 or 1 Gbit/s Ethernet OC 48 Fiber DS-n or OC-n OC 192 OC 768 Fiber channel FIGURE 3.49 Tributary aggregation scheme of a GFP signal. Relying on WDM, GFP considers the transmission trunk (the optical transmission chan- nel) divided into wavelengths, forming a sort of frequency frame. Each wavelength supports a TDM frame composed by a GFP "core header" and a "payload area" where different clients can be multiplexed. This structure of the optical transmission channel is shown in Figure 3.49. This figure is quite important, since we will see that the structure shown in the figure will recur also in the design of next generation control plane that we will consider in Chapter 8. The core header supports all the client independent functions related to layer 2 link management, such as payload length indication and delimitation of the frame. The core header also has an independent error control, called core header error control (CHEC). The payload incorporates not only the client data to be transported, but also a set of control data that are related to the specific client. These data are contained in the payload header, which also provides a payload-specific error control. Moreover, in the payload area there is also a frame alignment sequence. The separation of payload and common header error control is done to also allow the system to send to the end user frames corrupted in the payload if the SLA SO requires, while frames that are corrupted in the header have to be either repaired or somehow sub- stituted by the layer 2. In considering this error control policy two things have to be taken into account: first it is well possible that the upper layer has a powerful error correcting procedure that is capable of correcting corrupted frames in its end-to-end working. Moreover, in real-time services like TV or telephone, a corrupted frame is often better than no frame from the point of view of individual user perception. This is a point that layer 2 cannot cover: it will be the upper layer to judge if passing through corrupt