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 point code/CIC method of resolving glare is being used, SSP B takes control of the circuit because the CIC is even numbered and SSP B has a numerically higher point code.

Avoiding Glare

When provisioning trunks, glare conditions can be minimized by properly coordinating the trunk selection algorithms at each end of a trunk group. A common method is to perform trunk selection in ascending order of the trunk member number at one end of the trunk group, and in descending order at the other end. This minimizes contention to the point of selecting the last available resource between the two ends. Another method is to have one end use the "Most Idle" trunk selection while the other end uses the "Least Idle" selection. The idea is to have an SSP select a trunk that is least likely to be selected by the SSP at the other end of the trunk group.