Re: (ASCEND) Poor TNT performance?

[Simon Rainey <srainey@rmplc.net>]

Tim,

>> We typically see around one call coming up or going down per second on
>> each primary rate.
>...
>> Every site demands their full bandwidth and our observations are that
>> most of them do indeed saturate their links almost continously during
>> the day.
>
>Since there are 32 channels in an E1 circuit, this would mean that each
>connection is lasting only about 30 seconds on average.  This is at odds
>with the idea that the links are continously saturated.  If they are
>saturated, they should not be going up and down twice a minute.

We set the idle timer on the remote routers to 30 seconds because our users
have a capped charging scheme for the use of their ISDN lines. They will
never pay more than a set figure no matter how many calls they make or how
long they are connected for. In an attempt to reduce the number of backend
lines we have to buy we set the idle time to be something low, and 30
seconds seemed like a reasonable figure.

We have observed two class of users. The majority hammer their connections
for most of the day with web access, but even then there are usage patterns
which cause the bandwidth to drop to zero for 30 seconds or more. Then
there are users who do little else but pick up their mail, and these
account for most of the call transitions.

Analysing the RADIUS accounting logs we see a small but significant number
of users that come and go very rapidly. Some have call durations as low as
5 seconds and their line is going up and down all day. We're looking into
why this should be, but it means that averaged over all the PRIs we do see
about one call come up or go down per second per PRI.

>> am I right in assuming that the TNT's IP routing is all done by the
>> main CPU rather than by a hardware routing engine?
>
>No, the routing table is maintained by the shelf router and distributed
>to each slot card, where the IP routing is done.

That's reassuring.

>> Performing a throughput test revealed that we could only get 20kbps over
>> a 64kbps connection.
>
>What type of traffic (ICMP, TCP, UDP) with what size packets (64, 1500)?

We downloaded a 10MB file from one of our Unix FTP servers using the Win95
FTP client. The Win95 host was connected using a UTP crossover cable to an
Ascend P50. When the P50 was dialling into the loaded TNT we could not
achieve more than 20kbps. Repeating the test with the P50 dialling into a
less loaded TNT or a Cisco AS5300 we achieved 60kbps. The tests were
repeated multiple times. At one point we were seeing less than 2kbps
through the loaded TNT. Once the load dropped off (early evening) we
achieved 60kbps consistently.

Regards,
Simon.
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