Some Details of T-Carrier Systems
- Developed in the early 1960s. Here are the services provided:
| Service | Circuit |
Bit Rate (Mbps) | # Voice Channels |
| DS0 | | 0.064 | 1 |
| DS1 | T1 | 1.544 | 24 |
| DS2 | T2 | 6.312 | 96 |
| DS3 | T3 | 44.736 | 672 |
| DS4 | T4 | 274.176 | 2016 |
- A T1 line can be used channelized (24 voice channels) or unchannelized
(one data channel). It can also combine some of the channels for voice
and use some channels for data.
- A customer can lease a fractional T1 line, that is use only some of the
channels allowing another customer or customers to use the remaining
channels.
- T-carrier lines use Bipolar Alternate Mark Inversion to represent
0s and 1s.
- A T1 line transmits 8,000 frames per second at 193 bits per frame.
Here is the frame layout:
24 * 8 + 1 = 193 bits per frame.
F = Framing bit of the frame.
DS0#1 is an 8 bit piece of data or voice for channel 1,
DS0#2 is an 8 bit piece of data or voice for channel 2, etc.
-
12 Frames taken together constitute a SuperFrame, which has
12 * 193 = 2,316 bits. The framing bits (8,000 per second) are used for
(a) Synchronization, (b) Error Checking, (c) Network Management Messages.
-
In addition, the low order bit in DSO#1 through DSO#24 of the 6th and 12th frames
is used for signalling. If the bit is 1, this indicates that the channel is idle
(not in use). If the bit is 0, the channel is not in use. This is called bit robbing.
Changing the low order bit of a voice sample from 0 to 1, or from 1 to 0 normally
cannot be detected by the human ear. Bit robbing is not acceptable with data
transmission, so it is not used for dedicated data channel.
-
Since 1983, when T1 lines were first connected directly to the customer premises,
AT&T needed a way to monitor the quality of the connection. The error checking
and network management messages in the framing bits allow the phone company to
continuously monitor the quality of the line. If the line develops a serious
problem, the customer can be automatically switched to another T1 line.