8.1.1 Overview
Flow control is a protective measure for communications between the RNC and its peer equipment. Flow control provides protection in the following ways:
l It restricts incoming traffic to:
– Protect equipment from overload, thereby maintaining system stability.
– Ensure that equipment can properly process services even under heavy traffic.
l It restricts outgoing traffic to reduce the load on the peer equipment.
8.1.2 Panorama
During mass gathering events, the traffic volume may exceed the processing capability of the system. As a result, the system becomes overloaded, which may lead to messages being randomly discarded and NE resetting, as well as response failures, call drops, service access failures, and other unexpected events.
Resources in a WCDMA system are limited, so how they are used affects system performance.
The resources concerned here are:
l Equipment system resources, including CPU resources and memory l Air interface resources, including channels, codes, and power l Transmission resources
l Core network processing capabilities
To keep system stability and capabilities at the maximum possible level, Huawei RNCs perform flow control at four points in the system, which are numbered inFigure 8-1.
Base Station Controller Equipment Reliability Feature
Parameter Description 8 Flow Control
Figure 8-1 Four points in flow control
Flow control involves discarding originating messages (such as RRC connection requests) that overload the system when system resources are insufficient, refusing to process low-priority services, and rejecting access requests for low-priority services.
l To address problems caused by limited RNC resources (labeled 1 in Figure 8-1), the RNC performs flow control for RNC units. The software of each RNC board monitors the system resource usage. When necessary, the RNC starts basic flow control functions that suspend non-critical functions, such as recording logs and printing to reduce the system load. Then, based on the system load and the switch status of flow control functions, the RNC performs other flow control functions to ensure system stability and reliability.
l To address problems caused by limited air interface resources (labeled 2 in Figure 8-1), the RNC performs call attempt per second (CAPS) control, PCH congestion control, and FACH congestion control.
– When the cell is overloaded with services, the RNC limits the number of RRC connection requests admitted to a cell each second. This processing is implemented by CAPS control.
– When the paging channel is congested, the RNC allows CS-domain paging messages to preempt PS-domain paging messages in order to raise the paging success rate in the CS domain.
– When the forward access channel (FACH) is congested, the RNC restricts message retransmissions on the logical channels, rejects certain PS service requests, and triggers state transitions such as CELL_PCH to CELL_DCH (P2D) and CELL_DCH to idle (D2Idle). This gives priority to access requests for high-priority services such as CS services, keeps a high cell update success rate, and reduces call drops.
The RNC performs admission control, load reshuffling, and overload control on code and power resources. For details about admission control, see Call Admission Control Feature Parameter Description. For details about load reshuffling and overload control, see Load Control Feature Parameter Description and Overload Control Feature Parameter Description.
l To address problems caused by limited signaling bandwidth over the Iu interface (labeled 3 in Figure 8-1), the RNC works with the core network to perform flow control over the Iu interface. Based on link congestion conditions detected at the local end and congestion
indications reported from the peer end, the RNC performs flow control on initial direct transfer messages to reduce the signaling traffic over the Iu interface. This prevents severe congestion on the signaling link between the RNC and the core network and also reduces the load on the core network when it is overloaded.
l To address problems caused by limited transmission resources over the Iub interface (labeled 4 in Figure 8-1), the RNC supports user-plane congestion control over the Iub interface. Specifically, the RNC restricts the data transmission rates when there is transmission congestion over the Iub interface. This prevents packet loss and makes more efficient use of the bandwidth.
For RRC connection requests, the RNC supports control-plane load sharing and user-plane load sharing. This achieves dynamic resource sharing, balances the load among subracks and boards, and improves RNC service processing efficiency. For details, see Controller Resource Sharing Feature Parameter Description.
NOTE
The BSC6910 inherits the flow control function from the BSC6900. The only difference is in the RNC units that flow control works on. Unless otherwise stated, the following descriptions apply to both the BSC6900 and BSC6910.
8.1.3 E2E Flow Control
E2E Flow Control protects NEs in a RAN from being overloaded. The NEs that participate in flow control are the RNC and NodeB.
Without E2E flow control, when the CPU of the baseband board or WMPT is congested or overloaded, or when the cell power is congested, the RNC will not know. Therefore, the RNC continues to admit a large number of RRC CONNECTION REQUEST messages and send RADIO LINK SETUP REQUEST messages to the NodeB over the Iub interface even when the NodeB is congested or overloaded. In this case, the NodeB should reject or discard these RADIO LINK SETUP REQUEST messages, which lower the cell resource utilization. In addition, the access of high-priority services cannot be guaranteed because the NodeB is unaware of the service priority of each message. To address these issues, Huawei has introduced the following E2E flow control functions:
l E2E flow control based on NodeB CPU load – E2E flow control phase 1
– E2E flow control phase 2
l E2E flow control based on power congestion
E2E Flow Control limits the traffic flow that enters NEs and therefore ensures the stable operation of NEs when these NEs are overloaded. For details about other flow control measures, such as flow control for overloaded RNC units, see Flow Control Feature Parameter
Description. Compared with flow control performed on a single NE, E2E Flow Control has the following benefits:
l More reference information is provided for flow control because of cooperation between NEs. For example, if the RNC provides service priority information for the NodeB, the NodeB can implement differentiated flow control based on service priorities to
preferentially ensure the access of high-priority services.
l Better flow control effects can be achieved because of cooperation between NEs. In E2E Flow Control Phase 2, the RNC performs flow control on RRC CONNECTION REQUEST Base Station Controller Equipment Reliability Feature
Parameter Description 8 Flow Control
messages, and the NodeB performs flow control on RADIO LINK SETUP REQUEST messages. Consequently, if the NodeB is overloaded, the RNC reduces the number of unnecessary RRC CONNECTION REQUEST messages to be processed. This action reduces the NodeB Application Part (NBAP) signaling traffic on the Iub interface, increasing resources available to admitted UEs and RAN resource utilization.
For details about the engineering guidelines, see E2E Flow Control Feature Parameter Description.