ATM vs. DS3

Eric D. Madison madison at acsi.net
Fri Jul 11 05:08:03 UTC 1997


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


_______________________________________________________
      Eric D. Madison - Senior Network Engineer -   
 ACSI - Advanced Data Services - ATM/IP Backbone Group  
   24 Hour NMC/NOC (800)291-7889 Email: noc at acsi.net


On Thu, 10 Jul 1997, Peter Kline wrote:

> 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:jbeck at connectnet.com] 
> >> > > 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 (karl at 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
> >> > 
> >> 
> >
> >
> 




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