Model-Based Design for Embedded Systems- P19: This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. | 156 Model-Based Design for Embedded Systems the sending and the receiving node and a is an environment parameter typically in the range from 2 to 4 . If the received energy is below a user-defined threshold then no reception will take place. A node that wants to transmit a message will proceed as follows The node first checks whether the medium is idle. If that has been the case for 50 gs then the transmission may proceed. If not the node will wait for a random back-off time before the next attempt. The signal-to-interference ratio in the receiving node is calculated by treating all simultaneous transmissions as an additive noise. This information is used to determine a probabilistic measure of the number of bit errors in the received message. If the number of errors is below a configurable bit-error threshold then the packet could be successfully received. Example Constant Bandwidth Server The constant bandwidth server CBS 1 is a scheduling server for aperiodic and soft tasks that executes on top of an EDF scheduler. A CBS is characterized by two parameters a server period Ts and a utilization factor Us. The server ensures that the task s executing within the server can never occupy more than the Us of the total CPU bandwidth. Associated with the server are two dynamic attributes the server budget cs and the server deadline ds. Jobs that arrive at the server are placed in a queue and are served on a first-come first-serve basis. The first job in the queue is always eligible for execution using the current server deadline ds. The server is initialized with cs UsTs and ds Ts. The rules for updating the server are as follows 1. During the execution of a job the budget cs is decreased at unit rate. 2. Whenever cs 0 the budget is recharged to cs UsTs and the deadline is postponed one server period ds ds Ts. 3. If a job arrives at an empty server at time r and cs ds - r Us then the budget is recharged to cs UsTs and the deadline is set to ds r Ts. The first and .
