Emperical data shows that we are currently losing about 20.5% of capacity
to IP over ATM overhead on fairly aggregated traffic. This means that *IF*
your new connection is being measured as 4Mbps of cell bandwisth, you
will only be getting 3.18Mbps. You may want to verify from the company
providing this link what exactly are they limiting you to?
btw, the extra overhead is lost in things like the last cell of a packet not
being full, etc.
Chris A. Icide
Sr. Engineer
Nap.Net, L.L.C.
My God, someone admits it?
I've used 20% as the general ATM overhead now for almost two years, and have
been poo-pooed by lots of people claiming that it wasn't anywhere near that
bad.
Funny how it all comes out in the end. 
i've never heard anything *less* than 20% loss in ATM overhead.
Question: On a Cisco with an AIP card, how do you determain what the
overhead is? We connect to our upstream provider via ATM.
.stb
Question: On a Cisco with an AIP card, how do you determain what the
overhead is? We connect to our upstream provider via ATM.
Again, from experience, a DS3 AIP is "full" when the 5-minute-average
counters are reporting ~34Mpbs (in other words, the counters never went
higher). Performance wasn't terrific, but if you're looking for raw
quantity to compute "efficiency", or what some call goodput, then my crayon
on painted wall calculation is ~34Mbps (observed max) / ~45Mpbs (approx DS3
raw max) = ~75% "efficiency". Given that networks and data have a
measurable coefficient of friction, I'd say that's the max you could get
under optimal conditions is something less.
It's not an option with the AIPs, but when we ran switch to switch we could
get another 4 - 5 Mpbs out of a DS3 by turning off PLCP (not an
endorsement, recommendation, or even technically sound, but it worked).
Another way of looking at this is that we know an ATM PVC over a DS3 using
PLCP is configurable for a maximum of 96000 cells per second. 96000 cells
per second * 48 payload octets per cell * 8 bits per octet = 36864000 bps,
or 36.864 Mbps, not too far from what I observed above. With PLCP turned
off, the maximum PVC config was ~105000 cells per second, yielding ~40.3 Mpbs.
So AIP to AIP has the downside of all the protocol overhead with no benefit
in a point to point connection. But you're trading off for the convenience
of not having to use a HSSI port to a T3 CSU/DSU with the additional rack
space and power concerns. And if I had to pick one particular device which
I spent the most time fussing with and repairing/replacing, it would be T3
CSU/DSUs.
-peter
.stb
i've never heard anything *less* than 20% loss in ATM overhead.
> > > Hello,
> > > I just thought of something. We are in the process of purchasing a
> > > 4 Mb CIR from another backbone. Now, we have the choice of ATM or
standard
> > > T3 delivery (over a DS3 either way). Now, if we get ATM, that 4 Mb
CIR
> > > turns into:
> > >
> > > [ (53-5)/53 ] * 4 Mb/s = 48/53 * 4 Mb/s = 3.62 Mb/s
> >
> > Emperical data shows that we are currently losing about 20.5% of
capacity
> > to IP over ATM overhead on fairly aggregated traffic. This means
that *IF*
> > your new connection is being measured as 4Mbps of cell bandwisth, you
> > will only be getting 3.18Mbps. You may want to verify from the company
> > providing this link what exactly are they limiting you to?
> >
> > btw, the extra overhead is lost in things like the last cell of a
packet not
> > being full, etc.
> >
> > Chris A. Icide
> > Sr. Engineer
> > Nap.Net, L.L.C.
>
> My God, someone admits it?
>
> I've used 20% as the general ATM overhead now for almost two years,
and have
> been poo-pooed by lots of people claiming that it wasn't anywhere near
that
> bad.
>
> Funny how it all comes out in the end.
>
> --
> --
> Karl Denninger (karl@MCS.Net)| MCSNet - The Finest Internet Connectivity
> http://www.mcs.net/~karl | T1's from $600 monthly to FULL DS-3
Service
> > 99 Analog numbers, 77 ISDN, http://www.mcs.net/
> Voice: [+1 312 803-MCS1 x219]| NOW Serving 56kbps DIGITAL on our
analog lines!
> Fax: [+1 312 803-4929] | 2 FULL DS-3 Internet links; 400Mbps B/W
Internal
PLCP mapping is the only option on a Cisco AIP (DS-3), as you probably
already know.. if you have a ATM Switch that is current, you can change
the mapping to Direct Cell mapping, which will give you about 3 megs more
of bandwidth on the line rate.. All of our switch-switch DS-3's are direct
mapping, and all of our switch-routers are OC-3 SONET, so you don't have
the same loss of bandwidth as PLCP on a DS-3. (also easier to aggregate
multiple backbone circuits on one card).
Eric