Hungtran,
In Alcatel BSS, the SDCCH failure rate threshold is 20%.
Below 20% : it is good.
Subscribers will not “feel” anything wrong with your network, it is a perfectly normal network.
Trust me.
Now, you have to check the SDCCH Preparation Failure due to Congestion. If that is higher than 2% or 3% (at busy hour), then you have SDCCH congestion problems.