DVB MPEG-2 TS architecture.
Current signalling metadata relies on this TS packet format [
SD&S is a generic term that has been used to describe various discovery and selection procedures for, mainly, IP-based content metadata. The term Network Discovery and Selection (ND&S) is defined in this paper to describe the discovery and selection of network signalling metadata such as the acquisition of PSI/SI.
Figure
ND&S and SD&S procedures.
First, when a multiplex has been identified at a receiver, the receiver will need to perform a bootstrap of the signalling system, network bootstrapping. The same transmission multiplex may carry bootstrap information for more than one network, if the multiplex supports multiple logical networks. Bootstrap information could also relate to other network services transmitted over other multiplexes, possibly using different transmission technologies. Once the bootstrap has been performed, the receiver has the basic information required to discover the signalling stream—that is, the logical flow of signalling information relating to a network service from which it wishes to receive content. The receiver then needs to set a filter that extracts the appropriate signalling from the multiplex selected based on the required network service. The receiver can, then, be used to perform address/service resolution, identifying the required elementary streams or an IP stream that can be used to locate content.
In an all-IP system, the content may be directly accessed, or may be accessed via a content guide such as an ESG. The content guide may be discovered from content bootstrap information provided in a well-known IP stream. More than one network may reference the same content stream as in the case of duplicate multicast content. Similarly, more than one content guide may be active within a network service and the content bootstrap can then be used to select the appropriate content guide. DVB-H and DVB-SH systems follow this two-stage procedure once the PSI/SI information has been extracted.
Current broadcast transmission networks using the TS format can provide platforms for high-speed unidirectional IP transmission, not just for TV-based services. A convergent IP-oriented architecture will ease integration of transmission systems and enable development of multi-network service delivery platforms. The benefits of a DVB IP-based signalling architecture are discussed in Section
The remainder of the paper is divided as follows: a brief description of the current DVB signalling is given in Section
In current DVB systems, key-signalling information is sent, in the Programme Association Table (PAT) of PSI, using its well-known 13-bit Packet Identifier (PID) value in the TS packet header. This allows a receiver to readily extract this PID from a received TS multiplex. Figure
Initial acquisition of PIDs.
In many cases, equipment has hardware support to filter PID values, initially, set to well-known values defined by the MPEG standard, that is, the fixed PIDs of the PAT, the Conditional Access Table (CAT), and the Transport Stream Descriptor Table (TSDT). Once the PAT has been received and the respective PID of the Network Information Table (NIT) has been extracted (step 1 in Figure
As shown in Figure
The 2nd generation of DVB standards [
To support a converged approach, the 2nd generation of DVB transmission standards introduced the Generic Stream (GS) in addition to the TS. The GS may be used to carry packets of different sizes, eliminating the TS packet format. The GS is, primarily, expected to be used for network services, where IP packets and other network-layer protocols can be efficiently encapsulated using the Generic Stream Encapsulation, GSE, protocol [
GSE provides a network-oriented adaptation layer. Each network layer Protocol Data Unit (PDU) is prefixed by a GSE header, which is shown in Figure
GSE header.
While GSE defines the adaptation needed to support data transmission, there is no current specification for a signalling system that could replace the MPEG-2 TS signalling by a system using IP over GSE.
A transition to an IP-based content and signalling will enable common use of IP delivery techniques at the receiver, presenting new opportunities for integrating broadcast content with standard IP applications, and the introduction of value-added services. An IP-based transmission network design also enables the use of data networks (e.g., using wired/wireless Ethernet or mobile platforms) for onward delivery to the TV receiver. An IP-based approach allows reuse of existing techniques and protocol machinery (for configuration, management, accounting, encryption, authentication, etc.). This can support evolution of the services and be used to manage the network and monitor performance.
IP-based transmission products are already available for TV contribution networks and digital satellite news gathering. For example, IP satellite news gathering can significantly benefit from the improved efficiency of DVB-S2 while also utilising standards-based IP-based media codecs.
Broadcast transmission can supplement existing wireless infrastructure where sufficient capacity is not available, provide a resilient alternate path, or be used to roll out new services. Broadcast networks are especially suited to services that can exploit cost-efficient wide-area delivery using IP multicast.
This paper proposes a framework based on the reference model shown in Figure
Envisaged DVB IP/GSE signalling framework.
One simple solution is to encapsulate TS packets in GSE through its TS-Concat extension header [
Encapsulating the current TS-packed Tables into GSE packets could be an attractive transition method while both TS and GS multiplexes are in use. However, it is likely to constrain the evolution towards an all-IP network and it does not provide an efficient way to transmit PSI/SI Tables. The total overhead would consist of GSE and TS packet headers, and the TS packet padding. For example, if a 30B Table were sent in one TS Packet, the overhead comprised by the TS headers (5B), padding (153B), and the GSE base header (4B) would be 162B, if a Label field (Figure
However, this is only a partial solution. If signalling were transported using GSE packets instead of TS packets, there would not be a direct equivalent to the PID filters used for TS, that is, GSE does not contain a PID field. Thus, the receiver will need to identify which physical-layer frames or GSE packets carry the required network signalling information. Potential procedures to recognise GSE packets conveying signalling are proposed and discussed in this paper.
This section derives a set of requirements for transition to a GSE-only signalling framework.
The signalling system needs to support the IP protocol stack, as the envisaged system depicted in Figure
During transition, there is a need to allow the exchange of TS signalling information over a GS transmission network. Various options exist that may enable this transport, including the transmission of MPEG-2 Sections over UDP/IP using GSE encapsulation, or a direct mapping between MPEG-2 SI/PSI and GSE, for example, using the GSE TS-Concat extension [
The overhead arising from the protocols used in the envisaged IP-based signalling framework of Figure
When desired, signalling may be secured in an all-IP solution. The security requirements can be different for discovery functions (where all receivers may initially need access during bootstrap), and for individual signalling streams (which may be authorised to specific groups of users). Security of the signalling stream may be provided using a GSE security extension [
The new signalling system should enable a receiver to perform a “network scan” to discover the network and content, equivalent to the current PAT functionality. That is, it would allow a receiver to determine which networks and what content is available by decoding the GS without a priori information. The network discovery methods should identify the multiplex and resolve to a Network Point of Attachment/Medium Access Control (NPA/MAC) address at the GSE level. Supporting a “network scan” will place requirements on the repetition rates of the network signalling stream.
A receiver must quickly and efficiently identify the GSE packets carrying network signalling information within the GS. This is needed to provide fast service acquisition and may help in changing to a different service (e.g., to provide fast acquisition of signalling information when zapping between channels). The chosen mechanisms also need to ensure this procedure is not processing intensive at the receiver.
The delivery requirements for network signalling need to be considered. It is assumed that packet loss due to link corruption may be disregarded, since in most cases the physical-layer waveform will provide a quasi-error-free service using a combination of physical-layer parameters and Forward Error Correction (FEC) coding (e.g., a certain ModCod in DVB-S2). The repetition of signalling also improves robustness and allows fast PSI/SI bootstrap acquisition. The description syntax should allow easily inclusion of QoS descriptors for the network service. A/V timing needs to be synchronised, requiring mechanisms equivalent to the Program Clock Reference/Network Clock Reference (PCR/NCR), for example, using RTP timestamps. The GSE timestamp extension header [
The network signalling metadata syntax should provide a “user-friendly” description to facilitate modification, extension and/or enhancement of the signalling to support new formats and methods from a network/content provider. It should also enable easy addition of new signalling schemes that may be needed to support new applications and new services (requiring new descriptors or Tables).
Network and content signalling should be organised and sent independently from each other, so that a receiver can acquire network signalling faster than that of its content counterpart. This can also permit a receiver to acquire appropriate signalling without the need to parse the entire GS. That is, the identification of GSE packets carrying network signalling should not involve the filtering of all frames at levels GS-L1 or GS-L2. In addition, the method to achieve this separation should be applicable to any DVB standard, allowing sending network signalling with the same technique over any DVB physical frame, making it bearer-agnostic.
Together these requirements may be used to derive a new signalling framework. Requirements 4.2, 4.3, 4.6 and 4.9 involve network discovery procedures, while requirement 4.1 includes network selection and SD&S techniques. For a better understanding, Table
Relationships among the proposal requirements and applicability to network/content discovery and selection.
No. | Requirement | Network discovery | Network selection | Service discovery & selection |
---|---|---|---|---|
4.1 | IP interoperability | X | X | |
4.2 | Coexistence with MPEG-2 TS services | X | X | X |
4.3 | Similar, or higher, bandwidth efficiency as current TS signalling | X | X | |
4.4 | Signalling security | X | ||
4.5 | Enable service discovery and service description metadata | X | X | |
4.6 | Provides easy identification of signalling in GSE streams | X | X | |
4.7 | QoS and timing reconstruction | X | ||
4.8 | Extensible syntax | X | X | |
4.9 | Separation of network and content signalling | X | X | X |
This section analyses methods to provide GSE signalling identification and ND&S procedures. The signalling transport protocol and signalling syntax are also studied to identify which may be suitable for a GSE-only signalling framework and may meet the requirements stated in the previous section. Some methods are already used for IP-based signalling of content metadata, however, all current DVB systems use network signalling based on MPEG-2 encoded Tables.
Since there are no PIDs in a GSE-only signalling architecture, the first step towards this framework is to provide ND&S by filtering of signalling information at the GS-L1 or GS-L2 layers, to identify which GSE packets convey signalling information. Procedures for identification of packets carrying signalling metadata are needed to minimise receiver processing. Appropriate techniques can also assist in meeting the requirements for separation of network and content signalling.
A range of techniques is available, as presented in Table
Candidate methods for identifying GSE packets carrying network signalling.
No. | Candidate method | Filtering level |
---|---|---|
5.1.1 | Assignment of a dedicated transmission stream (e.g., DVB ISI) | GS-L1 |
5.1.2 | Assignment of fields in the transmission frame header | GS-L1 |
5.1.3 | Alignment of signalling transmission to a time-slicing frame | GS-L1 |
5.1.4 | Placement of a GSE packet at a known position within a frame | GS-L2 |
5.1.5 | Assignment of a dedicated GSE Type field value | GS-L2 |
5.1.6 | Assignment of a dedicated Label/NPA or IP address | GS-L2/GS-L3 |
5.1.7 | Assignment of a well-known UDP port | GS-L4 |
It is possible to reserve entire transmission frames at the physical-layer for use by a separate signalling stream. This stream could be identified by a physical-layer identifier, for example, a well-known Input Stream Identifier (ISI) value in DVB-S2/T2. A receiver performing a bootstrap may skip all frames with a different ISI, reducing the receiver information processing load. However, this method could reduce overall system efficiency when the frame size is large. Receivers need to be setup to process more than one ISI, this approach is being tested in some present systems.
Rather than dedicate a specific channel to signalling, the control information in the physical-layer header may be extended to carry network signalling information. This approach resembles the use of the Fast Information Channel (FIC) in ATSC Mobile Digital Television systems [
Simplified protocol stack for ATSC Mobile DTV.
The DVB physical-layer specifications do not provide an equivalent physical-layer signalling channel, although the DVB-S2/T2 frame headers currently have unallocated bits. A single bit in these frames could signal if a GSE packet conveying signalling is present in the frame, otherwise a receiver seeking signalling may ignore the frame. Additional bits could be used, if appropriate and available, to help define the type of signalling, for example, bootstrap or network services signalling. This would require an update to the present DVB transmission standards.
Time-slicing is a well-known method used for power-saving in DVB-H and DVB-SH systems. This technique could be applied to signalling to allow a receiver to know which frames may contain signalling information and allow a receiver to skip processing of frames that are known to not contain signalling PDUs. Timeslicing information (i.e., prior knowledge of times when signalling data is to be sent) would allow a synchronised receiver to disregard a proportion of physical-layer frames. Such an approach may be desirable for mobile applications, and could be extended to all signalling messages in any new system.
Transmission frames are typically long compared to the PDUs (signalling or data) that they carry. Processing of a frame that may contain one or more signalling PDUs could be simplified if the signalling information was inserted at a known position within a frame. The flexibility in the fragmentation algorithm of GSE would allow signalling packets to always be placed at the start of the frame payload. Although a receiver would need to inspect all frames, it may then skip any remaining payload after finding the first GSE packet in the S2/T2 frame that does not contain signalling information. This method does not require any change to the present physical-layer or GSE standards.
A receiver needs a simple way to demultiplex GSE signalling packets from data packets. One option is to use the GSE Type field. This may be performed in two ways:
A mandatory Type field directly precedes the GSE PDU (Figure
An optional Type field [
The Internet Assigned Numbers Authority (IANA) assigns Mandatory and Optional Type values. The Institute of Electrical and Electronics Engineers (IEEE) also register EtherTypes that can be used as mandatory Type values.
The demultiplexing of signalling from data packets may be aided by using well-known values of other protocol fields. Two methods have been identified at the GSE and IP levels that may assist in this process:
It is attractive to use well-known L2 addresses for bootstrapping, for example, an IANA DVB multicast IP address that maps to a MAC/NPA address [
This method would allow suppression of the NPA/MAC address but requires an IP packet format. Filtering using the IP address is not recommended, since it would preclude the use of link header compression or encryption of the GSE packet payload (since all packets would have to be decompressed and/or decrypted before filtering). The system would also increase complexity when other GSE extensions are present (e.g., timestamps). Well-known IP multicast destination addresses are used in many IP bootstrap procedures, and when present, these would normally result in a mapping to well-known MAC addresses [
This method requires an IP packet format, as in Section
This section proposes a two-stage approach which could be used for ND&S, in common with other IP-based systems to provide content discovery and selection. Once the GSE packets carrying signalling metadata are filtered at the GS-L1 or GS-L2 layers, a bootstrap will be performed to select the appropriate network signalling information. The network signalling information can then be used to select the required network service. The procedures below are based on IP satisfying the requirement for IP interoperability when enabling service discovery.
A bootstrap method eliminates the need to manually enter a bootstrap entry point, for example, the need to configure IP/NPA addresses out of band or using device configuration. Instead the device only has to be configured with the logical name for the network to which it is attached.
The format of network bootstrap information could be a Table structure that maps logical names to appropriate discovery entry points, that is, IP addresses where the discovery information can be found. Such a Table may be equivalent to the IP/MAC Notification Table (INT) used by DVB-H systems to signal the availability and location of IP streams. Another format could use a multicast Domain Name Server Service (mDNS SRV) record [
For broadcast networks, the bootstrap could be sent using a well-known IP multicast address. This approach is similar to that for DVB service discovery (
For bidirectional networks, ND&S entry point addresses may be found through the following three options: the Simple Service Discovery Protocol (SSDP) over UDP, SRV records via DNS over UDP or SRV records via DHCP option 15 over UDP. SSDP, defined by Microsoft and Hewlett-Packard, is specified as the Universal Plug and Play (UPnP) discovery protocol [
Selection of a transport protocol for the signalling metadata needs to take into consideration the requirements (similar efficiency than that of TS signalling) and characteristics (high repetition rates) of the metadata.
For unicast scenarios with bidirectional connectivity, HTTP over TCP is a commonly chosen method for unicast content metadata transport since it is used by DVB-IPTV, DVB-H, DVB-SH and OIPF architectures.
A/V data is transmitted over RTP via UDP/IP in DVB-IPTV and DVB-H systems. RTP with an extension header [
The DVB SD&S Transport Protocol (DVBSTP) [
The FLUTE [
Since the requirements for transport of network signalling metadata differ from those for content metadata, the transport protocols listed above may not be suitable. For example, ALC/FLUTE offers support for FEC-based reliability although this may increase processing overhead and is not required when data is repeated frequently. It also increases transmission cost. DVBSTP adds an overhead of at least 12B and provide reliability (also not required). DVBSTP does provide an indication of the type of XML-record carried through its 1B Payload ID field and the type of compression used through a 3-bit Compression field. These features, together with the ability to determine if content (Table) is encrypted before processing the payload are attractive for a transport protocol. Further work is needed to determine whether the overhead is justified and whether this choice of transport can be efficiently combined with the metadata encoding to optimise overall performance, or whether a new alternate lightweight protocol is preferable.
This section reviews a set of candidate methods for representing the metadata. It discusses existing SI/PSI, the Session Description Protocol (SDP) [
PSI, SI and FLS syntax has been standardised in MPEG-2 [
The IETF MMUSIC group standardised SDP [
The ESG in DVB-H, the OIPF framework and multicast sessions in ATSC use SDP records. Even though SDP is an IP-level method, it does not provide link-specific information to identify a network service or physical-layer tuning parameters for the transmission multiplex (e.g., frequency, transmission mode, and ISI). Hence, it would need to be extended to be suitable for network signalling.
The IETF started to develop an updated SDP protocol, SDPng. This was intended to address the lack of negotiation capabilities in SDP by providing alternatives for session parameter configurations. That is, an IP host would be able to negotiate session parameters according to its system capabilities. Proposals for SDPng used the XML syntax, Document Type Definitions (DTDs) and Schemas, to allow extensibility. It was one candidate method to convey session parameters for an IMG [
The eXtensible Markup Language, XML [
A Uniform Resource Name (URN) namespace has been defined for naming resources defined within DVB standards by [
In a GSE-only signalling framework, metadata syntax could be converted to XML. A simple, but effective method could retain the segmentation of the PSI/SI Tables, since the Section mechanism is an important element of the PSI/SI structure, to allow easy access to parts of the Table. In the XML encoding, the PID may be substituted by the IP destination address and UDP port number, similar to the approach proposed in [
Since encoding the signalling metadata in XML significantly increases the information rate (due to its inherent verbosity), this will decrease bandwidth efficiency. However, XML data may be readily compressed, for example, two compression algorithms are recommended for DVB-H content metadata: GZIP [
BiM compression can reduce the transmission cost up to 60% of the MPEG-2 encoded PSI/SI Tables [
GZIP presents lower complexity than BiM, since no Schemas are needed before decompression at the receiver. This makes it attractive for handheld terminals to minise the processing requirements. However, its compression gain is typically much lower than for BIM; PSI/SI Sections converted into XML and compressed with GZIP can increase the overall volume of data by 30% compared to the original binary encoded size [
Other XML compression algorithms are in the process of being developed. One is the Efficient XML Interchange (EXI) by W3C [
Table
Potential methods for a GSE-only signalling framework.
GSE-only signalling framework area | Prospective methods | Section |
---|---|---|
GSE Signalling Identification | Assignment of a dedicated transmission stream | 5.1.1 |
Assignment of fields in physical frame header | 5.1.2 | |
Alignment of signalling transmission to a time-slicing frame | 5.1.3 | |
Placement of a GSE packet at a known position in a frame | 5.1.4 | |
Allocation of a dedicated GSE Type field value | 5.1.5 | |
Allocation of a dedicated Label/NPA or IP address | 5.1.6 | |
Assignment of a well-known UDP port | 5.1.7 | |
ND&S | Bootstrap Table using well-known multicast addresses | 5.2 |
SRV record via DNS using well-known multicast addresses | ||
SRV record via DHCP option* | ||
SSDP | ||
Transport Protocol | HTTP/TCP* | 5.3 |
DVBSTP/UDP | ||
FLUTE/ALC/UDP | ||
RTP/UDP | ||
New lightweight transport/UDP | ||
Syntax | MPEG-2 | 5.4.1 |
XML | 5.4.3 | |
Compressed XML | 5.4.3 |
Table
Potential methods to fulfill the GSE-only signalling requirements.
GSE-only signalling requirement | Prospective methods |
---|---|
IP interoperability | Encapsulation of an IP packet as GSE payload. |
Any of the ND&S methods described in Section | |
Any unidirectional transport protocol. | |
Separation of network and content signalling | Any methods for GSE signalling identification described in Section |
Transport protocol with a payload type field. | |
Extensible syntax | XML syntax. |
Similar, or higher, bandwidth efficiency to that of current TS signalling | GSE extension headers to indicate compression. |
IP/UDP header compression. | |
XML compression. | |
New optimised lightweight transport protocol. | |
Coexistence with MPEG-2 TS services | Encapsulation of TS packet over GSE. |
Encapsulation of TS packet over UDP/IP/GSE. | |
Signalling security | GSE security extensions. |
Transport protocol with a field to indicate payload encryption. | |
XML encryption. | |
QoS and timing reconstruction | Timestamps in RTP extension header. |
QoS descriptors in XML. | |
Signalling repetition rates. | |
Enable service discovery and service description metadata | Any of the ND&S methods described in Section |
XML syntax. | |
Provides easy identification of signalling in GSE streams | Any methods for GSE signalling identification described in Section |
Several network signalling encapsulation options exist for a GSE-only system. The GSE TS-Concat extension [ To reduce overhead, the SI Table may be directly encapsulated as a PDU in the GSE payload. Since a Section should not be larger than 1024B [ Network metadata may be encapsulated as UDP datagram’s over IP, similar to the current encapsulation performed in DVB-H systems where ESG XML records are sent over FLUTE via UDP/IP. Recent techniques for IP/UDP header compression, such as ROHC [ The PDU-Concat extension [
This section compares the transmission cost for sending network signalling. The overhead respect to the Table size resulting from the candidate techniques is shown in Table
Overhead for different combinations of syntax and encapsulation procedure.
Overhead (%) | |||||||||
DFL (B) | 384 | 3216 | 7274 | 384 | 3216 | 7274 | 384 | 3216 | 7274 |
MPEG-2 Section in TS | 526 | 10.1 | 10.1 | ||||||
MPEG-2 Section in TS with dedicated ISI | 1146 | 10586 | 24113 | 119 | 213 | 609 | 22 | 63 | 84 |
MPEG-2 Section in TS/GSE with TS-Concat | 540 | 11.8 | 10.5 | 11.3 | 10.5 | 10.1 | |||
XML GZIP Section over DVBSTP/UDP/IP/GSE | 160 | 5.8 | 4.3 | 5.6 | 4.5 | 4.3 | |||
XML BiM Section over DVBSTP/UDP/IP/GSE | 146 | 5.2 | 4.3 | 4.9 | 4.2 | 4.2 | |||
XML BiM Section over FLUTE/ ALC/UDP/IP/GSE | 146 | 5.2 | 4.3 | 4.9 | 4.2 | 4.2 | |||
XML GZIP Section over DVBSTP/UDP/IP/GSE with HC | 76 | 3.3 | 1.8 | 3.0 | 2.1 | 1.9 | |||
XML BiM Section over DVBSTP/UDP/IP/GSE with HC | 63 | 2.8 | 1.8 | 2.7 | 2.0 | 1.8 |
The methods were compared to native transmission of MPEG-2 encoded Sections using the TS, as in current DVB signalling. Padding is added to each TS packet, as necessary. Table
Section
Encapsulation of TS packets in GSE, as a transition method, is also analysed. GSE TS-Concat is considered for Tables with multiple Sections. As expected, the overhead is higher than that for native TS transmission. This is also higher than the IP-based encapsulation methods.
The next set of methods considers IP-based protocols and XML-translated Sections. Each Section is encapsulated by DVBSTP/UDP/IP or FLUTE/ALC/UDP/IP, where the additional headers contribute 40B. The GSE PDU-Concat extension is used for the Table with four Sections. It is assumed that signalling identification is carried at GS-L1 (e.g., optional GSE Type fields are no considered).
The overhead for the medium and large Tables represents a trade-off with the benefits provided by an IP-based signalling system. Small Tables negatively impact the efficiency of an IP-based signalling framework regardless of the encapsulation technique and frame size, however the overhead is always lower than that of native MPEG-2 TS. This overhead is further reduced when header compression (HC) is considered. Estimates of the compressed size using either GZIP or BiM algorithms are provided. This assumes that BiM compression of the XML-encoded Section results in a reduction of 40% with respect to the size of the MPEG-2 encoded Section [
DVBSTP and FLUTE resulted in the same overhead. Although, DVBSTP was designed to ease processing of SD&S XML records at the receiver, it results in significant overhead for small PDUs. The 12B of overhead introduced above the UDP layer is seen as an upper bound. This overhead could be reduced further by design of a lightweight transport protocol to replace the DVBSTP header, or by combined optimisation of content-encoding and transport protocol.
The UDP/IP headers are assumed to be compressed to 3B when using a form of header compression, although no method has currently been specified for use with DVB. The use of header compression for signalling should be analysed further given the positive effect on reducing the overhead.
The convergence of DVB networks with IP infrastructure bridges the gap between broadcast transmission and traditional networks. Current MPEG-2 systems are already used to transmit IP packets, mostly using MPE or ULE, it is expected that future DVB transmission networks adopt an all-IP approach by gradually replacing the TS by the GS. Transition to IP-based content and signalling will enable common use of IP delivery techniques at the receiver, presenting new opportunities for integrating broadcast content with standard IP applications, and the introduction of value-added services.
One major challenge to transitioning broadcast services to the GS is the lack of a GSE-only signalling framework. IP-based procedures for content metadata exist in DVB systems, but current signalling is implemented through MPEG-2 TS Tables. This paper explains the need for a GSE-only signalling framework and formulates a set of requirements, reviews a range of candidate methods, including current IP-based methods and has derived their potential benefits.
The proposed methods can identify GSE packets carrying signalling and replace the role of PIDs. In addition, current IP-based methods may be used as prospective techniques for ND&S procedures and for the signalling syntax. Options were also presented for a signalling transport protocol. Methods for encoding metadata that allow extensibility and easy modification were examined. XML Schemas are strong candidates because of their extensibility characteristics and current common use for content metadata. Indicative performance data is used to compare the anticipated overhead for the various approaches.
This work is intended to guide and inform future standardisation work. As future work, we intend to select the optimal candidate methods and propose a GSE-only signalling architecture. The high-level requirements in terms of signalling for different scenarios, for example, fixed broadcast, interactive, will be also defined, as well as the specification for mapping current SI/PSI/FLS MPEG-2 encoded Tables to their XML-based counterparts.
The authors acknowledge the support of the European Space Agency (ESA) Contract 22471/09/NL/AD.