[nsp] 12000 OC12 ATM Card

Daniel Puka daniel.puka at gvt.com.br
Sat Mar 8 21:52:36 EST 2003

Hello All,

    My company is connected to a upstream provider using 1 dedicated
STM1 ATM port. The interface has only one ATM VC. The upstream provider
is actually connecting my STM1 to a ATM Switch and then connecting the
ATM switch to a Cisco 12000 OC12/STM4 ATM card. I found out that the
upstream provider has more two customers buying STM1 Access(full
capacity) connecting to the same cisco interface trough atm vcs.

    (Customer 1,2,3)-> (STM1-ATM SWITCH) <-> (OC12 CARD - CISCO 12000)

     The cisco documentation says that when one vc get congested it can
use resources from other vc in the same card because it has only one
single queue for all vcs.

     One week ago I started to experience very high packet delay during
the peak usage hours(about 150ms where it was usually 1ms). I am not
experiencing any packet, cell or crc errors. Is there any change that
one vc congested from other customer can be the cause of delayed
packets? My maximum usage is about 90Mbps.

I found on cisco docs the following...



The 1xOC12 supports a maximum PCR or SCR of 31000 kbps, or half the line
rate, on VBR-nrt VCs. Configuring a non-supported value produces the
following error message:

%ALC-4-HIRATE: Interface (Int#): SAR limits VBR VC PCR to 311000 kbps,
opening UBR VC instead

Although it supports VBR-nrt traffic shaping, the 1xOC12 is recommended
for use as a high-speed pipe in the network core.  The 1xOC12 does not
support per-VC queueing and the 4xOC-3 card's VC-isolation techniques
like segmentation and reassembly (SAR) pools and tx-queue-limits.  Thus,
one congested VC can fill the packet buffers shared by all other VCs
configured on the same card.  Symptoms of this problem include delayed
and dropped packets.

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