Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

why 10,000,000,000 is good

10,000,000,000 bps is the right choice for the next Ethernet data rate 

A) it is exactly 10 times faster than Gigabit Ethernet.  This is more
important than many people recognize.  In the switch business, products
are evaluated (in part) on their packet forwarding rate, and on the
fraction of wire speed performance they can achieve.  At 1 Gbps, an
ideal switch forward minimum size packets at a rate of 1,488,095
packets per second.  This is ten times faster than a 100 Mbps Ethernet
port (148,809 pps) and 100 times faster than a 10 Mbps Ethernet port
(14,880 pps).  These numbers have been ingrained in the heads of
customers, testers, writers, designers, managers, salesmen, indeed any
one who has ever gotten their hands on an Ethernet switch.  To these
people, 10 Gbps means that their switch should forward 14,880,952
packets per second.

This is a very strong point of competition between vendors, and a very
common metric for comparison.  Therefore, the customers have strongly
held expectations, because every sales guy they have ever met and every
comparison they have ever read stresses this figure.  A 4% difference
between products can, and does, make the difference in a sale.

If we adopt a signaling rate which does not yield a maximum small
packet forwarding rate of 14,880,952 packets per second, we will fail
to meet customer's expectations.  Thus, we will start out with a
negative image to overcome.

B) A "10 Gig" switch port that actually runs at only 9.584640 Gbps won't
quite keep up with 96 fast ethernet ports.  96 turns out to be a nice
number of ports, because it is a multiple of 24, which is the maximum number
of ports you can line up across the front of a line card in a chassis
that is mounted in a 19" rack.  Thus, you find a lot of 24, and even
48 port line cards out in the world.

C) I am very concerned about adopting a SONET signaling rate and the
associated scrambler and framing logic, because my experience with SONET
framing chips is that they are very big and very expensive.  ATM adopted
SONET framing for the physical layer, even in the LAN environment, 
precisely because the ATM proponents wanted "seamless connectivity" 
between the LAN and the WAN.  There was no other reason to use SONET 
framing in the LAN.  However, the SONET physical layer chips for
ATM were extremely costly, so ATM host adapters and switch ports were
very costly, and this is one of the factors that prevented ATM from gaining
any serious penetration in the LAN market.

The argument has been made that, however many gates it takes to do SONET,
it can't be any worse than 1000BASE-T.  This sounds compelling, until
you consider that the silicon represents most of the cost of a 1000BASE-T
physical layer.  With a SONET based 10 gigabit Ethernet physical layer,
you will incur the cost of a complicated piece of silicon, PLUS the cost
of the optics.

D) I am also concerned that simply adopting a data rate of 9.584640 Gbps
and the scrambling polynomial and the frame structure is not the end of
the story.  Once we set off down the road towards SONET compatibility, we
will wind up with something that looks more like POS than Ethernet, and
we don't need to write a standard for POS.

E) I think that there are other choices.  In the LAN, I see no reason
why we can't use a 10,000,000,000 bps data rate (which I will
henceforth shorten to 10 Gbps).  For WAN connectivity, I think that a
device can be built that will perform rate conversion between 10 Gbps
and 9.584640E9 bps.  This device would need a relatively small amount
of buffering, and it would need a mechanism to slow down the transmit
side of the 10 Gbps MAC.  This mechanism could be 802.3x frame based
flow control.  Since the data stream to the receiving side of the 10
Gbps MAC is carrying only 9.584640E9 bps worth of Ethernet frames,
there is plenty of bandwidth available to send Pause frames from the
rate converter to the 10 Gbps MAC.  Thus, a MAC designed to run at 10
Gbps in the LAN will be throttled back to 9.584640E9 bps when connected
to the WAN through one of these rate converters.

Howard Frazier