Archived from groups: comp.dcom.lans.ethernet (
More info?)
In article <vnmzu7rqje.fsf@kenny.ex.ac.uk>,
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 ;-)
>
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
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