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T1: A Survival Guide
This book explains the inner workings of T1 voice and data lines for network administrators, PBX techs, sales people

By: John Shepler

If you've gotten the impression that high speed telecommunications and data networking is a matter of life and death, then the title of this O'Reilly manual by Matthew S. Gast makes perfect sense. Yes, when the network goes down or all the PBX phone lines go dead, big bucks can be at stake. Here's your survival guide if you are a telephone technician, IT manager, network administrator, or involved in marketing PBX systems, frame relay networks, or T1 lines. It's also a great book if you interested in entering the field, or are a telecommunications manager who just wants to know enough to be able to specify your needs and solve simple line problems.

So, What's a T1?
You might think that fast & reliable T1 lines are a products of the tech boom of the late 1990's. After all, individual users and small businesses still deal with dial-up modems, DSL, and ISDN lines. T1 lines can transport 24 phone conversations at once, or give you an Internet connection that runs at over 1.5 megabits per second. Would you be shocked to know that T1 is a 1950's invention?

It's true. You may have seen old photos from the early 20th century that showed cities jam packed with telephone poles and wires so thick that you can hardly see the sky. That's what drove Bell Labs to invent the T-carrier system of combining multiple phone calls on a single line. T1 and T3 are the most popular variations. T1 can hold 24 calls. T3 can transport 672 voice channels or nearly 45 megabits per second of data.

How do they do that? Simple. Just convert the phone conversations from analog to digital and then stack 24 of those digital bit patterns into a single data frame one right after the other. Since the line is digital already, it can be used to transmit Internet data without the need of using a modem to force digital information down an analog phone line.

Oh, did I say simple? Well, it's not really quite so simple. Remember, the T1 circuit was intended for the phone company's use to send phone calls between their offices to save wires. In 1960, nobody expected to use it for Internet or other private uses (even Al Gore hadn't thought of inventing the Internet in 1960). Making that old standard sit up and do tricks for electronic data interchange, call center operations, and Internet service providers has added a lot of complexity to the original scheme. But, it's still better than what's in second place.

Why Use a T1 Line?
The beauty of T1 is that it is installed and maintained by a local phone company over the wires that they already own and already come into your facility. Specifically you need two pair, which is really 2 regular type telephone lines that have been cleaned up and perhaps boosted with repeaters between you and the central office. But that's the phone company's problem. They already know how to do it and can generally install T1 service anywhere within 50 miles of an AT&T point of presence. That's a lot farther than DSL, and a T1 line comes with a service guarantee to boot.

You pay for this wonderful service, but the speed, versatility and reliability are often worth it. So much so that T1 lines are gaining in popularity and dropping in price as we speak. So, expect that you'll be more involved with T1 and its ilk, such as T3, OC3, and so on.

The Scary Parts of T1
In order to transmit as much data as fast as they do, T1 lines use what is called synchronous transmission. That means that everything needs precise timing, traceable back to those atomic clocks the government maintains. Normal modems are asynchronous, which means that they add extra bits so they can send data whenever they want and the other end will know when a byte has arrived. Synchronous systems are fussy and intolerant of little peculiarities like not sending data for awhile or sending too many zeros in a row. So, the T1 standard has been expanded to include things like B8ZS encoding and extended superframes.

What's spooky about an extended superframe?  Nothing, until there are problems on the line and bits start getting scrambled or you lose the framing altogether. Do you know what to do when the red, yellow or blue alarms start going off? Better have your survival guide handy.

The other tricky matter is matching up what is going on at both ends of your T1 line. If you are installing or upgrading a PBX, PABX, or newer key telephone system, or setting up high speed data transmission for VoIP telephone, corporate data transmission, or Internet service, you absolutely, positively must have your equipment set up exactly the same as the data pattern on the T1 line. Otherwise, you'll have a big pipe with nothing coming out.

What's in the Survival Kit
T1: A Survival Guide lives up to its name because it is much more than just another explanation of technology. It starts off nicely with a history of the phone system and the how and why T1 came to be. Then you get the technical explanation, slowly at first, and then deeper and deeper into the bit patterns, timing, circuit connections, and so on.

You learn about the CSU/DSU equipment that you'll likely be responsible for, and how they connect to the telephone company's "smart jack" at the demarcation point. You'll need the right settings to match your particular T1 line installation.

You also get something sorely missing in most textbooks or technology primers. That's the vital section on T1 troubleshooting. Applying the theory to solve actual problems is just what you need in a survival guide.

The book also has additional chapters on the High-Level Data Link Control Protocol and PPP, which are the software glue that connect those bits flying down the T1 line to your applications. There's a chapter on Frame Relay technology, which is often used in conjunction with T1 lines to connect far flung locations at reasonable costs.

T1: A Survival Guide is one of those books that you should read once or twice, and then keep handy on your technical shelf for those times when you'll want to find the key information fast.


If you are a T1 line user and would like to see how much you could be saving on your T1 voice and data costs, click here for a free competitive T1 quote from T1 Rex.

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Book Cover: T1 A Survival Guide.
T1 A Survival Guide

 

 

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