Metro E stands for Metro Ethernet, is a computer network based on the Ethernet standard and which covers a metropolitan area. It is commonly used as a metropolitan access network to connect subscribers and businesses to a Wide Area Network, such as the Internet.
An Ethernet interface is cheaper than a SDH interface of the same bandwidth. Another 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. Therefore, bringing Ethernet in to the Metropolitan Area Network (MAN) introduces a lot of advantages to both the service provider and the customer (corporate and residential).
A typical service provider Metro Ethernet network is a collection of Layer 2 or 3 switches or routers connected through optical fiber. The topology could be a ring, hub-and-spoke (star), full mesh or partial mesh. The network will also have a hierarchy: core, distribution 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 10G or 100G speeds.
Ethernet on the MAN can be used as pure Ethernet, Ethernet over SDH, Ethernet over Multi Protocol Label Switching (MPLS) or Ethernet over Dense Wavelength Division Multiplexing (DWDM).
Pure Ethernet-based deployments are cheap but less reliable and scalable, and thus are usually limited to small scale or experimental deployments.
MPLS based deployments are costly but highly reliable and scalable, and are typically used by large service providers.
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.
An SDH based Ethernet MAN is usually used as an intermediate step in the transition from a traditional, time-division based network, to a modern statistical network (such as Ethernet). In this model, the existing SDH infrastructure is used to transport high-speed Ethernet connections. The main advantage of this approach is the high level of reliability, achieved through the use of the native SDH protection mechanisms. On the other hand, an SDH-based Ethernet MAN is usually more expensive, due to costs associated with the SDH/DWDM equipment that is necessary for its implementation.
Traffic engineering also tends to be very limited. Hybrid designs use conventional Ethernet switches at the edge of the core SDH ring to alleviate some of these issues, allowing for more control over the traffic pattern and also for a slight reduction in cost.
