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170 Chapter 6: Using ISDN and DDR Technologies
Figure 6-14
SF and ESF
T1 Line Code
Once the framing has been configured, the next step is to configure the line coding. In North
America, there are two types of line coding that are dominant: Alternate Mark Inversion (AMI)
and Bipolar with 8 Zero Substitution (B8ZS).
AMI is becoming increasingly rare in favor of B8ZS. AMI forces data bandwidth to 56 kbps
due to the enforcement of a 1s density rule stating that 12.5 percent of all bits transmitted must
be 1s. To enforce the rule, AMI line coded CSU/DSUs force every eighth bit (that is, the least
significant bit of each timeslot) to a 1. Effectively, this bit is lost. The rule may or may not
actually change the bit, but effective throughput is reduced nonetheless. The end result is that
there are still 8000 samples per second, but each sample has been reduced to seven bits, thereby
rendering 56000 bits per second.
B8ZS has a more effective way of dealing with the enforcement of 1s density. It alters bits only
when necessary, and then changes the affected bits back to their original values at the remote
side.
To verify the configuration of the framing and line code being currently in use, type the
command show controllers t1. Example 6-29 shows a sample output of this command.
T1
Frame
1
F
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T1
Frame
2
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T1
Frame
3
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T1
Frame
4
F
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T1
Frame
5
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T1
Frame
6
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T1
Frame
7
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T1
Frame
8
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T1
Frame
9
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T1
Frame
10
F
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T1
Frame
11
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T1
Frame
12
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12 T1 Frames = 1 SuperFrame
T1
Frame
1
D
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T1
Frame
2
C
R
C
T1
Frame
3
T1
Frame
4
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T1
Frame
5
T1
Frame
6
T1
Frame
7
T1
Frame
8
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T1
Frame
9
T1
Frame
10
T1
Frame
11
T1
Frame
12
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T1
Frame
13
T1
Frame
14
T1
Frame
15
T1
Frame
16
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T1
Frame
17
T1
Frame
18
T1
Frame
19
T1
Frame
20
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T1
Frame
21
T1
Frame
22
T1
Frame
23
T1
Frame
24
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24 T1 Frames = 1 Extended SuperFrame
D
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D
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C
R
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D
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D
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C
R
C
D
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C
D
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C
C
R
C
D
L
C
D
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C
C
R
C
D
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C
D
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C
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C
D
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C