11
Switches SW1.1 SW1.2, SW1.3, SW 2.7 & SW2.8
These switches set the starting channel. The starting channel is
the first channel that carries valid data. This channel can be set to any
value between 1 and 24. If the starting channel is other than 1, then
the maximum possible bandwidth will be less than 1536 kbps. For
example, if the starting channel is set to be 12, then the maximum
bandwidth is limited to 13x64 kbps (channels 12~24), or 832 kbps. This
must be kept in mind when setting the starting channel. The Model
2711 will flash the ERR indicator LED if the switch setting is invalid.
Refer to section 5.1 for a more detailed explanation of the ERR LED
function.
T1 STARTING CHANNEL
T1 Starting Channel
SW1.3
SW1.2
SW1.1
SW2.8
SW2.7
(maximum value of n)
Off
Off
Off
Off
Off
1 (24)
Off
Off
Off
Off
On
2 (23)
Off
Off
Off
On
Off
3 (22)
Off
Off
Off
On
On
4 (21)
Off
Off
On
Off
Off
5 (20)
Off
Off
On
Off
On
6 (19)
Off
Off
On
On
Off
7 (18)
Off
Off
On
On
On
8 (17)
Off
On
Off
Off
Off
9 (16)
Off
On
Off
Off
On
10 (15)
Off
On
Off
On
Off
11 (14)
Off
On
Off
On
On
12 (13)
Off
On
On
Off
Off
13 (12)
Off
On
On
Off
On
14 (11)
Off
On
On
On
Off
15 (10)
Off
On
On
On
On
16 (9)
On
Off
Off
Off
Off
17 (8)
On
Off
Off
Off
On
18 (7)
On
Off
Off
On
Off
19 (6)
On
Off
Off
On
On
20 (5)
On
Off
On
Off
Off
21 (4)
On
Off
On
Off
On
22 (3)
On
Off
On
On
Off
23 (2)
On
Off
On
On
On
24 (1)
12
Switches SW2.6 & SW2.5 Line Build Out
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 net-
work. 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.
SW2.6
SW2.5
LBO
Off
Off
0 dB, 0-133 ft
Off
On
-7.5 dB
On
Off
-15.0 dB
On
On
-22.5 dB
Switch SW2.4 Line Coding
This switch is used to set the line code. There are two line coding
options 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 consecu-
tive pulses. B8ZS is identical to AMI, except that, under certain circum-
stances, 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 desity) 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 net-
work 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, B8ZS may be preferred over AMI.
