Continuity Test

Continuity testing verifies the physical bearer facility between two SSPs. When CAS signaling is used, a call setup fails if the voice path is faulty. Using ISUP signaling, it is possible to set up a call using the signaling network without knowing that the bearer connection is impaired or completely broken.

The voice and signaling channels are usually on separate physical facilities, so a means of verifying that the voice facility is connected properly between the SSPs is needed. Many digital voice transmission systems provide fault detection on bearer facilities, which are signaled to the connected switching system using alarm indication bits within the digital information frame. However, these bits are not guaranteed to be signaled transparently through interconnecting transmission equipment, such as a Digital Access Cross Connect system (DACS) or digital multiplexers. Some networks require these alarm indications to be passed through without disruption, therefore, reducing the need for continuity testing.

Continuity testing can be considered part of the ISUP maintenance functions. It can be invoked to test trunks manually, as part of routine maintenance and troubleshooting procedures. Continuity testing can also be provisioned to take place during normal call setup and it has an impact on the flow of call processing. During call processing, the originating exchange determines whether a continuity test should be performed. Network guidelines vary concerning whether and how often continuity testing is performed. The determination is typically based on a percentage of call originations. For example, in the United States, the generally accepted practice is to perform continuity testing on 12 percent of ISUP call originations (approximately one out of eight calls). This percentage is based on Telcordia recommendations.

Loopback and Transceiver Methods

The actual circuit testing can be performed using either the loopback or the transceiver method. The loopback method is performed on four-wire circuits using a single tone, and the transceiver method is used for two-wire circuits using two different tones. The primary difference between the two methods is related to the action that takes place at the terminating end. When using either method, a tone generator is connected to the outgoing circuit at the originating exchange. Using the loopback method, the terminating exchange connects the transmit path to the receive path, forming a loopback to the originator. The originator measures the tone coming back to ensure that it is within the specified parameters. When the transceiver method is used, the transmit and receive path are connected to a tone transceiver that measures the tone coming from the originating exchange and sends a different tone back to the originating exchange. The tone frequencies vary between countries. The following tones are used for the continuity test in North America:

• 2010 Hz from the originating exchange

• 1720 Hz from the terminating exchange (transceiver method only)

Another example of the COT tone frequency is 2000 Hz, which is used in the U.K.

Continuity Check Procedure

The Initial Address Message contains a Continuity Check Indicator as part of the Nature of Connection field. When an ISUP trunk circuit is selected for an outgoing call and the exchange determines that a continuity check should be performed, the Continuity Check Indicator is set to true. A tone generator is connected to the outgoing circuit, and the IAM is sent to the SSP at the far end of the trunk. Timer T25 is started when the tone is applied, to ensure that tone is received back within the T25 time period. When the SSP at the far end receives the IAM with the Continuity Check Indicator set to true, it determines whether to create a loopback of the transmit and receive path, or to connect a transceiver. The transceiver receives the incoming tone and generates another tone on the outgoing circuit. The determination of whether to use a loopback or transceiver is typically based on provisioned data at the receiving exchange. Upon receipt of the IAM, Timer T8 is started at the terminating exchange, awaiting the receipt of a COT message to indicate that the test passed. The terminating exchange does not apply ringing to the called party or send back ACM until the COT message has been received with a continuity indicator of continuity check successful to indicate that the bearer connection is good.

The originating exchange measures the received tone to ensure that it is within an acceptable frequency range and decibel level. Next it sends a COT message to the terminating exchange to indicate the test results. If the test passes, the call proceeds as normal; if the test fails, the CIC is blocked, the circuit connection is cleared, and the originating exchange sends a Continuity Check Request (CCR) message to request a retest of the failed circuit. While ISUP maintenance monitors the failed circuit's retest, ISUP call processing sets the call up on another circuit. Figure 8-9 shows a successful COT check using the loopback method.

Figure 8-9. Successful COT Check Using the Loopback Method

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