8. Codec - Coder/Decoder

What is the function of Codec?

• Voice transmission is in analog signal, whereas the data network is digital.
  
• So codec is help to convert analog signal into digital form and transport it to the network. There are many standards to sample an analog signal and the process is done through PCM(Pulse Code Modulation or variations.
• How the PCM works? Firstly the analog signal is input to a low pass filter. After that, the filter signal is sampled using one of the codec. Next the waveform is converted into a discrete digital signal. This sample is than represented by a code that indicate the amplitude of the waveform at the instant the sample was taken.

The following show an example of PCM:


Types of codec

• G.711 - It is a 64kps PCM voice encoding method that encode voice that is ready in PCM format for digital voice delivery in the public network or through PBX.
• G.726 - Describe ADPCM coding at 40,32,24,and 16kpbs. It allow user to interchange ADPCM voice between packet voice and public phone or PBX.
• G.728 - A 16kpbs low-delay variation of CELP voice compression.
• G.729 - A CELP compression that enale voice to be coded into 8kpbs streams. There are two types of variation of this standard(A and B) that are differ in computational complexity and both also provide good speech quality.
• G.723.1 - It compresses speech or other audio signal components of multimedia service at low bit rate, as part of the overall H.324 Standards. There are two bit rates associated with this coder : 5.3 and 6.3kpbs.
  

7. Media Gateway Control Protocol (MGCP)

What is MGCP?

• It is protocol used within a VoIP system

• It was developed to address the demands of carrier-based IP telephone network.
• Also a complementary protocol for H.323 and SIP, which was designed as an internal protocol between Media Gateway Controller and Media Gateway.

What does MGCP consists of?

• It comprised o a Call Agent, one MG (media gateway) which performs the conversion of media signals between circuits and packets, and one SG (signaling gateway) when connected to the PSTN (Public Switched Telephone Network).
  
• MGCP is widely used between elements of a decomposed multimedia gateway. The gateway has a Call Agent, which is comprised of the call control "intelligence" and a media gateway boasting the media functions, for example conversion from TDM voice to Voice over IP.

MGCP Connection

MGCP connection can be point to point or point to multipoint. Once there is a connection, data transfer takes place between the endpoints. In multipoint connections, it is set up between endpoint and a multipoint session. In multipoint connection, it can be created over various types of bearer network.

The Architecture of MGCP

• The architecture does not involved in the frustrating work of encoding ,decoding and transfer of voice signals.
• The MGCP call agent works as a software switch for the VoIP network,.
• It does noting more than simply direct the media gateway and media signalling gateway which perform all the work.
• The main task of Call Agent is to implement numerous possibilities in the protocol.
• In every architecture, each and every components features a transaction ID, gets an anknowledgement and receives a responses.

The MGCP Packets

• MGCP packets are generally found wrapped in UDP port 2427.
  
• An MGCP packer can either be a command or a response.
  
• Commands start with a four-letter verb while "responses" start with a three number response code.