The IP over ATM Mailing List Archive by date

[rajeev@trillium.com]

> From: George Swallow <swallow@cisco.com>
> 
> >i don't remember how AAL3/4 implements MID allocation, but vci allocation
> >protocol could be something similar.
> 
> I've only seen the scheme used for SMDS/CBDS.  This assumes taht the
> links are all point to point.  The MIDs serve no purpose than to
> distinguish the cells of one packet from the next, so the assignment
> of MIDs can be arbitrary. 

When using SMDS with DQDB (IEEE 802.6), the links are not point-to-point,
in fact there is a shared, slotted bus. When two stations need to talk,
they can either have a reserved (prearbitrated) channel (VCI) or
they have to use the general purpose VCI (all 1's). In the former case,
the MID can be assigned arbitrarily since it is relevant only between
the two stations. However, when sending packets on the general purpose
VCI, all stations on the bus need to coordinate MID allocation, so
no two stations use the same MID simultaneously. I am not sure if
this distributed assignment algorithm is specified in the IEEE standard,
but it sure would be a good place to look.

+-------------------------------------------------------------------------+
|Rajeev Gupta                               Trillium Digital Systems, Inc.|
|rajeev@trillium.com                               Los Angeles, California|
|                  * Opinions expressed are mine alone *                  |
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