Optical Components

Optical components provide access to large quantities of optical bandwidth (several
hundred Terahertz). However, bit error rates need to be typically 1 in 1012 or better. This
places severe demands on component performance and effectively means that our
optical transport layer is power-limited (just like the copper access and radio access
parts of the network).
As our ability to generate and filter discrete optical frequencies improves, we can
use increasingly narrowband channels. This allows us to deliver differential quality of
service over the optical transport layer and to provide a measure of adaptive bandwidth
(by dropping optical channels in and out of the optical frequency multiplex).
This depends, however, on being able to have a fast optical cross connect and presently
this presents performance limitations.
One merit of the optical layer, shared with copper access, is that it is a consistent
transport medium. It does not suffer from the fading effects encountered on the radio
physical layer. Impairments tend to be steady-state. This means they increase with distance.
Thus, provided sufficient power can be made available (a sufficient number of
repeaters), it is reasonable to assume good consistent quality.
Even so, some wavelengths will provide better quality than others�"for instance,
impairments increase as optical frequency increases, so lower frequencies will generally
deliver better quality. This provides the basis for differentiated quality of service
using Internet traffic shaping protocols, such as Multiprotocol Label Switching.
One practical problem of packet routing in the core network is the sheer physical
speed at which packets have to be read. The answer tends to be to implement parallel
processing. As the number of optical wavelength channels increases, it becomes possible
to define different packet routing trajectories, which can be maintained both across
the copper transport and optical transport layer. However, this process becomes more
complex as the offered traffic becomes increasingly asynchronous over time.
An option is to extend ATM across the optical layer, hardware switching on a 10-ms
resolution, to manage and maintain the time domain properties of the rich media products
as they move across the copper and optical transport layer. We revisit these protocol
issues in Chapter 17, which is devoted to traffic shaping protocols. 333