Wi-Fi multicast has risen up the agenda for mobile operators and pay-TV operators alike, given the proliferation of hotspots, the emergence of carrier Wi-Fi and the rapidly growing level of offload from 3G or 4G cellular networks. The use of multicast over Wi-Fi holds great promise to deliver further savings in expensive cellular spectrum and backhaul infrastructure, while also enabling a range of revenue generating applications, especially in zones with temporarily high device densities such as sports stadia.
Far from being new, Wi-Fi Multicast has been around for over a decade, but it was originally proposed for shared applications such as desktop conferencing among users of corporate wireless Lans, which were then based on an early version of Wi-Fi. That would have been hopeless for streaming HD video at data rates of say 5Mbps, and so further refinements and technical developments have been necessary to make Wi-Fi multicasting anywhere near robust enough to deliver premium video among large concentrations of users also trying to do other things over the network. Even now there are many who dispute that Wi-Fi multicasting is yet fit for purpose, but Cisco for one strongly disagrees.
Cisco, as far as we can tell, has the only Wi-Fi multicasting platform that has been deployed commercially at scale, at several stadia including the Barclays Center, a multi-sports arena in New York, and Sporting Kansas City in the US, along with Real Madrid football club at its Bernabeu stadium in Europe. This, however, is a rather heavy duty system costing $1m to $2m, so it is a moot point whether it has yielded great savings compared with a small cell 4G deployment. In these cases multicast is just playing a part, albeit a lead role, in a complete broadcasting system including IP head end.
Cisco claims that these deployments are in fact far more cost effective than 4G small cells, while overcoming problems experienced by previous attempts at dense Wi-Fi in such places. Typically given large crowds in a small area, users would either get adequate capacity but poor coverage punctuated by frequent dead zones, or else there would be no problem getting a signal but performance and QoS would be terrible.
Cisco has developed enhanced directional antenna techniques combined with intelligent RF handling to optimize signal power to enable very dense configurations. At the Barclay Center, for example, it installed 275 Wi-Fi access points during the arena’s construction, including 100 in the seating bowl to provide bandwidth for thousands of fans connecting to Cisco’s Wi-Fi multicast app to watch live video. This provides insider views from the scorer’s table, players’ bench and basket stanchion (the post for basketball), all free. Similarly Real Madrid wants to enable fans to see instant replays of incidents such as goals scored from different camera angles, which could be multicast to avoid swamping the network, even if this meant there might be slight delays.
Launched in February 2013, Cisco’s StadiumVision Mobile uses its Connected Stadium Wi-Fi network, incorporating multicast. Although this is taking Wi-Fi multicast to a new level, Cisco says it has been able to build on its earlier work in the enterprise WLAN arena to overcome some of the well known issues.
The first point to note is that multicasting over a radio network is not the same as over fixed lines. For one thing the signal is broadcast and received by all devices within range of an access point, so the actual multicasting joining and pruning process really only applies over the backhaul serving these. So in the stadium case only those mini cells containing users that have joined a particular session would receive the transmission.
The point of multicast at the cell level is that it can be matched to the capabilities of the devices that have joined in. There is no point transmitting at 4Mbps if the only users wanting to access a stream have ancient smartphones that can only take 100Kbps.
At the same time the multicast transmission must get guaranteed bandwidth ringfenced from other unicast processes going on at the same time. Cisco catered for these requirements with its Resource Reservation Control protocol developed for enterprise WLANs, managing admission, allocating resources, measuring traffic levels and executing policy decisions. All this has been carried across to the new platform with suitable modifications and extensions.
It is clear that Cisco’s StadiumVision Mobile does make Wi-Fi multicasting work on a large scale. What is not quite clear yet is whether it completely avoids one of the other problems, which is hogging all the bandwidth so that there is not much over for normal unicast activity such as sending emails. It also offers no remedy for the problem of congestion for unicast applications in general.