There is OSN 1500B in between of RNC-NodeB link. It is connected to RNC over IDQ1 external port and to NodeB over 6xVC12 IMA group. Topology is given in the attachment.
IMA group is finished on internal port of IDQ1 card as VCTRUNK7 and after PVP cross-connect traffic is sent to IDQ1 external port 1, connected to RNC. On side of NodeB IMA is also configured as 6xE1 and IMA is functional. Traffic is going normally. Problem occurs after one E1 is disconnected from OSN-NodeB link. After that all traffic is down, until E1 is returned back to OSN-NodeB link.
OSN 1500B version is: V100R006C02B012
NMS T200 version is: V2R4C01B01cSPC009
IDQ1 board version is in accordance with version matching table of V100R006C02B012.
NodeB and RNC are NOKIA-Siemens vendor.
During normal traffic functioning there are no alarms present on OSN, RNC or NodeB equipment. After one E1 disconnection from DDF following appears:
1) OSN1500B IMA group is working normally with remained 5xVC12 capacity, and it can be seen in port status which VC12 is not functional. No alarms are present on OSN1500B;
2) NodeB equipment can not be connected remotely from RNC because whole link is in down state. After on-site check of NodeB it appears that IMA is working normally with 5xE1 rate and alarm is present only for certain E1 which is disconnected;
3) RNC is receiving AIS on VPI and VCI towards NodeB.
VC AIS or VP AIS can be sent over an ATM link either trough VCI=3 for F4 O&M ATM flow, either trough PTI=5 or PTI=4 for F5 O&M ATM flow. Trough this link ATM alarms similar to SDH alarms can be transferred (such as VCI/VPI RDI, VPI/VCI AIS). We have assumed that RNC is receiving O&M alarms from OSN equipment trough those links. In ATM O&M flow we can distinguish end point and segment point. End point is placed on both ends of ATM link and O&M info is going in between them. Segment point can be present in between end points and it is transferring data trough itself. Picture is given in the attachment. Left end point will ignore O&M data received from left side and right end point will ignore O&M messages received from right side.
At first we have assumed that IMA protocol of OSN and NodeB is not in accordance with each other and that fault appears because of protocol mismatch. We have checked IMA protocol versions of NodeB and OSN as well as IMA protocol settings. It was consistent. IMA 1.1, packet length is the same.
Then we have deleted problematic VC12, which was removed from the link, from VCTRUNK7 which has IMA group configured over it. After that, link was established again. That was evidence that IMA is working properly between NodeB and OSN.
ATM layer was not operational and for some reasons RNC was receiving VC AIS alarm from OSN equipment.
By default OSN is set as segment point regarding ATM O&M. We have decided to set it as end point and to cut so called embedded ATM O&M link between RNC and NodeB, because it is not needed for customer. Customer is using self defined user plane VCI=32 for establishing O&M link between RNC and NodeB.
After cutting embedded O&M link RNC was not receiving VPI/VCI AIS in case of one VC12 down from IMA link, so it was not breaking down the link between itself and NodeB. Traffic is working normally also after E1 is disconnected from IMA link.