[c-nsp] ATM over a Serial Interface question

[Michael Sokolov]

Nick Voth <nvoth at estreet.com> wrote:

> We have a number of channelized DS3's that are terminating on to a 7206VXR
> with the standard old PA-MC-T3 cards. We have a need to deliver ATM based
> DS1's out to Adtran DSLAMs in the field. Customers will connect to the
> Adtran units via ADSL and their local telco loop and their data is then
> aggregated by the Adtran and on to the ATM based T1 coming back to us.

Let me see if I've understood you correctly: you are planning on feeding
a whole DSLAM with a single T1?  In other words, the aggregated bandwidth
usage of all subscribers you are going to serve is going to fit into a
single 1.5 Mbps T1?  I thought today's spoiled ADSL users demand a heck
of a lot more than that...  (The latter thought is based on my personal
experience of the level of ridicule I get for promoting my beloved SDSL
which also maxes out at 1.5 Mbps.)

> SO, my question is if we can deliver these circuits to the Adtran units
> using the "encapsulation atm-dxi" on each serial interface. I know this
> encapsulation is for delivering ATM through an ATM ADSU, but in this
> application, does the far end Adtran function as the ADSU or are we missing
> something on our end?

I invite someone from Cisco to prove me wrong, but my understanding is
that "encapsulation atm-dxi" is another term for what's also known as
ATM FUNI:

http://ifctfvax.Harhan.ORG/OpenWAN/l2conv.html

With ATM FUNI aka DXI the line carries HDLC frames which have the AAL5
packets encapsulated in them, so the hardware is still HDLC.  True ATM
over DS1, OTOH, would mean running actual 53-octet ATM cells over the
DS1 bits, which calls for significantly different hardware.  I would
expect that a DS1 feeding a DSLAM would need to be true ATM, but I'm not
familiar with Adtran DSLAMs specifically.

(The DSLAMs which I do know want to be fed with DS3 or higher, not DS1,
and they do want ATM cells over that DS3, not FUNI or DXI.)

HTH,
MS