minimum header is two bytes, comprising three flags fields, and a 12-bit payload length field. The diagram below shows all possible fields. Fragmentation and Reassembly The basic mechanism of GSE payload fragmentation uses the Start and End Flags, where the Start flag indicates the beginning of a payload frame, and the End flag indicates its end. This is shown in the diagram below. On DVB-S2, DVB-T2, and DVB-C2 the ACM/VCM modes may cause the Base Band frames to vary in size depending on the transmission conditions. Hence there may be situations where the first fragments of a payload frame have been sent, but the encapsulator is forced to set aside the current payload frame, and start working on a new one. This may e.g. occur when large fragments have been prepared while transmission conditions were fine, but suddenly the conditions deteriorate, and only small Base Band frames are available. This is when the Fragment ID field becomes important. It is a short-term identification of the payload frame. Whenever the encapsulator needs to move on to the next payload frame, without having finished transmitting the previous one, it uses the next available Fragment ID. That way, up to 256 payload frames can be "kept open" at any time. The decapsulator uses the Fragment ID to pick the reassembly buffer in which to store the fragment. GSE addresses The "Label Type" (LT) bits determine how the GSE packet address is encoded according to the following table: CRC-32 Trailer Each GSE Packet containing the last fragment for a payload frame, carries a CRC-32 checksum over the payload frame. The checksum is used to detect loss of intermediate fragments. The checksum is a 32 bit value calculated according to the generator polynomial represented by 0x104C11DB7: If the last fragment of a payload frame is lost, the decapsulator can not directly detect that fact. It never sees the GSE frame with the End flag set and containing the CRC-32. For this situation, the decapsulator must choose a suitable time-out based on the data-rate and application. GSE Implementations Products Supporting GSE Since GSE packets are directly inserted into base-band frames of the modulation scheme, GSE products come in the form of "GSE Routers" or "GSE Modems", which - from the outside - act very much like a DSL Router or DSL Modem used by consumers. More generically these devices are also referred to as "GSE Encapsulators".