Muni fiber: L1 or L2?

Jay Ashworth jra at
Thu Jan 31 03:21:23 UTC 2013

----- Original Message -----
> From: "Leo Bicknell" <bicknell at>

> The Cable Modem is in many ways very similar to a FTTH ONT. It takes
> one media (cable, fiber), does some processing, provides some security
> and a test point to the provider, and then hands off ethernet to the
> customer. A majority of customers then plug in a Home Gateway (router,
> one of those linksys/netgear/belkin things), although some plug in a
> single device.

It's actually not.  A cablemodem is similar to a DSU, while an ONT is
similar to a CSU.  Layer 2 vs layer 1.  I think.

> What goes wrong? Well, the Home Gateway sees a 1000Mbps GigE to the
> cable modem, and tries to send at that rate. The cable modem is only
> allowed to transmit to the plant at maybe 10Mbps though, and so it
> must buffer and drop packets, at what appears to be L2. At which point
> virtually any ability the customer had to do QoS is gone! I believe
> some Verizon FIOS customers had similar issues with GigE to the ONT,
> and then 100Mbps upstream service.

Sure.  So you hard-throttle the ETH PHY to whatever speed they're 
paying for; if it's 50Mbps, then you set it as 100BaseT, not Gig.

> Havng the two separate devices significantly degrades the customer
> experience in many cases, particularly where there is a speed mismatch.

I'm all about not buffering, but this seems baby/bathwater, to me.

> I want to chuck the cable modem and/or ONT out the window never to be
> seen again, and let the customer plug their home gateway in directly.
> No middle box to buffer or drop packets, or otherwise mangle the data
> stream in bad ways.

Nothing at their end that you have any hope of being able to manage
when they call in with trouble...
> I have no issues with the Home Gateway responding to OAM testing from
> the provider. I have no issues with it learning part of its config
> (like a maximum transmit speed) from the provider.
> A Cable Modem or ONT is a glorified media converter which should not
> exist.

If by "Home Gateway", you mean CPE, then no; the field is *way* too big
to be able to get reliable remote management to work properly, not to
mention the combinatorial explosion of multiple ISPs on the back end.

Whether you can get by without carrier gear at the customer site is 
pretty much directly proportional to the complexity of the service.

If a T-1 needs it, fiber sure does.

-- jra
Jay R. Ashworth                  Baylink                       jra at
Designer                     The Things I Think                       RFC 2100
Ashworth & Associates         2000 Land Rover DII
St Petersburg FL USA               #natog                      +1 727 647 1274

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