Читать книгу Wireless Connectivity - Petar Popovski - Страница 39

One can easily think of creating new random access algorithms by combining the principles of ALOHA and probing. An advantage of framed ALOHA is avoidance of collisions and economic usage of feedback messages. ALOHA uses memoryless randomization: the random choice of a slot made to access at a given time is independent of the random choice made at another time. Contrary to this, probing or splitting tree algorithms create memory in the randomization process and utilize the history of collisions to resolve the set of colliding sensors more efficiently. Here is an example of what a hybrid ALOHA probing algorithm could look like. Basil sends a probe-to-all packet, which, instead of a single slot, is followed by a frame that has slots. Each sensor selects randomly a slot to send its packet. We can now interpret as a self-assigned random address, as we did in the probing algorithm. If Basil observes a collision in slot , then he proceeds to resolve this collision recursively. At first, he sends a probe addressed to the sensors that have collided in the th slot or, equivalently, all sensors that have self-assigned the address . This probe is followed by a frame with slots. Hence, the generalization is that a probe can be followed by more than a single slot. This would require generalization of the tree structure used to describe the basic probing algorithm.