Storage Technology In Depth - DWDM Page 2


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MAN-Based Applications

Many types of applications can benefit from a MAN-based SAN configuration. The most common applications include those for remote storage centralization (such as a service provider model), centralized remote backup, and business continuity.

For example, an optical DWDM ring topology which provides redundant paths, has the ability to fail over from a disconnected path to an alternate path. Let's say Site B has a 70 km connection (primary path) to Site C. When that connection goes offline, Site B uses the alternate path (other direction) over DWDM to restore Site B's connection to Site C. This path (let's say from Site- B to A to C) spans 100 km (50 km plus 50 km). Because of the extended buffering at the Fibre Channel switch E_Ports, the primary and alternate paths provide nearly the same level of data access performance during testing.

Storage Centralization Over A SAN/DWDM Infrastructure

Enterprises can centralize storage across a campus or a geographically dispersed environment, or even remotely outsource the work to a Storage Services Provider (SSP). You can also have an SSP configuration where a designated site (Site C, the SSP) provides storage to multiple sites over MAN-based SANs in heterogeneous environments.

In this example, Sites A and B subscribe to Site C (the SSP). Here, zoning (a feature in Fabric switches or hubs that allows segmentation of a node by physical port, name, or address) can also be used to isolate heterogeneous fabrics, thereby controlling the amount of storage each customer site can access. Two fabric zones (one for Site A and the other for Site B) isolate storage for the two sites.

Centralized Backup Over A SAN/DWDM Infrastructure

Centralized remote backup enables multiple sites to back up data to a single shared tape library by using fabric zoning. Sites A and B can share the tape library provided by Site C, which allows the tape library into both sites' respective zones. As a result, each site can perform data backup with any tape device in the library.

Business Continuity Over A SAN/DWDM Infrastructure

A business continuity solution provides synchronous data mirroring to a remote location. In the event of a disaster, a redundant system can take over for the main system and access the mirrored data. This solution also facilitates the recovery from the redundant remote system back to the main system after it is operational again. Two sites can utilize this type of solution concurrently.

For example, Sites A and B are the primary sites (running different operating systems), and Site C is the remote business continuance site for both Sites A and B. If either Site A or B goes down, it can fail over to Site C.

A Multinode DWDM Configuration

Finally, as previously mentioned, let's briefly look at a multinode DWDM configuration that spans four sites (DWDM 1, 2, 3 and 4) and provisions optical services. For example, let's say that there are four switches, with each switch's E_Ports connected over a DWDM channel that includes dual paths for transmitting and receiving. Each path has its own wavelength. The DWDM passthrough feature enables non-contiguous sites to connect over an intermediate site as if they were directly connected. The only additional overhead of the passthrough is the minimal latency (5 usec/km) of the second link. The passthrough has no overhead since it is a passive device.

Each of the links can operate in protected mode, which provides a redundant path in the event of a link failure. In most cases, link failures are automatically detected within 50 msec. In this case, the two wavelengths of the failed link reverse directions and reach the target port at the opposite side of the ring. If the link between DWDM 1 and 4 fails, the transmitted wavelength from 4 to 1 would reverse direction and reach 1 through 3 and 2. The transmitted wavelength from 1 to 4 would also reverse direction and reach 4 through 2 and 3.

Calculating the distance between nodes in a ring depends on the implementation of the protected path scheme. For instance, if the link between DWDM 2 and 3 fails, the path from 1 to 3 would be 1 to 2, back from 2 to 1 (due to the failed link), 1 to 4, and finally 4 to 3. This illustrates the need to utilize the entire ring circumference (and more, in a configuration with over four nodes) for failover.

Another way to calculate distance between nodes is to set up the protected path in advance (in the reverse direction) so the distance is limited to the number of hops between the two nodes. In either case, the maximum distance between nodes determines the maximum optical reach. An example of this specification is 80 to 100 km for a maximum distance between nodes and 160 to 400 km for maximum ring size.

But, suppose, you have a couple of SAN switches: one at location X and one at location Z. The sites are approximately 5K apart.. You attempted to link the two switches using a dedicated dark fiber across a DWDM system. Each of the line cards you are using is specified for 850nm fiber. It failed. And, you were told that it failed because you were using short haul gigabit interface converters (GBICs (a transceiver that converts electric currents (digital highs and lows) to optical signals, and optical signals to digital electric currents)). But, the GBICs and band cards are rated for 850nm so it shouldn't be an issue.

The questions you need to ask here are: Is there some configuration feature that you need to set? Or, is there anything else you can do to establish these links across DWDM? The answers to these two questions follow next.

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