[c-nsp] 6500s/XFPs (Was XENPAK 10GB-LW (WAN PHY))

[Richard A Steenbergen]

On Wed, May 31, 2006 at 11:01:52PM +0200, Gert Doering wrote:
> Hi,
> 
> On Wed, May 31, 2006 at 09:29:37AM +0200, Asbjorn Hojmark - Lists wrote:
> > I believe it's just about the only way you can put eight ports
> > on a C6K blade and still support LX4. Since they have customers
> > who insist on multimode, the choice must have been pretty easy.
> 
> Naive question from someone who has not looked into 10GE *that* much
> yet - "what is LX4, and why can it not be done with XFPs"?

XENPAKs connects to the host device via a standard called XAUI, which has 
4x serial data links running at 3.125Gbps each. The PHY itself is located 
on the pluggable (which is why you need seperate XENPAKs for LAN and WAN 
PHY, and they don't do OC192 FC etc). XFP connects via a single 
native 10G serial data (between 9.95G-10.7G iirc), moves the PHY onto the 
host board, and eliminates the need for seperate mux/demux of 10G to 
4x3.125G signals. This cuts the power consumed by the XFP in 1/5th, cuts 
the pluggable size in 1/10th, and results in a significant cost reduction 
for manufacturing and integration. It also makes the XFP completely 
protocol agnostic, so you can use the same optics in 10GE, OC192, FC, etc.

Unfortunately there is a lot of low grade multimode plant out there, with 
low modal bandwidths, and a lot of enterprises/campuses want to run 10GE 
over it. A native 10G signal like 10GBASE-SR doesn't get very far on this 
type of fiber (26 meters on the low end), but LX4 on the other hand runs 4 
parallel optical signals at 3.125G/ea (in 8b/10b encoding instead of 
64b/66b), roughly in the 1310nm range (I think its 4 channels between 
1270nm and 1355nm). This lets you get much longer distances over the 
existing low-grade MMF plant, up to 300 meters.

Of course LX4 lends itself naturally to XAUI because there are already 4 
native lower speed signals (plus XENPAK is big enough to easily stuff a 4 
channel WDM mux and all the other required components into it). XFP being 
a much smaller module, there just isn't room for all the LX4 components 
and an XAUI to XFI transceiver today. Thus XFP is the clear technology 
winner and is better for everyone in the long term, but XENPAK and its 
smaller (but interface-wise exactly the same) cousin X2 hang around to 
support the interfaces which use 4 channels to transport 10G over legacy 
connections (LX4, CX4 which is the same gist but over copper, etc). Being 
bigger, it also gives you first crack at new technologies like 80km reach 
optics, DWDM tuned optics, etc. That will probably change over time too.

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)