Basic clarification on QoS - EF - VoIP

Unanswered Question
Jan 7th, 2012

Imagine you have SIP trunking services and 3 sites connected via ISP MPLS.

Then Branch1 bandwidth is T1. Gold CAR in place is 340K (it supports therefore 10 concurrent calls, G.729 as per provider confirmation). Assume no burst mechanism has been enabled.

Question:

a)What happens if phone users on Branch1 place 11 concurrent calls? Is the 11th call going to be dropped? Does it depend on the PE policing policies regarding how the EF traffic is treated? Please advise.

I have this problem too.
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lgijssel Sun, 01/08/2012 - 01:05

When the bandwidtg is exceeded, the policer kicks in.

This means traffic from all active session will become subject to packet loss.

Depending on your PBX, it may be possible to set a limit to the number of concurrent VOIP calls.

In this case, the 11th call would be routed over PSTN if available or have its call setup denied.

You cannot configure this kind of setting on the router as it has no knowledge about phone conversations, only about IP.

regards,

Leo

JosephDoherty Mon, 01/09/2012 - 06:26

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What exactly happens depends on how QoS polices are defined on your devices, whether there's interface congestion, and your MPLS provider's bandwidth enforcement policies.

Generally, assume you'll be limited to agreed bandwidth.  This limitation is usually "global", i.e. not just the 11th call flow would be impacted, but all 11 calls might be impacted.

Ideally, as Leo noted, some systems will track number of calls and block new calls that would over subscribe your allocated bandwidth.  As an alternative, I have had some luck by marking down oversubscribed traffic, but your mileage might vary.

PS;

BTW, when working with compressed CODECs, you should allow for transitory bandwidth spikes.

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Posted January 7, 2012 at 7:00 PM
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