Lotus Sametime Unified Telephonys's CAC (Call Admission Control), also known as IRM (Internal Resource Management) is the mechanism by which new calls may be refused by Lotus Sametime Unified Telephony, if the IP network does not have the capacity (bandwidth) to handle the call with a acceptable quality of service.
In the traditional telephony world, a circuit (line or trunk) is either busy or idle. A circuit is dedicated to a single call and so voice quality is predictable and assured. If all available circuits are busy, new call attempts are rejected.
In contrast, IP links are shared resources and hence can be "overbooked". If too many calls are routed over a bandwidth-limited LAN or WAN link, this doesn't result in call blockage, as in the circuit-switched world, but in reduced quality due to delayed or lost media packets.
In an enterprise network congestion may occur
on access links between the enterprise core network and the subnets serving its branch offices
on dedicated access links between branch offices (that may exist in addition to other links present in the enterprise network)
at the aggregation layer between branch office LANs and the backbone WAN.
This can happen when the total bandwidth capacity of the related access links is overbooked to an extent that forces the access routers to drop even high-priority real-time media packets. The result is a poor quality connection for all multimedia calls that are routed over these access links.
Consider the following simple scenario:
Figure 1. Branch Office with Bandwidth-Limited link
A branch office is connected to the core network (WAN) via an access router. The LAN within the branch office is over-provisioned and has the capacity to guarantee good QoS for real time media. However, the bandwidth for access to the core network is limited, and the access router may start dropping media packets if the capacity is exceeded.
Real-time media calls should not be routed over networks that cannot guarantee an acceptable QoS (Quality of Service). CAC provides the means to prevent these poor-quality connections from being established. A new call (of type T
= voice, video or fax) can only be established, if the maximum allowed number of calls and/or the bandwidth limit (for type T
) hasn't been reached on all involved access links.
When used in conjunction with effective VLAN and packet prioritization schemes to segregate call media and and data traffic, CAC provides an effective means to assure good quality.
Proper segregation and prioritization of voice and data traffic is important because CAC only manages calls, and does not see or control the amount of other data traffic on the network.
The network planner must determine how much bandwidth is required between the sites, and how much of that bandwidth can be used for voice, video and fax traffic.
It is also necessary for the real-time media packets to be correctly classified so that the network routers can provide the appropriate priority processing through their queues.
If a call cannot be established due to missing resources, it may be either blocked or rerouted.
Figure 2. Rerouting Based On Insufficient Bandwidth
CAC Administration in Lotus Sametime Unified Telephony
CAC can be administrated via Telephony Control Server Assistant. In simplest terms, the administrator can
- enable resource management, set the parameters used for bandwidth calculation and enable "high bandwidth usage" alarms
- create groups of subscribers and gateways sharing the same bottleneck links
These CAC Groups may be defined based on DNs (Directory Numbers), IP addresses or Location Domains.
- define media-stream related policies to be applied to
calls traversing the bottleneck link between a CAC Group and the core network (Regular CAC Policies)
calls traversing a dedicated bottleneck link between two CAC Groups (Group-to-Group CAC policies)
These CAC Policies may also restrict the allowed codecs.
For scenarios involving two or more Lotus Sametime Unified Telephony systems, each system is responsible for the bandwidth management of its own endpoints. The only requirement is that there cannot be endpoints that belong to two (or more) Lotus Sametime Unified Telephony systems served by the same bandwidth-limited link-for example, in the same branch location. All endpoints served by a particular bandwidth-limited link must be configured in the same Lotus Sametime Unified Telephony system.
Supported Network Topologies - Star NetworkParent topic: User Telephony
A star network is a LAN in which all nodes are directly connected to one central node (hub or a switch).
Supported Network Topologies - Tree network
The tree network topology integrates multiple star topologies together onto a bus.
Parent CAC (Call Admission Control) Group
A parent CAC (Call Admission Control) Group is required if a tree network topology is present. Its purpose is to establish relationships with higher- and subordinate-level CAC Groups. A parent CAC Group is just like any other CAC Group, except that it is defined based on CAC Groups instead of IP addresses, subnets, or DNs (Directory Numbers).
Supported Network Topologies - Mesh Network
A mesh network consists of several nodes that are all connected to each other.
An IRM (Internal Resource Manager) function within Lotus Sametime Unified Telephony's UCE (Universal Call Engine) integrates bandwidth management with call processing in order to provide robust call handling, such as the rerouting of a call via the PSTN (Public Switched Telephone Network) when there is insufficient bandwidth in the enterprise network to carry the call, based on bandwidth availability.
CAC Traffic Data in Switch
The measurements are written into files. One file is created for every collection interval that contains all the CAC data for that interval. The file names indicate that the data is for CACs and have a timestamp indicating the time and interval length for the collected data.The file naming is consistent with the existing OMM file naming for other measurement groups, such as Trunk Group and PRI data. Logging for CAC is done using Time Base Logging.
CAC Group Measurements
The CAC measurements are stored in a log file for post-processing. Although Lotus Sametime Unified Telephony does not offer a mechanism to read these measurements in real time, Lotus Sametime Unified Telephony Assistant allows the administrator to view the information in the stored log files in a table format.
CAC Record Format
Traffic measurements are collected and recorded by the Operational Measurements Manager (OMM) in the form of CSV files. Using FTP (actually Secure FTP) these files may be downloaded to any Telco platform concerned with the collection of performance data. The files may be transferred in either binary or ASCII format.
Bandwidth Calculation Settings
Lotus Sametime Unified Telephony uses the bandwidth control parameters specified in Telephony Control Server Assistant to perform bandwidth calculations for CAC.
Bandwidth Calculation Factors
The IRM (internal resource manager) calculates only the IP bandwidth required to transport the media payload. It does not take in consideration the overhead added by the Layer 2 (L2) transport protocol, for example, Ethernet, ATM, Frame Relay, and so on. The IRM considers the transport of UDP media packets over Internet Protocol version 4 (IPv4) or IPv6. When IPv6 is present, the IRM is able to consider the additional overhead for the IPv6 header when performing bandwidth calculations.
Bandwidth Requirements for Audio Codecs
Bandwidth requirements for audio codecs vary depending on the codec type, the link speed, and whether payload encryption is used. Use of codecs that provide compression introduces a trade-off of speech quality against additional capacity.
Bandwidth Fax Considerations
The internal CAC solution in Lotus Sametime Unified Telephony allows the system administrator to limit the voice codecs which can be used over a specific link.
Video Considerations - Lotus Sametime Unified Telephony
Other than the video codec, all other characteristics for a video call - for example picture size, frame rate, and the like - are not negotiated via the SDP offer and answer mechanism. In the SDP offer and answer negotiation, each party only informs the other what they can receive, not what they are going to transmit.
Enhanced Video Call Support
With legacy system, the Telephony Control Server provides specific video service support allowing video calls to be identified and handled separately if required in the server. With this feature the Lotus Sametime Unified Telephony will be able to enable users for Video support and allow routing of calls based on the video media type.
Establishing Codec Restrictions
The internal CAC solution in Lotus Sametime Unified Telephony restricts the voice codecs for calls between SIP/SIP-Q endpoints. Since legacy system this feature is also applicable for MGCP connections to the Media Server for announcements, tones or conference.
Overriding Codec Restrictions
When codec restrictions are in place, they are applicable to all calls on the bandwidth-limited link. However, the administrator can override, on a per-endpoint and/or per-subscriber basis, the codec restrictions the CAC policy imposes.
Dynamic Handling of Link Failures
Lotus Sametime Unified Telephony permits optional provisioning of primary and secondary link capacities for each CAC (Call Admission Control) Policy. The ability permits the support of an access router that can switch over to a backup link (with a different bandwidth capacity) than the primary link, if the primary link fails.
CAC Administration from Telephony Control Server Assistant
The CAC/Resource Management feature can be administrated from the Telephony Control Server Assistant. It is enabled and configured at the switch level. Codec restrictions defined in CAC Policies can be overridden at the endpoint or subscriber level.
CAC (Call Admission Control) Group
A CAC (Call Admission Control) Group represents the group of endpoints being served by each bandwidth-limited link which needs to be monitored. A CAC group is required regardless of the type of network topology present. It represents the group of endpoints being served by the bandwidth-limited link which needs to be monitored. A group is the entity to which the CAC policies are applied.
CAC (Call Admission Control) Policies
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).
Group-To-Group CAC (Call Admission Control) Policies
A group-to-group CAC policy is required if a mesh network topology is present. This policy is used to represent the properties of a dedicated bandwidth-limited link between two CAC groups—for example, between two branch offices that have a dedicated link to one another.
CAC (Call Admission Control) Rerouting
This feature provides the rerouting of calls to SIP gateways, SIP-Q gateways or SIP subscribers if Lotus Sametime Unified Telephony receives a SIP response code indicating a bandwidth restriction.
Rerouting a Call to a SIP Subscriber
One of the benefits of the integrated CAC (Call Admission Control) solution is Lotus Sametime Unified Telephony's ability to provide rerouting via the PSTN (Public Switched Telephone Network) in case there is not enough bandwidth in the applicable bandwidth-limited link, whether this link is between branch offices or from the branch office to the WAN.
Subscriber Location Identification