Going Mobile: Streaming to Mobile Devices
The challenges of streaming audio to mobile devices and moving vehicles
Once the stream is generated it's clearly very important to make it easily accessible to all the end-users. You could plant a server yourself at an ISP, and make use of the ISP's high-speed connection and peering with other ISPs, but that isn't typically how it's done. In order to provide the best user-experience, you need to minimize the number of hops the stream must transit on its way to the user. Let me explain why.
The public Internet is basically made up of connections between ISPs (and other large users). These connections are made between larger routers - or peers - at many, many locations. In certain circumstances there may be multiple routes between organizations and ISPs; some connections may be done via load-sharing, meaning that packet streams are broken up, and literally routed over more than one route between points A and Z.
Streaming such as we are talking about uses UDP - the best effort methodology in IP communications. Packet sequences are numbered, so that at the far end they can be re-assembled in the right order. A couple things can happen along the way for these packets as they transit the Internet. For one thing, it takes a finite amount of time to make the trip, and transiting more hops extends the time. Secondly, in the event that the packets are sent along different routes (load sharing) they may arrive at the far end way out of order. Of course the decoder at the far end uses a buffer stage to give itself a little time to re-organize packets that come in out of order, but it cannot make up for packets that are simply lost and never make it. More hops increase the chances of lost packets; too many lost packets means a noticeable drop-out in the audio. No user likes that of course.
- continued on page 4
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