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I have
a problem with the concept of a "channelized MAC". The 802.3 MAC is defined to
operate on a single logical interface. The state machines operate on one packet
at a time, and with the exception of the WIS data rate synchronizer, there are
no storage buffers defined nor implied between the PHY and the
MAC.
I have
no problem with someone choosing to implement multiple MAC state machines, or a
single MAC state machine operating on the far side of some arbitrated FIFOs
whose inputs are multiple PHY channels. That sounds like a great product where
the line costs greatly outweigh the silicon costs (traditionally telecom), but
that should not cause increased costs in an environment where silicon costs
outweigh line costs and bandwidth efficiency is not as paramount (traditionally
data center).
Just a
gentle reminder that we're attempting to define (the proposal for) a Standard
here, not an architecture nor implementation.
-Larry
Rubin
-----Original Message-----
From: Marcus
Duelk [mailto:duelk@xxxxxxxxxx]
Sent: Thursday, August 17, 2006 9:05
AM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: [HSSG]
channelized vs non-channelized B-MAC
Hi,
I was thinking a
little more about that scalable "B-MAC"
that we have discussed here on the
reflector over the past
days and I think that there are two main
options:
1) Channelized B-MAC
The MAC has a total throughput of N*10 Gb/s
(for example N=4
or N=10) but is able to control
1..N logical interfaces with rates
between N*10 Gb/s
and 10 Gb/s. The electrical interface of the MAC device
towards the network processor would have to be channelized,
for
obvious reasons, for example SPI-6 or something similar.
As an example,
a 100 Gb/s B-MAC could control two logical
40G ports and two 10G ports.
That would mean that this B-MAC
would receive full packets on four ports
at the same time. It would
require some buffering to reassemble the
packets on the logical 40G ports,
which are constituted physically by
four 10G ports across which bit or byte
striping is performed. The B-MAC
would also require additional buffer
because it would send packets in
either interleaved or non-interleaved mode
over that channelized
interface to the NP. For a non-interleaved interface the
buffer size
would be larger because the B-MAC would need to be able to
buffer N*jumbo
packets per port. Furthermore, it would require a scheduler.
Basically, a
channelized B-MAC would not only be a traditional MAC that
is controlling
one logical interface but it would become sort sort of little
traffic
manager / scheduler. This is an additional complexity that has to be
considered.
2) Non-Channelized B-MAC
The MAC has a total throughput of N*10 Gb/s but is able to
control
only one logical interface. The service rate on that interface
can be anywhere
between 10 Gb/s and N*10 Gb/s. If a 100G B-MAC connects
say to a 40G
B-MAC then the negotiated max. service rate is obviously
40G. The 100G B-MAC
would not be able to reuse any of the remaining 60G
capacity. These would be
wasted, similarly probably to wasting 90 Mb/s of
bandwidth when connecting a
100Base-T device to a 10Base-T device.
Functionality and complexity of the MAC
would be more or less what we are
used from MAC devices. Buffers would be needed
to compensate differential
delay among lanes that belong to that one logical port. The
electrical
interface of this type of B-MAC would be non-channelized, of course.
The benefits of the second approach might be lower
because even on day one you would have to
pay for 100G MAC and a 100G
PMD/PHY even if the max. service rate you need is only 40G,
for example.
With the first approach, you would still be able to use 100% of the throughput
of
your MAC/PMD devices in this example. In any case, these scalable MACs
will allow vendors to
practically offer N*10G MAC and PMD devices with N
being practically any number. This, as
Roger pointed out, may lead to a
less distinct MAC/PMD standard and may fracture the overall
100G market
even more.
Marcus
--
___________________________
Marcus Duelk
Bell Labs / Lucent Technologies
Data Optical Networks Research
Crawford Hill HOH R-237
791 Holmdel-Keyport Road
Holmdel, NJ 07733, USA
fon +1 (732) 888-7086
fax +1 (732) 888-7074
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