| VoIP(Voice over IP) is a new technique to transmit voice asdata packets over the internet with Internet Protocol.Its basicprinciple is: compressing and coding voice with some codectechnique such as G.723,G.729,then transmitting the digital dataover the internet ,after arriving at the endpoint , uncompressedand turned back to analog audio signals.VoIP is a combination of telephone communication andcomputer technique. Compared with the traditional PSTN, theadvantage of VoIP is that it can provide more flexible but low-costservice for the customers.At first, this article introduces the development of VoIP andgives a profile of VoIP. Then it tells the key technology of VoIPincluding signaling to create the session, codec technology ofvoice, real-time transmitting protocol and quality of service.The signaling technique is the core technique because itassures the setup of call. There are mainly two kinds of signalingtechnique: H.323 and SIP.Then this article introduces some concept and principle aboutH.323 and SIP. It analyzes and compares them.H.323 works in acentralized mode, it fits for the traditional telephone networks,especially for the "Phone to Phone" mode. However,SIP works ina distribute mode and it put complexity to the endpoint. Theadvantage of SIP is simple,flexible and it has tight relationshipwith the current internet.Gateway is an important device in VoIP system.It is thebridge to connect the PSTN to internet.It is responsible fortranslation between the different signal and voice in PSTN andinternet.Gateway consists of three parts:Media Gateway,SignalGateway and Media Gateway Controller.Media Gateway performsthe media translation and connects the PSTN and internet.SignalGateway performs singal translation.Media Gateway Controllermanages and controls the Media Gateway and they communicatewith MGCP or H.248 protocol.At the end, this article introduces the implementation of amedia gateway ISAMV.It consists of two boards:POLT andIVPS.POLT is responsible for the access of phones.POLT consists of four modules:ZIPS,LAPV5,PacketHanlerand PotsProxy.ZIPS is the signal interface,and it communicateswith IVPS by XLES signal.It uses the service of LAPV5 to sendand receive signals.Pots Proxy module manages the voice chips Vinetic to controlthe phone.It get the state and events of the phone,then reports toZIPS.It also operates the phone according to the commands ofZIPS.It gets RTP packets of the phone,then sends to PacketHanlermodule,and puts the RTP packet that sent by PacketHanler toVinetic chips.PacketHanler module receives and sends IP packets from theethernet.It defines an kind of internal packets to perform thetranslation between voice packets , signal packets and IPpackets.And it defines an route table to map IP packets tocalls .When it receives an IP packet from internet,it looks for theroute talbe to see which call this packet belongs to,then adds theinternal packet header and passes it to the related moduleaccording to its type and the router talbe item .It gets signalpackets from LAPV5 message queues and voice packets by usingthe interface that PotsProxy provides.Then it checks the routetable,and fill the related IP packet header and sent it to theinternet.It runs a HPT to perform its work.HPT rus once per 1 ms. |
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