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A metropolitan-area Ethernet, Ethernet MAN, or metro Ethernet network is a metropolitan area network (MAN) that is based on Ethernet standards. It is commonly used to connect subscribers to a larger service network or the Internet. Businesses can also use metropolitan-area Ethernet to connect their own offices to each other. An Ethernet interface is much cheaper than a SONET/SDH (Synchronous Digital Hierarchy) or PDH (Plesiochronous Digital Hierarchy) interface of the same bandwidth. Another distinct advantage of an Ethernet-based access network is that it can be easily connected to the customer network, due to the prevalent use of Ethernet in corporate and, more recently, residential networks. A typical service provider's network is a collection of switches and routers connected through optical fiber. The topology could be a ring, hub-and-spoke (star), or full or partial mesh. The network will also have a hierarchy: core, distribution (aggregation), and access. The core in most cases is an existing IP/MPLS backbone but may migrate to newer forms of Ethernet transport in the form of 10Gbit/s, 40Gbit/s, or 100Gbit/s speeds or even possibly 400Gbit/s to Terabit Ethernet network in the future. Ethernet on the MAN can be used as pure Ethernet, Ethernet over SDH, Ethernet over MPLS, or Ethernet over DWDM. Pure Ethernet-based deployments are cheaper but less reliable and scalable and thus are usually limited to small scale or experimental deployments. SDH-based deployments are useful when there is an existing SDH infrastructure already in place, its main shortcoming being the loss of flexibility in bandwidth management due to the rigid hierarchy imposed by the SDH network. MPLS-based deployments are costly but highly reliable and scalable and are typically used by large service providers. ==Metropolitan area network (MAN) topology== Familiar network domains are likely to exist regardless of the transport technology chosen to implement Metropolitan area networks: Access, aggregation/distribution, and core. * Access devices normally exist at a customer's premises, unit, or wireless base station. This is the network that connects customer equipment, and may include ONT and/or Residential gateway, or office router. * Aggregation occurs on a distribution network such as an ODN segment. Often Passive Optical Network, microwave or Digital Subscriber Line technologies are employed, but some of them using point-to-point Ethernet over "home-run" direct fibre. This part of the network includes nodes such as Multi Tenanted Unit switches, Optical line terminals in an outside plant or central office cabinet, Ethernet in the First Mile equipment, or provider bridges. * A MAN may include the transport technologies MPLS, PBB-TE and T-MPLS, each with its own resiliency and management solutions. * A core network often uses IP-MPLS to connect different MANs together. Much of the functionality of Ethernet MANs such as virtual private lines or virtual private networks is implemented by the use of Ethernet VLAN tags that allow differentiation of each part of the network. Logical differentiation of the physical network helps to identify the rights that the traffic has and to ease the management of hosts' access rights with respect to other users and networks. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Metro Ethernet」の詳細全文を読む スポンサード リンク
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