T1: A Survival Guide
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?
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?
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
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
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.
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Copyright 2003 - 2016 by John E. Shepler, All