3.2
Understanding The T1 Options
(SW2–1 Through SW2–8 And LIU–1 Through LIU–4)
The following T1 operating modes are described to help you understand the many options that you can select with
DIP switches SW2–1 through SW2–8 and LIU–1 through LIU–4 on the DXPT1 board. DIP switches designated
LIU are associated with the DXPT1 board’s line interface unit.
3.2.1 Loopback
Loopback is the maintenance routine used to verify the receive function (local or remote) and to isolate system
problems. When using the superframe format, the customer interface (CI) loopback option from the network, if
provided, can be activated from the network via in-band signaling (per ANSI T1.403, 1989, SECTION 8.3.1.1).
Framed code words activate and deactivate loopback routines when the option is enabled with DIP switch
SW2–1. The default setting is off, but many central offices will support SF loopback. See your central office
representative to ensure that you are complying with their requirements.
You access remote and payload loopback operation via the facility data link when using the extended superframe
format.
You can manually make three different types of loopback requests depending on the option you select using DIP
switches SW2–7 and SW2–8. These are: local, remote, and payload loopback. When using payload loopback
(ESF format only), all channel bits maintain bit-sequence integrity but the framing bit is re-established.
3.2.2 Customer Service Unit (CSU) Identification Bit
You can set the message-oriented (C/R) bit transmitted over the ESF data link by DIP switch SW2-2 to be a zero
(0) bit or a one (1) bit. The C/R bit is used to identify the T1 signal origin as being either from customer premise
equipment (CPE) or carrier equipment such as a CSU.
Set C/R to be a zero (0) bit for customer premise equipment (CPE) or when using an external CSU (default).
Set C/R to be a one (1) bit if the network requires the T1 internal CSU to be classified as carrier equipment.
3.2.3 Cyclic Redundancy Check (CRC-6)
This is a method for checking the accuracy of data transmissions when using the ESF format. This method uses a
polynomial algorithm based on the content of a superframe of data. When CRC-6 checking is enabled via DIP
switch SW2–4, an alarm will be activated whenever a CRC code (checksum) is received over the ESF data link
that does not match the locally calculated code (checksum).
3.2.4 Yellow Alarm During ANSI Messages On The Facility Data Link
When using the ESF format, the facility data link is available for reporting transmission performance information
once a second. Normally, these transmissions are inhibited by yellow alarms. However, you can set DIP switch
SW2-5 to allow these transmissions during yellow alarms.
3.2.5 Green B8ZS/COFA LED Definition
The green LED on the DXPT1 board’s edge-panel labeled “B8ZS/COFA” indicates either that B8ZS code words
are being received or that a change of frame alignment (COFA) occurred during the last synchronization by the
external T1 source or carrier equipment. You can determine which condition you want reported by how you set
DIP switch SW2–6. The default setting is for “COFA” reporting.
3.2.6 Receive Sensitivity
The equalizer gain logic (EGL) determines how sensitive the receive circuits are to the incoming signals. You set
the level for -26 dB or -36 dB by using DIP switch LIU-1. (Normally, the -26 dB level setting is the better choice.)
IMI89–193
Installing The DXPT1 Line Board
8 – Installing The DXPT1 Line Board
