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RE: Long distance links


At 02:11 PM 8/30/99 -0600, DOVE,DANIEL J (HP-Roseville,ex1) wrote:
>Hi All,
>Have we come to concensus yet? 
>I keep seeing things flying across the reflector about how we need SONET
>OAM&P, SONET's 9.58464 data rate at the MAC/PLS interface, etc...  I thought
>that the initial issue was having SONET's signalling at the PHY PMD

I believe management and rate are separate topics. They are coupled in the
sense that both become an issue when considering Ethernet in the WAN.

>For those who want a 10.0000 Gigabit Ethernet that operates as a "Local Area
>Network" (LAN) and allows us to hook our switches together in wiring
>closets, buildings, and campuses cost effectively, we have an architecture
>that will work.
> [ 10 Gig MAC ]->10 Gig MII-> 10 Gig PHY (SMF/MMF/Cu)
>For those who want their long-distance WAN solution
> [ 10 Gig MAC ]->10 Gig MII-> 9.58464 SONET PHY (SMF)

The WAN PHY is not SONET. SONET does not specify a method for carring
Ethernet frames. SONET requires full management. SONET has very tight clock
requirements. The reason we are getting confused is that we are calling the
WAN PHY SONET and, in so doing, are carrying more bagage than Ethernet can
stand. I believe what is being proposed is the use of some OC-192
technology for the WAN PHY which might be called 'SONET light' or 'Thin
SONET', but is not OC-192. The issue we are debating is how much of SONET
needs to be retained to fill the market requirement for easy interface and
operation over existing WAN networks. To make it clear we are not talking
about SONET we should refer to the PHY as the WAN PHY.

>which uses HOLD as a mechanism to prevent overflow of the SONET PHY
>on outbound traffic. Now, I have a few questions about this SONET PHY that
>are due to my inexperience with SONET signalling. I am willing to
>expose my naivete in the interest of gaining knowledge on the specific
>problems with my vision on this matter.
>1) Assuming the HOLD mechanism, what is essentially wrong with using a
>10.0000 Gbps MAC/PLS interface?

What is wrong with this is it forces the PHY to use a different transmit
clock than the MAC since the PHY wants to send a 9.584640 Gbps data stream
on a 9.953280 Gbaud carrier. A better solution is to use a 9.953280 Gbaud
clock at the MAC with a HOLD mechanism. This will allow the MAC and PHY
transmitters to be synchronous.

If a word-by-word HOLD is used, then the PHY will not require any
buffering. Using any of the IPG stretching techniques, including IPG HOLD,
will force the PHY to have about 1 packet worth of transmit and 2 packets
worth of receive queue. The reason these queues are much larger than
required for the rate match is because the transmitter must synchronously
transmit overheads mixed in the data stream. While the overheads are being
transmitted the PHY must either be able to HOLD the MAC or must buffer as
many bytes as the overhead area. On reception the PHY must either be able
to put the receiver in HOLD over the overhead area or must have sufficient
packet data buffered to feed an entire packet to the MAC synchronously.

>2) Why can't OAM&P functionality be performed in the SONET "PHY" where
>it renders the least impact on the overall system architecture?

Yes, the WAN PHY could perform OAM&P without impact on the upper layers.
The issue is not implementation, but is cost. For connections over photonic
networks OAM&P function are required, however for LAN connections they are
an overkill. It is very desirable to have a PHY which can operate with or
without OAM&P so those who need it pay for it, but those who do not can
have cheap interfaces.

> 2a) Can't we use MDIO/MDC to allow management access to the necessary
>     PHY parameters in case processing is required?


>Now I totally realize that I am using the term "PHY" loosely as the SONET
>PHY function may require extensive processing capability. But from an
>architectural/specification perspective, it makes it much more clean than to
>burden a short-haul system with the same complexities as a long-haul system.

This is why SONET is not proposed for the PHY. The OAM&P function does
require considerable processing which is expensive for in building
applications while very valuable for WAN applications.

>Now, if I have a vision that is workable (subject to debate) why are we
>continuing to discuss a 9.58464 MAC/PLS interface or OAM&P functionality in
>the MAC?
>Dan Dove
>HP Workgroup Networks
>PS: Is there a good white-paper on SONET signalling on the web?

Have a look at:

Paul A. Bottorff, Director Switching Architecture
Enterprise Solutions Technology Center
Nortel Networks, Inc.
4401 Great America Parkway
Santa Clara, CA 95052-8185
Tel: 408 495 3365 Fax: 408 495 1299 ESN: 265 3365
email: pbottorf@xxxxxxxxxxxxxxxxxx