When to implement QoS inside the LAN network

We are a partnering with a carrier to ready networks for implementing VoIP. I understand the carrier implements QoS on their Router at the demarcation point at client site. That is where they stop and we begin. We are planning on implementing a VLAN for phone traffic. My question is this: At what point is it necessary to implement QoS INSIDE the network architechture (other than the router going out to the world)? We are working with small to medium businesses, 30-175 users.

It was recommended that this would be of little benefit to the customer considering the time and cost to implement QoS. I just want to confirm this from a professional.
eidebaillyAsked:
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FrabbleConnect With a Mentor Commented:
Bear in mind that if this solution is to be their telephony system, internal call traffic will be on the local network between end points.

According to http://www.3com.com/other/pdfs/legacy/en_US/3Com_503139.pdf, audio quality is one of the five critical considerations when implementing IP telephony. I think the carrier recommendation shows a total lack of consideration, especially since it will be you who will be responsible after their demarcation point. So the cost benefit perspective is your call, since it will need to be based on not only QoS, but network design, equipment comparison costs and the results obtained after testing a network's readiness.
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FrabbleCommented:
I would disagree with that recommendation.

The number of users is immaterial, QoS should be introduced if there is any type of traffic requiring priority handling, especially when there is contention.
While this is usually the case with slower router links, it also should also apply to links between switches, especially those connecting to the desktop.

The simplest example is a fast ethernet switch port carrying both data and voice to an IP handset with a workstation connecting to the handset switch port. A sustained file transfer to the workstation will easily use all the bandwidth of 100 mb. The problem here is that as part of the normal data stream, the voice packets to the handset will be delayed and the resulting audio will occassionally break up - very undesirable and believe me, users will get very annoyed when this persists.

So, QoS should be part of the packakge used to implement VoIP.
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eidebaillyAuthor Commented:
This is an outsourced softPBX solution and this is hosted at the carriers site outside the client site. Typically they're connect with a 1.5 - 2Mbps link. We are thinking the bottleneck will be this link vs. inside the network. From a cost benefit perspective does the additional investment in QoS capable switches represent a significant enough increase in quality to justify the cost?
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FrabbleCommented:
Thanks for the points. Good luck with the project.
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