Overview

As networking technology and services have evolved, one consistent characteristic is that much of the traffic requires a bit-rate less than that of a full wavelength. For example, while many backbone networks support 10-Gb/s wavelengths, most of the network demands require rates of 2.5 Gb/s or lower. Demands at the bitrate of a wavelength are referred to as line-rate traffic or wavelength services, whereas demands at a lower bit-rate are referred to as subrate traffic.

With SONET/SDH framing at the physical layer, the wavelength line-rate has evolved in accordance with the SONET/SDH rate hierarchy (see Section 1.4). When used to carry subrate SONET/SDH services, the wavelength partitioning is straightforward. For example, with a SONET-based system, a line-rate of OC-N carries a maximum of N effective OC-1 (51.8 Mb/s) units. Thus, one OC-192 wavelength can carry, for example, a combination of three OC-48s and four OC-12s. Similarly, an SDH line-rate of STM-N carries a maximum of N effective STM-1 (155.5 Mb/s) units. With data-oriented services such as IP and Ethernet, the rates of the guaranteed-bandwidth traffic are more arbitrary and of finer granularity. Additionally, data services typically include bursty best-effort traffic, where there is no pre-negotiated bandwidth dedicated to carrying the traffic. Efficiently packing such bursty traffic into wavelengths is a challenge and will not be the focus of most of this chapter.

There are several options for carrying subrate traffic in a network. The most simplistic approach utilizes a full wavelength to carry a...