3com MSR 20-20 참조 매뉴얼

Use the e1 set frame-format command to set the framing format of an E1 
channel.

Use the undo e1 set frame-format command to restore the default, that is, 
no-CRC4.

# Set E1 channel 1 to use framing format CRC4.

<Sysname> system-view

[Sysname] controller cpos 1/0

[Sysname-Cpos1/0] e1 1 set frame-format crc4 

e1 e1-number set loopback { local | payload | remote }

undo e1 e1-number set loopback

CPOS interface view

e1-number: Number of an E1 channel on the CPOS interface, in the range 1 to 63.

local: Enables internal loopback on the E1 channel.

payload: Enables external payload loopback on the E1 channel.

remote: Enables external loopback on the E1 channel.

Use the e1 set loopback command to configure the loopback mode of the E1 
channel.

Use the undo e1 set loopback command to remove a loopback.

By default, loopback is disabled.

You may test E1 channels in different depths by using the loopback command 
with different keywords.

In an internal loopback, data of the sender is directly looped to the receiver.

In an external payload loopback, data received by the receiver is looped back at 
the E1 framer as payload.

In an external loopback, data received by the receiver is looped back directly 
without passing through the E1 framer.

display controller cpos e1.

# Enable external payload loopback on E1 channel 1.

<Sysname> system-view

[Sysname] controller cpos 1/0

[Sysname-Cpos1/0] e1 1 set loopback payload