ATM Link Utilisation

[habeebk]

Hi All,

One basic question to you about bandwidth utilization measurement of ATM links !!

We are using the MRTG to measure bandwidth Utilization of all our WAN links, and in all cases we could see the utilization growing upto 100% in peak time except on ATM based connections. In ATM links, MRTG shows maximum utilizations upto 86% and not growing beyond even we pump more data through the links.

Complaints to our service provider given the answer of "remaining 14% is ATM Overhead" !!. Yes, we do understand about the overhead of ATM (which is around 13-20%). But my question is

--Does MRTG measures bandwidth after these overhead

--where does (at what point or layer) MRTG measures bandwidth in Cisco Router?

--is there any overhead after this measuring point (for instance the encapsulation of AAL5SNAP, or technology overhead of 5 bytes header in 53 bytes)

It will be great if you could shed some on this matter,

Thanks In Advance

Habeeb

[ebreniz]

Did you try with anyother monitioring tools.

[habeebk]

I didn't try any other tools, but I am expecting the same result with all tools, because all these tools measures the in/out octect rates given by the SNMP MIBs.

Thanks

[marikakis]

ATM switches have per-VC counters in cells/sec.

Routers don't, due to driver limitations.

They count only a part of the AAL5 overhead

(layer 3 packet and 8-byte LLC/SNAP header).

AAL5 trailer and ATM Cell header are not included in the router counters.

The variable-length padding is not counted either.

Simply put, a 100 byte layer-3 packet needs 3 cells,

which means some part of the 3rd cell is wasted.

See the following documents for the details:

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a00800c940a.shtml#octet_packet

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080093c9a.shtml#pervc_stats_routers

Depending on the mean packet size of traffic traversing a VC,

overhead can be significant.

I have come across voice traffic scenarios (small packets)

with ATM overhead reaching 33,3 %.

If the router counter based rate was x, the real one was: x + 0.5*x.

The real was measured with ATM switch per-VC counters.

What your provider says seems realistic to me.

You can either use ATM switch per-VC counters (if you administer the switch),

or you can conclude to a formula that would tell you what the real rate is,

so you can plan your network for capacity.

Having reached an estimation of the overhead being 14%

is a very important conclusion you can use.

If the displayed utilization is x and mean overhead is 14%,

then the real utilization is approximately : (100*x)/86

[habeebk]

Thank You Very much,

Habeeb