Multicasting
When viewers watch TV, they don’t care about where the program originated, what route the signals take, or how much bandwidth is used. What they demand is that when the remote is clicked, the program or event they want immediately appears and the picture and sound are clear. Today’s on-demand culture has been the catalyst for service providers and cable operators to look for ways to conserve bandwidth so capacity is always available to meet multiple subscribers’ simultaneous requests for digital and video on-demand services. However, as broadcast entertainment makes inroads into the Internet, the ante goes up for cable operators and service providers to quickly adopt new technology because the Internet cannot broadcast.
When cable operators started deploying video-on-demand (VoD) services, the only option available was to purchase a VoD server with either an ASI (Asynchronous Serial Interface) or a QAM RF interface. These options were costly, requiring the VoD servers and ASI interfaces to be collocated at the hub site and the use of expensive ASI transport systems.
VoD equipment vendors responded by adding GigE output ports to their servers. The video signals were transmitted in a single program MPEG-2 transport stream, encapsulated in a UDP packet. (There are seven MPEG packets inside a GigE frame, inside an IP frame, inside a UDP frame.) The output of the GigE port was unicast, meaning the transport stream or set of streams traveled only as far as the Ethernet mapping layer allowed (point to point). The VoD stream or set of streams can only go to the VoD QAM service group located at a hub site. The only way to modify this was to use VLANs, which has been shown to be problematic.
For service providers to respond to the demands video over IP places on networks, new sets of standards, network architectures, and quality requirements were introduced. One new technology that is now considered to be most effective for delivering bandwidth-efficient video and VoD content to multitudes of new subscribers is multicasting.
When first introduced in the early 1990s, multicasting was a concept ahead of the industry’s existing technology and market awareness. Now, the level of intelligence and agility built into the physical network and the application and service layers has generated an industry-wide initiative for integrating multicast technology into IP networks. With video traffic at a few large headends compiled into multicasting groups that are transported over IP, personalized digital and VoD services can more quickly evolve from their niche market status into mainstream product offerings.
A network that supports multicasting only transmits those programs requested by the subscriber. When video services are delivered over the Internet, there could be nearly 1,000 subscribers connected to a single access system. IGMP (Internet Group Management Protocol) is the signaling protocol used to access multicast video services. A subscriber who requests a program joins the appropriate multicast group, which is comprised of other subscribers receiving the same content. This conserves bandwidth because there is only one channel traversing the access network.
To deliver the quality, responsiveness, and resiliency that digital and VoD services require, cable operators and service providers must understand where and what to test in a multicasting network. With multicasting, the transport stream is sent to a router where it is split and then sent to a DSLAM, which uses an IGMP snoop function to see if the requested program material is available. If the DSLAM has the program, the subscriber’s IP address is added to the list of addresses embedded in the video stream. If the requested program is not available at the DSLAM, IGMP messages requesting the program continue upstream until reaching a location such as a video hub office that has the material. After the requestor is added to an existing broadcast channel flow, the video stream travels to the program router and then to the DSLAM, which sends the program to the subscriber destination point.
QoS begins by ensuring that the program reaches all destination points that should receive the channel, which could number in the millions. Of equal importance is identifying the sites that should have received the program, but did not, and investigating other segments of the network to ensure that the multicast configuration is not broadcasting the program everywhere.
In the not-too-very-far-future, individualized, differentiated video programming will reach a massive scale. Multicasting technology has turned mass customization into commodity status by making differentiated programming available on a widespread basis. However, this requires continued commitment to testing, monitoring, and analyzing the multicast stream.
