Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Does only decimal system work for data rate specification?


I have seen some of the emails, and also missed some. 
I am a designer for a single-chip solution of gigabit Ethernet switcher. 
I was a little bit confused by this discussion group. I have the following 

1) Why we prefer to only specify 10GE rather than suggest other 
reasonable data rates related to gigabit Ethernet, say  2.5/5 gigabit 
2) Why we need Hari coding to tie four individual links together instead 
of sending data separately?
3) Does our data rate specification fit with market requirement and 
technology progress well? 
I think the last question is very critical. If a specification does not 
match the market and technology progress, it will die! 

I also have the following points: 
1. From a customer point of view, only 1GE and 10GE give less choices 
for one to build a good switching structure. He has to chose between two
possible data rates, either 1GE or 10GE. It might be very difficult for him
to upgrade his data links, in other words, 10GE is too luxurious and 1GE 
is insufficient for his requirement. What is his best solution? Perhaps
he has to wait until the price of 10GE comes down to an acceptable value. 
This acceptable value may never come if the standard does not fit to the 
market requirement. However, if there is a 2.5/5 gigabit Ethernet data rate,

he could flexibly upgrade his network according to his real requirement.

2. Data rate evenly distributed networks are seldom met in reality, and it 
exists only in people's imagination. Designing a switcher fabric based on
this imagination will cause a big waste of resource. For data rate unevenly 
distributed networks, multiple data rates can give one more solutions, 
therefore a relative optimum is easy to achieve. The future 10GE switcher 
will face a serious challenge of limited I/O capacity. How to efficiently
of the I/O pads is a very important issue. If we could use 2.5/5/10 GE data 
rates, we can individually assign a data rate for a path in a network 
according different requirements. Multiple data rates can be very helpful
optimal I/O pad usage.

3. There is no reason why we only specify 10GE, which bonds 4 individual 
links together. Probably there isn't any information source or sink today 
which has to be specified at 10Gb/s. Why we have to glue 4 individual links 
together just for making more problems in design, test, and maintain? The 
only clue is the historical reason that Ethernet had migrated from 10ME to 
100ME, and from 100ME to 1GE. Does only decimal system work for data 
rate specification?

 4. If 4 individual links at 2.5Gb/s are glued together to form a 10GE,
we need word synchronization, as someone suggested using Hari coding.
Transmission at 10Gb/s (or 12.5Gb/s with 8B10B coding) are more difficult 
than sending 4 links individually. From reliability point of view, an 
encapsulation of several links will greatly reduce network reliability. 
For example, if one of the link is broken the whole frame of data is 
destroyed, therefore, nothing could pass even if the rest three links 
function well.

5. At today's technology, few devices could support 10GE within one link
for a meaningful distance. From technology point of view, there isn't any
remarkable breakthrough so that we can utilize a new and cheap device 
to implement 10GE links. For most of the transmission media, what we 
can utilize are the lower speed links. 10Gb/s data can only be transmitted 
at distance about a few dozens cm in PCB.  Long distance optical fiber link 
is an exception, and that means we only need 10GE at switcher interface 
to the long distance trunks.

6. In fact, we could simply extend the Gigabit Ethernet standard to 
2.5/5 GE specification by adapting new speed as a smooth move. If we 
use 8B10B encoding, the speed will be 3.125/6.25 Gb/s, respectively. 
Simple and efficient! Why not?

Thank you for your attention

Fenghao Mu	

SwitchCore AB		phone: +46 46 270 15 85(o)
Scheelevägen 32		fax: +46 46 270 15 81
S-223 63  LUND 
SWEDEN			homepage: