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Re: (ASCEND) shared D-Channels on PRI

At 09:09 AM 10/14/98 , John T. Farmer wrote:
>
<snip>
>
>Related to this, I had a 3com vendor tell me yesterday that NFAS was a 
>BAD thing, that above 1 PRI's worth of ISDN calls, that performance
>would drop off, both for call setup and throughput.  He also claimed
>that this was true for all equipment including Ascend MAXs.

Call setup? Even if all 4 PRI were in use and for some reason all 96
channels were "connecting" at the same time - you have a 64K D-channel
there, more than adequate.

Throughput? I see absolutely no relationship between the two - the 
D-channel *ONLY* plays any part in the whole scheme during call setup
and tear-down.

>My first thought was to politely tell him how wrong he was, and then
>realized that I didn't have enough real world data to form an opinion.

Right.

>So, assuming that with standard PRI's in MAX boxes the call setup time
>is "X", and the average link throughput is "Y"; how are these values
>effected when using NFAS for 2PRIs?  3 PRIs? 4 PRIs?

I would suggest that you just consider the fact that the PRI D-channel
is a full 64K channel - so there is never going to be a delay or bottle
neck issue there handling call setup/tear-down messages, even for 4
T's worth of channels. 

Also consider the alternative of 1 D-channel per
span - nice to have 256K worth of 'signalling' channel(s) - but is it
necessary? If the *MAX* was unable to keep up with the 64K channel setup/
tear-down requests, then why would it make any difference to split them
over 4 D-channels since the MAX processor is still involved whichever 
scheme was used.

The only valid argument against NFAS that I've heard so far is the
'disaster' scenario - if the D-channel goes down, it takes down 4
spans rather than 1. But, how often has anyone lost even 1 span?

>Also are these values effected by wheither or not the PRIs are 
>terminated in 1 chassis or multiple chassis?  Is there an effect 
>that can be tied to type of calls, Modem vs. ISDN?

The above discussion only relates to single chassis comparison. NFAS
scheme is not supported across chassis. 

Kevin Smith			(kevin@ascend.com)
Ascend Communications...
	...where Network Solutions never end.

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