Fibre Channel Back-End Redundancy - EMC Symmetrix DMX-3 Product Manual

Data Integrity, Availability, and Protection
LCC B LCC A
SYM-000422

Fibre Channel back-end redundancy

Figure 38
Data integrity for Fibre In order to ensure that the data transmitted by the disk director is the
Channel arbitrated
same as the data received by the disk drive, a multilayer protection
loops
strategy is employed. At the highest level, as data is transmitted
between the global memory and the disk, a connection referred to as
an exchange is established.
Logical parameters that identify this connection are initialized, and
then all information transferred as part of this exchange is identified
with these parameters. In addition, as the data in global memory is
encapsulated into Fibre Channel frames, it is marked with a sequence
number that identifies the order of the frame within the exchange
and, in addition, a cyclic redundancy checksum is computed over the
data and appended to the data frame. The receiving port (after
validating the exchange parameters) will recompute the checksum
for the data received and compare it to the transmitted checksum to
determine if there are any bytes-in-error. It will also check the
sequence number to ensure that the received frame is the next one
expected for that exchange.
The next level of protection is designed in at the transmission layer of
the Fibre Channel protocol. The data frame, with the additional
information appended, is translated into special 10-bit data
transmission symbols. These symbols are chosen for their ability to
increase the reliability of the transmitted data, whether it is through a
fiber-optic or copper cable.
Reliability and availability features 167