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Chapter 3The DVB/DAVIC protocol pThen, the sign-on, ranging and calibration are performed in order to adjust the internalclock and the transmission power of the NIU according to the specific transmissiondelay and cable attenuation. The initial connection is also established by default. TheMAC protocol also carries out the establishment and release of logical connections andallows readjustment of parameters, as well as performing an exchange of keys andestablishment of a secure connection. Here Diffie-Hellman and Data Encryption System(DES) are used.Once an NIU has initialised and registered with the INA, bandwidth reservation isprovided by both per-packet and per-session (or connection) basis. The former is knownas Reservation Access and is provided by sending reservation requests to the INA. Thefirst version of EuroModems (class A) will support this functionality as a mandatoryaccess mode. The latter is known as Fixed-rate access, where the INA provides either afinite amount of slots (in the fixed-rate access region) to a specific NIU or a given bitrate requested by an NIU until the INA stops the connection on NIU's demand. Thisaccess mode will be an optional functionality for EuroModem Class A, and mandatoryfor EuroModem Class B.3.5.1 Reservation access mode operationThe reservation-access mode with its dynamic slot-allocation feature is the main accessmode of the DVB MAC protocol for the transmission of data packets via the upstreamchannel. For this access mode, the DVB/DAVIC group has adopted two contentionresolution algorithms, which are used to resolve collisions: the exponential backoffalgorithm and splitting tree algorithm. The splitting tree algorithm takes advantage ofthe exponential backoff algorithm in the sense that feedback and allocation informationallow a station, (with new incoming arrivals) to compete for contention-based slotswithout risk of collision with backlogged stations. In addition, this algorithm makes useof minislots, which decreases the risk of collisions, since one contention-based slot isdivided into three minislots of 21-bytes long transferring shortened reservation requestmessages, increasing the probability of successful request transmissions andconsequently improving the system performance.3-6