Eytan ModianoSlide 1Digital Communication NetworksMIT PROFESSIONAL INSTITUTE, 6.20sJuly 24 - 28, 2006Professor Muriel Medard, MITProfessor Eytan Modiano, MITEytan ModianoSlide 2Digital Communication NetworksIntroductionEytan ModianoEytan ModianoSlide 3Course syllabus• Monday– AM: Introduction to layering, the physical layer– PM: The data link layer• Tuesday– AM: Delay models and queueing– PM: Higher layer protocols: TCP/IP, ATM• Wednesday– AM: Routing– PM: More routing, flow control• Thursday– AM: Multiple access– PM: LANs, MANs, SANs, and switching• Friday– AM: Wireless Networks– PM: Optical NetworksEytan ModianoSlide 4Logistics• 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 eveningEytan ModianoSlide 5Reference textbooks• Computer Networks, by Peterson and Davie– Most current on the internet• Data Networks, by Bertsekas and Gallager– Strong on probabilistic modeling and analysis• Network Optimization: Continuous and Discrete Models, by DimitriP. Bertsekas– Network algorithms and routing, network flows, optimization theory• Introduction to Probability, by Dimitri P. Bertsekas and John N.TsitsiklisEytan ModianoSlide 6Data 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 TheoryEytan ModianoSlide 7Network 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– Email, FTP, Telnet, Web access– Video conferencing– DB access– Client/server applicationsEytan ModianoSlide 8Network 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• Metro Area networks (MANS)• Storage area networksEytan ModianoSlide 9Network 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– One time transaction (e.g., email)• QoSEytan ModianoSlide 10Switching Techniques• Circuit Switching– Dedicated resources• Packet Switching– Shared resources– Virtual Circuits– DatagramsEytan ModianoSlide 11Circuit Switching• Each session is allocated a fixed fraction of the capacity on eachlink 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., 64Kbps) Difficult to support variable data ratesEytan ModianoSlide 12Problems 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 thedelay requirement. This allocated capacity is idle when the sessionhas nothing to send• If communication is expensive, then circuit switching isuneconomic to meet the delay requirements of bursty traffic• Also, circuit switching requires a call set-up during whichresources are not utilized. If messages are much shorter than thecall set-up time then circuit switching is not economical (or evenpractical)– More of a problem in high-speed networksEytan ModianoSlide 13Circuit Switching ExampleL = message lengths λ = arrival rate of messagesR = channel rate in bits per secondX = message transmission delay = L/R– R must be large enough to keep X small– Bursty traffic => λx << 1 => low utilization• Example– L = 1000 bytes (8000 bits)– λ = 1 message per second– X < 0.1 seconds (delay requirement)– => R > 8000/0.1 = 80,000 bps Utilization = 8000/80000 = 10%• With packet switching channel can be shared among manysessions to achieve higher utilizationEytan ModianoSlide 14Packet Switched NetworksPacket NetworkPSPSPSPSPSPSPSBufferPacketSwitchMessages broken intoPackets that are routed To their destinationEytan ModianoSlide 15Packet 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 fromlink 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 representedand stored at each node– E.g., ATM (Asynchronous transfer mode)Eytan ModianoSlide 16Virtual Circuits Packet Switching• For datagrams, addressing information must uniquely distinguisheach network node and session– Need unique source and destination addresses• For virtual circuits, only the virtual circuits on a link need bedistinguished by addressing– Global address needed to set-up virtual circuit– Once established, local virtual circuit numbers can then be used torepresent the virtual circuits on a given link: VC number changes fromlink to link• Merits of virtual circuits– Save on route computation