VoIP sessions require the support of many protocols, including
H.323
SIP
Megaco
MGCP
These protocols, in turn, rely on several other protocols.
VoIP Gateways
Gateways implement the n-way conversation abstraction, for n
> 1.
A gateway performs two main functions:
Interface the phone to the cloud and vice versa.
Talk amongst themselves to implement calls.
Gateway Structures
Gateways comprise several hardware and software subsystems.
Hardware: media, media control, and signaling.
Software: protocols to control hardware, service protocols.
Physically, a gateway is zero or more boxes.
Zero boxes: in the user's phone.
More boxes mean more flexibility and complexity.
Gatekeepers
Gatekeepers are another set of boxes to control the gateways.
Gatekeepers may rely on still more boxes.
Databases, billing systems, security monitors.
Oh My
Gateways are the interface elements between users and the network.
Gatekeepers define and provide services.
Partially by controlling gateways.
The Gateways
Media gateway.
Signaling gateway.
Media gateway controller.
Call Processing Protocols
The principle protocols are
H.323
SIP
Megaco
MGCP
Politics
The two main standards setting bodies are
International Telecommunications Union (ITU).
Internet Engineering Task Force (IETF).
The ITU and IETF are vastly different organizations.
They coordinate, but have almost diametric outlooks.
References
RFC 4566, SDP: Session
Description Protocol by Mark Handley, Van Jacobson, and Colin Perkins,
Network Working Group, Internet Engineering Task Force, July 2006.
