Digital wireline STLs


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So to review quickly: Each of these systems is mainframe-based, modular and configurable. Each system has 24 timeslots (24 ds0s) or a grand total of 1536kb/s of payload capability.

Beyond T1

What do you do if a single T-1 isn't enough for you?

I wrote earlier that if you buy in quantity, you get a better deal on a per-unit basis. Turns out here in NY that the cost of five T1s is the same as an entire T3, which is 28 T1s. Because we have five stations in New York we went with T3 all the way.

Let's say though for the sake of argument that you don't need 28 T1s, although you do need more than one. What could you do in that case? Let's make up an example and then look at one potential solution. Say you have two stations on a mountain top, and through budget analysis, you've determined you can afford four T1s through your local telco. You want to use linear PCM for your audio on both stations, and you also want the most bandwidth for the transmitter site LAN as you can get a hold of. Of course you don't want the failure of any particular T1 to take either station off the air; in fact, it would be really sweet if you could stay on the air even if two of the T1s were to fail — no matter which two.

For station A you purchase a frame-based T1 system. In that frame, you configure two audio cards to use 18 timeslots. That becomes your primary STL for station A. In the same frame, you configure a data-reduced audio path that uses four timeslots. That becomes the backup path for station B. For station B, you do the same thing after buying a second frame-based system; 18 timeslots as the main station B STL, and a data-reduced path of four timeslots that is a backup for station A. Station A uses T1-1, and station B uses T1-2.

Adtran 4305

Adtran 4305

For T1-3 and T1-4 you may need some help from your IT people (unless that of course is you). For this example, you could spec a set of routers that have multiple T1 interfaces. One example of that is the Adtran 4305. With a router such as this, you can literally bundle multiple T1s in to one network link. The cool thing also is that the link will continue to operate even if one of the T1s goes down. Obviously the data throughput will be reduced.

The two bundled T1s make up your LAN extension now, giving you a LAN bandwidth of 3072kb/s.

Taking this one step further, you could develop an audio stream for stations A and B (though the data rate can't be that high unless you want the streams to hog your network) that go through the routers to reach the far-end site.

A system such as this can easily continue to operate (albeit at reduced audio quality) through the failures of two of the four normally connected T1s.

I wrote earlier about using a T3 data circuit for data transport between the studio and the transmitter site (assuming that makes financial sense for your station). Again, let me point out that Adtran makes equipment for just such purposes. The MX2800 (as the name implies) takes 28 T1s and aggregates (or muxes) them together to build up a T3 data stream, and provides the network clock reference. The interface for both send and receive directions (since this is also a full-duplex link) are in the form of 75-ohm BNCs. You'll hand off your T3 to your local telco via these cables.

The great thing about T3 is the scalability; just because you have 28 T1s to start doesn't mean you have to turn all of them on right way. You can use what you need at first, and turn more on as needed. Remember, the cost is the same whether you use 5, 10 or 25 of the T1s. Using a router such as the one I mentioned earlier allows you to bundle multiple T1s together to form a high-bandwidth link.

If you were to build a high-bandwidth LAN extension to the transmitter site, another set of possibilities comes in to being for your station's STL (and TSL of course) systems.



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