5.10.1 Boundary Traversal

Traditional telephony calls are transmitted over a dedicated network. However, IP-based systems carry all sorts of data traffic, with which the voice traffic has to compete for carriage, and they are guarded by various devices to protect customers' valuable data, such as firewalls and network address translators (NATs).

Any VoIP call could have to negotiate an infrastructure that is constructed with zones of different administrative policy and control, separated by firewalls or similar devices. At the boundary points between these zones, it may be necessary to map IP addressing ranges and/or IP header parameters (e.g. type of service (ToS) field values) for both media and signalling packets. Failure to correctly map and police IP telephony traffic results in asymmetric transmission, inability to complete call signalling, and poor end-to-end performance due to the absence of appropriate packet admission control criteria. Crossing boundaries therefore has to address:

• firewalls;

• network address translation;

• quality of service;

• border gateways;

• security.

A common theme to these problems is the need to be able to route and map packets from one IP network to another within a profile determined by some call control messages. Without this, the real-time characteristics of the application cannot be ensured, except by resorting to massive over-provision of bandwidth which may not be sustainable.

The two key pieces are the entity that can...