100FX SFP: Whats the difference between normal ones and SGMII?

[Steffen Koepf]

Hello,

for 100FX SFPs there seem to be two different versions: Normal ones and
ones with SGMII interface on the electrical side.

As far as i understand, 100FX means on electrical side PECL signaling,
serialized 5B4B encoded NRZI data at 125MHz.
SGMII in 100FX mode should output every bit 10 times.
But a normal 100FX SFP seems not to work in a SFP Slot that uses SGMII.
Does one know what the difference is?

Thanks,

Steffen

[Allan Herriman]

The "normal" SFP modules just have a straightforward optical to
electrical and electrical to optical conversion. What you get on the
optic fibre is what you get on the SFI (the electrical side). For
100Base FX this is 4B5B encoded as you said, and for 1000Base-X this will
be 8B10B encoded.

The ones that have an SGMII also contain an Ethernet PHY chip to do the
conversion.

SGMII runs its physical layer at 1.25Gb/s, 8B10B encoded at all times,
regardless of the negotiated data rate (100Mb/s in this case). As you
said, it will repeat the same symbol once (for 1Gb/s); ten times (for
100Mb/s) or one hundred times (for 10Mb/s)


Now, if you have an SFP socket backed by a PHY that only understands
SGMII, it will only understand 1.25Gb/s, 8B10B.
If you plug your normal SFP into this it will only work if the link
partner produces 1000BaseX ('cause that is encoded the same as SGMII).
It will not work if the link partner produces 100Base-something, as the
SFI signal will be 4B5B at 125Mb/s, rather than 8B10B at 1.25Gb/s.


Whilst I have seen SFP modules that produce SGMII (in particular the
"Copper" "RJ45" SFPs work this way), I have never seen an SFP socket that
only accepted SGMII. I'm curious, which equipment is this? Can it be
configured to accept normal SFI rather than SGMII?

Regards,
Allan

[Steffen Koepf]

Hello Allan,

thanks for your response.
So, the SGMII implementation used here is a bit off-standard because it
does not have clock lines - both sides have to use CDR.

Allan Herriman <allanh...@hotmail.com> wrote:
> Whilst I have seen SFP modules that produce SGMII (in particular the
> "Copper" "RJ45" SFPs work this way), I have never seen an SFP socket that
> only accepted SGMII. I'm curious, which equipment is this? Can it be
> configured to accept normal SFI rather than SGMII?

Some models of the HP Procurve switches have SFP slots that can either
be poulated with a Gigabit SFP or a 100 MBit one.
When i try to get a 100FX SFP working in this switch, the Link LED on
the other side (a 100FX Media Converter) is only flickering. The original
HP 100FX SFP is a SGMII SFP as i found out.
I do not know if there is a way to get the SFP Slot to operate in SFI mode.

Then i found this:
https://docs.oracle.com/cd/E25618_02/E25355/html/sr3-plus-nw-sfp.html
So other devices seem to have this problem, too.

Btw, the standard document INF-8074i does not tell anything about
SGMII or SFI signalling.


Steffen

[Allan Herriman]

Hi Steffen,

It's been a while since I've read them, but I understand that the SFF
standards (e.g. INF-8074i) only describe the physical layer, e.g. voltage
levels, mechanical dimensions, etc.

The protocols are mostly described in IEEE 802.3 except that the IEEE-
defined MII connections are too expensive to implement. This gives rise
to manufacturer-defined "standards" such as SGMII or RGMII, which use far
fewer connections. IIRC, SGMII ("serial" GMII) is a Cisco baby, whilst
RGMII ("reduced pincount" GMII) came from HP.

Regards,
Allan

[Cydrome Leader]

Steffen Koepf <Taxman...@opaya.de> wrote:
> Hello Allan,
>
> thanks for your response.
> So, the SGMII implementation used here is a bit off-standard because it
> does not have clock lines - both sides have to use CDR.
>
> Allan Herriman <allanh...@hotmail.com> wrote:
>> Whilst I have seen SFP modules that produce SGMII (in particular the
>> "Copper" "RJ45" SFPs work this way), I have never seen an SFP socket that
>> only accepted SGMII. I'm curious, which equipment is this? Can it be
>> configured to accept normal SFI rather than SGMII?
>
> Some models of the HP Procurve switches have SFP slots that can either
> be poulated with a Gigabit SFP or a 100 MBit one.
> When i try to get a 100FX SFP working in this switch, the Link LED on
> the other side (a 100FX Media Converter) is only flickering. The original
> HP 100FX SFP is a SGMII SFP as i found out.
> I do not know if there is a way to get the SFP Slot to operate in SFI mode.

have you played with autonegotiation or forcing the port speeds on the
switch, the media converer or even the remote copper port attached to the
media converter? This would be the first thing to do.

Media converters are really obnoxious devices that cause all sorts of
problems that can be hard to troubleshoot.