O Preventing Signalingbased Denial of Service

The key to solving QoS issues like latency and bandwidth congestion is speed; thus every phase of network traversal must be completed quickly in VoIP, and the latency often associated with tasks in data networks cannot be tolerated. Chief among these latency and delay variation (jitter) producers are various security measures, most notably Network Address Translation (NAT) as implemented by firewalls and traffic encryption/ decryption. Inserting traditional firewall and encryption products into a VoIP network is not feasible, particularly when VoIP is integrated into preexisting data networks where QoS is not a standard feature. These and other security architecture components, such as intrusion detection and prevention systems, must be specialized and adapted to support the new, fast world of VoIP.

Not only does VoIP require higher performance than most data systems, but also availability is a central issue, and critical services such as Emergency 911 (911 in North America; 112, 999, or other numbers internationally) must also be accommodated. Conventional telephones operate on 48 volts supplied by the telephone line itself. This allows home telephones to continue to work even during a power failure. Office PBXs typically have backup power systems in place for this scenario. These backup systems will continue to be required with VoIP but in many cases will need to be expanded. A careful assessment must be conducted to ensure that sufficient backup power is available for the office VoIP switch, as well as each desktop instrument. To help with this task, many modern switches now support Power over Ethernet (PoE) technology, allowing IP phones to take their needed power directly from the Ethernet lines. With such a configuration, backup power only needs to be provided for the PoE-enabled switch.

Thus, in addition to the other signaling attack countermeasures introduced previously, you should take other general availability guidelines into careful consideration.

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