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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: ATM vs. DS3
At 04:06 PM 7/10/97 -0400, Stephen Balbach wrote: > >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 > >On Thu, 10 Jul 1997, Ben Black wrote: > >> i've never heard anything *less* than 20% loss in ATM overhead. >> >> On Wed, 9 Jul 1997, Karl Denninger wrote: >> >> > On Tue, Jul 09, 1996 at 10:38:57PM -0500, Chris A. Icide wrote: >> > > On Wednesday, July 09, 1997 9:34 PM, Josh Beck [SMTP:[email protected]] >> > > wrote: >> > > > 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 ([email protected])| 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 >> > >> > >
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