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The Fundamentals of T1 Testing
A business acquaintance of mine has substituted their DSL connection with a T1 line. They possess an internal network installation, a few workstations and one file server connected to a router through DHCP and NAT service. After disconnecting the DSL modem from the router, they tried to connect the T1 communicator (CSU/DSU modem) to this router with the help of a crossover cable.
It necessitated T1's connection to a node, then entry via the static IP, Subnet Mask, Default Gateway, DNS1 and 2, into the router to restart everything, but unfortunately, it did not work as it was supposed to work. There was absolutely no web connection via the router, when it bypassed to connect the T1 communicator to a PC.
Presently, the T1 lines serve as the role model for softwares designed specifically to enhance business Internet connectivity. Just like the veins and arteries of the human body, they help in transporting the Internet traffic to its final location. But have you ever tried to determine the cause of breakdown when the circuit fails the bit-error-rate-test (BERT) on one pattern, but passes the another, or which pattern to adopt when all traffic bound to NATed workstations come via one T1 line, leaving the others unutilized or underutilized.
You will always face these ordinary problems whenever you wish to install new T1 lines for accelerating your data transmission processes. I have witnessed disruptions occurring several times whenever I contemplate using a standardized link layer protocol to make the T1 physical layer compatible with data networks.
Or when the amount of data to be transported is huge, then until the largest packets are transported, no other packet would be able to get through. Consequently, it will lead to increased delay in the transportation of other packets.
At first sight, it appears that T1 lines can breakdown in situations where something is wrong with its installation or when all the clocks are constantly left unsynchronized. These ordinary breakdowns are part of any kind of technological imprint, which can be avoided only through proper care at the time of hardware installation.
You can avoid these breakdowns if you take into account these simple considerations:
The first step in this direction will be to identify the particular problem. For this purpose, you have to get to the desktop screen without starting any programs or minimizing them. If you are unable to go to the screen, then the main problem lies with your PC and/or your network.
In order to determine it, just click on your Internet browser icon (which can be either Internet Explorer or Netscape or anything else). After this, if your default home page appears on the desktop screen without any external messages, it means that your T1 line is functional. Once you are sure about it, you can browse through other sites that are not cached in the memory of your PC.
In case, your browsing does not respond this time or you see a message on the screen that informs that it is using a saved copy of the home page, then it means that there is a problem in the T1 line.
The next step would be to check the telephone enclosure where the CSU/DSU, the router, and the hub (or switch) are located. There, check if all the tools are appropriately plugged in or not. If all the tools are functioning properly, then the lights of every tool should be lit.
The CSU/DSU (beige colored) is accompanied by four green lights in a row and one green light above them on the left side. In normal circumstances, other three lights are not lit. In case there is a problem, you have to locate any of the lights that has turned red or is constantly flashing red.
On the switchboard, every plug with a cable and switched on corresponding computer would emit green light. On the other hand, all the lights on the router (black colored) are red. This is the right time to recycle, or in other words, switch off and on the CSU/DSU (beige colored).
The rocker switch that helps in switching it on and off is located on the same side where the cable of the computer and plugs are linked to it. Switch it off, wait for a minimum of 15 seconds and then, switch it on again.
Then it is the time to switch off the router (black colored). The rocker switch that helps in switching it on and off is located on the same side where the cable of the computer and plugs are linked to it. Switch it off, wait for a minimum of 15 seconds and then, switch it on again. Then wait for a minimum of five minutes and try to access the Internet from your workstation.
This exercise will certainly solve most of the problems. Sometimes, you may be required to follow the same procedures for a second time as well to establish the Internet link again. So, just go ahead and don't hesitate in trying it for the second time, but beware not to resort to it several times in a row.
The most popular way to test T1 cables and signal problems is the Bit Error Rate (BER) tests. It is very simple and you can easily configure each T1 line to run BER tests. Yet, you must remember that there is only one BER test circuit for all 28 T1 lines. So, you have to fix the testing period from one minute to 14,400 minutes or 240 hours.
This test enables you to retrieve the error statistics at any time in the course of the BER test. When a BER test is continuing, your system is supposed to get back the same pattern that is being transmitted by it.
In order to make it feasible, you can utilize a loopback at your preferred site in the T1 link or network or alternately, you can configure remote testing tool to transmit simultaneously the same BER test pattern. Moreover, a T1 test pattern is a predestined bit sequence in the DS-1 data.
These patterns are made to stress T1 tool or to simulate the client data traffic. Due to these stress patterns, the tools function at the limits of details with regard to consecutive zeros, one's density and line coding. The main function of traffic simulation patterns is to simulate the random nature of client traffic.
An appropriate loopback in any of the two directions helps in locating a VIT problem (though, you may detect the problem in the Network Interface only). These VIT problems can be rectified easily by resetting the multiplexer at either end or the CSU at RC.
In this context, it is important to consider that several T1 lines are usually employed without any proper or thorough testing. Therefore, it is the duty of the service provider to balance the risks of any problem erupting in the future against the possible expenditures in test equipment.
Luckily, T1 testing equipment sellers are constantly improving their products along with maintaining affordable prices.
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