[EFM] Of mice and modems
From whence came this definition of a demarc?
A PHY device, in the context of 802.3, and the ISO reference model,
has a media interface on the bottom (known as the Medium Dependent
Interface, or MDI) and an interface to the data link layer (such as
the MII) on top. In particular, PHY devices have only one MDI.
PHY devices perform functions like signal conversion, pulse shaping,
filtering, clock recovery, serial to parallel conversion, encoding/decoding,
and in some cases, scrambling and Forward Error Correction. PHY devices
are aware of packet boundaries, but not the packet contents.
So how can the demarcation point be a PHY device? These days, PHYs are
buried inside larger pieces of silicon. Can we really have a demarcation
point inside of a chip?
Okay, so maybe the model that some of us have in mind is the DSL modem,
or DOCSIS modem, or a T1 DSU/CSU, or an ISDN terminal adapter. These
two port devices have an interface to the access link (DSL, cable, T1,
ISDN) on one side, and an interface to the customer's equipment
(Ethernet, RS-422, V.35, etc) on the other side. For the sake of
discussion, I think that it is confusing to call these devices "PHYs",
or "demarcation points, or "demarc devices". For the sake of discussion,
why don't we call these "modems".
An EFM modem would be a two port device with an EFM port on one side (either
opper, point to point fiber, or point to multipoint fiber), and an
Ethernet|Fast Ethernet|Gigabit Ethernet port on the other side.
You could put different levels of functionality in between the two ports.
Some might try to build it as a layer 1 device and call it a modem or a media
converter. Some might build it as a layer 2 device and call it a bridge.
Some might build it as a layer 3 device and call it a router. Others might
make it a whizbang, content-aware, caching, load balancing, encrypting,
tunneling, firewalling, network address translating, who knows what else
device, with VoIP and an MPEG decoder, and call it any number of catchy
names like an Integrated Access Device, an IP Services device, or a
Residential Gateway.
It seems that the question we must deal with is whether or not we are
going to write specifications for modems in 802.3ah. In my opinion, this
is not what we are supposed to be doing. We're supposed to be writing
specs for PHYs, plus minimal augmentation of the 802.3 MAC, plus far
end OAM for subscriber access networks.
Howard
Bob Barrett wrote:
>
> Harry et al
>
> yup, all the IP 'stuff' is payload as far as the demarcation point is
> concerned.
>
> The demarc is a PHY that carries packets at the end of the day. Some demarcs
> may be buried inside a bigger system, however, the standard must also cater
> for stand alone demarc devices. My expectation as a user would be that at
> the demarc the bandwidth was the same capacity as my enterprise MAC and PHY
> of the same spec.
>
> Would I miss 10k per second on a 1GE, I doubt it.
>
> Would my test gear pick it up on an end to end private circuit test, I don't
> know, anyone on the reflector tried this?
>
> Bob
>
> > -----Original Message-----
> > From: Harry Hvostov [mailto:HHvostov@xxxxxxxxxxxx]
> > Sent: 27 September 2001 17:41
> > To: 'fmenard@xxxxxxxxxxxxxxx'; 'Denton Gentry';
> > bob.barrett@xxxxxxxxxxxxxxx
> > Cc: 'stds-802-3-efm'
> > Subject: RE: [EFM] 1 Gbps != 999.9 Mbps
> >
> >
> > And how about the ICMP and IGMP traffic from the same CPE devices?
> >
> > Harry
> >
> > -----Original Message-----
> > From: Francois Menard [mailto:fmenard@xxxxxxxxxxxxxxx]
> > Sent: Thursday, September 27, 2001 6:05 AM
> > To: 'Denton Gentry'; bob.barrett@xxxxxxxxxxxxxxx
> > Cc: 'stds-802-3-efm'
> > Subject: RE: [EFM] 1 Gbps != 999.9 Mbps
> >
> >
> >
> > Or for that matter, what about ARP traffic unsolicited from my CPE
> > devices ?
> >
> > -=Francois=-
> >
> > -----Original Message-----
> > From: owner-stds-802-3-efm@majordomo.ieee.org
> > [mailto:owner-stds-802-3-efm@majordomo.ieee.org] On Behalf Of Denton
> > Gentry
> > Sent: September 26, 2001 3:12 PM
> > To: bob.barrett@xxxxxxxxxxxxxxx
> > Cc: stds-802-3-efm
> > Subject: [EFM] 1 Gbps != 999.9 Mbps
> >
> >
> >
> > > Service providers have a desire to offer a full 1GE service and not
> > > use any of it's bandwidth for OAM. The rule of conservation of
> > > bandwidth means the OAM needs to go somewhere other then in the
> > > bandwidth reserved for the 1GE payload. I take it as read that 100%
> > > utilisation of a 1GE is unlikely, but that is not the point. The point
> >
> > > is that service providers want to offer 1GE service period, not a
> > > 999.9Mbit service.
> >
> > Does the existence of the Mac Control PAUSE frame therefore make
> > Ethernet unsuitable for service providers?
> >
> > Denton Gentry
> > Dominet Systems