code words. At this point, the header signal can be reconstructed by digital low-pass filtering. In the following paragraphs, we will neglect the impact of nonidealities, and we will assume that the terms containing convolutions of code words are rigorously zero, while the signal term gives a pseudo-noise contribution that in the worst case has a power equal to Ps RH/R. Besides interference with the signal, the header is affected by the ASE noise, whose spec- tral density is NASE. The optical signal-to-noise ratio required for the header detection depends on the modulation format. Assuming using a differential phase shift keying (DPSK), it has to be SNH = 14dB. Since the overall noise spectral density is NTOT = NASE + PS/R, the ratio between the required power of the header signal and of the information bearing signal in the same point where the header is read, has to be (10.20) where SNo is the signal optical SN in front of the header receiver and it is assumed SNo >> 1 to neglect its inverse in the parenthesis the terms Mhe and represent the ratio between the unilateral optical bandwidth and the electrical bandwidth (supposed equal to the bit rate) for the header signal and the information bearing signal, respectively Assuming the data used in the previous example and SN = 17 dB, since it is a pre-FEC value, we have PH/Ps = 20 dB. This number is important since, when detecting the data at the end of the wavelength path, the header signal is an unwanted InB disturbance; thus, penalty on data transmis- sion exists. Considering that the code spreads the header over the whole data bandwidth, when the data bandwidth is detected by a conventional receiver, the optical signal-to-noise ratio can be written in a first approximation as (10.21) From Equation 10.21, it is easy to evaluate in a first approximation the penalty induced by the signaling method in conditions of high SN (10.22) where SNoi is the required data SNo is the absence of signaling. Next Generation Networking: Enabling Technologies, Architectures, and P