VoIP bandwith one way or two way

For VoIP with G.711, you generally need 64kbps + x in either direction (64+x (up) AND 64+x (down)). That’s because you have to build up 2 separate voice paths (one from subscriber A to subscriber B and another one in the opposite direction).
By the way: if you assume a usual ip network, “x” (in the above calculation) is NOT 16 (add up header sizes of rtp, udp, ip + (eventually) ethernet)! Or did you mean a plus of 16 kbps for signalling? usually you don’t have to count the signalling bandwidth for voip because signalling is done in a totally differenz way than it’s done in isdn. there’s no need for a certain signalling bandwidth. have you heard about the session initation protocol (SIP)? That’s just one possibility to do signalling in voip.

ISDN uses ONE voice path (one so called B-channel) for bidirectional talking, which means that the voice data from A and B are added up and transported in an overlaid manner in the same data stream. That’s a main difference between voip and isdn. in voip, you open up two unidirectional “voice channels” (one from A to B and one vice versa), in isdn you just have ONE channel for both directions.

Hi dmck,
for E1/T1 trunk lines you are right. I just figured it out from the ISDN BRA subscriber line (Uk0) point of view. There you’ve got 2 B-Channels (which in fact are both bidirectionally used) with 64kbps each plus one 16kbps D-Channel for signaling.
That kind of bidirectional voice transmission over one 64 kbps channel is realized by the use of echo cancellation (ITU-T G.165/G.168 I think). Unfortunately I don’t know the english expression for that kind of technique that is used to transmit the voice stream bidirectional A B on just one 64 kbps B-channel (in german it’s called Gleichlageverfahren). But it really works that way, otherwise it would not be possible to have two independent calls on just one ISDN BRA subscriber line (which in fact is possible).
I just didn’t think about E1/T1 because VOIP Beginner wrote about (64+16) kbps. I think with the “+16” he means the bandwidth of the D-Channel that’s used for signaling on a BRA subscriber line.

Best regards

Frank

Hi dmck,

the overhead b/w (dimensioning: kbps) for voice data carried over IP can’t be calculated in general. Amongst others it depends on the codec used and on how many bytes of voice data you carry per packet (this number is not codec-specific, so it can vary even for the same codec).
The number of overhead bytes that are transported with each voice-carrying packet can be calculated if you know the header length of all protocols that are used. RTP header length is minimum 12 bytes, UDP header is 8 Bytes, and so on.

Either way, “16” is not a number that I would say is typical for voip overhead as far as I know.

I had a look for the ISDN standards in the last days but unfortunately I couldn’t find the right passages by the time that I could affort. In fact on copper-wire based ISDN BRA (and as far as I know even on PRA) subscriber lines that I know definetely no FDM is used to provide 2 independent full duplex B-channels. Each subscriber NT adds up his outgoing voice data on the wire and filters out (using echo cancellation) these data in real time to make sure the TE only sees the voice of the opposite subscriber. As far as I found it this method is not necessarily true for all countries, so there might be other solutions around the world.

Best regards

Frank

Hi dmck,
ITU-T G.961, Chapter 5, ECH, explains what I was talking about concerning ISDN BRA bidirectional transmission by overlay.

Best regards

Frank

each call on voip requires 10 to 13 Kbits/s of bandwidth
i guess so

Can you tel me you people, When we use ISDN for voice, Is thier any codec technique involved?