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Ametropolitan area network (MAN) is acomputer network that interconnects users with computer resources in a geographic region of the size of ametropolitan area. The term MAN is applied to the interconnection oflocal area networks (LANs) in a city into a single larger network which may then also offer efficient connection to awide area network. The term is also used to describe the interconnection of several LANs in a metropolitan area through the use ofpoint-to-point connections between them.[1][2]
By 1999,local area networks (LANs) were well established and providing data communication in buildings and offices.[3] For the interconnection of LANs within a city, businesses relied primarily on thepublic switched telephone network. But while the telephone network was able to support the packet-based exchange of data that the various LAN protocols implemented, the bandwidth of the telephone network was already under heavy demand fromcircuit-switched voice, and thetelephone exchanges were ill-designed to cope with the traffic spikes that LANs tended to produce.[4]: 11
To interconnect local area networks more effectively, it was suggested that office buildings are connected using thesingle-mode optical fiber lines, which were by that time widely used in long-haul telephone trunks. Suchdark fibre links were in some cases already installed on customer premises and telephone companies started to offer their dark fibre within their subscriber packages. Fibre optic metropolitan area networks were operated by telephone companies as private networks for their customers and did not necessarily have full integration with the publicwide area network (WAN) through gateways.[4]: 12
Besides the larger companies that connected their offices across metropolitan areas, universities and research institutions also adopted dark fibre as their metropolitan area network backbone. InWest Berlin the BERCOM project built up a multifunctional broadband communications system to connect themainframe computers that publicly funded universities and research institutions in the city housed. The BERCOM MAN project could progress at speed because theDeutsche Bundespost had already installed hundreds of miles of fibre optic cable in West Berlin. Like other metropolitan dark fibre networks at the time, the dark fibre network in West Berlin had a star topology with a hub somewhere in the city centre.[4]: 56 The backbone of the dedicated BERCOM MAN for universities and research institutions was an optical fibre double ring that used a high-speedslotted ring protocol developed by the GMD Research Centre for Innovative Computer Systems and Telephony. The BERCOM MAN backbone could thus support two times 280 Mbit/s data transfer.[4]: 57
The productive use ofdense wavelength-division multiplexing (DWDM) provided another impetus for the development of metropolitan area networks in the 2000s. Long haul DWDM, with ranges from 0 to 3000+ km, had been developed so that companies that stored large amounts of data on different sites could exchange data or establish mirrors of theirfile server. With the use of DWDM on the existing fibre optic MANs of carriers, companies no longer needed to connect their LANs with a dedicated fibre optic link.[5]: 14 With DWDM companies could build dedicated MANs using the existing dark fibre network of a provider in a city. MANs thus became cheaper to build and maintain.[5]: 15 The DWDM platforms provided by dark fibre providers in cities allow for a single fibre pair to be divided into 32 wavelengths. One wavelength could support between 10 Mbit/s and 10 Gbit/s. Thus companies that paid for a MAN to connect different office sites within a city could increase the bandwidths of their MAN backbone as part of their subscription. DWDM platforms also alleviated the need for protocol conversion to connect LANs in a city, because any protocol and any traffic type could be transmitted using DWDM. Effectively it gave companies wishing to establish a MAN choice of protocol.[5]: 16
Metro Ethernet uses a fibre optic ring as aGigabit Ethernet MAN backbone within a larger city. The ring topology is implemented usingInternet Protocol (IP) so that data can be rerouted if a link is congested or fails.[6] In the US theSprint was an early adopter of fibre optic rings that routed IP packets on the MAN backbone. Between 2002 and 2003 Sprint built three MAN rings to coverSan Francisco,Oakland andSan Jose, and in turn connected these three metro rings with a further two rings. The Sprint metro rings routed voice and data, were connected to several local telecom exchange points and totalled 189 miles of fibre optic cable. The metro rings also connected many cities that went on to become part of theSilicon Valley tech-hub, such asFremont,Milpitas,Mountain View,Palo Alto,Redwood City,San Bruno,San Carlos,Santa Clara andSunnyvale.[7]
The metro Ethernet rings that did not route IP traffic instead used one of the various proprietarySpanning Tree Protocol implementations; each MAN ring had aroot bridge.[8] Becauselayer 2 switching can not operate if there is a loop in the network, the protocols to support L2 MAN rings all need to block redundant links and thus block part of the ring.[5]: 41 Capsuling protocols, such asMultiprotocol Label Switching (MPLS), were also deployed to address the drawbacks of operating L2 metro Ethernet rings.[5]: 43
Metro Ethernet was effectively the extension ofEthernet protocols beyond thelocal area network (LAN) and the ensuing investment in Ethernet led to the deployment ofcarrier Ethernet, where Ethernet protocols are used inwide area networks (WANs). The efforts of theMetro Ethernet Forum (MEF) in defining best practice and standards for metropolitan area networks thus also defined carrier Ethernet.[9] While the IEEE tried to standardise the emerging Ethernet-based proprietary protocols, industry forums such as the MEF filled the gap and in January 2013 launched a certification for network equipment that can be configured to meetCarrier Ethernet 2.0 specifications.[10]
Internet exchange points (IXs) have historically been important for the connection of MANs to the national or globalInternet. TheBoston Metropolitan Exchange Point (Boston MXP) enabled metro Ethernet providers, such as the HarvardNet to exchange data with national carriers, such as theSprint Corporation andAT&T. Exchange points also serve as low-latency links betweencampus area networks, thus theMassachusetts Institute of Technology and theBoston University could exchange data, voice and video using the Boston MXP. Further examples of metropolitan Internet exchanges in the USA that were operational as of 2002[update] include the Anchorage Metropolitan Access Point (AMAP), theSeattle Internet Exchange (SIX), the Dallas-Fort Worth Metropolitan Access Point (DFMAP) and the Denver Internet Exchange (IX-Denver).[11]Verizon put into operation three regional metropolitan exchanges to interconnect MANs and give them access to the Internet. TheMAE-West serves the MANs ofSan Jose,Los Angeles andCalifornia. TheMAE-East interconnects the MANs ofNew York City,Washington, D.C., andMiami. While the MAE-Central interconnects the MANs ofDallas,Texas, andIllinois.[12]
In larger cities several local providers may have built adark fibre MAN backbone. In London, themetro Ethernet rings of several providers make up the London MAN infrastructure. Like other MANs, the London MAN primarily serves the needs of its urban customers, who typically need a high number of connections with low bandwidth, a fast transit to other MAN providers, as well as high bandwidth access to national and international long-haul providers. Within the MAN of larger cities, metropolitan exchange points now play a vital role. TheLondon Internet Exchange (LINX) had by 2005 built up several exchange points across theGreater London region.[13]
Cities that host one of the international Internet exchanges have become a preferred location for companies anddata centres. TheAmsterdam Internet Exchange (AMS-IX) is the world's second-largest Internet exchange and has attracted companies to Amsterdam that are dependent on high-speed internet access. The Amsterdam metropolitan area network has benefited too from high-speed Internet access.[14]: 105 Similarly Frankfurt has become a magnet for data centres of international companies because it hosts the non-profitDE-CIX, the largest Internet exchange in the world.[14]: 116 The business model of the metro DE-CIX is to reduce the transit cost for local carriers by keeping data in the metropolitan area or region, while at the same time allowing long-haul low-latency peering globally with other major MANs.[15]
A MAN is optimized for a larger geographical area than is a LAN, ranging from several blocks of buildings to entire cities.
Then one day, hardware was ridiculously cheap, software knew about the hardware, and you could actually plug a couple of machines together and they'd talk to each other. The real year of the LAN had quietly happened.
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