Edge caching and multicast
One important aspect of streaming protocols like MPEG-DASH is that they change the fundamental nature of video delivery, and, to some extent, end the old debate over the relative merits of unicast, multicast and broadcast packet delivery — at least as far as the mobile or Wi-Fi cell. At first, all OTT and mobile video was unicast as it had to be, since it required a one-to-one path between a content server and end-user device to deliver the audio and video as requested on demand. But, it quickly became clear this was not scalable to large numbers of users all consuming popular content, whether via live streaming or on demand, and especially not at the higher bit rates required for broadcast-quality HD. This is where streaming came in, breaking video down into chunks and distributing these out to edge caches close to the points of consumption to avoid clogging core bandwidth with multiple unicast streams or downloads.
An alternative approach for linear content consumed simultaneously by many users is IP multicasting implemented within the network. It is set up to transmit video just once over each network hop, and only when there is at least one user consuming the content at the time it is on a downstream TV or other end device. IP multicast has been deployed by some IPTV operators but is of no use for on-demand content, which, in such cases, has to be unicast because people consume it at different times. This makes IP multicast useless for mobile video applications, where most content is consumed on demand.
However, adaptive streaming with edge caching can save dramatically on core bandwidth for both linear and on-demand content. In either case, video can be transmitted just once to a network of edge caches. This way, when users request it, the core network does not have to be touched. Also, latency is reduced because the content has a shorter distance to travel, traversing fewer router hops. This, as Springall pointed out, means that MPEG-DASH avoids the need for multicast because it provides the same function of avoiding unnecessary bandwidth consumption, but at a higher application level rather than at the IP network level.
“Protocols like MPEG-DASH change the meaning of ‘unicast’ because with edge caches, it becomes a layer 7 multicast, which is much easier to implement and does not need big changes in the layer 3 network,” Springall said.
But, there is no way of avoiding unicast delivery of on-demand video within a mobile cell or Wi-Fi network, with potential for using up the available spectrum and ruining performance. The only alternative is mobile broadcast, but this only applies to popular linear channels watched by a number of people at the same time within a given cell. So far, the history of mobile broadcast is littered with the corpses of failed technologies such as DVB-H, and Qualcomm’s MediaFLO in the U.S. But, this was largely because sufficient bandwidth was not available, and devices were incapable of displaying at suitable quality. The advent of 4G LTE services, combined with the arrival of larger-screened smartphones and tablets, has renewed interest in mobile broadcast as a way of avoiding a spectrum crunch. But, the area is still contentious, with a dividing line between traditional broadcasters, (arguing that unicast delivery of linear content within the radio access network is a ludicrous waste of spectrum) and the mobile operator camp (that contends that there is limited demand for broadcast TV for mobile handsets).