A basic question about QoS calculation

[eksantrik]

We are trying to configure QoS over a T1 link for G.711 calls.
We have some nortel baystack 5520s.
I am personally stuck at some really basic thing. If you do the math for G.711 calls you can say that a T1 link can carry 17 simultaneous calls. However isnt this only one way. So what about the voice traffic coming from the opposite side.

Is this a right statement:

A T1 link can carry 8 simultaneous bidirectional calls.

The more I think the more I get stuck. How shall think here

 

[Crowtalks]

Yes, the calculations would be for one half of the conversation, IP to IP, so you would have 8 concurrent calls available.

I have several IP Network links active and I always use G.729 because it offers the best balance between voice quality and bandwidth.

Off the top of my head, I think the total overhead for a Frame link is 16 mg compared to 38 mg for a serial ATM link. That's a consideration to take into account.

You probably want to plan your maximum number of concurrent calls to gobble up no more that 50-60% of the available bandwidth in a converged network and weighted fair queuing is a good choice for your QoS protocol.

NCSS NCTS NCTE

 

[eksantrik]

thanks for the reply.

I dont know why but this has been confusing for me. Lets say you have a T1 link which means your clock rate is 1,5Mbps. You can not exceed this.
In other words, the total number of received and transmitted bits for that single interface can not exceed 1,5Mbps per second. Is this a correct statement.

Or can we say since this is a full duplex link, the real speed is actually 3Mbps for that interface.

thanks

 

[Crowtalks]

That's an interesting question...a T1 link is max 1.5 down & 1.5 up.

Therefore you lift the receiver and start speaking, your packets are flowing downlink. The remote speaks and you receive their voice as an uplink. You could conceivably have 3 mg full duplex traffic traveling from and to your network in a perfect world. Many variables cut that down to a nominal 1.35 to 1.4 each direction in the real world though.

NCSS NCTS NCTE

 

[eksantrik]

so that means I can have 17 simutaneous and bidirectional G.711 calls on a single T1.

because I thought when you do the math for bidirectional calls the number is 8 calls.

correct?

Thanks again by the way

 

[Crowtalks]

You caught me speaking without asking what type of network you are building, so I retract my first statement about halving the numbers of concurrent calls. I was thinking of a converged network with PC and VoIP sharing the bandwidth.

As you said with a T1, up and down are the same. Conversations, for the most part, are half duplex, one speaks and the other listens. Even if you had a couple trying to yell over each other, you still have the bandwidth available at full duplex, as long as there are no other devices competing for the bandwidth. Apparently your network is solely VoIP and not converged, so therefore you should be able to handle your 17 calls.

If you had a converged network, the rule of thumb is to plan on using no more than 50-60% of the available bandwidth and using QoS.

NCSS NCTS NCTE