A CAC (Call Admission Control) Policy is assigned to a CAC Group and represents the characteristics for the bandwidth-limited link being monitored. Lotus Sametime Unified Telephony supports CAC Policies for voice-RTP (Real-time Transport Protocol), SRTP (Secure Real-time Transport Protocol ),video-RTP and/or T.38 Fax-UDPTL (User Datagram Protocol Transport Layer).
A CAC policy can limit the calls over a bandwidth-limited link based on number of calls, bandwidth limit, or both.
Lotus Sametime Unified Telephony Assistant can be used to display the current status for each provisioned CAC policy. This means displaying the number of concurrent calls and the actual bandwidth usage applicable to the CAC policy at any given time.
Both the number of concurrent calls and the actual bandwidth usage are DND displayed independent of whether the CAC policy is limiting the calls based on number of calls, bandwidth limit, or both.
Each CAC Policy contains the following information:
The CAC Group or parent CAC Group to which the policy applies. The CAC Policy applies to all calls to and from the CAC Group.
The traffic type controlled by the CAC Policy. This can be:
Only Voice (RTP)
Only Fax (T.38 over UDPTL)
Only Video (RTP)
The capacity limits the CAC Policy enforces for a primary link and optionally for a secondary (backup) link. The primary and secondary capacities can be defined based on the number of calls, bandwidth limit, or both, as follows
- Number of calls:
The concurrent calls per CAC Policy are counted. When the limit is reached, no new calls are admitted.
- Bandwidth limit:
Lotus Sametime Unified Telephony calculates the used bandwidth based on the negotiated codecs in the SDP (Session Description Protocol). The bandwidth limit is the common limit for the traffic types associated with the CAC Policy — for example, if the CAC Policy is only applicable for voice, the bandwidth limit is exclusive for voice traffic.
The bandwidth limit must be entered considering the common limit reserved for voice/video/fax for upstream and downstream traffic. For instance, if a value of 1 Mbps is entered, it indicates that the upstream bandwidth is 1 Mbps and the downstream bandwidth is 1 Mbps as well.
Figure 1. Branch office with bandwidth-limited link
If both number of calls and bandwidth limit are defined, the limit is enforced as soon as one is reached.
The primary and the secondary link capacities must use the same criteria. For example, if the primary capacity is based on bandwidth limit, the secondary capacity must also be based on bandwidth limit.
Whether to generate alarms when usage increases above the applicable threshold value. If alarm generation is enabled, the IRM ...
sends an alarm when the usage gets above the high threshold.
clears the alarm when the usage gets below the low threshold.
Default values may be used for both thresholds; the administrator can also specify custom values if desired.
The permitted voice codecs that may be used for SIP and SIP-Q voice calls routed over the bandwidth-limited link represented by the CAC policy. Limiting permitted voice codecs in this manner optimizes the usage of the bandwidth and allow more simultaneous connections, while still guaranteeing an acceptable quality of service.
- In legacy system: The permitted video codecs that may be used for SIP and SIP-Q voice calls routed over the bandwidth-limited link represented by the CAC policy. Limiting permitted video codecs in this manner provides benefits comparable to the ability to limit voice codecs as described above.
A flag that indicates whether connections to the media server for announcements/tones are ignored for bandwidth calculations.
Some features require a media server to collect DTMF (Dual Tone Multifrequency) digits or play a tone/announcement. The connection to the media server may be routed over a bandwidth-limited link when the media server is located elsewhere in the network. Such media server connections can occur even when the feature involves endpoint devices that are located within the same CAC Group.
- When this option is disabled:
Insufficient bandwidth to the media server can result in the feature to either be blocked or to continue without progress tones, depending on the feature scenario.
- When this option is enabled:
Bandwidth used for media server connections are ignored for the purpose of bandwidth management. This allows the media server always to play tones/announcements and collect DTMF digits, even when bandwidth limitations exist. Occasional degradation in speech quality may occur due to temporary overbooking of a bandwidth-limited link when this option is enabled.
Only connections for tones and announcements are ignored when this option is set. Connections for media server applications, such as conferencing and unified messaging, are always counted in the bandwidth calculations.
A flag that indicates whether answered calls are allowed even if insufficient bandwidth is present. This can occur in scenarios in which the resource reservation only takes place when the destination answers (when the SDP offer is included in the SIP 200 OK response).
This option, when set, eliminates the possibility of a situation in which a bandwidth limitation could prevent the media stream from being connected after a subscriber has already answered a call. Occasional degradation in speech quality may occur due to temporary overbooking of a bandwidth limited link when this option is enabled.
A flag that indicates whether Lotus Sametime Unified Telephony should allow a video call to proceed as an audio-only call in case there is not enough bandwidth for the video stream. This flag is only applicable for CAC Policies which include the video traffic type.
One CAC Policy can only be related to one CAC Group. A CAC Group, on the other hand, can be related to multiple Policies as long as the same traffic type is not used
by more than one policy.
One CAC Policy for audio, another CAC Policy for fax (OK)
or one CAC Policy for both audio and fax, another CAC Policy for video (OK).
But not one CAC Policy for audio and video, another CAC Policy for fax and video (not allowed because video is defined twice).
With the introduction of the Video traffic type, there can be up to 3 CAC Policies assigned to the same CAC Group (one policy for Voice, one for FAx and one for Video).
Figure 2. CAC Policy Relationships to CAC Groups
Lotus Sametime Unified Telephony supports the provisioning of up to 6000 CAC Policies; this limit includes group-to-group CAC Policies, which are described in the following section.
When a new CAC policy is provisioned, Lotus Sametime Unified Telephony does not apply the policy to any existing calls. Only new calls after the policy has been provisioned are monitored by Lotus Sametime Unified Telephony.
In case of bandwidth-based CAC Policies, the Telephony Control Server behavior is as follows:
- For new calls, the Telephony Control Server calculates the required bandwidth (BWREQUIRED) based on the worst-case codec in the call request. The Telephony Control Server then adds the required bandwidth of the call to the currently used bandwidth (BWUSED) and compares it to the limit provisioned for the CAC Policy (BWLIMIT). If the limit is not reached (BWUSED + BWREQUIRED =< BWLIMIT), the call is allowed and the required bandwidth is reserved (BWUSED = BWUSED + BWREQUIRED).
- After the call is established, the Telephony Control Server calculates the actual bandwidth (BWACTUAL) used by the call based on the negotiated codec in the call answer. The Lotus Sametime Unified Telephony server then updates the currently used bandwidth with the actual bandwidth used by the call (BWUSED = BWUSED + [BWACTUAL - BWREQUIRED]).
- When the call is disconnected, the Telephony Control Server releases the actual bandwidth (BWACTUAL) used by the call (BWUSED = BWUSED - BWACTUAL).
System Specific Information
The current CAC/bandwidth management solution performs bandwidth reservations based on the endpoint's signaling address, as opposed to the media address. This is because the resource reservation must be done before the called party starts alerting and because the Telephony Control Server does not know the media address for the called party until it answers.
The signaling address and the media address usually match. One exception may be in scenarios in which there is a non-transparent proxy — for example, another Telephony Control Server — in the way. In these scenarios, the EPs (Endpoints) behind the proxy are considered to be in a different domain, and this feature only performs the reservation up to the proxy.
For instance, consider the network topology shown in figure below, in which Telephony Control Server A serves Branch Office 1, and Telephony Control Server B serves Branch Office 2. In this case, the bandwidth management for the bottleneck link to Branch Office 1(L1) is performed by Telephony Control Server A. The bandwidth management for the bottleneck link to Branch Office 2 (L2), on the other hand, is performed by Telephony Control Server B.
Figure 3. Endpoints Behind Another Telephony Control Server
Parent topic: CAC (Call Admission Control)