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    Call Admission Control Overview

    Call admission control (CAC) on the MobileNext Broadband Gateway ensures that required network resources are available for real-time data traffic such as voice and video. CAC maintains information about all resources available on the broadband gateway and resources that have been allocated to bearers. Call admission is based on resource availability and the priority of the bearer, and allows the broadband gateway to reject or downgrade (Create or Modify) bearer requests when the system, CPU, memory, or bearer load for upstream or downstream traffic exceeds configured CAC thresholds.

    This topic covers:

    Enforcing Call Admission Control

    Call admission control is enforced only when a local policy profile is configured at the system level or access point name (APN) level on the broadband gateway.

    Managing Bandwidth

    A bandwidth pool limits the number of guaranteed bit rate (GBR) bearers that can be supported on the broadband gateway (at the APN level or system level) per traffic class. Because a broadband gateway provides a limited amount of bandwidth, it must keep track of the amount of allocated bandwidth when receiving create/update PDP context requests with GBR requirements.

    Note: You configure bandwidth pools to provide GBR requirements for 3G networks.

    When admitting bearers, and especially bearers with GBR requirements, the broadband gateway must reject requests when the bandwidth requirements cannot be guaranteed. However, the bandwidth guarantees are only soft guarantees in that the broadband gateway can only restrict the total bandwidth guaranteed to the bearers; no hardware resources are allocated in the system for a bearer with a GBR.

    Bandwidth is reserved at the system level or access point name (APN) level based on where the local policy is configured. A local policy configured at the system level specifies a bandwidth pool for all APNs that do not have an explicitly configured bandwidth pool. A bandwidth pool associated with multiple APNs is shared among all bearers of those APNs. A local policy configured at the APN level specifies a bandwidth pool reserved for bearers associated with the specific APN.

    Managing the Number of Bearers

    A broadband gateway provides resource control for the number of bearers. In the control plane and data plane, a set of resources is allocated to each bearer regardless of the bandwidth requirements for the bearer, and the broadband gateway should always specify the maximum number of bearers allowed at the system level, or APN level, or both. When the number of bearers at the system level or APN level reaches the maximum limit, no bearer requests other than delete bearer requests are allowed.

    Managing Resource Thresholds

    You configure the following parameters for resource thresholds to control traffic flow at either the system level or APN level:

    • Bearer load—Specifies a more precise level of admission control when bearer load reaches a configured lower or upper threshold.
    • System load—Specifies a level of traffic flow control when memory utilization, CPU load, and queue depths (for GTP, RADIUS, and CDR) reach a configured lower or upper threshold.

      Note: System load is an average of memory and CPU load, so the configuration you specify for the system load should take into consideration both memory and CPU load.

    • Memory load-–Specifies a more precise level of admission control when memory utilization reaches a configured lower or upper threshold.
    • CPU load—specifies a more precise level of admission control when CPU load reaches a configured lower or upper threshold.

    Each threshold parameter includes a low and high threshold setting that is associated with an allocation and retention priority (ARP).

    Note: When subscriber traffic on the broadband gateway exceeds the configured low or high resource threshold settings, only Create Session requests with a higher-priority ARP (GTPv1) or PL (GTPv2) are allowed. When the limits for bearer, system, CPU, or memory load exceed the configured threshold limits, the broadband gateway can preempt bearers with a lower priority.

    Default Resource Threshold Settings

    If you do not explicitly configure resource threshold settings on the broadband gateway, the following resource threshold default values apply:

    • Low threshold—70 percent for all parameters
    • Low threshold ARP—10 (GTPv2) and 2 (GTPv1)
    • High threshold—85 percent for all parameters
    • High threshold ARP—5 (GTPv2) and 1 (GTPv1)

    Published: 2011-11-17