Testing a Span With a T1 Test Set
I was asked about how to set up a T1 test set.
The T1 circuit is terminated at each end with one of two devices: a CSU, or a Smart Jack. The Smart Jack allows testing from a remote access point (like the tester person who is on shift at the telephone company's Central Office). With either a CSU (Channel Service Unit) or a Smart Jack, you can also test the circuit locally. So, let's say that you are the person who has been sent to the equipment room to find out if the T1 between your Seattle office and your Redmond office is working.
1. You turn on your test set.
2. You connect it to the (in my hypothetical example) CSU.
3. And now -- you have to know which of two kinds of signaling is being used on the T1 you are testing. Is it using B8ZS protocol, or AMI? It's one or the other -- and your test set will show the line as bad -- even if it's good -- unless you set up your test set for the same protocol as your T1 is using.
4. Now, suppose the dreaded Murphy's Law sneaks up on you, and no one has good records on the circuit. Try them both. You won't break it. If the tester shows "bad" on both B8ZS and AMI, then the circuit is bad. On the other hand, if the circuit checks good with one of the coding types, then you know the line is good, and you also know which protocol is being used. My advice: label it.
For your convenience, I have copied and pasted a definition of B8ZS and AMI from http://whatis.techtarget.com/
B8ZS (bipolar 8-zero substitution, also called binary 8-zero substitution, clear channel, and clear 64) is an encoding method used on T1 circuits that inserts two successive ones of the same voltage - referred to as a bipolar violation - into a signal whenever eight consecutive zeros are transmitted. The device receiving the signal interprets the bipolar violation as a timing mark, which keeps the transmitting and receiving devices synchronized. Ordinarily, when successive ones are transmitted, one has a positive voltage and the other has a negative voltage.
B8ZS is based on an older encoding method called alternate mark inversion (AMI). AMI is used with Dataphone Digital Service, the oldest data service still in use that uses 64 Kbps channels. AMI, however, requires the use of 8 Kbps of the 64 Kbps of each channel to maintain synchronization. In a T1 circuit, there are 24 channels. This loss adds up to 192 Kbps, which means that in reality only 56 Kbps is available for data transmission. B8ZS uses bipolar violations to synchronize devices, a solution that does not require the use of extra bits, which means a T1 circuit using B8ZS can use the full 64 Kbps for each channel for data. B8ZS is not compatible with older AMI equipment.
T1 technology is used in the United States and Japan. In Europe, a comparable technology called E1 provides 32 channels instead of 24 and uses an encoding scheme called high-density bipolar 3 (HDB3) instead of B8ZS.