Metropolitan area network

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Metropolitan area network

Pinning down the definition of a metropolitan area network (MAN) is a bit more difficult than pinning down the definition of a LAN. However, a few themes surface from the varied definitions of the term. In general, the MAN lives up to its name: it scales to the metropolitan area and is most commonly found in large metro areas. Although there is no technical reason a MAN could not be deployed in Huntington, Vermont, until the local cows have higher bandwidth requirements, it is unlikely to happen. But MAN technologies are commonplace in New York, Philadelphia, Atlanta, Los Angeles, and even smaller cities with populations as low as 50,000–100,000. This also makes a MAN largely an intraLATA service.

The scale of a MAN means it is fairly consistently a carrier service. LANs are not. While some carriers have begun offering LAN services to their customers, this generally takes the form of an outsourced IT activity. The carrier comes in and deploys a LAN on the customer’s behalf. A MAN, on the other hand, is typically a shared network service. The carrier builds out the backbone service, and multiple customers share the facilities. In this respect, a MAN is a closer cousin to the WAN than to the LAN. The metro scale of MAN services also opens the door to significant competition. A carrier does not need to build a vast national network to effectively compete in a given metro area.

Historically, customers have used various services to deploy metro-scale networks, including private line, SONET, frame relay, and ATM services. These services are also found at WAN scales. For a brief period in the mid 1990s, the industry flirted with a defined MAN service called switched multimegabit data service (SMDS). SMDS died an inglorious death, however.

Another MAN service emerged in the early-to-mid 1990s, however. These so-called transparent LAN services gave the customer a native LAN handoff on their premises, but used a variety of proprietary technologies to carry the traffic across the metro area. They were popular with the customer because they combined the familiarity of their LAN environments (mostly Ethernet and token ring) with the metro scale the customers were seeking.

By the early 2000s, advances in Ethernet made it possible for carriers to begin to morph these services into true end-to-end Ethernet services, which is the trend today. In fact, Ethernet is rapidly becoming the MAN technology of choice and native Ethernet capability is being added to traditional carrier technologies, such as SONET. Emerging technologies like resilient packet ring (RPR) are further enhancing these metro offerings.

Responsibilities of the MAN

Metropolitan area networks are typically switched Ethernet offerings that provide interconnection of LANs and CANs from a single or cluster of switches. These service provider-based network offerings provide security and quality of service capabilities that are limited to the lowest common denominator of the equipment deployed. In most cases, the integrity of the LAN or CAN security or QoS tagging is maintained over the MAN.

The responsibilities of the MAN are listed below.

  • Business continuation and disaster recovery: Via fast convergence and survivable options (dual diverse access links)
  • Inter-site communications: Allow the connection of locations miles apart at local speeds and reasonable prices.
  • High-speed connectivity: MAN offerings today at gigabit speeds, plenty for most connectivity needs