domingo, 14 de febrero de 2010
The metropolitan network market is being driven by demand for new application services and the introduction of high speed access. Taken together, these forces are creating a bottleneck in the MAN.
New applications include e-commerce transactions, packetized voice, and streaming multimedia. New services, primarily to the enterprise, include interconnecting and consolidating data centers, transparent extension of the LAN across the MAN by connecting geographically disparate locations using wavelengths over dark fiber, a trend towards SAN architecture, the server-less office, real-time transactions backup, and high-speed disaster recovery. For service providers, new services include support for access technologies such as DSL, cable, and wireless (which still requires a land-based transport infrastructure) and wavelength leasing or wavelength-on-demand.
Two of the most important applications for DWDM technology in the MAN are in the areas of SANs and SONET migration.
Storage Area Networks
Storage area networks (SANs) represent the latest stage in the evolution of mass data storage for enterprises and other large institutions. In host-centric environments, storage, as well as applications, was centralized and centrally managed. With the advent of client/server environments, information that was previously centralized became distributed across the network. The management problems created by this decentralization are addressed in two principal ways: network attached storage (NAS), where storage devices are directly attached to the LAN, and SANs.
Composed of servers, storage devices (tapes, disk arrays), and network devices (multiplexers, hubs, routers, switches, and so on), a SAN constitutes an entirely separate network from the LAN (see Figure 3-3). As a separate network, the SAN can relieve bottlenecks in the LAN by providing the resources for applications such as data mirroring, transaction processing, and backup and restoration.
A number of types of interfaces have been used to connect servers to devices in a SAN. The most prevalent is IBM's Enterprise System Connection (ESCON), a 17-MBps half-duplex protocol over fiber.
Fibre Channel, on which IBM's FICON is based, is also frequently employed in SANs and has a much higher capacity than ESCON. Both technologies, however, have significant distance limitations. For example, the standard maximum distance without repeaters is around 3 km (1.9 mi) for half duplex ESCON and around 10 km (6.2 mi) for full duplex 100-MBps Fibre Channel. There is performance degradation as distances increase beyond these numbers.
This distance limitation can be overcome by transporting data between one or more enterprise locations and one or more SANs over the optical layer using DWDM. In Figure 3-4, for example, the distance separating the enterprise location and the SAN sites can be greatly extended. Access to the ring is by way of "satellite" OADMs with Fibre Channel or ESCON interfaces at each SAN location (one of these could provide data mirroring). These interfaces can also support Sysplex Timer and Sysplex Coupling Link interfaces, used in IBM environments for distributing loads across the members of a server complex.
In addition to overcoming distance limitations, DWDM can also reduce fiber requirements in SANs. Both ESCON and FICON require a pair of fibers for every channel. By multiplexing these channels over DWDM transport, significant savings can be realized.
Migration from SONET/SDH
As a transport technology, SONET is an "agnostic" protocol that can transport all traffic types, while providing interoperability, protection schemes, network management, and support for a TDM hierarchy. Although SONET may continue to be the interface standard and transport protocol of choice well into the foreseeable future, upgrading it is expensive, as line-rate specific network elements are required at each point of traffic ingress or egress.
Using DWDM to increase the capacity of embedded fiber, while preserving SONET infrastructure, offers an alternative to expensive SONET upgrades. Migration from SONET to DWDM may in fact be the single most important application in the near term. In general, this migration begins by replacing backbones with DWDM, then moves toward the edges of the network.
In one common scenario, bandwidth on a SONET ring can be increased greatly by replacing SONET ADMs with DWDM equipment. In the example shown in Figure 3-5, there are three options for upgrading the ring:
· Replace or upgrade the equipment; for example, from OC-48 to OC-192.
· Install a new ring on new or existing fiber.
· Install one or more new rings by deploying DWDM.
The third option is shown in Figure 3-6. By using DWDM to increase the capacity of the existing ring, one fiber can essentially act as many.
In a second type of scenario, DWDM can be used to remove an entire class of equipment, the SONET ADMs. This change, which might constitute a second phase of SONET migration, allows routers and other devices to bypass SONET equipment and interface directly to DWDM, while simplifying traffic from IP/ATM/SONET to POS to eventually IP directly over the optical layer (see Figure 3-7).
In this phase of migration, end user sites are served by OADMs rather than SONET ADMs. In this way DWDM rings and mesh networks can eliminate the increased cost and complexity of introducing more SONET elements into the network to meet demand. The advantage here for carriers is the ability to offer bit-rate-independent services, freeing them from the DS1/DS3/OC-n framework. Such a scheme would also allow enterprise LAN access to be extended across the MAN or WAN without a SONET infrastructure.
A further advantage in migrating from SONET to the optical layer is that protection and restoration becomes less susceptible to failure of electronic components; a common survivability platform for all network services is created, including those without built-in protection.
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Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 14:03
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