Understanding the network.pdf - Back to Home

permits the entire circuit to be segmented for testing without any interruption to
service.
ESF accomplishes all this by increasing the frame size to 24 M24/D4 frames and
reallocating the use of the available framing bits, only 6 of which are used for actual
frame alignment. The framing bits are in frames 4, 8, 12, 16, 20, and 24. The CRC
error checking facility uses 6 bits, which appear in frames 2, 6, 10, 14, 18, and 22.
The FDC utilizes the last 12 bits, which are used to carry management information
and alarm information. Figure 5.4 illustrates the assignments of each of the framing
bits in the ESF frame format.
Figure 5.4. ESF framing bit assignments.
ESF management and data collection information is stored on the CSU, where it can
be accessed by the carrier when non-obtrusive circuit testing is enabled. Because
remote testing is one of the big advantages of using ESF, some kind of testing
system must be employed by the carrier. One of the most commonly used is AT&T's
Automated Bit Access Test System (ABATS).
T1 Line Coding
Formatting the bitstream is only half of the data transport process. The other half is
the line-coding scheme, which determines how the actual bits are interpreted.
Digital data transmission is a basically simple process, because there are only two
data types: ones and zeros. In a basic transmission scheme, a binary one is
interpreted as the presence of a signal, and zero is interpreted as the lack of signal.
Transmission would be easy if all your data was like this: 10101010. Digital data