Today’s web is full of articles and discussion about Mobile Broadband. We like to read Om Malik, whose crew of subject matter experts frequently add value on the topic. But what exactly is Mobile Broadband? How do wireless operators provide it? What makes it fast? Or slow? We’ve implemented stat-of-the-art 3G Mobile Broadband for a tier 3 regional CDMA operator in the US, and we’ll be providing insight into how this fiendishly complicated technology works, and what, if anything, you might do to improve your experience.
First, a warning. Telecoms discussions, as much as any other high-tech industry in the early part of the 21st century, is riven with acronyms. This is not simply jargon spoken by Illuminati intended to befuddle the masses. But rather a means to communicate that allows dauntingly complex topics to be discussed intelligibly amongst people familiar with the field. In this regard it’s no different than molecular biology, particle physics or rocket nozzle design: acronyms and jargon allow deeply complex topics to be discussed without distracting interruptions by the frequent long-winded expansion of commonly used terminology. But I’d be guilty of the same if I drone on, so on to the discussion.
The end-to-end technology for delivering Mobile Broadband starts at the cell tower nearest the wireless users, connect that equipment to the central office mobile telephone switch, and thence onto the Internet. That would be your 25 words or less summary.
At the cell tower, aka “Base Transceiver Subsystem” (BTS,) there is equipment dedicated to providing Mobile Broadband service. This equipment is responsible for negotiating the wireless connection to the client, determining the speed of that connection and allocating resources amongst all the clients accessing that particular BTS. Metrics that can influence the throughput experienced by the user are the distance to the BTS, the number users trying to access the same BTS, the geographic distribution of users throughout the coverage footprint of this BTS and the speed of the connection (aka “Backhaul”) back to the cellular telephone switch. Entire books are dedicated to these topics. But what you need to know is that the key factors influencing the speed experienced are the capacity of the backhaul, the number of simultaneous users accessing this BTS, where those users are located relative to the tower itself.
Most important of these are the speed of the backhaul.Â Many mobile operators, Tiers 1 and 2 included, continue to use T1 “pipes as the backhaul connection.Â A T1 is a legacy of hierarchical voice telephony service that provides a total capacity of 1.544 megabits per second (mbps).Â For a single mobile subscriber, 1.5Mbps is nothing to write home about.Â When shared, it can provide a very pokey Mobile Broadband experience.Â Multiple T1s can be connected to a single BTS.Â But the leased T1 monthly recurring cost to the Mobile service provider is generally high, and the user experince suboptimal.Â This solution does not scale well, and is a primary motivation for the use of Ethernet backhaul inherent in the next generation LTE (aka “4G”) specification.Â Ethernet is already a solution for current mobile backhaul technology, but is not commonly deployed.
Continuing on to the central office mobile telephone switch, there are several interconnections that govern the speed perceived by users. Generally there may be one or more Ethernet layer 2 switches in the backhaul path.Â Each of these could gate the overall speed, but are unlikely to do unless multiple BTS are aggregated onto a single Fast Ethernet port, which wouldn’t happen when the backhaul is T1.
The Ethernet switches must talk to a router and to a controller.Â The routerÂ handles all the actual subscriber data, such as emails sent and received, or web pages requested and returned. The controller can force the individual session to authenticate for access to the network.Â This rarely causes a throughput bottleneck. From the router the subscriber data will pass to what is called a Packet Date Serving Node (PDSN) in the CDMA world, and a Serving GPRS Support Node (SGSN) in GSM.Â This network element manages the interaction with a RADIUS (in a CDMA network) or DIAMETER server (GSM) to authenticate and authorize a given mobile broadband session.Â The mobile Operator will use these elements to implement policies which determine the experience of what individual subscribers.
Lastly the PDSN routes the subscriber data to the Internet, usually through a router connected to 2 or more Internet Service Providers.Â Typically these connections are in a BGP arrangement which routes traffic to the Internet by determining the shortest path to get to the specified destination.Â BGP does not distribute the load evenly amongst the various ISPs.
It should now be clear that Mobile Operators have a huge number of parameters to manage and levers to pull when managing their Mobile Broadband bandwidth.