tailieunhanh - Lecture Digital communication systems - Lecture 17

The learning objectives for this chapter include: distinguish among the types of informal controls on the media, explain the most important ethical principles, explain what the standards departments and performance codes are, discuss the relationship between the media and their advertisers vis-à-vis ethical practices, understand the pros and cons of pressure groups. | 11 7 12 INTRODUCTION TO EECS II DIGITAL COMMUNICATION SUSTEMS Fall 2012 Lecture 17 Communication networks intro Packet switching Delays queues and Little s Law Fall 2012 Lecture 17 Slide 1 Fall 2012 Multi-hop Networks Network topology modeled as a graph Lecture 17 Slide 3 From Links to Networks Have digital point-to-point Fall 2012 We ve studied channel coding and modulation we know how to build a communication link Want many interconnected points Image by MIT OpenCourseWare. Lecture 17 Slide 2 MIT Network y I Enhy Switches Buddingq- _______________ Y I Edge Switches I Budding Aggregation 1 11 7 12 Sharing the Network Circuit Switching We have many application-level communications which we ll call connections that need to mapped onto a smaller number of links How should we share the links between all the connections Two approaches possible Circuit switching isochronous Packet switching asynchronous First establish a circuit between end points - . done when you dial a phone number - Message propagates from caller toward callee establishing some state in each switch Then ends send data talk to each other After call tear down close circuit - Remove state Callee 1 2 3 Fall 2012 Lecture 17 Slide 5 Fall 2012 Lecture 17 Slide 6 Multiplexing Demultiplexing TDM Shares Link Equally But Has Limitations Switch Frames Slots 0 1 2 345 0 1 2 345 Switch frames 0 1 2 3 4 5 0 1 2 3 4 5 One sharing technique time-division multiplexing TDM Time divided into frames and frames divided into slots - Number of slots number of concurrent conversations Relative slot position inside a frame deter mines which conversation the data belongs to - . slot 0 belongs to the red conversation - Mapping established during setup removed at tear down Forwarding step at switch consult table Suppose link capacity is C bits sec Each communication requires R bits sec frames in one epoch one frame per communication C R Maximum number of concurrent communications is C R What