TM interface. This field is typically not used and is set to its default value. Transmission Network Planning and Optimisation 8 6 5 4 3 2 1 8 6 5 4 3 2 1 Stypte Stayte header Service Information field Information field (48 bytes) (48 bytes) (a) User-network interface (b) Network-network interface Figure 3.33 ATM cell format Virtual path identifier (VPI). In conjunction with the VCI, this identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination. Virtual channel identifier (VCI). In conjunction with the VPI, this identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination. Payload type (PT). This field indicates the type of information in the information field. A value of 0 in the first bit indicates user information. In this case, the second bit indicates whether congestion has been experienced. The third bit, known as the service data unit (SDU) type bit, is a one-bit field that can be used to discriminate two types of ATM SDUs associated with a connection. The SDU is the 48-byte payload of the cell. A value of 1 in the first bit of the payload indicates that this cell carries network management or maintenance information. This indication allows the insertion of network management cells on to a user's VCC without impacting the user's data, thus providing the in-band control information. Congestion loss priority (CLP). This indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. If the CLP bit equals 1, the cell should be discarded in preference to cells with the CLP bit equal to zero. Header error control (HEC). This calculates the checksum only on the header itself. The ATM Adaptation Layer (AAL) With SO many services requiring greater speeds it would be suitable if ATM provided them with a common platform to be transferred. However, in spite of using different versions of ATM layers for different services, another layer was introduced