Storage Technology In Depth - DWDM Page 3


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Troubling Cables Across A DWDM

Having the correct cable connections will make or break your link to a remote site. To help ensure you have the right configuration, lets now take a look at the specs for fiber connections, including the maximum distance each can have between devices.

The Problem

The problem it appears, is related to how your connections are set up. The correct specifications for fiber connections are as follows:
  • Shortwave (850 mm) GBIC using 50u cables have a maximum distance or 500 meters between end devices.
  • Longwave (1300 mm) GBIC using 9u cables have a maximum distance of 10 kilometers between end devices without an extender/repeater.
  • By using DWDM on dark fiber (9u cables), the distance can be extended to up to 100 kilometers between end devices.

The Solution

So, for you to create an extended fabric between your sites at 5Km apart, your configuration should look like this:
  • Host to switch=50u Multi-Mode Cable to short wave GBIC (850mm).
  • Switch host port uses short wave GBIC (850mm).
  • Switch to DWDM= 50u Multi-Mode cable to short wave GBIC (850mm).
  • DWDM to remote site connection=long wave GBIC (1300mm) to 9u single-mode cable (dark Fibre).

Thus, your line cards have short wave multi-mode connections. That's fine for the local connections, but you need longwave single mode connections for the link between the sites.

Finally, you would actually have the same setup on the remote side. You may also need the SAN extension license for your switches, as that pumps up the number of available buffer credits between the switches. The bottom line here is, that all of these distances should continue to increase as fiber optic technology advances.

Summary And Conclusions

DWDM is ideal for reliable metro area connectivity between two data centers, while SONET provides high Time Division Multiplexing (TDM) bandwidth over longer distances. Both technologies provide excellent transport options for remotely replicated data over Fibre Channel or Fibre Channel Over Internet Protocol (FCIP).

Finally, by internetworking SANs over distance across MANs using DWDM, enterprises can implement a highly reliable environment; allowing enterprises to replicate business-critical data to remote locations; and, supporting business continuance applications such as data mirroring, data replication, electronic tape vaulting, and remote server clustering. These business continuance applications, and associated storage-area networking (SAN) technology, such as Fibre Channel and ESCON, requires a fault-tolerant, high-bandwidth, and low-latency network. For synchronous mirroring, the low latency of a DWDM optical network is critical to avoid a negative impact on application performance.

About the Author :John Vacca is an information technology consultant and author. Since 1982, John has authored 36 technical books including The Essential Guide To Storage Area Networks, published by Prentice Hall. John was the computer security official for NASA's space station program (Freedom) and the International Space Station Program, from 1988 until his early retirement from NASA in 1995. John can be reached at jvacca@hti.net.

» See All Articles by Columnist John Vacca

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