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PORTABLE USB MODEM
5. Type AT&T6 and press ENTER. The local modem responds to this command
by transmitting an unscrambled marking signal, which causes the remote
modem to place itself in digital loopback mode. Then the local modem exits
online command mode and enters data mode.
6. Enter data from your keyboard. For this test, typing multiple uppercase U
characters is a good way to send an alternating test pattern of binary ones and
zeroes. Data received by the remote modem enters its analog receiver, is
converted to digital data, is reconverted into analog, and then is transmitted
back to your modem. Your modem passes this test if the data received on your
monitor is the same as the data entered from your keyboard.
7. To exit the test, type the escape sequence +++AT and press ENTER. This puts
the modem in online command mode. The modem should respond with an
OK message. If you wish to stay on line with the remote modem for normal
data transmission, type AT&T and press ENTER to exit the test, then type
ATO and press ENTER to return on line. If you wish to terminate the call,
type ATH and press ENTER to hang up.
B.4 Local Digital Loopback Test (V.54 Loop 2)
The local digital loopback test is identical to the remote digital loopback test with
one exception. Instead of using your modem to signal a remote modem to place
itself in digital loopback mode, your modem is placed in digital loopback mode
while the remote modem is not. Data is entered and transmitted from the remote
modem, sent across the phone line to your modem, and looped back to the
remote modem.
Figure B-3. Local digital loopback test.
UUUUUUUUU
UUUUU
Computer or Terminal Local MultiModem Remote MultiModem Computer or Terminal
Digital Analog DigitalAnalog
AT&T3
OK
Local Portable
USB Modem
Remote Portable
USB Modem