Tips on Processing Basics of VOIP

A/D and D/A conversion is achieved by add on cards called the sound card. (ADC and DAC). Voice occupies 4Khz of bandwidth space. To convert this into digital it has be sampled at regular intervals of time. The corresponding electric voltages values during the samples are converted into binary numbers (0’s and 1’s). The technical requirement for this process would be a sampling frequency of 8Khz according to Nyquist criteria.

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If we take stereo into consideration as an extreme then the sound card on the computer allows you to convert a band of frequencies of 22050 Hz with a sampling frequency of 44100Hz. If you are using a 16 bit sample output then the throughput is 2 * 44100 = 88200 bytes/second. In case of a stereo system it doubles giving 176400 bytes. Only voice would not require such a high throughput. The throughput will be around 16000 bytes/second. For a 8 bit sample it will be 8000 bytes/second or 8000 * 8(1byte is 8bits) = 64,000 bits/sec which is that of a typical digital telephone line.

Coding Techniques

The digital data is to be converted into a format that can be quickly transmitted. The respective coding format and specifications are now recognized industry standards.

64 Kbps for PCM, Pulse Code Modulation, Standard ITU-T G.711
32 kbps for ADPCM, Adaptive differential PCM, Standard ITU-T G.726
LD-CELP, Standard ITU-T G.728
CS-ACELP, Standard ITU-T G.729 and G.729a
6.3kbps for MP-MLQ, Standard ITU-T G.723.1 (True speech)
5.3kbps for ACELP, Standard ITU-T G.723.1 (True speech)
2.5 kbps for LPC-10

The entire above protocols are implemented using codecs (coders and decoders).

Encryption

Eavesdropping on the internet is not new and if you need to encrypt all voice conversations from end-point to end-point then vendor specific applications are available. Also encryption provided by Virtual Private Networks is also welcome. Secure Real time Protocol using SSL techniques is being fine tuned for VoIP.

Protocols

VoIP uses two types of protocols

Signaling protocols
Transport Protocols

Signaling protocols

SIP- Sessions Initiation Protocol is developed by IETF and is primarily used for Voice over IP calls. It is a text based protocol based on HTTP- Hyper Text Transfer Protocol and MIME-Multipurpose Internet Mail Extensions. SIP can be used for video or any other media type or instant messaging. It is designed for real time transmission. It depends on SDP-Session Description Protocol and RTP- Real time Transport Protocol for actual transport.
H.323 provided a mechanism for multimedia applications over LAN and is rapidly evolving to the growing needs of VoIP. It is a relatively early model for basic call and supplementary services. It is very comprehensive and supports voice, video, data, application sharing, and whiteboarding. H.323 defines various elements for conferencing like media gateways for the conversion to packets and gatekeepers for call control and multipoint control units
MGCP- Media Gateway Control Protocol is also known as MEGACO-MEdia GAteway COntroller. MGCP is the original protocol for IP telephony. This eveolved into MEGACO. The advantages is that IP phones designed using these protocols are cheaper in cost that SIP or H.323. This uses softswitche that resemble circuit switching of PSTN. It is a text based protocol and uses “terminations” and “contexts”.

Transport Protocols

Real Time Transport Protocol supports real time transmission of video and voice. RTP rides on top of UDP. It includes a timestamp and synchronization in its packets in its information header so the proper reassembly can take place at the receiving end.
RSVP- Reservation Protocol signals the router to reserve bandwidth for real time transmission. This is advantages in the sense of constant increase of voice and video traffic on the internet

There are many voip tutorial and voice ip voip tutorials on the net which tell you what is voip and give you more detailed information on the basics of voip and the various processes. You could look at the benefits and products of voip for a better understanding.

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