Metropolitan area network (MAN) is a concept that can mean several things depending on the context. For example, a MAN could be ? A telco service that provides data transport across an urban area using ATM over SONET technology. ? Several local area networks (LANs) connected across an urban area using a high-speed fiber-optic backbone. ? Several connected networks within a city, forming a citywide network for a specific government organization or corporation. ? Any network bigger than a LAN but smaller than a wide area network (WAN). ? Any network that spans more than a single building. ? A Distributed Queue Dual Bus architecture network based on the IEEE 802.6 standard of Project 802 (this is the formal definition of a MAN, but it is now obsolete). The first definition on this list is the one most familiar to enterprise network architects. To build a WAN consisting of LANs in different geographic locations, enterprises lease MAN services from their local telco. These MAN services thus enable LANs to be joined together into a WAN, and the MAN thus functions as the demarcation point between LAN and WAN. Common MAN services offered by telcos include Integrated Services Digital Network (ISDN), T1 and T3 lines, and Digital Subscriber Line (DSL) services. Enterprises typically interface their LANs with the telco MAN by deploying an access device (also called a WAN switch or access multiplexer) in the basement of their equipment room. The access device then aggregates company LAN traffic and backhauls it from the customer premises to the Point of Presence (POP) of the telco, where it typically traverses the metropolitan area over a dual SONET ring. Prospects A new development is the emergence of metropolitan Ethernet service providers, which compete with traditional telcos for hauling data traffic by building their own fiber MANs and provisioning fiber to businesses in downtown areas. Metropolitan Ethernet is cheaper than traditional telco WAN services that rely on ATM over SONET, is easier to deploy and manage, and is fast becoming a viable alternative for the MAN demarcation point between LAN and WAN. See Also: ATM over SONET, local area network (LAN), metropolitan Ethernet, wide area network (WAN) Metropolitan Ethernet A service that uses Gigabit Ethernet (GbE) to provision metropolitan areas with high-speed data services. Overview Metropolitan Ethernet represents a new kind of data service now offered in many dense urban areas. Traditionally wide area network (WAN) and metropolitan area network (MAN) services have been provisioned by telcos, particularly by Incumbent Local Exchange Carriers (ILECs) that own the local loop telephone wiring connecting customer premises to their services. Telecommunication services offered by telcos have included dial-up Internet access, Integrated Services Digital Network (ISDN), T1 and fractional T1 lines, and recently Digital Subscriber Line (DSL). The monopoly enjoyed by telcos as a result of local loop ownership has enabled them to maintain high prices for many of these services. For example, a T1 line typically costs around $1,000 a month and provides only 1.5 megabits per second (Mbps) bandwidth, but a T3 line offers a much higher bandwidth of 45 Mbps but at a correspondingly higher price of tens of thousands of dollars per month, which is beyond the budget of most businesses except the largest enterprises and large service providers. DSL is a relatively new technology that offers multimegabit data speeds, and competition with cable modem technologies has kept DSL prices low for residential customers, but the absence of cable television infrastructure buildout in business parks and downtown areas has meant that DSL prices for business customers can be kept artificially high. The result of all this is that the MAN has become the bottleneck between the corporate local area network (LAN) and the WAN. Although LAN speeds have risen to 100 Mbps Fast Ethernet and higher and WAN speeds within the Synchronous Optical Network (SONET) backbones of carriers is also 100 Mbps or higher, companies trying to join their LANs into a WAN using traditional T1 lines find that these lines have become the bottleneck in the equation and that upgrading to T3 is too costly a solution. Enter the metropolitan (or metro) Ethernet service providers, companies who are building their own separate MANs to compete with the existing SONET rings owned by telcos. These new service providers are building their MANs using GbE switching gear, laying their own fiber backbones in dense urban areas to connect their points of presence (POPs) with industrial parks, campuses, and downtown multitenant units (MTUs). The demarcation point at the customer premises now becomes as simple as an RJ-45 connection, and customers can simply use a Category 5 (Cat5) patch cord to connect their Ethernet LAN to their carrier’s GbE backbone to build a WAN or receive high-speed Internet access. Other services supported by some metro Ethernet providers include high-speed connectivity to storage area networks (SANs), Web and application server hosting, and multicast Internet Protocol (IP) video services. Implementation Metro Ethernet is made possible by the use of new carrier-class GbE switches that can be deployed at either the service provider’s POP or the basement of the customer’s MTU, depending on the scenario. At the provider’s end, these switches interface with telco SONET rings to provide connectivity for long-distance WAN links; within a downtown area of a city, however, the metro Ethernet provider maintains end-to-end Ethernet connectivity for customers without the need of telco SONET services. Metro Ethernet service providers may either lay their own fiber downtown or lease dark fiber from other carriers. To get the most out of this fiber, providers are starting to employ Wavelength Division Multiplexing (WDM), which enables as many as eight full-duplex GbE channels to run over a single fiber pair. Dense Wavelength Division Multiplexing (DWDM) offers the potential of squeezing even greater bandwidth out of individual fiber strands. With GbE now supporting distances up to 150 miles (240 kilometers) over single-mode fiber, metro Ethernet can be deployed across even the largest cities to provide a high-speed alternative to SONET for data transport. The emergence of 10 Gigabit Ethernet (10 GbE) promises to provide even greater capacity for this service. Advantages and Disadvantages Metro Ethernet offers a number of advantages over traditional SONET rings for building WANs: ? Simplicity: The fact that Ethernet is running on both sides of the access device at the customer premises makes installation, configuration, and management of WAN links easier. ? Cost: Metro Ethernet connections are typically only 25 to 50 percent of the cost of traditional telco data services, even taking into account the cost of laying new fiber. The cost of GbE switching equipment used for WAN access to metro Ethernet services is about a tenth the prices of traditional ATM over SONET access devices, which adds up to another cost savings for metro Ethernet over telco WAN services. Finally, GbE is a well-known technology to LAN administrators, and so the cost of hiring or training IT (information technology) people skilled in ATM over SONET technologies is eliminated, which further reduces the cost. ? Scalability: Metro Ethernet is typically provisioned in 1 Mbps increments up to 1 gigabit per second (Gbps), with prices scaled accordingly. Although upgrading a traditional T1 line to T3 can take weeks, increasing bandwidth on a metro Ethernet connection usually takes only a few hours. Some service providers even offer customers a Web interface from which they can provision additional bandwidth for themselves as required. ? Manageability: Metro Ethernet simplifies the task of data management by enabling packets to be processed at Layers 3 through 7 of the Open Systems Interconnection (OSI) reference model. This contrasts with SONET, where it is difficult to process packets on the backbone. ? Fault-tolerance: Traditional dual SONET rings deployed by telcos offer fault-tolerance that GbE networks do not intrinsically possess. By combining GbE with Dense Wavelength Division Multiplexing (DWDM), however, SONET’s dual-ring architecture can be imitated to provide a robust, fault-tolerant backbone comparable to SONET. The main disadvantage of metro Ethernet is availability: for the foreseeable future, this service will probably be available only in the downtown sections of large urban areas. Another disadvantage is that although dual SONET rings deployed by telcos are self-healing (typically in less than 50 milliseconds), GbE networks do not have such built-in recoverability features. Furthermore, while ATM over SONET is a cell-based connection-oriented technology that is optimized for voice but can also carry data easily, Ethernet is a connectionless packet-switched technology that suffers unpredictable delay and jitter, factors that have little effect on data traffic but which make it difficult to transport voice traffic (unless bandwidth is far below saturation point). Finally, SONET networks traditionally offer five-nines (99.999 percent) uptime, which is typically an order of magnitude better than the best metro Ethernet networks available today. Marketplace Metro Ethernet service providers offer a wide range of services from LAN-LAN interconnection within urban areas to high-speed Internet access and even Voice over IP (VoIP) services. They can provision a single fiber link to an MTU in such a way that each client within the building has its own secure connection and is billed separately according to usage. Popular service providers in the metro Ethernet marketplace include Yipes Communications, Telseon, FiberCity Networks, Cogent Communications, Everest Broadband Networks, Intelli-Space, XO Communications, and many others. Although some of these providers offer services only in a few urban centers, many of them are scaling out to offer services nationally in large cities across the United States. Some metro Ethernet service providers specialize only in high-speed Internet access or WAN connectivity, but others offer a full range of services comparable to telcos. Carrier-class GbE switches for metro Ethernet rollouts are now available from a number of vendors, including Extreme Networks, Foundry Networks, Cisco Systems, Nortel Networks, and Riverstone Networks. Prospects Just as GbE has won out over competing LAN backbone technologies such as Fiber Distributed Data Interface (FDDI) and Asynchronous Transfer Mode (ATM), the same thing may happen in the WAN, at least for data services. ATM over SONET still offers a much higher Quality of Service (QoS) for voice communications, but the simplicity and price advantages of end-to-end Ethernet in the WAN make it the leading technology as far as data services are concerned. Unfortunately, metro Ethernet is likely to remain exactly that—Ethernet restricted to metropolitan areas where dense clusters of MTUs make laying fiber a cost-effective decision for service providers. Metro Ethernet is unlikely to be available in the foreseeable future in smaller urban or rural areas, which must continue to rely on telco services as their only option. Nevertheless, metro Ethernet is having a profound impact on the enterprise WAN cost model and may even eat into the traditional voice market of telcos as VoIP technologies become more standardized and widely deployed. See Also: Asynchronous Transfer Mode (ATM), ATM over SONET, dense wavelength division multiplexing (DWDM), Digital Subscriber Line (DSL), Fiber Distributed Data Interface (FDDI), Gigabit Ethernet (GbE), Incumbent Local Exchange Carrier (ILEC), Integrated Services Digital Network (ISDN), local area network (LAN), metropolitan area network (MAN), storage area network (SAN), Synchronous Optical Network (SONET), T-carrier, Voice over IP (VoIP), wide area network (WAN)