Central offices, also called switching offices, have telephone lines branch out to individual customers. And a metro network largely links all central offices in a metropolitan area such as San Francisco or New York. The part of the network from the central office to the customer is the "access network", while that from the central office inward is the "metro network".

A large metro network can have two layers of network loops deployed. The inside layer, called "metro access loops", connect groups of central offices, and these inside loops,in turn, link to a "metro core" network loop which links the whole metropolitan area together. Simply put, it looks like a big circle with more small circles inside it.

:: How do customers lease capacity in metro networks?

The holders of these metro networks often lease bulk capacity to other companies. Examples include: telephone companies making connections among their central offices, cellular companies, big businesses leasing their private lines, or even competitive phone companies that don't have their own networks.

Let's see an example here. A cable company may lease capacity between a local television station and the cable company's headquarter. So the local television station can send its signal directly to the cable company and thus give them a very clean signal for distribution through its cable network.

Another example could be the same cable company leasing capacity between an antenna farm in the suburb to its headquarter. So the satellite signals collected by the antenna farm can be sent directly to the cable company which guarantees the signals' quality and speed of delivery.

:: What capacities can you lease from a metro network?

Most new metro network carries offer very high speed services for leasing. The capacity can range from 155Mbit/s up to 10 Gbit/s. Or you can even lease a whole channel in the optical fiber link and run whatever transmission rate you choose.

:: How do you access a metro network

Two types of metro network access exist: a hub or an add-drop multiplexer.

A hub is a point where all signals in the system get switched and organized. Hubs include local central offices.

An add-drop multiplexer is a point where some signals get picked up and others dropped off. An add-drop multiplexer diverts only part of the signals to the node.

:: Metro network transmission distances

Metro network transmission distance can range from a few kilometers up to 200 kilometers but most are in the shorter range. Most spans are actually under 100 kilometers and thus do not require optical amplifiers in most cases unless the coupling loss are too high.

No optical amplifiers required allows light wavelength operation outside of the readily available optical amplifier band. Such that light wavelengths can be spread out across a broader range of less-expensive optics and transmitters.