Question

Using multiple VLANs to separate VoIP from data traffic and testing QoS

Asked by: norcotek

We have just completed a network design that is intended to run both data traffic and VoIP traffic from a Call Centre. The backoffice which has the call centre workstations and the VoIP phones (polycom) connect with the Data Centre over an Ethernet fibre link, between the sites we have Cisco 3750 swtiches.

The network was designed so that we have separate VLANS: one exclusively for the VoIP related traffic and others for different data traffic. Only the VoIP phones will use the VoIP VLAN for the SIP messages and the RTP traffic.

We just completed a QoS certification process with a subcontractor that used VoIP call simulators to load the VoIP VLANS. They placed these simulators at each end (Data Center and Backoffice). And they pumped a lot of VoIP calls over the link for several days ane they did QoS measurements.

At the same time, we were asked to load the network with data traffic to try to reach congestion and "stimulate" the QoS setting in the switches to ensure that QoS was being done properly. We did pump a lot of data traffic on the network using data simulators, but of course, this traffic was on the data VLANS (not on the VoIP VLANs).

The result from the QoS tests was the followong:  almost perfect QoS results, no delay, no jitter, etc. Almost too good to be true.

The question I have is this:  Is it possible that even if we were loading the network with data traffic that this traffic was unable to "congest" the VoIP links becuase they are on totally separate VLANS?

The VLANs have been defined at the port level, so from a logical perspective the VoIP traffic should never "mix" with the data traffic.

Shoudl I be suspicious of having such "clean" QoS results?

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Asked On
2009-09-09 at 14:58:29ID24719745
Topics

Voice Over IP

,

IP PBX Systems

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Answers

 

by: lrmoorePosted on 2009-09-09 at 21:06:56ID: 25297346

considering gigabit uplinks between the switches would be the only place that congestion could occur, it is awfully hard to push that much data. Even 100 simultaneous calls would take no more than 6.4Mb of that Gig of bandwidth, while the QoS mechanisms actually work as designed. So yes, you can expect near perfect results on a pure LAN.
it really has little to do with the fact that you are using spearate VLAN's..

 

by: norcotekPosted on 2009-09-11 at 14:57:17ID: 25313777

Thank you for for your comment.  I guess I am still trying to understand a bit on the VLAN issue.

Say for example:
1.- We have one big "physical" Gig pipe between two 3750 swtiches
2.- We have three VLAN's on this pipe: one is for data, the other two are for VoIP only.
3.- Only ONE of the VoIP VLANs has been properly setup to do QoS
4.- We hypotetically use up CLOSE to all the available bandwith with traffic on the data VLAN

Then:
A: If we setup voice traffic using the VoIP VLAN WITH QoS, I assume that the VoIP traffic will be properly marked and have priority over the data traffic and hence we will have good voice quality (hence QoS is doing its job)

B: If we setup voice traffic using the VoIP VLAN WITHOUT QoS, VoIP traffic will NOT be marked, however, will the fact that they are on separate VLANs help allow the VoIP traffic to "travel clean" on the non QoS VoIP only VLAN?  Or will it compete for the overall Gig bandwidth even if VoIP and data are on separate VLANs?

I am not exaclty sure if they way I am asking this question makes sense, perhaps it is my lack of understanding of how VLANS work.  I guess it is also a bit more complex than just thinking about two pipes with different "water pressure" analogy.

I gues I have one more hypotetical scenario:

Say for example:
1.- We have one big "physical" Gig pipe between two 3750 swtiches
2.- We have only one VLAN on this pipe (I guess almost the same as "no VLAN" ?)
3.- We pump a LOT of data traffic using up, say 80% of the bandwith. We setup VoIP traffic (no QoS) and also use up say another 5% of the available "free" bandwith.

Then: will VoIP traffic hove as good quality as though we had QoS enabled?  OR, is it possible that the fact that VoIP and data packets are travelling "mixed up" can still collide and retransmit, etc. and we would have bad VoIP quality?

So to finish off, if we take the previous ONE VLAN only example, and mark the VoIP traffic (at the phone level, and let the swtich just "pass on " QoS policies) would we achieve then perfect VoIP quality?

And the million dollar question: would we gain anything more, aside of marking VoIP packets, to put the VoIP traffic on a dedicated VLAN?

I hope these questions make sense.

Cheers.




 

by: lrmoorePosted on 2009-09-18 at 05:42:54ID: 25365297

I will re-iterate what I said before. QoS mechanisms only have an effect if and when there is contention for traffic at a choke point in the network.
>the fact that VoIP and data packets are travelling "mixed up" can still collide and retransmit, etc
This is a physical layer property of Ethernet and how Ethernet transport handles the. In a perfect switched network, there are no possibility of collisions that require retransmits. When a device is full-duplex, the collision avoidance/collision detection mechanisms are disabled.

>VoIP traffic will NOT be marked, however
VoIP traffic will almost always be marked. Whether or  not the transport devices (switches/routers) actually look for and do anything according to that marking is up to you.

>And the million dollar question: would we gain anything more, aside of marking VoIP packets, to put the VoIP traffic on a dedicated VLAN?
Yes. Remember that a VLAN is a broadcast domain and all voice devices would see and have to stop to process all broadcast packets never meant for them. Windows NetBIOS for example.. So yes, segregating Voice into its own broadcast domain makes them more efficient for better service.

20120131-EE-VQP-002

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