Nortel Networks NTRN10AN, 3500 manual Storage over Sonet

2-96Operation, administration, and maintenance (OAM) features

Figure 2-38

Storage over Sonet

EX1495p

Disk arrays Disk arrays

	Fiber	SONET	Fiber
		WAN
SAN	channel		channel	SAN

	Fiber	OM3500	OM3500	Fiber
	channel			channel
Servers	switches			switches	Servers

Data center Data center

Storage extension

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.

OPTera Metro 3500 Multiservice Platform NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004