White Paper - QoS Bandwidth Management - White Paper
Media Policing (continued)
The policing mechanism restricts data and packet rates based on the negotiated codec type with excessive data/packets being discarded to control the consumption of network resources. This is an extreme, but effective, action where the discarding of data will substantially disrupt the media of any call that exceeds its negotiated bandwidth.
Additionally, the 1460's policing mechanism will police packet sizes to protect against some forms of malicious attack. For example, if the agreed codec type is G.711 with a 10 ms sample, larger packets, such as G.711 with a 20 ms sample, will be discarded.
The 1460's policing policy is independently set for each call from each subscriber, even if the subscribers are behind the same firewall or using the same IP-PABX.
Anti-Tromboning
If a group of subscribers have their own private network, it is often desirable to keep bandwidth-intensive media flows within the subscribers' network, yet maintain signalling control of the call by the Service Provider. Enabling only the media to flow locally is termed 'anti-tromboning' (it is also refered to as 'media release' and 'anti-hairpinning'). IP-PABX installations naturally achieve this, but the media traffic of IP-Centrex subscribers is normally 'tromboned' from the private network, through the access network and back to the private network, unless the session controller uses precautionary measures to avoid this.
The 1460 maintains a 'context' for all registered IP-Centrex subscribers that might require the anti-tromboning feature. When a new call is established, the calling and called party's registration contexts are examined to determine if the call's media should be released. If the 1460 detects that both parties reside on the same private network, media release is enabled, causing the media to flow directly between the terminals and freeing up capacity on the Service Provider's access network.
Figure 3 – Anti-Tromboning in Operation
Conclusion
Session controllers occupy a unique position in the network architecture enabling them to protect the quality of communications. The carrier-class 1460 session controller, with its advanced hardware accelerated architecture, performs the three aspects of effective bandwidth management; Session Admission Control, policing and anti-tromboning, in a coordinated and effective way with little impact on total performance.
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IPsec and VoIP
