What Are Metropolitan Area Networks?

Metropolitan area networks (MANs) are communication networks designed to interconnect computers, devices, and local area networks (LANs) across a geographic area roughly the size of a city or large campus, typically spanning distances from a few kilometers to roughly 50 kilometers. They occupy the middle tier of the traditional three-tier network hierarchy, sitting above LANs (which serve individual buildings or floors) and below wide area networks (WANs) that link cities, countries, or continents. MANs provide the shared infrastructure that connects enterprise branches, government offices, universities, and public institutions within a metropolitan region to a common high-speed backbone.

The MAN concept emerged in the 1980s alongside the growth of corporate distributed computing, when organizations with facilities spread across a city required something faster and more private than early leased telephone lines but did not need intercontinental reach. Early implementations used fiber-optic rings based on standards such as the Fiber Distributed Data Interface (FDDI) and Synchronous Optical Networking (SONET/SDH). These were later supplemented, and in many deployments supplanted, by Ethernet-based and wireless technologies as speeds and cost-efficiency improved.

MAN Standards and Architecture

The IEEE has developed several standards relevant to metropolitan area networking. The IEEE 802.6 standard, defined in the 1990s, specified a Distributed Queue Dual Bus (DQDB) topology intended for MANs, enabling simultaneous voice and data over dual fiber rings. The most significant modern wireless MAN standard is IEEE 802.16, the basis for WiMAX technology, which defines point-to-multipoint and mesh wireless access for broadband service delivery over distances up to 50 kilometers. IEEE 802.16 specifies physical layer options covering frequency bands from 2 GHz to 66 GHz, including both licensed and license-exempt spectrum. Wired MAN deployments today typically use Carrier Ethernet services or dense wavelength division multiplexing (DWDM) over fiber, with metro Ethernet providing a familiar Layer 2 interface to enterprise customers while the service provider operates a Layer 3 IP backbone.

Media Access and Protocol Design

Controlling access to a shared MAN medium presents different challenges than LAN access because the propagation delay across city-scale distances introduces timing asymmetries that LAN protocols do not encounter. Early token-passing ring protocols such as FDDI addressed this through a timed token rotation discipline that maintained bounded latency for time-sensitive traffic. The IEEE 802.16 standard employs a connection-oriented MAC layer based on a scheduling architecture in which the base station controls uplink and downlink allocations on a per-connection basis, ensuring quality-of-service differentiation for voice, video, and data traffic. This centralized scheduling approach contrasts with the contention-based access of LANs and is specifically designed to handle the cell sizes and multiuser interference conditions of wireless metropolitan deployments.

Regional Area Networks and Scale

Regional area networks (RANs) extend the MAN concept beyond city limits to cover regional distances of 100 kilometers or more, bridging MAN and WAN tiers. The distinction is not always sharp, and the term is sometimes used to describe interconnection fabrics serving multiple metropolitan areas within a geographic region, such as a state or province. Internetworking between MANs and the broader internet relies on peering arrangements at internet exchange points (IXPs) and on border gateway protocol (BGP) routing. NIST guidelines on securing metropolitan wireless networks address authentication, encryption, and management plane protection requirements specific to the scale and shared-access nature of MAN deployments.

Applications

Metropolitan area networks have applications in a wide range of fields, including:

• Municipal broadband and last-mile internet access
• Utility smart grid communications and metering infrastructure
• Public safety and emergency communications networks
• Distributed enterprise and campus connectivity
• Transit and transportation management systems