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New Member

Questions about Gateways and VOIP

First~~~ please do not be mad with me.since my questions sound some how stupid.

1. We use gateway between PSTN and IP network. But in PSTN, the voice is analogue. But in IP network, the data is digital. Which role does gateway act? The D/A converter, beside some necessary protocol translation?

2. What is the difference of PSTN and ISDN? I think there is no requirement of gateway between the connection of ISDN and IP network.

3. In PSTN, the signaling protocol is SS7. In IP network, the signaling protocol is TCP for data transmission, and SIP/H.323 for voice transmission such as VOIP. What is the signaling protocol for ISDN?

4. When we talk each other in MSN or ICQ,by microphone and speaker, which protocol in use in this case? still H.323? I do not think so. I guess still TCP?!

thanks for your answer~~~~


Re: Questions about Gateways and VOIP

Hi lily dont worry we r here to learn!

1. PSTN and IP

A gateway is the point at which a circuit-switched call is encoded and repackaged into IP packets.

For that we use DSP (Digital Signal Processor)

The Gateway converts media provided in one type of network to the format required for another type of network. For example, a Gateway could terminate bearer channels from a switched circuit network (i.e., DS0s) and media streams from a packet network (e.g., RTP streams in an IP network). This gateway may be capable of processing audio, video and T.120 alone or in any combination, and is capable of full duplex media translations. The Gateway may also play audio/video messages and performs other IVR functions, or may perform media conferencing.

In VoIP, the digital signal processor (DSP) segments the voice signal into frames and stores them in voice packets. These voice packets are transported using IP in compliance with one of the specifications for transmitting multimedia (voice, video, fax and data) across a network: H.323 (ITU), MGCP (level 3,Bellcore, Cisco, Nortel), MEGACO/H.GCP (IETF), SIP (IETF), T.38 (ITU), SIGTRAN (IETF), Skinny (Cisco) etc.

I always think in Analog (FXO, FXS) and Digital T1, E1, E&M, etc) and IP packets.

2. PSTN = Public Switch Telephony Network

The worldwide voice telephone network. Once only an analog system, the heart of most telephone networks today is all digital.

ISDN (Integrated Services Digital Network) is an all digital communications line that allows for the transmission of voice, data, video and graphics, at very high speeds, over standard communication lines. ISDN provides a single, common interface with which to access digital communications services that are required by varying devices, while remaining transparent to the user

Yes, you need a GW for the conversion.

3. ISDN for voice transmission Q921/Q931 (Layer 2 and Layer 3)

4. I believe now MSN uses SIP and SIMPLE.



New Member

Re: Questions about Gateways and VOIP

thanks very much for your answers in such a detail.

but I am still a little bit confused about the difference of POTS/PSTN/ISDN

my understanding is that POTS is totally analog and circuit switched. ISDN is totally digital and packet switched.

how about PSTN, digital and packet swtiched????

One difference of them is the speed. from POTS to ISDN, the data rate is faster and faster. right?!

thanks a lot!

New Member

Re: Questions about Gateways and VOIP

by the way, the DSP resides in Gateway???

i suppose it resides in the end device-telephones

New Member

Re: Questions about Gateways and VOIP


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