T1 refers to a Time Division Multiplex based carrier routinely used in North America. This interface carries 24 voice channels. Each voice channel uses 64 Kbits/sec. A T1 line also has an 8 Kbits/sec signaling/framing channel. Therefore the bit rate for a T1 carrier is 24 x 64K + 8000 = 1.544 Mb/s.
There are two types of framing formats used with T1 carriers. These are called D4 and ESF.
The T1 carrier needs to maintain a certain 1’s density so that the network equipment at each end can recover timing clock even if no traffic is being carried. There are two mechanisms in use for that purpose. These are referred to as AMI (Alternate Mark Inversion) or B8ZS (bipolar 8-zero substitution).
The older T1 trunks (> than 10 years in service) are generally set up for D4/AMI and those in last 5-10 years are generally set up for ESF/B8ZS.
The XOP Networks equipment supports both flavors of T1s.