Diagnostics & Test Facilities

		Diagnostic & Test Facilities

Failure in a point-to-point communications link can be very expensive and can be attributed to a fault in the local DTE, the local modem, the telephone network, the remote modem or the remote DTE. The modem's diagnostic and test facilities are provided to enable you to efficiently locate the faults through looping procedures. There are four loops defined by ITU-T recommendation V.54. These are illustrated below:

A symmetrical set of test loops could exist from DTE (b). However, only loops 2 and 3 directly relate to the modem's operation.

Using the diagnostics

Start performing a local analogue loopback test with both the local and remote modem and their respective DTE' s. If they pass this test perform a remote digital loopback with or without self test to verify the complete communications link from one modem to the other. The success of the local analogue loopback test and the failure of the remote digital loopback test may indicate a fault in the telephone network, especially if many attempts for a satisfactory connection failed. All the tests are initiated using the &T commands and are only available when the modem is set for non-error corrected operation i.e. the \N0 command has been issued. The remote and local digital loopback tests are only available after a successful connection has been established. In this instance, the test command is issued after returning to command mode by using the escape sequence.

&T0 Terminating a test :

Register S18 determines the duration of the loop tests. The default value 0 causes the test to be conducted continuously. However, these tests can be terminated at any point by issuing the escape sequence (+++) to return the modem to command mode, then issuing the &T0 or H0 command.

&T1 Local analogue loopback (loop 3) :

This test is used to verify the operation of the path which comprises the local DTE and your modem. The transmitted data from the DTE is modulated by the modem transmitter and fed back into its receiver where it is demodulated and sent back to the DTE. This test is only available in off-line mode where anything typed from the DTE should be echoed back if the data path is operational.

&T3 Local digital loopback :

This test is used to loopback the received data stream from the remote modem. Test data must be sent, received and analysed at the remote modem. The local digital loopback test is used to engage the remote modem in a remote digital loopback test without the latter initiating a V.54 handshake sequence. To initiate this test both your modem and the remote modem must be on-line.

&T4 Grant RDL request from remote modem :

The &T4 command enables your modem to accept a request for an RDL (request digital loopback) test from a remote modem and acknowledge it according to ITU-T Recommendation V.54. This is the default condition.

&T5 Deny RDL request from remote modem :

The &T5 command prevents your modem from responding and granting a request for a remote digital loopback test.

&T6 Remote digital loopback (loop 2) :

The test is used to check the integrity of the local DTE, your modem, the telephone network and the remote modem from the local site. Data transmitted from the local DTE is sent by your modem through the telephone network to the remote modem which, in turn, loops it back to the local DTE. The local DTE can then compare this received stream with its transmitted data to check the performance of the communications link. After establishing a connection and issuing an escape sequence to return to command mode, the test is initiated by entering the &T6 command. The remote digital loopback is requested by your modem through a special ITU-T handshake signalling sequence (ITU-T Recommendation V.54) with the remote modem. If enabled, the remote system will automatically acknowledge the request and loop its received data into its transmitter and send it back to the local DTE for comparison. The remote modem is conditioned to accept or refuse a request for this test with the &T4 or &T5 command.

&T7 Remote digital loopback (loop 2) with self test :

This test is the same as the previous one with the addition of a self test, in which an internally generated data pattern is transmitted to the remote modem, which in turn loops it back and sends it to your modem. The received stream is compared to the transmitted data and the number of errors counted. When the timer expires (or &T0 is issued), the number of recorded errors is returned. If errors are encountered, you can initiate a local analogue loopback test to isolate the fault which can be attributed to your modem, the telephone network or the remote modem.

&T8 Local analogue loopback (loop 3) with self test :

This test is the same as the previous one with the addition of a self test in which an internally generated data pattern is applied to the transmitter that loops it back to the receiver and errors are recorded. The modem responds with the OK message. After the timer expires (or &T0 is issued), this response is given:

000
OK (where 000 is the number of errors detected).