VoIP general concepts

By | 03/11/2014

Voice Gateway Protocols

MGCP – Media Gateway Control Protocol – Standard protocol based on a Master (Call Manager, CUCM) and a Slave (Voice ports of the gateway). Call manager decides where the call should be forwarded.

H.323 – Standard and distributed protocol. The gateway decides where the call should be forwarded.

H.323 is composed by H.225 used for signaling and H.245 for the media.
H.323 version 1 had a limitation called “slow connect”. Version 2 resolved it with the “fast start/connect”

 

SIP – Standard and distributed protocol. The gateway decides where the call should be forwarded.

 

Codecs

NIQUIST – defines 64kbps required to digitalize the voice

 G.711 – codec without compression (64kbps of payload) – 80kbps

  • alaw – used in Europe
  • ulaw – used in US and Japan

both sides of the communication should use the same xlaw version.

G.722 – more quality (16bits) but same bw than G711 (64kbps) because it uses compression. – 80kbps

G.729 – 8kbps de payload  – 24kbps

Bandwidth calculation: Codec Payload + Headers (usually 16kbps)

Headers includes L2 (Ethernet), L3 (IP), L4 (UDP), L5-7 (RTP, RTCP)

DSP – Digital Signal Processor – Used to transcode the audio

PSTN

PRI – PRimary – Channels from 0 to 31, but channels 0 and 15 are used for signaling. 30 channels for data/voice. It requires a clock to synchronize both sides of the link.