r a payload FCS field is present. Extension Header Identifier (EXI) is a 4-bit identifier that indicates the type of extension that is present. A null header extension implies that an extension header and its associated HEC check (eHEC) are not present. A null header extension is generally used in situations where all GFP client data frames are mapped into a single transport path (SDH container/virtually concatenated container) and carry a single type of payload, which is demapped at the receiving end to a single physical port/link. The linear extension header signifies multiple types of payload from multiple ports/links mapped in a framewise multiplexed manner into a single transport path. In this case, the extension header contains two octets. One octet is the Channel ID (CID), which is a single byte for a destination port/link ID. The other byte is unused. Two bytes of the eHEC are also required for a linear extension header. User Payload Identifier (UPI) is an octet set according to the type of client that is trans- ported and whether the mapping is frame based or transparent. When PTI = 0, it identifies the type of payload and mapping in the data frame. When PTI = 4, the UPI defines the management frame type. Any idle time between GFP frames carrying client data is filled with GFP idle frames. A GFP idle frame consists only of a core header with PLI = 0. Framed GFP Framed GFP (GFP-F) is optimized for packet-switching environments and operates on signals that have been packetized or framed at layer 2 or higher by a client service. The diagram in Figure 11.19 illustrates its use in transporting multiple-client IP router traffic Chapter 11 Next Generation Synchronous Digital Hierarchy Customer IP Customer IP GFP-F Routers GFP-F Routers Packets Mapper STM Frames Mapper Packets STM-N Customer ID in Queues GFP Payload Header Queues Inside VC Container Fig. 11.19 Example of GFP-F usage in transporting multiple-customer IP packets over a single SDH link. GFP Frame GFP Header GFP Payload Payload Ethernet Frame Payloa