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QoS required on PRI?

Hi Everyone,


     I wanted to ask if QoS is required on a PRI circuit?  the scenario is that SIP will be coming to the building but, it will be converted to PRI. therefore, it will not be on our MPLS cloud.   Therefore, i am trying to find a definitive answer if QoS is required still at this point.

thank you in advance.


The short answer is no.

ok, and long answer on why "no" ?

I haven't had any coffee yet.  Was hoping to get away with that.

ISDN PRI's don't operate within the OSI model which is where QoS lives.  QoS is essentially classifying traffic and then taking some action based on those classifications.  There's no similar construct with PRI's, and if you think about it there doesn't need to be QoS there.  There are a finite number of bearer channels to carry the media.  You can't squeeze more in to it, so no concept of congestion.  No need to define traffic types (as only voice is carried), and no need to prioritize voice traffic (for the same reason).

As correctly pointed out you do not enable QoS on PRI itself as PRI is not IP routed. However, you still want LAN QoS on your switches to ensure proper voice quality. Designing LAN QoS is not just slapping few commands, you need to identify all hardware across the network as every Cisco switch, every line card, etc have different capabilities and you need to design comprehensive QoS that will work across all of the devices.

HTH, please rata all useful posts!


Actually, I think the answer is sort of a "yes and no". If I follow your description you have something like the following:

[ITSP] ---(SIP)---->[Gateway]-----(PRI)------>[Gateway]----(MGCP/H323/SIP)----->[?unspecified entity?]

Or you have:

[ITSP] ---(SIP)---->[Gateway]-----(PRI)------>[PBX - TDM line card]

Further, I assume that the carrier owns (i.e. is responsible) for delivery of the trunk up to the [Gateway] entity.

So, the carrier is assumed to be providing QoS for the (SIP) trunk path. If the first call flow is representative of your enviornment then you will need to provide QoS between the [Gateway] and the [unspecified entity] as I assume this is on your prem.

If your environment is more like the second call flow then you do not need to worry about QoS provisioning on your gear.

Getting back to the (PRI) trunk, you don't need to configure network QoS. A T1/E1 PRI basically provides a dedicated communications channel with a fixed number of sub-channels (B-channels). Each B-channel has a constant bandwidth assigned. Each B-channel corresponds to a timeslot and the timeslots are allocated in a fixed order. This is different than packet-switched networks where packet reception is unpredictable and transmission is scheduled on a packet-by-packet basis.



HTH -Bill (b) (t) @ucguerrilla

Please remember to rate helpful responses and identify

I took the question too literally -- looking only at the PRI connection itself.  Thanks for catching that guys.

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