[Bloat] SDSL speeds (was: the side effects of 330ms lag in the real world)

[Michael Spacefalcon]

Steinar H. Gunderson <sgunderson at bigfoot.com> wrote:

> His email address might give a hint? :-)

Yes, I am quite familiar with that domain name, hence my ??? comment:
I know what speeds are supported by Megapath's DSLAMs (got a DSLAM
just like theirs in my own lab), and 256 kbps isn't one of them.

Hal Murray <hmurray at megapathdsl.net> wrote:

> Thanks for the heads-up.  I wonder when my brain warped the numbers?  Yes, 
> I'm running at 384.

Thanks for the reassurance. :-)

> I remember that many years ago I had something slightly faster.  Maybe 416?  

That must have been Copper Mountain's flavor of SDSL, which indeed had
416 kbps instead of Nokia/Covad's 384.  CM DSLAMs (I've got one in my
lab too) were operated by Northpoint (mine came from NP's demise, from
BCRTFLMA) and by Rhythms.  NP was before my time, but I used to have a
416 kbps (billed as 384 kbps) line from a CM DSLAM, back when Rhythms
was part of WorldCom.  Then I had to move to a different physical
location that had only Covad, and I've been running on the Nokia D50
flavor of SDSL ever since.

I miss CM's flavor of SDSL.  It is nicer than the Nokia one served by
Covad/MP, because CM sent an HDLC bit stream (FR encapsulation) to the
subscriber, which is much more bit-efficient (fewer bits needed for
the same packet) than ATM cells to the subscriber as in Nokia SDSL.

> I think that went away when Covad got in financial trouble and 
> Megapath had to switch to somebody else.

It was not Covad or Megapath who killed CM SDSL, as neither of these
two companies ever owned any of that stuff.  Instead the bad guy is
Verizon, who swallowed up WorldCom/MCI and killed the CM network that
used to be Rhythms.

Covad and Megapath are now one company, but on the technical side
nothing changed from that merger: our SDSL lines (mine and yours,
probably the last two active SDSL circuits in the world) are served
from the same Nokia D50 DSLAMs as before.

> In addition to your list, my modem's list also includes 2320.

My modem of my own design and make supports 2320 kbps too (I'm using
the RS8973 transceiver chip; your modem uses the same chip if it's any
model other than Siemens 5890, which uses M28945 instead), but
Covad/MP's DSLAMs don't support this high speed.  Nokia supported SDSL
at 2320 kbps only with the SDSL8+ line card, which was never deployed
by Covad.  All of Covad/MP's D50 DSLAMs have the original SDSL8 line
cards, which use the older Bt8970 transceiver chip that maxes out at
1.5 Mbps, give or take a few kbps.

Both CM and DLCC/Nokia DSLAMs used these Bt8970 bitpumps; CM set their
top speed at 1568 kbps, whereas DLCC went for plain 1536 kbps.  That
Bt8970 chip requires external clock switching logic to change from one
SDSL speed to another, and DLCC made that logic a little simpler by
their restricted choice of data rates.

How do I know all this?  Intense dissatisfaction with the idea of
having to use a CPE device not designed by me drove me to learn
everything I could about SDSL (including a lot of reverse eng etc),
and to design and build my own SDSL CPE device, an SDSL->V.35 CSU/DSU
of my own design and make.  It's all written up here:

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

Those pages haven't been updated in a few years, so some parts are a
little out of date, but not by much.

SF