VoIP Gateways and Routers

VoIP gateways and IP routers move RTP voice datagrams through an IP network. VoIP
gateways provide a connection between the VoIP network and the PSTN. These devices
therefore play a key role in the migration path towards VoIP. There are few totally VoIP
phone networks in the world today. It is necessary to connect to the PSTN to place calls to
PSTN users. VoIP gateways must talk the SS7 protocol (remember that protocol from the
PSTN section?). SS7 is used by the VoIP gateway to signal switches in the PSTN when a
phone call is originating from the VoIP network with the callee in the PSTN. VoIP gateways
may also provide conversion between different codecs, which is called “transcoding.”
If a codec other than G.711, let’s say G.729, is used on the VoIP network, the voice data
must be converted to G.711 before being transferred to the PSTN.
In a corporate environment, VoIP gateways can interconnect with traditional PBXs to provide
a migration path and allow for staged VoIP deployments. Gateways are typically very
smart in terms of the number of protocols that they speak. They have to be, to handle the
variety of signaling and data protocols of the VoIP network and PSTN.
By examining the IP packet headers, IP routers make the necessary decisions to move
packets to the next router or hop along the path to the destination. Tracing the route of a
voice packet through the network can be useful for problem identification and diagnosis;
we’ll discuss techniques like this in later chapters. However, router technology itself is wellunderstood
and is not discussed in detail in this .