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Circuit Glare (Dual-Seizure) Circuit glare (also known as dual-seizure) occurs when the node at each end of a two-way trunk attempts to set up a call over the same bearer at the same time. Using ISUP signaling | Circuit Glare Dual-Seizure Circuit glare also known as dual-seizure occurs when the node at each end of a two-way trunk attempts to set up a call over the same bearer at the same time. Using ISUP signaling this occurs when an IAM for the same CIC is simultaneously sent from each end. Each end sends an IAM to set up a call before it receives the IAM from the other end. You will recall from our discussion of the basic ISUP message flow that once an IAM is sent an ACM is expected. When an IAM is received after sending an IAM for the same CIC glare has occurred. Resolving Glare When glare is detected one node must back down and give control to the other end. This allows one call to complete while the other call must be reattempted on another CIC. There are different methods for resolving which end takes control. For normal 64-kb s connections two methods are commonly used. With the first method the point code and CIC numbers are used to determine which end takes control of the circuit. The node with the higher-numbered point code takes control of even number CICs and the node with the lower-numbered point code takes control of odd numbered CICs. This provides a fair mechanism that allows each node to control approximately half of the calls encountering glare. In the United States an example of this use would be two peer End Office exchanges. The second method of glare resolution is handled by prior agreement between the two nodes about which end will back down when glare occurs. One node is provisioned to always back down while the other node is provisioned to take control. A typical example of this arrangement in the U.S. network would be a hand-off between non-peer exchanges such as an IXC to AT. The method to use for glare resolution can usually be provisioned at the SSP typically at the granularity level of the trunk group. Figure 8-8 illustrates a glare condition when SSP A and B have both sent an IAM before receiving the IAM from the other end. Assuming that the
