Nortel Networks OPTera Metro 3500 - Manual (Page 134)

2-96 Operation, administration, and maintenance (OAM) features
OPTera Metro 3500 Multiservice Platform NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
Figure 2-38
Storage over Sonet
EX1495p
Storage networks cannot tolerate data discard. Fibre Channel devices employ
a credit-based flow control mechanism to guarantee delivery between storage
devices in the network and to ensure that the rate at which the data is sent by
the source does not exceed that at which it can be received at the destination.
To establish a link by link flow control, ports of the two connected storage
devices first exchange and agree on the number of frames each can receive.
This is called the Buffer-to-Buffer Credit (BBC). Whenever the source device
sends out a frame, it increments the credit counter (or BBC counter) by 1. The
receiving device will send back an acknowledgement message, called R_RDY,
upon receipt of each transmitted frame. Once the source device receives the
R_RDY, it lowers the BBC counter by 1. If the BBC counter reaches the
previously agreed credit threshold, the source device simply stops sending
frames until the BBC counter is lowered below its threshold by receiving
another R_RDY from the destination.
When storage needs to be extended over distance, the storage devices need to
provide a sufficient number of buffer credits to compensate for the latency
introduced by the link so that the system can still achieve maximum link
efficiency
OPTera Metro 3500 storage extension solution offers the ability to transport
Fibre Channel traffic over 1,000's of kilometers. OPTera Metro 3500 solution
enables service providers to offer a fully managed service, independent of the
end user devices. Fibre Channel client traffic can be mapped into both full-rate
and sub-rate SONET signal.
OM3500
Data center
Storage extension
Data center
Fiber
channel
Fiber
channel
Disk arrays Disk arrays
Servers Servers
Fiber
channel
switches
Fiber
channel
switches
SAN SAN
OM3500
SONET
WAN