Module 15 Network Structures.ppt

Module 15: Network Structures Background Topology Network Types Communication Communication Protocol Robustness Design Strategies A Distributed System Motivation Resource sharing sharing and printing files at remote sites processing information in a distributed database using remote specialized hardware devices Computation speedup – load sharing Reliability – detect and recover from site failure, function transfer, reintegrate failed site Communication – message passing Network-Operating Systems Users are aware of multiplicity of machines. Access to resources of various machines is done explicitly by: Remote logging into the appropriate remote machine. Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism. Distributed-Operating Systems Users not aware of multiplicity of machines. Access to remote resources similar to access to local resources. Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task. Computation Migration – transfer the computation, rather than the data, across the system. Distributed-Operating Systems (Cont.) Process Migration – execute an entire process, or parts of it, at different sites. Load balancing – distribute processes across network to even the workload. Computation speedup – subprocesses can run concurrently on different sites. Hardware preference – process execution may require specialized processor. Software preference – required software may be available at only a particular site. Data access – run process remotely, rather than transfer all data locally. Topology Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: Basic cost. How expensive is it to link the various sites in the system? Communication cost. How long does it take to send a message from site A to site B? Reliability. If a link or a site in the system fails, c