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ip precedence and voip

Hi everybody.

Tcp/ip was standardized around 1982, and we started using tcp/ip until very recently. But here is my question why did it take us that long to use tcp/ip for voice?

   I heard many reasons, the big one being we could not use qos then. But ip header has always " ip precedence"  so that reason does not hold any water

Another reason I have heard our frame relay switch can not support ip precedence i.e they can not provide qos to transit traffic based on ip precedence.

But have we not been using voice over frame relay for a while ? Now back to my original question we always had " ip precedence" in ip header but yet it took us decades to start using it for qos. What are some limitations that prevented us from using Ip precedence" for qos? .

thanks and have a great day

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Hall of Fame Master

ip precedence and voip

Sarah,

I am not sure what you mean we have not been using tcp/ip for voice.  TCP has been used for voice signaling since its inception, the reason we still dont use TCP for RTP (Voice stream packets) is because the nature of how TCP works. If TCP packets are not delivered they are resent (read on TCP windowing) which for voice is BAD as it would cause jitter (voice packets arriving out of order). Voice is real time application, which is better off with loss of some packets than having the resent out of order hence UDP is used for RTP. Both TCP and UDP are using IP protocol (think IP address). IP precedence is not a function of one or the other, but a function of IP protocol itself.

HTH,

Chris

Hall of Fame Master

Re: ip precedence and voip

Traditionally , We used to have separate network for voice and separate network for data.  For example PSTN is one example used for voice communication.  .At that time, two decades ago,were we using tcp/ip for voice communication? I guess not.

No, TCP is IP network, PSTN as it has been known until SIP trunks became available was TDM signaling over T1/E1 channalazed lines, think PCM encoding (64 kbps per channel), nothing to do with IP networking.

IP precedence is still used today for QoS, but DSCP is more prelevant (addition to 3 more bits to IP precedence), here are couple of good docs on that topic:

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

http://blogs.manageengine.com/netflowanalyzer/2012/04/24/understanding-ip-precedence-tos-dscp/

HTH,

Chris

4 REPLIES 4
Hall of Fame Master

ip precedence and voip

Sarah,

I am not sure what you mean we have not been using tcp/ip for voice.  TCP has been used for voice signaling since its inception, the reason we still dont use TCP for RTP (Voice stream packets) is because the nature of how TCP works. If TCP packets are not delivered they are resent (read on TCP windowing) which for voice is BAD as it would cause jitter (voice packets arriving out of order). Voice is real time application, which is better off with loss of some packets than having the resent out of order hence UDP is used for RTP. Both TCP and UDP are using IP protocol (think IP address). IP precedence is not a function of one or the other, but a function of IP protocol itself.

HTH,

Chris

Contributor

Re: ip precedence and voip

Hi Chris.

Traditionally , We used to have separate network for voice and separate network for data.  For example PSTN is one example used for voice communication.  .At that time, two decades ago,were we using tcp/ip for voice communication? I guess not.

So what were some limitations  that prevented us from using tcp/ip for voice communication two decades ago the way we are using tcp/ip for voice today.? The reason I mentioned ip precedence is to  give an idea how we used this field in ip haeder to provide qos to our voice traffic. We could have done this decades ago because ip precedence was and still part of ip header.

thanks Chris. Please ask if you want me to further explain my question

Hall of Fame Master

Re: ip precedence and voip

Traditionally , We used to have separate network for voice and separate network for data.  For example PSTN is one example used for voice communication.  .At that time, two decades ago,were we using tcp/ip for voice communication? I guess not.

No, TCP is IP network, PSTN as it has been known until SIP trunks became available was TDM signaling over T1/E1 channalazed lines, think PCM encoding (64 kbps per channel), nothing to do with IP networking.

IP precedence is still used today for QoS, but DSCP is more prelevant (addition to 3 more bits to IP precedence), here are couple of good docs on that topic:

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

http://blogs.manageengine.com/netflowanalyzer/2012/04/24/understanding-ip-precedence-tos-dscp/

HTH,

Chris

Contributor

Re: ip precedence and voip

Thanks Chris

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