IEC: On-Line Education: WPF: Introduction to Optical Transmission in a Communications Network

Two different transmission scenarios—one for the metro environment and one for the long-haul environment—are significant. Broadly speaking, the long haul is about creating big pipes. The types of services there are not numerous. The network shape is relatively stable; however, as traffic grows dramatically, the size of the pipes must be able to grow easily, too. On the metro side, many different services of different sizes are observable, and the shape of the network is changing rapidly as new, big customers come on board. Metro means short distances between nodes, and this is important when it comes to optical solutions (optical signals going shorter distances are cheaper than the ones going longer distances).

Apart from the traditional voice and leased-line services not represented here, new services appearing in the metro environment include the following:

data storage—This service connects disc with storage medium.
distributed application—This is made up of functions residing in separate geographical locations, cooperating together.
video link—This is a large data pipe to carry computer traffic or a large pipe that can carry anything (leased wavelength).

Figure 2. New Metro Services

The backbone network is the traditional long-haul network that has been around for many years. Typical backbone networks have the following characteristics:

There are an extensive number of points where traffic is going onto or leaving the network.
Distances of a circuit (a service) transported on this network are less than 600 km.
A new type of network is appearing: the express or super-express network, largely driven by Internet protocol (IP) traffic.
The networks tend to be characterized by nodes that pick up a huge amount of traffic (much more than in the backbone).
Mostly end-to-end traffic is involved, with less add/drop.
Distances of a circuit (a service) transported on this network are greater than 1000 km.

Network Requirements

To build an economical network, the following elements are required:

flexibility—This involves significant interchanges to offload traffic in various ways and is present in both metro and long-haul scenarios.
optics adapted to the distances—This means going to two extremes. In the long-haul express, systems must allow the optical signal to go several thousand kilometers without requiring regeneration. In the metro, however, cheap optics for the shorter distances are preferred. Also, to save cost, each road where the traffic is heavy must be able to carry maximum traffic, which leads to the final requirement.
option for a number of signals to share the same optical fiber—This may enable the best use of the fiber, if this is important. Installing cable is expensive, especially on long distances. Getting the right of way and digging takes time and money, and this is not an attractive proposition for a telecom carrier that hopes to get its network running and producing revenues.

5. Overview of Multiplexing >>