d Ethernet Frame Header Header Header Header 64-1518 bytes 4 bytes 64-1518 bytes 4 bytes 4 bytes 4 bytes Fig. 11.20 Example of Ethernet frame encapsulation into GFP-F frames. Container (e.g., C-4, C-4-Xc, C-4-Xv) Fig. 11.21 GFP frames transported in SDH containers. GFP Idle GFP Frame across a common SDH infrastructure. Data is queued before being mapped onto a TDM channel, and the GFP CID field (8 bits) in the payload header is used to de-multiplex at the far end. Because one client frame is encapsulated into a single GFP frame (see Figure 11.20), there is a delay as the frame is stored and encapsulated prior to transmission. In addition, any preamble or interframe gaps are removed to improve transport efficiency, and these are later reinserted at the receiving equipment. GFP idle frames are inserted to maintain a Constant Bitrate (CBR) at the output stream. The process of mapping GFP frames into an SDH virtual container is shown in Figure 11.21. The choice of container is determined by the predicted amount of client data. GFP idle frames are inserted to adapt the datarate to the CBR stream of the SDH container and to fill the container in periods of low data activity. Conclusion Transparent GFP Transparent GFP (GFP-T) is a protocol-independent encapsulation method in which all client codewords are decoded and mapped into fixed GFP frames. Unlike GFP-F, the frames are transmitted immediately without waiting for the entire client data packet to be received. For this reason, it is regarded as a layer-1 transport mechanism because it doesn't distinguish between data, headers, or control information and simply transmits the received client bits. The result is a very-low-delay transport mechanism. GFP-T only accepts protocols based on 8B/10B line coding and immediately transcodes to the more efficient 64B/65B superblock version prior to encapsulating into fixed-length GFP-T frames. Several important data pro- tocols use the 8B/10B line code at the physical layer. Fibre Channel, Fiber Connection (FICON) and Enterprise 