Re: [EFM] Re: OAM - To side-band or not to side-band
Roy,
Perhaps you'd better get your 802 terminology straightened out before we
continue this discussion - see my comments interspersed below. Reading
the latest rev of the 802 Overview & Architecture would seem like a
terribly useful first step - perhaps also reading the 802.1D MAC Bridge
standard would also be of help to you in understanding what a MAC Bridge
is, and what a switch is not. You will find the latest rev of the 802
Overview & Architecture at:
http://www.ieee802.org/1/pages/802.html
Username: p8021
Password: go_wildcats
802.1D is freely available via the "Get 802" scheme - go look
at the IEEE "Standards Online" webpage:
http://standards.ieee.org/getieee802/
Regards,
Tony
At 17:37 26/09/2001 -0500, Roy Bynum wrote:
Tony,
Again, I think you may not be understanding what I am talking
about.
Actually, according to Geoff Thompson, there is no such thing as a
"full duplex repeater".
I believe that is true in the context of what 802.3 has defined to date.
The 802 architecture certainly doesn't preclude such a device, if there
is a need for one to be defined (see below).
A hub is not a repeater, it is a half
duplex bridge.
Not true, and I very much doubt that Geoff would have said
otherwise. A hub (at least in the context of 802) is nothing more
than a multi-port repeater.
To quote from the latest revision of the 802 Overview and
Architecture:
"Standard methods of interworking
fall into three general categories, depending upon the layer at which
the
corresponding interconnection devices operate:
— Physical-layer interconnection, using devices usually termed
repeaters
or
hubs
(6.3.1);
— MAC-sublayer interconnection, using devices termed
bridges
(6.3.2);
— Network-layer interconnection, using devices usually termed
routers
(6.3.3)."
And quoting further from 6.3.1:
"A repeater is a device used to
interconnect segments of the physical communications media, for example
to extend the range of a LAN when the physical specifications of the
technology would otherwise be exceeded, while still providing a single
access domain for the attached LAN stations. Repeaters used in support of
multiple end stations attached by star-wired network topologies are
frequently referred to as hubs."
So, according to 802 usage, a hub *is* a repeater, and *is not* a
Bridge. Hubs can be (and were/still are) built with no link layer
functionality whatsoever, although managed 802.3 hubs (as I pointed out)
do also exist, managed using MAC frames. As Andrew Smith points out
in his response, you should not make the mistake of confusing the
abstract objects that we write standards about with what goes into actual
product specifications; if I choose to put an 802.3 MAC in the same box
as an 802.3 hub in order to allow it to be managed, that is up to me as a
product developer.
An 802.3 managed hub is therefore logically nothing more than a repeater
that happens to be packaged with a management end station in the
same box, the end station being connected to one of the hub's ports. The
repeater functionality of such a product is still just a layer 1
function.
A switch is a full duplex
"bridge". Both have 802.1
functionality.
Actually, "switch" is not a defined term in any 802 standard
that I am aware of; it is certainly not defined in the 802.1D Bridge
standard. It is a colloquial term that, to many, means "A Bridge,
having only full duplex Ports"; however, I have also seen/heard it
used to refer to hubs, and also to multi-port Bridges that have
half-duplex ports. 802.1 makes no distinction between Bridges that have
half duplex Ports, Bridges that have full duplex Ports, and Bridges that
have a mixture of the two. Whether the MACs are full or half duplex is of
no concern to the Bridging function. Again, to quote from the latest
revision of the 802 Overview and Architecture:
"The term
switch
is often used to refer to some
classes of bridge. However, there is no consistent meaning applied to the
distinction between the terms bridge and switch, and ISO/IEC 15802-3 does
not make any such distinction. Hence, this standard only uses the term
bridge."
(ISO/IEC 15802-3 is, of course, just the ISO version of IEEE
802.1D - the MAC Bridge standard.)
What I am referring does not have any 802.1 or
802.2 functionality.
If you decide to outlaw the use of link layer functionality in a product
that performs a repeater function, then the only way to manage it would
be to use a PHY-based communications channel; however, as I point out
above, this is an implementation choice.
Actually, while we are being picky about definitions, the OSI reference
model, or specifically, the OSI Management Framework (part 4 of the OSI
RM) defines management to be an application layer activity; it is
therefore arguable that what you want to do (manage a device that has no
functionality above layer 1) is itself a contradiction in terms, as the
fact that you are performing management means that, by definition, you
are using all 7 layers of the OSI RM (although some of the intervening
layers might have only rudimentary functionality, as necessary to support
the management application). Putting it another way, if you are talking
about a device that has no functionlity above layer 1, then that device,
by definition, is not, and cannot be, manageable, because if it is, the
device will have layer >1 functionality of some kind.
It has no visibility into the revenue data
traffic stream. It can not get access to the revenue traffic data
stream to put upper level application management traffic such as SNMP
into the revenue traffic data stream in either direction.
Thank you,
Roy Bynum
At 07:47 PM 9/26/01 +0100, Tony Jeffree wrote:
Roy -
Managed Ethernet repeaters (more commonly known as hubs these days) have
been around for some while, and they use MAC frames to carry their (SNMP)
management exchanges. I would therefore hesitate to use that particular
argument either for or against the use of a side-band for OAM.
Having said that, in networks with repeaters there may be distinct
advantages in *not* using such a side-band for OAM - for example, where
it is the device the other side of the repeater that you want to manage.
Unless, of course, you start putting some form of addressing into this
PHY-based side channel, which rapidly starts to look like you're
replicating MAC functionality in the PHY, which begins to look like a
waste of time & effort.
As to T1 and T3, there's no doubt that some of the EFM participants
(myself for one) are not intimately acquainted with the management
entrails of these technologies, and with the thinking behind why they are
that way. I'm sure that some of that information may be useful in
informing what we do in EFM. However, I'm equally sure that
re-inventing T1 or T3, giving it a bit-rate that is a power of 10, &
then badging it "Ethernet", would be a total waste of our
time. After all, if you continue to do what you have always done,
you inevitably end up with what you have always had.
Regards,
Tony
At 11:31 26/09/2001 -0500, Roy Bynum wrote:
There are a lot of other reasons to have the
OAM ou-of-band to the MAC traffic, such as being able to support OAM on
an intelligent "transparent" full duplex repeater in the
future. When this group took on the task of adding subscription
network support for edge access infrastructure into Ethernet, they took
on applying most all of the functionality that is being used today.
There is a long history of why the functionality for these types of
services is what it is. How many of the EFM Task Force people have
looked at how the OAM overhead of T1 or T3 framing works today? How
many of the EFM Task Force people have looked into why the OAM overhead
of T1 or T3 framing works the way that it
does?