ed frames, correcting them or even discard them. Among possible payloads, the GFP distinguishes between client data frames (CDF) and client management frames (CMF). A CDF transports application data; a CMF transports data exchanged among management units on the management plane. This distinction is quite useful since it allows the GFP to manage differently the QoS for the different classes of payload. The high-level structure of the GFP frame is reported in Figure 3.50. In order to perform such a flexible multiplexing, GFP distinguishes two client types, assigning to each of them a corresponding transport mode: the frame mapped GFP (GFP-F) and the transparent mapped GFP (GFP-T). Even if these transport modes are quite different, they can coexist in the same frame, SO that they provide more efficient and client- oriented adaptation without any rigidity. Telecommunication Networks 8 bits Payload header Payload frame Payload header FIGURE 3.50 alignment Payload data Schematic representation of the GFP envelop. The GFP-F has been optimized for packet-based applications, primarily transport of IP datagrams, but also transport of Ethernet frames and even support to an MPLS upper layer. In order to adapt to these types of clients in a better way, the GFP-F has, among the other specific features, the following characteristics: Variable frames Data multiplexing at packet level Variable data rate Can work at VT level It is MAC aware Requires buffering and thus introduces latency The GFP-T is optimized for applications that require sensitivity to delay and real-time operation including all Storage Area Network Protocols. GFP-T has, among the others, the following features: Constant length frames No buffering, thus no latency No MAC is required Retain idle frames having a lower bandwidth efficiency The topological architecture of a network integrating different data clients over the same SONET transport using GFP is shown in Figure 3.51. From Figure 3.51, it is clear that GFP allows different virtual networks to be supported