Metro Ethernet

Metro Ethernet is a service offered by providers and carriers that traditionally had only classic WAN offerings, such as dialup, T1, and Frame Relay. Metro Ethernet blends the capabilities and behavior of WAN technologies with those of Ethernet. Carriers offer Metro Ethernet to customers who are looking for a cost-effective method to interconnect campus networks and to access the Internet.

One of the many advantages of Metro Ethernet is that the customer can use a standard 10/100-Mbps Ethernet interface, or a Gigabit or 10-Gbps interface, to access the service provider's network. The customer can set up virtual circuits to reach other sites and to reach an ISP. Metro Ethernet supports a copper or fiberoptic interface, but uses fiber-optic cabling inside the provider's network. It can use a variety of transport protocols, including SONET, ATM, dense-mode wave-division multiplexing (DWDM), and Multiprotocol Label Switching (MPLS).

Metro Ethernet service providers allow subscribers to add bandwidth as needed. Compared to adding WAN bandwidth, adding bandwidth to a Metro Ethernet service can happen extremely quickly, sometimes as quickly as a few minutes or hours. Metro Ethernet providers offer a wide range of bandwidth options. In addition to the standard Ethernet speeds, many providers offer a 1-Mbps service and allow customers to add bandwidth in 1-Mbps increments.

Some Metro Ethernet implementations use what is called an Ethernet virtual circuit (EVC). In Chapter 11, "Selecting Technologies and Devices for Enterprise Networks," you will learn about Frame Relay permanent virtual circuits (PVCs). An EVC uses the same concept. An EVC is an association of two or more user network interfaces (UNIs), where a UNI is a standard Ethernet interface at a customer's site. Two types of EVCs are supported: point to point, and multipoint to multipoint. There are also two basic service types:

• Ethernet line service (E-line service) provides a point-to-point EVC between two UNIs.

• Ethernet LAN service (E-LAN service) provides multipoint connectivity. It may connect two or more UNIs.

Metro Ethernet is a blending of WAN and LAN technologies. It supports virtual circuits like a WAN and also provides support for a committed information rate (CIR), committed burst size, peak information rate (PIR), and peak burst size, like a WAN often does. In a similar fashion to a LAN, Metro Ethernet also supports VLANs, 802.1X port authentication, and the Spanning Tree Protocol.

Metro Ethernet is used to connect intranet and extranet sites. ISPs are also starting to use Metro Ethernet. The ISP typically multiplexes multiple subscribers over a highspeed Ethernet UNI. An ISP's customer uses an EVC to connect to the ISP's local point of presence (POP).

The most common method for using Metro Ethernet for dedicated Internet access is to use the E-line service. If the customer wants to use the same UNI to support both Internet access and an intranet or extranet connection, this is also possible with the use of separate EVCs. A customer can also use multiple EVCs to access multiple ISPs for redundancy, thus multihoming the Internet connection.

Metro Ethernet has not caught on in a big way as of this writing, but it holds many promises and probably will catch on in one form or another. Although it may be called something else in the future by some providers, it is clear that Ethernet MAN and WAN technologies provide many benefits, including interoperability, high bandwidth, and low cost. For more information about Metro Ethernet, see the Metro Ethernet Forum's website at www.metroethernetforum.org.

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