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DSL frames (was: Jumbo Frames)
From: William Allen Simpson <wsimpson () greendragon com>
Date: Mon, 19 Jun 2000 21:25:35 -0400


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Bill Woodcock wrote:
This problem is also one that plagues DSL, as many DSLAMs expect to see a
full Ethernet packet, including Ethernet header, inside the frame/atm
packet that comes in from the network.  They strip the frame/atm header,
and dump the contents onto the DSL/Ethernet wire.

If you're using frame relay on the backhaul, a max-length Ethernet packet
with both Ethernet and frame relay headers will obviously exceed the
1500-byte limit, and get truncated.  If you pull the Ethernet header out,
you need to get the DSLAM to generate a new one on the other side, which
many of them verge on not being smart enough to do.

As a small ISP, we've been using Wailan.com boxen, which has both 
bridging and routing (RIPv2), DHCP and NAT.  We turn bridging off, 
which seemed to fix our problems.  

However, I've found no documentation on their wire discipline.  I'd 
not thought of using FrameRelay in the backhaul.  (You clever fellow!)  

Instead, I'm trying stringing them together (that is, one of the 
lines into the DSLAM router is really the upstream to the city POP), 
putting the DSLAM into one customer site, and then running our own 
wires from each DSLAM to other customers in the area....  This only 
works because Wailan allows both 7:7 SDSL and 7:1 ADSL in the same box, 
and the DSLAM seems to be a special purpose self-contained router.

Can you believe that BellSouth wants $2000 for installation and $91 
per month to setup an unloaded alarm circuit?

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