Exchange point peering

I have some questions regarding an IX point.
If I go to PacBell and sign the multilateral peering agreement it anyone and
everyone can peer with me and I can peer with everyone else. Now if I buy a
DS3 UBR port then my cells/packets have to contend with everyone else who is
trying to egress out to an upstream provider. If there is contention then
my cells can be dropped and my packets are unable to be reconstructed by the

Does anyone know how this works at the Chicago NAP? Is my PCR for my ATM
port set at line speed for DS3 OC3 and OC12. How do they prevent random
cell discard at the egress port to upstream providers, is there a graceful
discard so that cells are dropped to maximize good "packets".

How does Pacbell ensure that there is a level of fairness for packet
traffic. I understand that it will be UBR, but if Pacbell is sending only
cells to upstream that randomly make it through the egress queues, there is
really no way to ensure that ANY packets can get through. All the cells
could be partial packet info. Any insight into this would be helpful so
that I do not purchase a service that is worthless.