algorithm for transmission decisions, and a technique called Cell Segmentation and Reassembly. Since the majority of frames passed to an ATM network are not the required byte size, the ATM protocol must segment the frames into the proper cell size prior to transmission and subsequently reassembling the ATM cells back into their original state at the receiver.ATM cells are fixed size data packets of 53 bytes each. The ATM cell size is always 5 bytes of header and 48 bytes of payload. Therefore, this technology has the beneficial side-effects of a reduction of complex hardware for the switches themselves, improved frame queueing, and uniform switching activities all taking the same amount of time to accomplish. ATM cells have two possible formats dependent upon where the cell happens to be in the network. The names for these two formats are User-Network Interface (UNI) for the user to network interface, and Network-Network Interface (NNI) for the network to network interface. Figure 2 below shows the format for a UNI version of the ATM cell. The NNI version of the cell differs only in that the Generic Flow Control (GFC) of the UNI version is replaced by 4 additional bits for the Virtual Path Identifier (VPI). As can be seen from Figure 2, the cell starts with the GFC, which is intended to be used to arbitrate access to a link in the event that the local site uses a shared medium to attach to an ATM configuration. Following the GFC are the VPI and Virtual Circuit Identifier (VCI) bits used to identify the path (channel) created for this transmission. Next, we see the Type bits which are used to facilitate management functions and indicate whether or not user data is contained in the cell. The final two fields prior to the 48 byte payload are the Cell Loss Priority (CLP) bits and the Header Error Check (HEC) bits. The CLP establishes a priority value in case the network becomes congested and needs to drop one or more frames. (Se...
