Network Types - 2

Metropolitan Area Networks

A metropolitan area network (MAN) is a hybrid between a LAN and a WAN. Like a WAN, it connects two or more LANs in the same geographic area. A MAN, for example, might connect two different buildings or offices in the same city. However, whereas WANs typically provide low- to medium-speed access, MANs provide high-speed connections, such as T1 (1.544 Mbps) and optical services.

The optical services provided include SONET (the Synchronous Optical Network standard) and SDH (the Synchronous Digital Hierarchy standard).With these optical services, carriers can
provide high-speed services, including ATMand Gigabit Ethernet.

These two optical services provide speeds ranging into the hundreds or thousands of megabits per second (Mbps). Devices used to provide connections for MANs include high-end routers, ATMswitches, and optical switches.

Storage Area Networks

Storage area networks (SANs) provide a high-speed infrastructure to move data between storage devices and file servers. A storage device, sometimes referred to as a storage unit, includes disk drives, disk controllers, and any necessary cabling. This infrastructure can be dedicated to just these devices or can include other devices.

Typically, fiber channels are used for the connections. A fiber channel is an optical cable that connects the file servers, disk controllers, and hard drives at rates exceeding 1 gigabit per second (Gbps). The advantages of separating the storage device from the file services are more flexibility and centralization of storage, which eases management.

SANs are becoming very popular in LAN environments, and some ISPs and carrier companies are starting to offer these services in MAN environments. However, SANs are not currently being used inWAN environments, because of the connection types and access speeds required.

Since optical connections are used, you gain the following advantages over normal
storage techniques (keeping the data local to the server):

■ Performance is fast.
■ Availability is high because of the redundancy features available.
■ Distances can span up to 10 kilometers.
■ Management is easy because of the centralization of data resources.
■ Overhead is low (uses a thin protocol).

The main disadvantage of SANs is their cost. If you are using fiber channels, you must buy special disk controller cards for your file servers and buy a SAN storage unit, and you must lay down the necessary fiber.

Of course, if you are using your own network infrastructure, you only need to buy a storage unit and lay down any necessary cabling for it. Plus, if you are concerned about redundancy, your cost will increase because you’ll need to ensure that your network infrastructure has redundant paths between your servers and the SAN storage unit(s).