17-09-2012, 01:20 PM
An Introduction to Voice over IP
VoIP_Introduction200310.ppt (Size: 475 KB / Downloads: 33)
What is VoIP?
VoIP (Voice over Internet Protocol), sometimes referred to as Internet telephony, is a method of digitizing voice, encapsulating the digitized voice into packets and transmitting those packets over a packet switched IP network.
Voice over IP - the basics
Most implementations use H.323 protocol
Same protocol that is used for IP video.
Uses TCP for call setup
Traffic is actually carried on RTP (Real Time Protocol) which runs on top of UDP.
H.323 Multimedia Standard
H.225 RAS - Registration, Admission, Status
Q.931 - Call Signaling (Setup & Termination)
H.245 - Call Control (Preferences, Flow Control, etc.)
Lots of G.7XX CODECS for audio
SIP – Session Initialization Protocol
Covered in next presentation
Codec ITU G.711
G.711 is the international standard for encoding telephone audio on a 64 kbps channel. It is a pulse code modulation (PCM) scheme operating at a 8 kHz sample rate, with 8 bits per sample, fully meeting ITU-T recommendations. The module is designed and tested on the TI TMS320C54x platform but can be ported to other DSP and RISC platforms, as well as MS Windows.
Codec ITU G.723.1
G.723.1 is a speech compression algorithm standardized by ITU. G.723.1 has dual coding rates at 5.3 and 6.3 kbps. The vocoders process signals with 30 ms frames and have a 7.5 ms look-ahead and low distortion while passing DTMF tones through. The input/output of this algorithm is 16 bit linear PCM samples.
Middle bit rate G.723.1 vocoder delivers one of the highest compression ratios of any of the current ITU standards without compromising speech quality. This vocoder can perform full duplex compression and decompression functions for multimedia, visual telephony, wireless telephony, and videoconferencing products.
Issues with Firewalls and NAT
H.323 requires the use of specific static ports for RAS messages, and a number of dynamic ports for RTP.
SIP has one port (5060) for SIP messages, as well as dynamic ports for RTP.
For these protocols to pass the firewall, the specific static and the range of dynamic ports must be opened for all traffic.