RE: 16-bit 625Mbaud XGMII
Curt, Jaime - But in this discussion, pin count is not the only
consideration. Other important consideration is the length of the wire that
you can drive. Isn't LVDS 16 bit interface is better than half speed single
ended interface?
Can someone refer me to some discussion where we have comparison of
different options done in one place
for example something like this ..
XAUI LVDS
singled ended
speed 3 GHz 625 MHz 312 MHz
pin count 20+ 70+ 70+
distance drive ability 20" ?? 2"
coding scheme requried yes no no
design complexity
etc...
also did any proposal consider something in between 4 wide and 16 wide, for
example 8 wide differential signaling interface?
Millind Mittal
Level One Communications/An Intel Company
mmittal@level1.com
> -----Original Message-----
> From: Curt Berg [SMTP:cberg@extremenetworks.com]
> Sent: Monday, March 20, 2000 3:24 PM
> To: 'Jaime Kardontchik'; stds-802-3-hssg@ieee.org
> Subject: RE: 16-bit 625Mbaud XGMII
>
>
> Hi Jaime,
> If I recall correctly, last time a 16 bit interface
> was proposed, it was a differential interface. This would
> take about the same amount of pins as a single ended 32 bit XGMII.
> (Anyone know about a single ended PMA to PMD interface?)
>
> For design simplicity a single ended 312.5 Mbaud is very much preferred.
>
> -Curt Berg-
> Extreme Networks
>
>
> -----Original Message-----
> From: Jaime Kardontchik [mailto:kardontchik.jaime@ulinear.com]
> Sent: Monday, March 20, 2000 1:57 PM
> To: stds-802-3-hssg@ieee.org
> Subject: 16-bit 625Mbaud XGMII
>
>
>
> Hello 10G'ers,
>
> I do not remember any detailed discussion in the HSSG
> about the optimum number of octets for the XGMII,
> except for a brief discussion in Idaho, June 99, at the end
> of the presentation's section (the so called motion
> madness period), where people brainstormed about whether to
> choose a 32, 16 or 8 bit wide MII interface.
>
> An 8-bit XGMII was considered too advanced and 32 was
> considered a very matured CMOS technology (it needs a
> symbol rate of only 312.5 Mbaud).
>
> It seems that 32-bit wide was chosen by default because
> it also fitted nicely with the HARI interface that was
> being developed in parallel in another forum (Infiniband).
>
> However, 32-bits means an interface consisting of
> 2*(32+4+1) = 74 I/Os. This is very different from the
> small MII interfaces we were used to in previous versions
> of Ethernet. An interface with two many I/Os leads people
> in a natural way to try to hide it and avoid implementing
> it. This gave rise to the second interface, XAUI, with
> only 8 lanes (4 for Tx, 4 for Rx). This interface has
> the advantage that can be easily exposed. However, it
> comes with an added weight that the XGMII did not have:
> the I/Os in the 4-lane XAUI are not raw data but encoded
> symbols. And everyone proposes a different encoding: 8b/10b,
> 64/66, PAM-5, etc., etc. Hence, the 4-lane XAUI creates
> a serious problem of compatibility and interoperability.
>
> The solution seems to be to go back to the XGMII and
> use a 16-bit wide interface with a symbol rate of 625 Mbaud.
> This amounts to 2*(16+2+1) = 38 I/Os, total. This is an
> interface that can be exposed. In addition, since the I/Os
> send/receive raw unencoded data, this interface is
> compatible and interoperable with any coding scheme applied
> later in the chain to the raw data.
>
> A 16-bit wide interface is a well known interface for the
> WAN PHY proponents that already use 16-bit wide interfaces
> between the PMA and the PMD.
>
> For those that need to carry the 10 Gbps over long Copper
> traces in the backplane before reaching the PMD and wish
> to use in the Copper traces the 4-lane scheme shown in
> H. Frazier's presentation "10Gig MII update", Nov 99,
> they only have to add at the input of their PCS/PMA
> chip a simple 1:2 serial to parallel converter to go
> from 16-bits wide at 625 Mbaud to 32-bits wide at
> 312.5 Mbaud, and from then on use the 4-lane architecture
> they wish (see page 19 in H. Frazier's presentation).
>
> The same applies to those that want to run the MAC at only
> 312.5 MHz.
>
> Finally, for those that would like to integrate in the
> future the MAC with their specific PCS, they can eliminate
> the XGMII and run both the MAC and the PCS at 312.5 MHz,
> without the need to upconvert and downconvert at the
> (deleted) XGMII.
>
> Using a 32-bit wide exposable XGMII eliminates, in practice,
> the need to define an additional XAUI interface, which is
> in fact a PHY transceiver over Copper for backplanes using
> a specific coding scheme.
>
> 10 Gbps PHY transceivers over Copper backplanes should be
> left outside the scope of the 10 GbE Standard.
>
> Jaime
>
> Jaime E. Kardontchik
> Micro Linear
> San Jose, CA 95131
> email: kardontchik.jaime@ulinear.com