Storage Technology In Depth - DWDM


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The explosive growth of the Internet and enterprise business applications is placing tremendous demands on global enterprise and service provider networks. Mission-critical applications such as e-business, customer relationship management, storage networking, and emerging applications such as streaming media, are affecting all parts of the network from access to metropolitan- (MANs) and wide-area networks (WANs). These technology challenges are affecting every industry from financial services, healthcare, and education to telecommunication service providers.

As business services become critical to their daily lives, consumers expect instant, uninterrupted access to corporate systems and data. At the same time, unprecedented growth in storage requirements is forcing companies to reassess how and where to meet this steadily increasing demand. New storage-area networking (SAN) and network-attached storage (NAS) technology has emerged to address this issue. These technologies allow enterprises to scale their storage capabilities, providing extended geographical access while improving overall manageability of their storage resources.

Carrier deployment of fiber-optic cables in the metro area laid the groundwork for dramatic dark-fiber and high-bandwidth availability. Network connections once handled by T1 and T3 facilities, now require Fibre Channel, Enterprise Systems Connection (ESCON), Gigabit Ethernet, and in the future 10-Gigabit Ethernet, to satisfy the demand. This demand, coupled with advances in optical technology such as dense wavelength-division multiplexing (DWDM), has dramatically increased transmission capacity and reduced costs, making it economically attractive for carriers to offer dark-fiber and high-bandwidth services in the metro market.

With the preceding in mind, this article discusses storage consolidation used by storage service providers (SSP) over metropolitan area networks (MAN) using dense wave division multiplexing (DWDM). It also includes an explanation of why this technology is needed, what the advantages are, possible impact on the storage environment and some of the barriers to implementation.

So what really is DWDM? Let's take a look.

What Is DWDM?

Short for Dense Wavelength Division Multiplexing, DWDM is an optical technology used to increase bandwidth over existing fiber optic backbones. More specifically, it is multiplexing using close spectral spacing of individual optical carriers (wavelengths) to take advantage of desirable transmission characteristics (e.g., minimum dispersion or attenuation) within a given fiber, while reducing the total fiber count needed to provide a given amount of information-carrying capacity.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex eight optical carrier (OC) -48 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s. As vendors add more channels to each fiber, terabit capacity is on its way.

A key advantage to DWDM is that it's protocol and bit-rate independent. DWDM-based networks can transmit data in Internet protocol (IP), Asynchronous Transfer Mode (ATM), Synchronous Optical Network/ Synchronous Digital Hierarchy (SONET /SDH), Ethernet, and handle bit-rates between 100 Mb/s and 2.5 Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel. From a QoS (Quality of Service) stand point, DWDM-based networks create a lower cost way to quickly respond to customers' bandwidth demands and protocol changes.

Dense Wave Division Multiplexers (DWDM) Devices

DWDM devices are used for multiplexing multiple 1 Gbit/sec (or higher) channels on a single fiber. These optical multiplexers are transparent to the underlying protocols, which means that enterprises can use a single DWDM device to transfer Gigabit Ethernet, Gigabit Fibre Channel, ESCON, and SONET on a single fibereach with its own wavelength.

Enterprises can configure DWDM devices as point-to-point configurations or cumulative point-to-point configurations to form a ring. Most DWDM devices support immediate automatic failover to a redundant physical link if the main link is inaccessible. In a ring topology, only a single link is needed between nodes - if a link fails, the light is switched to the reverse direction to reach its target. Certain types of DWDM equipment can add and drop wavelengths - enabling wavelength routing in or out of a ring at 70 km to more than 160 km.

DWDM equipment is available in two basic classes - edge class (for enterprise access) and core class (for carriers). For the edge class, DWDM devices are usually smaller and less expensive, and provide fewer channels.

With the preceding in mind, an enterprise can connect two sites over 50 km by using the dual Inter-Switch Links (ISLs, a connection between two switches using the E_Port (an expansion port connecting two switches to make a Fabric)). The ISLs that lie between the switch and DWDM devices provide greater bandwidth (2 Gbits/sec instead of 1 Gbit/sec), but are not required. The DWDM devices can have a hot standby-protected link that is automatically invoked if the main link fails. The protected link should reside on a separate physical path.

For the core class, the DWDM equipment is larger and more expensive, and provides more channels. This DWDM equipment enables ring configurations and provides add and drop capabilities (later in the article an example of service provisioning across four sites is briefly discussed).

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