This book presents another approach of implementing quality of service (QoS) differentiation in physical layer, particularly the optical domain. Spectral amplitude coding techniques of optical code division multiple access (OCDMA) was used as the means of differentiating signal quality based on bit error rate. This approach is not a complete alternative to the current network layer QoS. However, it is believed that algorithm complexity in the higher layer can be reduced by implementing QoS provisioning at the prescribed bit level. Practical application of the proposed OCDMA codes as the add-drop multiplexer has enabled signal dropping based on different service requirements. There is a thorough explanation which includes mathematical analysis, software simulation and hardware experiment to support the feasibility of implementation of the proposed techniques.