MIT 6 02 - Digital Communication Networks (82 pages)

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Digital Communication Networks



Previewing pages 1, 2, 3, 4, 5, 39, 40, 41, 42, 43, 44, 78, 79, 80, 81, 82 of actual document.

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Digital Communication Networks

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Pages:
82
School:
Massachusetts Institute of Technology
Course:
6 02 - Introduction to EECS II: Digital Communication Systems
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Digital Communication Networks MIT PROFESSIONAL INSTITUTE 6 20s July 24 28 2006 Professor Muriel Medard MIT Professor Eytan Modiano MIT Eytan Modiano Slide 1 Digital Communication Networks Introduction Eytan Modiano Eytan Modiano Slide 2 Course syllabus Monday Tuesday AM Multiple access PM LANs MANs SANs and switching Friday Eytan Modiano Slide 3 AM Routing PM More routing flow control Thursday AM Delay models and queueing PM Higher layer protocols TCP IP ATM Wednesday AM Introduction to layering the physical layer PM The data link layer AM Wireless Networks PM Optical Networks Logistics Morning lectures 9 00am 12 00pm break around 10 15 Lunch on your own 12 00 1 30pm Afternoon lecture 1 30 4 30 break around 2 45 Friday start at 9 00 and end at 3 30pm lunch 11 45 12 30 Social Dinner on Thursday evening Eytan Modiano Slide 4 Reference textbooks Computer Networks by Peterson and Davie Data Networks by Bertsekas and Gallager Eytan Modiano Slide 5 Strong on probabilistic modeling and analysis Network Optimization Continuous and Discrete Models by Dimitri P Bertsekas Most current on the internet Network algorithms and routing network flows optimization theory Introduction to Probability by Dimitri P Bertsekas and John N Tsitsiklis Data Networks Fundamental aspects of network Design and Analysis Architecture Layering Topology design Protocols Pt to Pt Multiple access End to end Algorithms Error recovery Routing Flow Control Analysis tools Probabilistic modeling Queueing Theory Eytan Modiano Slide 6 Network Applications Resource sharing Computing Mainframe computer old days Today computers cheaper than comm except LANS Printers peripherals Information DB access and updates E g Financial Airline reservations etc Services Eytan Modiano Slide 7 Email FTP Telnet Web access Video conferencing DB access Client server applications Network coverage areas Wide Area Networks WANS Span large areas countries continents world Use leased phone lines expensive 1980 s 10 Kbps 2000 s 2 5 Gbps User access rates 56Kbps 155 Mbps typical Shared comm links switches and routers E g IBM SNA X 25 networks Internet Local Area Networks LANS Span office or building Single hop shared channel cheap User rates 10 Mbps 1 Gbps E g Ethernet Token rings Apple talk Eytan Modiano Slide 8 Metro Area networks MANS Storage area networks Network services Synchronous Session appears as a continuous stream of traffic e g voice Usually requires fixed and limited delays Asynchronous Session appears as a sequence of messages Typically bursty E g Interactive sessions file transfers email Connection oriented services Long sustained session Orderly and timely delivery of packets E g Telnet FTP Connectionless services Eytan Modiano Slide 9 QoS One time transaction e g email Switching Techniques Circuit Switching Packet Switching Eytan Modiano Slide 10 Dedicated resources Shared resources Virtual Circuits Datagrams Circuit Switching Each session is allocated a fixed fraction of the capacity on each link along its path Dedicated resources Fixed path If capacity is used calls are blocked E g telephone network Advantages of circuit switching Fixed delays Guaranteed continuous delivery Disadvantages Circuits are not used when session is idle Inefficient for bursty traffic Circuit switching usually done using a fixed rate stream e g 64 Kbps Difficult to support variable data rates Eytan Modiano Slide 11 Problems with circuit switching Many data sessions are low duty factor bursty message transmission time message interarrival time 1 Same as message arrival rate message transmission time 1 The rate allocated to the session must be large enough to meet the delay requirement This allocated capacity is idle when the session has nothing to send If communication is expensive then circuit switching is uneconomic to meet the delay requirements of bursty traffic Also circuit switching requires a call set up during which resources are not utilized If messages are much shorter than the call set up time then circuit switching is not economical or even practical Eytan Modiano Slide 12 More of a problem in high speed networks Circuit Switching Example L message lengths arrival rate of messages R channel rate in bits per second X message transmission delay L R R must be large enough to keep X small Bursty traffic x 1 low utilization Example L 1000 bytes 8000 bits 1 message per second X 0 1 seconds delay requirement R 8000 0 1 80 000 bps Utilization 8000 80000 10 Eytan Modiano Slide 13 With packet switching channel can be shared among many sessions to achieve higher utilization Packet Switched Networks Messages broken into Packets that are routed To their destination PS PS PS Packet Network PS PS PS Buffer PS Eytan Modiano Slide 14 Packet Switch Packet Switching Datagram packet switching Route chosen on packet by packet basis Different packets may follow different routes Packets may arrive out of order at the destination E g IP The Internet Protocol Virtual Circuit packet switching All packets associated with a session follow the same path Route is chosen at start of session Packets are labeled with a VC designating the route The VC number must be unique on a given link but can change from link to link Imagine having to set up connections between 1000 nodes in a mesh Unique VC numbers imply 1 Million VC numbers that must be represented and stored at each node Eytan Modiano Slide 15 E g ATM Asynchronous transfer mode Virtual Circuits Packet Switching For datagrams addressing information must uniquely distinguish each network node and session Need unique source and destination addresses For virtual circuits only the virtual circuits on a link need be distinguished by addressing Global address needed to set up virtual circuit Once established local virtual circuit numbers can then be used to represent the virtual circuits on a given link VC number changes from link to link VC7 3 Merits of virtual circuits Save on route computation Need only be done once at start of session Eytan Modiano Slide 16 Save on header size Facilitate QoS provisioning More complex Less flexible 9 VC4 VC13 5 VC3 8 VC3 VC7 6 2 Node 5 table 3 5 VC13 5 8 VC3 3 5 VC7 5 8 VC4 6 5 VC3 5 8 VC7 Circuit vs packet switching Advantages of packet switching Efficient for bursty data Easy to provide bandwidth on demand with variable rates Disadvantages of packet switching Variable delays Difficult to provide QoS assurances Best effort service Packets can arrive out of order Switching Technique


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