Issues about COT

Issues about COT

1. Acronym
CCR Continuity Check Request
COT Continuity
IAM Initial Address Message
CLF Clear Forward
RLG Release Guard

2. Background
The application of the continuity check depends on the type of the transmission system used for the circuit. For transmission systems having some inherent fault indication features giving an indication to the switching system in case of fault, a continuity check is not required. However, a per call continuity check may be needed on fully-digital circuits when circuits or bundles of circuits in primary multiplex groups are dropped and inserted en route between switches, and alarm indications carried on bits of the primary multiplex frame structure are lost in passing through an intermediate transmission facility that does not relay them transparently. Typical, per call continuity checks may be needed when the transmission link between switches contains a TDMA satellite system, a digital circuit multiplication system or a digital access and cross connection system, where fault indications are lost.

During the continuity check process, TC( Transceiver ) resources are needed. During the continuity check process, echo cancellation function should be stopped.

3. Messages
The related signalling messages are IAM, CCR, COT, CCF. These 4 messages are all forward-direction messages.

3.1. IAM
In IAM message, there is an IE named NOC( Nature of connection ). 2 bits are COT related in the IE, and its name is Continuity-check-indicator. Three possible states of Continuity-check-indicator separately represents "continuity check not required", "continuity check required on this circuit" and "continuity check performed on a previous circuit".

3.2. COT
In COT message, there is an indicator which shows the continuity-check is successive or failed. This message is corresponding to IAM.

3.3. CCR
CCR message is an indication to the succeeding exchange that the former exchange is performing continuity checking.

3.4. CCF
CCF message is an indication to the succeeding exchange that the continuity check has failed. This message is corresponding to CCR.

4. Procedure
4.1. continuity test
When the MSC determines that continuity-check is needed, it will send IAM with "Continuity check required" indicator to the opposite exchange. At the same time, a TC is connected to the speech circuit.

After received the IAM, the opposite exchange will connect a transceiver, too. Thus, continuity check loop is formed.

Specific tone with specific frequency is sent to the opposite exchange.

If a reply tone is received from the opposite exchange in permitted times, a COT with "Continuity check succeed" indicator is sent to the opposite exchange. Otherwise, a COT with "Continuity check failed" indicator is sent to the opposite exchange.

When the MSC which sponsors the continuity check detects that the continuity check is finished ( MGW report events ), it will send COT to the opposite exchange.

After the MSC which received COT from the former MSC which sponsors the continuity check, it'll also send COT to the succeeding exchange.

If COT succeed, call will continue and an ACM may received.

If COT failed, continuity-retest is needed.

Whether succeed or failed, TCs are removed.

4.2. Continuity retest
On first time COT failure, COT retest is needed.
The MSC send CCR to the opposite exchange. Issues about TC's application are the same with the first time COT.

If the COT succeed, CLF/RLG process will followed, which will bring the circuit to idle state.

If the COT failed, CCF message will be sent. COT retest will repeated infinite times.

5. Reference
ITU-T Q.723
ITU-T Q.724
ITU-T Q.763
ITU-T Q.764