[c-nsp] ATM-DXI on E1

[sthaug at nethelp.no]

> We get a new leased line from one of our carriers, and for some weird
> reason, als the "usual" stuff cannot be delivered - we have to take the
> line as ATM encapsulation on E1 (ATM UBR, G.703/G.704, 30 timeslots).
> 
> My current plans are to get a Cisco 26xx with an NM-4E1-IMA or a
> AIM-ATM + VWIC-1MFT-E1 combo, which should nicely support this
> (the other end is delivered on an existing ATM-OC3 going into a 
> 7200 with PA-A3-OC3).
> 
> Now our carrier has declared "you can hook up a standard E1<->X.21
> converter to that line, and use your normal X.21 routers", which *does*
> surprise me.

It's not that simple.

> I know that there is "ATM-DXI" encapsulation on Cisco serial interfaces,
> but I have to admit that I'm not 100 per cent sure what it is - it's some
> way to map ATM VCs onto a normal serial interface, but is it "just plain
> ATM, all bits forwarded 1:1"?  Or is there some special processing going
> on inside the ATM-E1 <-> ATM-DXI-on-X.21 converter?

DXI is a way to map several logical channels to one physical link - a
bit like Frame Relay DLCIs. You need an external box which understands
the DXI encapsulation and can perform the actual ATM segmentation and
reassembly.

> >>From reading http://www.cisco.com/warp/public/121/atmdxi.html, I understand
> that the converter *does* need to do special processing, because the
> SAR part of the ATM framing is done in the converter.  Correct?

Yes.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no