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Going Mobile: Streaming to Mobile Devices
The challenges of streaming audio to mobile devices and moving vehicles
One way to overcome these issues is to make use of a content delivery network (CDN). CDNs make use of private connections (often spread out over the entire globe) so that they can provide a much greater level of control in how the packets get from point A to a point very near the end-user. This to a very great extent mitigates the packet loss and/or delay issues just described. Plus, it's a very practical way to serve thousands upon thousands of end users, something that is hardly practical for radio station to do (via IP anyway). A few of the more well known CDNs are Akamai, Limelight, Level 3 and CDNetworks.
Now let's look at streaming to mobile devices in greater detail. We have a stream of packets that originate at your studio location, and find themselves peered from a CDN in to the IP network of one of the large cell phone providers, such as AT&T, Verizon or Sprint. They're just about ready to make that last-mile journey to a listener's smartphone. Verizon and Sprint use EVDO, and AT&T uses UMTS. In either case, the backend of the network is built to handle IP traffic. The way this is done has changed as well; in the days of 2G and even for 3G, many base stations were connected back to the base station controllers via T1s. Today that isn't really fast enough. The cell companies are building new IP-based backhauls to accommodate more and more users with more and more smartphones.
Even though the terminology is slightly different from system to system, they each use three common devices. First, and what's most familiar to us, is the base station. This is the last mile link done via radio. The base stations find themselves under a radio network controller, which performs several different functions, not least of which is instructing the base stations to hand-off the phone from one cell to the next as required. (For LTE, we have two devices: the serving gateway, and the mobility management entity.) Finally the data itself is connected to and from the IP network by way of a gateway. Routing of that data all the way back to the peering connection is what gets us back to the CDN (or perhaps the public Internet).
When the data itself makes its way to your smartphone, you could of course listen to the tiny built-in speaker, ear-buds or a headset. To make the connection to your car's audio system you could simply make a short cable connection, by way of a mini-stereo jack on the radio itself, or an adaptor of some sort (such as an FM modulator). But let's go with the most up-to-date way: Bluetooth. Many vehicle audio systems now come with Bluetooth as a means of connection to other devices. Bluetooth isn't really new (it was developed back in the mid-1990s) but it clearly has a lot of marketing presence now.
Bluetooth is simply a technology used to make a small ad-hoc network (known more regularly as a piconet) that can consist of up to eight devices. The devices communicate via frequency-hopping spread-spectrum in the 2.4GHz ISM band. One of the devices operates as the master, and the others are slaves. The master synchronizes the system, addressing each slave in turn, in a round-robin fashion. With a transmit power of 0dBm, the range is expected to be less than 15'; data throughput is on the order of 2Mb/s.
To complete the connection, it's necessary that the car audio system and smartphone have A2DP (Advanced Audio Distribution Profile) capability. A2DP is a Bluetooth profile that allows for streaming stereo audio between devices that are members of the piconet. Makers of car audio systems that support A2DP include the well-known brands Pioneer, JVC, Sony, Alpine and several others.
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