Application Level Gateway (ALG):
Application Level Gateways (ALGs) are application specific translation agents that allow an application (like VOIP) on a host in one address realm to connect to its counterpart running on a host in different realm transparently. An ALG may interact with NAT to set up state, use NAT state information, modify application specific payload and perform whatever else is necessary to get the application running across disparate address realms.

Call Processor:
component that sets up and monitors the state of calls, and provides phone number translation, user authorization, and coordination with media gateways.

Codec:
coder/decoder, which converts analog voice into digital data and back again, and may also compress and decompress the data for more efficient transmission.

Firewall Control Proxy:
component that controls a firewall’s handling of a call. The firewall control proxy can instruct the firewall to open specific ports that are needed by a call, and direct the firewall to close these ports at call termination.

H.323:
The International Telecommunications Union (ITU) standard for packet-switched network voice and video calling and signaling.

Jitter:
non-uniform delays that can cause packets to arrive and be processed out of sequence

Latency:
time delay in processing voice packets.

Media Gateway:
the interface between circuit switched networks and IP network. Media gateways handle analog/digital conversion, call origination and reception, and quality improvement functions such as compression or echo cancellation.

Media Gateway Control Protocol
common protocol used with media gateways to provide network management and control functions.

PSTN
the public switched telephone network.

QoS – Quality of Service
A network property that specifies a guaranteed throughput level.

Session Initiation Protocol (SIP)
A standard for voice and video over a packet switched network developed by IETF.

Sniffer
A network monitoring tool, usually a software tool running on a PC.