| Author |
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Stephen Brown
Guest
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 Posted:
Fri Feb 11, 2005 3:39 pm Post subject:
10Gbit Ethernet Half-Duplex |
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| Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed? |
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James Knott
Guest
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| Posted:
Fri Feb 11, 2005 6:06 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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Stephen Brown wrote:
| Quote: | Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
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Or more to the point, why bother. Examine the issues with half duplex, as
bit rate increases, to find your answer. Then try to come up with some
reasons why you'd even want HD, when FD is so common. |
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glen herrmannsfeldt
Guest
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| Posted:
Mon Feb 14, 2005 8:00 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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James Knott wrote:
| Quote: | Stephen Brown wrote:
Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
Or more to the point, why bother. Examine the issues with half duplex, as
bit rate increases, to find your answer. Then try to come up with some
reasons why you'd even want HD, when FD is so common.
|
When gigabit was new, before switches were easily available, I had
thought it would be useful to have NICs and repeaters that didn't do
carrier extension for clusters of machines in a single room. Especially
when a large amount of communication is broadcast (such as loading the
same code into many machines) it would have seemed useful. Now with
gigabit prices low, it is hard to argue that, but it might be for 10Gb.
-- glen |
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James Knott
Guest
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| Posted:
Mon Feb 14, 2005 6:47 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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glen herrmannsfeldt wrote:
| Quote: | James Knott wrote:
Stephen Brown wrote:
Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
Or more to the point, why bother. Examine the issues with half duplex,
as
bit rate increases, to find your answer. Then try to come up with some
reasons why you'd even want HD, when FD is so common.
When gigabit was new, before switches were easily available, I had
thought it would be useful to have NICs and repeaters that didn't do
carrier extension for clusters of machines in a single room. Especially
when a large amount of communication is broadcast (such as loading the
same code into many machines) it would have seemed useful. Now with
gigabit prices low, it is hard to argue that, but it might be for 10Gb.
|
But again, what advantage is there to going with half duplex? With HD,
you've got all the problems with collision detection, reduced throughput
etc. While there were techniques developed for gigabit, to allow HD with
reasonable segment lengths, the problems are 10x worse at 10 Gb. |
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John Rowe
Guest
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| Posted:
Mon Feb 14, 2005 10:37 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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"Stephen Brown" <sd_brown@ntlworld.com> writes:
| Quote: | Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
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Surely the real question is why 1 Gbit Ethernet at Half Duplex
WAS allowed ;-)
John |
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Rick Jones
Guest
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| Posted:
Mon Feb 14, 2005 11:45 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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John Rowe <rowe@excc.ex.ac.uk> wrote:
| Quote: | "Stephen Brown" <sd_brown@ntlworld.com> writes:
Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
Surely the real question is why 1 Gbit Ethernet at Half Duplex
WAS allowed ;-)
|
"It ain't Ethernet if it don't do CSMA/CD?" :)
rick jones
--
oxymoron n, Hummer H2 with California Save Our Coasts and Oceans plates
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to raj in cup.hp.com but NOT BOTH... |
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Manfred Kwiatkowski
Guest
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| Posted:
Tue Feb 15, 2005 12:01 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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In article <1l6Qd.8$X46.6@news.cpqcorp.net>,
Rick Jones <foo@bar.baz.invalid> writes:
| Quote: | John Rowe <rowe@excc.ex.ac.uk> wrote:
"Stephen Brown" <sd_brown@ntlworld.com> writes:
Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
Surely the real question is why 1 Gbit Ethernet at Half Duplex
WAS allowed ;-)
"It ain't Ethernet if it don't do CSMA/CD?" :)
|
The VG-AnyLan trauma? ;-)
SCNR
--
Manfred Kwiatkowski kwiatkowski@zrz.tu-berlin.de |
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Rick Jones
Guest
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| Posted:
Tue Feb 15, 2005 3:14 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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| Quote: | "It ain't Ethernet if it don't do CSMA/CD?" :)
The VG-AnyLan trauma? ;-)
|
Nah, I think I'm mostly over that one by now. Just feeling irreverant
today :)
rick jones
--
a wide gulf separates "what if" from "if only"
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to raj in cup.hp.com but NOT BOTH... |
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Rich Seifert
Guest
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| Posted:
Tue Feb 15, 2005 6:39 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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In article <vnmzu7rqje.fsf@kenny.ex.ac.uk>,
John Rowe <rowe@excc.ex.ac.uk> wrote:
| Quote: | "Stephen Brown" <sd_brown@ntlworld.com> writes:
Any reason why 10Gbit Ethernet at Half Duplex is dis-allowed?
Surely the real question is why 1 Gbit Ethernet at Half Duplex
WAS allowed ;-)
|
Here is a complete explanation, excerpted from Chapter 10 of my book
"Gigabit Ethernet" (Addison-Wesley, 1998):
----begin excerpt----
Do we even need a half duplex (traditional) Ethernet MAC at gigabit
rates? This is a reasonable question, especially since it is clear that
we had to make some significant changes to the Ethernet design in order
to support half duplex operation at this speed.
There is only one advantage of half duplex over full duplex operation:
Half duplex (shared) LANs can use a repeater hub rather than a switching
hub, thereby potentially saving some system cost. A switching hub will
always cost more than a repeater hub (on a per-port basis), however,
over time the difference shrinks considerably with competition and
availability of high-integration silicon switching components. The cost
of the station attachment (NIC) is identical whether using shared or
switched hubs.
So, to the extent that the hub constitutes a cost factor for the system
as a whole, the system cost will be greater when using full duplex vs.
half duplex operation, since full duplex LANs require switching hubs.
There is a premium paid to use full duplex mode, but it is not very
large, and the premium decreases over time. In return for this price
premium, we can avoid:
- Changing the Ethernet MAC algorithms,
- The performance degradation of carrier extension (for short frames),
- The performance limitations of CSMA/CD in general, and
- Any need to reduce network extent (distance).
[So] why did the industry bother to develop a half duplex Gigabit
Ethernet standard at all?
The answer is more political than technical. Gigabit Ethernet was
developed under the auspices of the IEEE 802.3 Working Group. By
definition [at that time], 802.3 networks must include the capability of
CSMA/CD operation. If Gigabit Ethernet offered a full-duplex-only
solution, it would have been difficult to justify its development within
the IEEE 802.3 Working Group. This would have resulted in some important
political difficulties:
(1) A new Working Group would have to have been formed within IEEE
802; this takes considerably more time than a new project within an
existing Working Group and would have delayed the development of the
standard.
(2) The resulting standard would have had difficulty calling itself
"Ethernet", since it would neither use CSMA/CD (even as an option), nor
would it have been developed as part of IEEE 802.3, the recognized
"owner" of the Ethernet name.
So, rather than fight the system, the IEEE 802.3z committee chose to
work on both half duplex and full duplex Gigabit Ethernet, making
modifications to the CSMA/CD algorithm to support reasonable distances
in half duplex mode. The real rationales for the specification of half
duplex Gigabit Ethernet are standards-committee machinations and market
positioning. Few industry observers believe[d] that there will be much
(if any) half duplex Gigabit Ethernet deployment.
----end excerpt----
(C)1998, Addison-Wesley Longman and Networks & Communications Consulting.
All rights reserved.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
Send replies to: usenet at richseifert dot com |
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glen herrmannsfeldt
Guest
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| Posted:
Wed Feb 16, 2005 8:01 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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|
James Knott wrote:
| Quote: | glen herrmannsfeldt wrote:
|
(snip)
| Quote: | When gigabit was new, before switches were easily available, I had
thought it would be useful to have NICs and repeaters that didn't do
carrier extension for clusters of machines in a single room. Especially
when a large amount of communication is broadcast (such as loading the
same code into many machines) it would have seemed useful. Now with
gigabit prices low, it is hard to argue that, but it might be for 10Gb.
But again, what advantage is there to going with half duplex? With HD,
you've got all the problems with collision detection, reduced throughput
etc. While there were techniques developed for gigabit, to allow HD with
reasonable segment lengths, the problems are 10x worse at 10 Gb.
|
But I am specifically interested in the case where the segment lengths
are known to be short. That is, even 10Gb without carrier extension.
And yes, in a fairly short time 10G switches will be cheap, but that day
isn't here yet.
How many clusters of machines packed together as close as they can be
exist today that could use faster network interconnects? How much
faster would 10Gb half duplex be than 1Gb full duplex?
Those are the questions. I am not interested in carrier extension and
long segments.
-- glen |
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James Knott
Guest
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| Posted:
Wed Feb 16, 2005 7:02 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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glen herrmannsfeldt wrote:
| Quote: | But again, what advantage is there to going with half duplex? With HD,
you've got all the problems with collision detection, reduced throughput
etc. While there were techniques developed for gigabit, to allow HD with
reasonable segment lengths, the problems are 10x worse at 10 Gb.
But I am specifically interested in the case where the segment lengths
are known to be short. That is, even 10Gb without carrier extension.
And yes, in a fairly short time 10G switches will be cheap, but that day
isn't here yet.
How many clusters of machines packed together as close as they can be
exist today that could use faster network interconnects? How much
faster would 10Gb half duplex be than 1Gb full duplex?
Those are the questions. I am not interested in carrier extension and
long segments.
|
But again, why bother? What possible advantage is there, that you'd want to
go backwards? If you want faster, why waste time with HD? You'd be back
to the overhead of collision detection, collisions, cutting the bandwidth
in half etc. If you want to maximize bandwidth, FD is the way to go. |
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J. Clarke
Guest
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| Posted:
Wed Feb 16, 2005 8:13 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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James Knott wrote:
| Quote: | glen herrmannsfeldt wrote:
But again, what advantage is there to going with half duplex? With HD,
you've got all the problems with collision detection, reduced throughput
etc. While there were techniques developed for gigabit, to allow HD
with reasonable segment lengths, the problems are 10x worse at 10 Gb.
But I am specifically interested in the case where the segment lengths
are known to be short. That is, even 10Gb without carrier extension.
And yes, in a fairly short time 10G switches will be cheap, but that day
isn't here yet.
How many clusters of machines packed together as close as they can be
exist today that could use faster network interconnects? How much
faster would 10Gb half duplex be than 1Gb full duplex?
Those are the questions. I am not interested in carrier extension and
long segments.
But again, why bother? What possible advantage is there, that you'd want
to
go backwards? If you want faster, why waste time with HD? You'd be back
to the overhead of collision detection, collisions, cutting the bandwidth
in half etc. If you want to maximize bandwidth, FD is the way to go.
|
I think perhaps that his reasoning is that 10G HD would be cheaper to
implement than FD but still give higher performance than gigabit. Which,
while it might be true, is mostly a matter of degree--10G right now is
going to cost you whether it's full or half and using a repeater instead of
a bridge wouldn't be likely to save that much. I suspect that the same
performance level as 10G HD would provide can be achieved for less cost
with trunked gigabit.
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net) |
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Rainer Nagel
Guest
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| Posted:
Thu Feb 17, 2005 3:42 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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Hi John,
On Wed, 16 Feb 2005 10:13:14 -0500,
J. Clarke <jclarke@nospam.invalid> wrote:
| Quote: | I think perhaps that his reasoning is that 10G HD would be cheaper to
implement than FD but still give higher performance than gigabit. Which,
while it might be true, is mostly a matter of degree--10G right now is
going to cost you whether it's full or half and using a repeater instead of
a bridge wouldn't be likely to save that much. I suspect that the same
performance level as 10G HD would provide can be achieved for less cost
with trunked gigabit.
|
On the other hand the cost of half duplex (collission detection) will
follow you even if only full duplex will be deployed. So in the long
term it will be more expensive.
Although i have no real idea of the relative costs of collision
detection to rest.
But another interesting point is the delay, an repeater will introduce.
A switch is allowed to have a delay greater than the time of a collision
domain...
Ciao
--
Rainer Nagel
Rainer.Nagel@tashrah.com
Duesseldorfer Linux User Group - http://www.dlug.de |
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Stephen Brown
Guest
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| Posted:
Fri Feb 25, 2005 5:29 pm Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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I think the size of the padding in each frame would be detremental to using
half duplex
especially at the distances it would be implemented over. The cost thing
might be only
partly responsible for it. But both seem worth commenting on.
- Thanks everyone.
"Rainer Nagel" <rainer@angor.de> wrote in message
news:slrnd17j1s.9rv.rainer@ramoth.angor.de...
| Quote: | Hi John,
On Wed, 16 Feb 2005 10:13:14 -0500,
J. Clarke <jclarke@nospam.invalid> wrote:
I think perhaps that his reasoning is that 10G HD would be cheaper to
implement than FD but still give higher performance than gigabit. Which,
while it might be true, is mostly a matter of degree--10G right now is
going to cost you whether it's full or half and using a repeater instead
of
a bridge wouldn't be likely to save that much. I suspect that the same
performance level as 10G HD would provide can be achieved for less cost
with trunked gigabit.
On the other hand the cost of half duplex (collission detection) will
follow you even if only full duplex will be deployed. So in the long
term it will be more expensive.
Although i have no real idea of the relative costs of collision
detection to rest.
But another interesting point is the delay, an repeater will introduce.
A switch is allowed to have a delay greater than the time of a collision
domain...
Ciao
--
Rainer Nagel
Rainer.Nagel@tashrah.com
Duesseldorfer Linux User Group - http://www.dlug.de |
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glen herrmannsfeldt
Guest
|
| Posted:
Sat Feb 26, 2005 8:03 am Post subject:
Re: 10Gbit Ethernet Half-Duplex |
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|
Stephen Brown wrote:
| Quote: | I think the size of the padding in each frame would be detremental to using
half duplex
especially at the distances it would be implemented over. The cost thing
might be only
partly responsible for it. But both seem worth commenting on.
|
I would be interested in one where the maximum link distance was 2m.
Consider clusters of machines, MPI or PVM, for example. You can put a
lot of machines within a 2m radius of a repeater.
-- glen |
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