15
CHAPTER 3: Configuration
S
WITCHES
SW2-6
AND
SW2-5 L
INE
B
UILD
O
UT
These switches are used to set the line build out (LBO). The LBO controls the
pulse shape and attenuation of the signal sent to the network. The amount of LBO
depends on the distance to the nearest repeater. The telephone company
providing the T1 service will advise you of the required LBO setting.
Table 3-5. LBO Setting.
SW2-5
SW2-6
LBO
Off
Off
0 dB, 0–133 ft.
On
Off
-7.5 dB
Off
On
-15.0 dB
On
On
-22.5 dB
S
WITCH
SW2-4 L
INE
C
ODING
This switch is used to set the line code. Two coding options are available through
the DIP switches: B8ZS and AMI. The line code refers to the way that the signal—
the sequence of ones and zeros sent to the network—is encoded. AMI reverses the
polarity of consecutive pulses. B8ZS is identical to AMI, except that, under certain
circumstances, the alternate polarity rule is deliberately violated.
AMI
: This stands for “Alternate Mark Inversion.” The CSU/DSU transmits data as a
sequence of ones and zeros. Ones are usually sent as pulses, and zeros as spaces
(no pulse). In order to maximize transmission range, every pulse is of the opposite
polarity of the preceding pulse. AMI does nothing else. Maintaining network
integrity requires a minimum pulse density (ones density) of the signal being
transmitted on the network. AMI does not inherently provide for this feature.
Thus, if a long sequence of zeros happen to be sent, the network may suffer. To
meet this requirement using AMI requires one of two methods: Reduce the rate of
each time slot (DS0) to 56 kbps, so that the last bit can be used to guarantee the
minimum ones density. Or, make sure that the DTE sends data in such a way that
ones density is always maintained. For this reason, you may prefer B8ZS over AMI.
B8ZS
: This stands for “Bipolar 8 Zero Substitution.” This line code ensures
minimum ones density. Long sequences of zeros are specially encoded. This line
code allows any data pattern to be transmitted without causing ones density
problems. Thus, it allows the use of 64-kbps timeslots.
Table 3-6. Line Codes.
SW2-4
Line Code
Off
B8ZS
On
AMI
