1. It created 2 traffics (one for CBR, one for UBR), and UPC/NPC function disable. Using tester send the CBR and UBR traffic, the time delay of UBR will be increase if the incoming bandwidth of CBR and UBR exceeds the out port bandwidth.

2. If UPC/NPC function enable, the ATM traffic will lose package easily.

Cause Analysis:

1. This phenomenon is normal. Because of ATM traffic, the leak bucket will be close and only QoS module operate after UPC/NPC function disable. It means the traffic will base on the PRI to attemper it after the traffic using QoS module. If the incoming bandwidth less or equal to out port bandwidth, the QoS module will attemper all traffic directly. If the incoming bandwidth is larger than out port bandwidth, the QoS module will attemper high PRI traffic first, and then attemper low PRI traffic. For the low PRI traffic which can not attemper completely at once, the QoS module will save the traffic at memory first and waiting for out port has enough bandwidth, then attemper it. The phenomenon of this condition is the time delay of low PRI traffic increase more and more if the incoming bandwidth large than out port bandwidth.
If the incoming bandwidth large than out port bandwidth at all time, it will lead to the low PRI traffic occupied all memory size, then the QoS module will reject the cell.
And the phenomenon of this condition is the time delay of low PRI become normal but the package will lose.

2. The parameter of CDVT is too small. Generally, the parameter of CDVT will set more than 1000ms. If the setting parameter is too small, the arrived time of cell has a little tingle. And it will lead to traffic lose package.

Handling Process:

1. Decrease the bandwidth of CBR traffic, the time delay of UBR will become normal.
2. If UPC/NPC function enable, the parameter of CDVT should be setting more than 1000ms. 

Thank you for sharing.

Thank you for sharing.

Thanks for sharing !

Good post.

Nice sharing! why do I think ATM traffic is so far away from us  ?...