Technical Blog
IPTV Instant Expert Part 3: IPTV Content Distribution Network
13th December 2011
As discussed in Part 2 of this IPTV Instant Expert series, VoD and broadcast content will be generated at the video Super Head End and distributed via a suitable IP transport network. Each IP datagram generated at the head end will be carrying a payload of video, audio or ancillary data (subtitling etc). The format of an IPTV packet is outlined in the diagram:
The packet's contents are detailed as follows (bottom to top):
- Ethernet - the defacto data link layer standard of IP networks, capable of mapping IP multicast addresses to MAC multicast addresses when necessary (typically last hop distribution). Provides a 1500 octet MTU (Maximum Transmission Unit) which restricts the total volume of audio/video content which can be carried in a single packet.
- IP - unicast or multicast addressing, depending on the situation. QoS marking in the form of DiffServ could be used in the IP header to ensure that real time video services are not severely delayed in transit.
- UDP (User Datagram Protocol) - a connectionless protocol which provides port addressing and little else. However, retransmission of packets in a real time service is often not required and the MPEG layers provide packet sequencing in the form of timestamps.
- RTP (Real time Transport Protocol) - when used, this will provide timestamping and sequencing of packets, which can be used to correct jitter and detect dropped packets respectively. Use of RTP is encountered when the Pro MPEG CoP3 standard for FEC (Forward Error Correction) is being used.
- MPEG-TS (Transport Stream) - elementary streams of audio, video and ancillary data are multiplexed together at the MPEG-2 TS layer in order to form a service (a TV program or movie). Several services may be multiplexed together in a MPTS (Multi Program Transport Stream), although in IPTV only SPTS (Single Program Transport Streams) will be encountered. Multiplexed elementary streams carry large volumes of additional data which support a variety of tasks. For example:
- Timestamps to decode and play out the audio.
- PMT (Program Map Tables) to demultiplex the elementary streams.
- Conditional Access information to allow decryption of services.
- Clock references to keep the STB in sync.
Restrictions at the Ethernet layer mean that an individual IP datagram typically contains 7 MPEG-2 TS packets. Each individual TS packet carries a sample of audio, video or ancillary data. Hence, in an individual IP datagram, you could be carrying any combination of audio, video and ancillary data samples, depending on the contents of each individual TS packet.
Regional Head End
Before this packetized content actually gets to the STB, it's likely that a RHE (Regional Head End) will be encountered. RHEs contain much of the same functionality as a Super Head End, albeit on a slightly smaller scale. There are several reasons for this:
- Regional content such as local news feeds could be injected at the RHE.
- Local advertising injection.
- VoD efficiency - housing VoD content at the Super Head End is not particularly efficient due to the fact that lots of unicast sessions will be flowing across the end to end network. Moreover, if signalling (RTSP) associated with controlling a service has to flow across the end to end network, service control could be desensitised, ie when you press pause, the pause effect isn't instantaneous. Hosting VoD content on a regional basis is more efficient, since all streaming and signalling traffic has a shorter network journey.
- ADSL2+ (Asymmetrical Digital Subscriber Line 2+).
- VDSL2+ (Very high data rate DSL 2+).
- FTTx deployments e.g. PON (Passive Optical Network).
- Ethernet.
- HGI (Home Gateway Initiative) - founded by a number of telecommunications service providers who are working towards technical and interoperability specifications for Home Gateways, often providing input to other standards bodies.
- DSL Forum - responsible for the creation of technical reports that provide technical specifications and standards. In particular, TR-069 is a common standard which many Home Gateways support in order to provide remote management features.
- DLNA (Digital Living Network Alliance) - this body focuses on the home network side of the gateway and deals mainly with interoperability and networking of media devices within the home.
- UPnP Forum - focused mainly on the home environment, this organization strives to introduce auto discovery and remove the requirement for manual configuration in order to connect network devices.
Security
Since the Home Gateway is acting as the perimeter device on the subscriber's home network, it will be required to host a selection of security features. In particular, the STB could have Firewall capability and may also contain support for malware detection. In terms of the media passing through the device, the STB will be responsible for enforcing DRM (Digital Rights Management) and CA (Conditional Access). In addition, the device may have DPI (Deep Packet Inspection) capability in order to filter malicious traffic.
The one crucial factor to bear in mind with respect to Home Gateway/STB features is cost! The more features, the more costly the platform is will be.
The Customer Premises Equipment
Eventually, IPTV content will arrive at the STB. The STB's primary responsibility is to demultiplex, decrypt and decode the incoming content before playing it out to the TV. However, over and above these basic requirements, the STB could support a wide range of features and as such, the term "Home Gateway" is also commonly encountered when referring to a STB. Typical features are outlined below.
Interface to Multiple Broadband Technologies
The IPTV service provider cannot guarantee that a subscriber will have a particular type of broadband connectivity and as a result, the Home Gateway may be capable of terminating a wide selection of broadband connectivity types. Examples include:
Remote Management
Remote management is a critical feature of Home Gateways, allowing the service provider to remotely configure the device in support of areas such as security updates, firmware updates, service changes and billing procedures. A key advantage to remote management capabilities is the reduction in operations and customer support costs, since problems can be dealt with without the requirement for an on-site engineer. TR-069 is a standard from the DSL Forum which is widespread as a remote management mechanism.
Service Support
As the main interface between the customer network and the service provider’s network, the home gateway will support the necessary features and protocols required to deliver services such as IPTV and VoIP. As such, the Home Gateway will have the relevant middleware functionality embedded into the device, providing the user interface to the service in question. In the case of IPTV, this will be features such as the EPG. It should be noted that as telecommunications service providers roll out IMS (IP Multimedia Subsystem), the Home Gateway may also act as a SIP gateway for devices on the home network.
Media Storage
With services such as VoD IPTV, media storage capability would allow the subscriber to download content and store it at the Home Gateway for future viewing. In addition, an attractive feature for the customer would be a storage facility for their own digital media, with media stored on the Home Gateway itself and optionally backed up remotely in the service provider’s network. Naturally, with respect to stored media, security in terms of media access control and privacy is a significant consideration. The popularity of the PVR (Personal Video Recorder) should not be overlooked here; a very attractive feature of a Home Gateway is the ability to remotely access previously recorded content or alternatively, remotely configure the Home Gateway to record a particular programme. Many STBs are available with this feature today, with iPhone and Android apps available to supplement this service.
Home Network Interface
If the STB is to be the interface point to the broadband access network, it must also act as the interface to the customer’s home network. This will undoubtedly involve Fast Ethernet (100Mbps) connectivity, in addition to support of Wi-Fi standards such as 802.11. Interestingly, with the recent popularity surge of Femto Cell technology, it is possible that the STB may also support radio functionality.
QoS Enforcement and Traffic Management
Due to the fact that traffic passing through the Home Gateway is of an extremely varied nature, QoS support at the gateway is extremely beneficial. This may include mapping DiffServ marked packets arriving from the service provider network to the relevant QoS mechanisms employed on the home network e.g. IEEE 802.1p or 802.11e. Conversely, the Home Gateway may mark packets coming from the home network which are destined for the service provider network. Traffic Management will be used by the STB at times of congestion, when traffic prioritization may be required.
Standards Conformance
In order to support the standards based approach to NGN, service providers will want to deploy STBs that conform to the standards set by the various standards bodies. With respect to the Home Gateway, the following standards bodies are involved: