rtual channels and thus both the VPI and VCI translation is performed. The various characteristics of the virtual path/virtual channel connections as given in the Recommen- dation I.150 are: Quality of service. A user of a virtual channel is provided with a quality of service specified by parameters such as the cell loss ratio (ratio of cells lost to cells transmitted) and cell delay variation. Switched and semi-permanent virtual channel connections. Both switched connections, which require call control signalling, and dedicated connections can be provided. Cell sequence integrity. The sequence of transmitted cells within a virtual channel is preserved. Traffic parameter negotiation and usage monitoring. Traffic parameters can be negotiated between a user and the network for each virtual channel. The network to ensure that the negotiated parameters are not violated monitors the input of cells to the virtual channel. The ATM Cell Structure The asynchronous transfer mode makes use of small fixed size cells of 53 bytes, consisting of a 5-byte header field and a 48-byte information payload field. An ATM cell header can be one of two formats: user-network interface (UNI) or the network-network interface (NNI). The UNI header is used for communication between ATM end points and ATM switches in private ATM networks. The NNI header is used for communication between ATM switches. Figure 3.33 depicts the basic ATM cell format, the ATM UNI cell header format and the ATM NNI cell header format. Unlike the UNI, the NNI header does not include the generic flow control (GFC) field. Additionally, the NNI header has a VPI field that occupies the first 12 bits, allowing for larger trunks between public ATM switches. ATM Cell Header Fields In addition to GFC and VPI header fields, several others are used in ATM cell header fields. The following descriptions summarise the ATM cell header fields shown in Figure 3.33: Generic flow control (GFC). This provides local functions, such as identifying multiple stations that share a single A