27
improvement as compared with Management (ECM) uses In principle, FCoE is a simple
classic Ethernet is the another technology to achieve “encapsulation” of all Fibre
TECHNOL
elimination of the risk of packet reliable flow control between Channel frames in one Ethernet
loss. To do this, Fibre Channel endpoints and to maximise the frame. One difficulty here is
uses a buffer management available bandwidth. The IEEE that the maximum size of an
system based on “Buffer-to- 802.1Qau Group is concerned Ethernet frame is 1518 bytes,
OGY
buffer credits” with with the recognition and while a Fibre Channel frame can
corresponding confirmation by deactivation of overload be as long as 2112 bytes.
the “R-RDY” frame. Such input statuses in network nodes by Splicing the FC frames would
buffers are not used in classic using “mirroring” of frames at result in unnecessary time and
F
Ethernet. Instead, “Media the starting point (instead of cause delays. IBRE CHANNEL
access control” (MAC) frames, “frame dropping”), if these
which enable step-by-step frames cannot be immediately For this reason, “jumbo frames”
control of incoming frames, are resent in case of overload. The are used. Jumbo frames can be
used. The “IEE 0802.3X flow sender slows down the transfer up to 9 KB – large enough for
control standard” used for CEE of frames until no more an entire Fibre Channel frame.
is based on such a “Pause” “reflected” frames are received. The size of these “payloads”, in
frame, which can hold the link’s In this way, overloads can be other words, the “freight
transmission port for a certain dealt with without losing capacity” of the jumbo frames,
period of time. A second frames. is unrestricted, so that
“Pause” frame with the value potentially larger Fibre Channel
“0” can reactivate the data In what ways are CEE frames can be later
transfer before the pause time and standard Ethernet “encapsulated”.
expires. similar?
CEE makes available more In FCoE, the Fibre Channel
In conjunction with an advanced and improved frame is packaged in a
appropriate input buffer, physical and transport layers completely unaltered state in
lossless transport of frames is for standard Ethernet frames. the payload of an Ethernet
possible, although it may not Frames can be transferred as jumbo frame.
be quite as efficient as with usual, including Top Level
Fibre Channel. As with Fibre protocols. However, layers 1 What is still missing are the
Channel, IEE 0802.3X must be and 2 are significantly altered, “fabric services” that are
implemented bi-directionally to which means the existing included perforce in Fibre
ensure performance Ethernet infrastructure can no Channel environments. The
comparable to Fibre Channel. longer be used. most important ones are the
Fabric Controller and the Simple
The IEEE initiative “Priority- Let’s take a closer look at the Name Server (not to be
based flow control” (FCF) deals transfer of Fibre Channel confused with the Name Server
with the prioritisation protocol using this “new familiar to TCP/IP
of frames (PFC). This Ethernet,” i.e. “Fibre Channel environments). This means that
standard IEEE 802.1Qbb allows over Ethernet (FCoE)”. In this FCoE environments do not
application-specific bandwidth case, ANSI T11 is responsible for consist of just a single CEE
reservations in CEE. PFC can standardisation. network, but rather that the
ensure that time-critical corresponding fabric services
protocols and applications can FCoE from the infrastructure must
work at a higher priority, and Actually, it should be called also be kept readily available. If
that sufficient bandwidth for “FCoCEE.” “FcoE” gives the fabric services are not available
these protocols and impression that the Fibre in a given CEE environment,
applications is reserved. Channel protocol can be used then FCoE is impossible.
on any Ethernet. As previously
Ethernet Congestion described, this is not the case! www.brocade.com
WWW.SNS-UK.CO.UK JUNE 09
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40