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Considerations for > 5 kms.


I would like to suggest a couple of topics for discussion, in case you
haven't already included them in your agenda.

1. Use of WWDM in 1550 nm range for dark fiber, single mode, metropolitan
area applications: It seems to have some advantages. A combination of DFB
Isolated lasers and low attenuation will permit the link to support useful
distances (25 kms.?). If wavelengths are spaced widely enough, use of
uncooled lasers will reduce cost and complexity of transmitters. These
lasers can be directly modulated. If four wavelengths are selected, each
wavelength needs to carry data only at 2.5 Gbps (3.125 Gbaud for 8B10B),
which makes jitter issue more manageable compared to the 10 Gbps serial

2. Flow Control Implications: I am perplexed as to why no one is talking
about the impact of extended distances on the buffer size of 10GbE
switches. Compared to Gigabit Ethernet, as we consider longer distances and
ten times the bit rate, it will take a much larger buffer in a switch to
operate properly. This is because a switch must accommodate the greater
number of bits that can still be in the pipeline after the 802.3x flow
control is invoked. (Propagation delay through one kilometer of fiber is
the equivalent of more than 6 kilobytes of data at 10 Gbits/sec.) The
longer the distance, the greater the buffer size required, or the switch
has to drop the packets. Until now, this hasn't been an issue because
Ethernet was not expected to work at long distances so modest buffer size
was adequate. If we want 10GbE to work over MAN or even WAN distances, the
required increase in buffer size for switches may be so large that at some
point, packets have to be dropped. That may well be the basis of a
cost-throughput tradeoff, but we should decide if we want to address that
tradeoff or leave it to implementers.


Vipul Bhatt
Finisar Corporation
274 Ferguson Drive
Mountain View CA 94043
Phone: (650)691-4000 x113
Fax: (650)691-4010
Email: vipul.bhatt@xxxxxxxxxxx