046335 Design of Computer Networks Prof. Ariel Orda Room 914, ext 4646

1. 046335 Design of Computer NetworksProf. Ariel OrdaRoom 914, ext 4646  A. Orda, R. Rom, A. Segall,

2. Introduction • Computer Network: • A set of autonomous connected computers • connected = can transmit information between computers • autonomous = independent ( not Master-Slave) • Related concepts: • computerized communication = computers aid to communication of a different type ( e. g. telephony ) • distributed system = the network is transparent to the user and the operating system takes care of the communication ( the difference between this and a computer network is minimal ) • communication system = there is exchange of information, but there is no communication network ( e.g. Master -Slave )  A. Orda, R. Rom, A. Segall,

3. Computer Network: reasoning and usage • Information Sharing • Resource Sharing: Files, Databases, Printers, Applications. • Reliability: Resource backup • Efficiency: work in parallel on different parts of the problem. • Cost: changes in relative cost of computation / communication • Network versus point-to-point communication • Most of the time no need for session between any two given users • While a session is in progress, actual communication is not continuous • Every node can connect to any other node  A. Orda, R. Rom, A. Segall,

4. Network Components: • End systems and computers ( hosts ): network users • Communication sub-network: • transmission of information between users • does not generate information ( except to support communication ) • Communication sub-network links: • Point-to-Point: twisted pair, coaxial cable, optical fiber, infra-red, wireless (Bluetooth, WiFi, etc). • Broadcast: radio, microwave, bus, satellite • source-destination data transmission: switching (to be explained later) • other network examples: • transportation network, phone network • first part in network design is network topology  A. Orda, R. Rom, A. Segall,

5. Network types, by distance between switches Note: Distance between switches normally determines the data transmission speed  A. Orda, R. Rom, A. Segall,

6. Z S E F B C M L J A G H D K I N Network topology types Point-to-Point Topologies Broadcast Topologies Wireless Ad-Hoc  A. Orda, R. Rom, A. Segall,

7. Logical design of networks ( architecture ) • Layered architecture • each layer is responsible for a collection of functions and provides service for upper layers • Modular architecture facilitates design and maintenance • Protocol: conversation between identical layers at different locations • Interface: conversation between adjacent layers at the same site  A. Orda, R. Rom, A. Segall,

8. OSI Reference Model - layer description • Physical Layer - bit transmission, electrical and mechanical problems • Data Link (DLC) - Reliable data transmission on links, overcomes noise problems. Normally uses data frames and ack frames. • Network Layer - Responsible for Operation of the Communication Sub-Network: • Routing: data flow in the network • Flow Control: stops network overflow • Inter-network transmission • Transport Layer • Reliable end-to-end data transmission • Differentiates between types of traffic, provides for each: reliability, order, delay • Session Layer • Different types of machines can maintain a conversation • Call control ( unidirectional or bi-directional), token control, synchronization • Presentation • Encryption, compression, etc. • Application: everything else • In common channel networks, MAC layer, an additional sub-layer under DLC, to control channel access  A. Orda, R. Rom, A. Segall,

9. Switching Methods • Circuit Switching • Needs setup • used in phone systems • reserved fixed bandwidth • no congestion problem • Message Switching • messages are forwarded in one piece ( store & forward ) • no fixed path between source and destination • maximum message size not specified • no need for preparation phase in the network ( setup) • large memory requirements ( to accommodate large messages) • Packet Switching • packets are forwarded individually, possibly on different paths • efficient bandwidth use • low delay and low memory requirements • may produce traffic jams • packets may arrive out of order  A. Orda, R. Rom, A. Segall,

10. Switching methods ( continued ) • Virtual Circuit Switching • Circuit Switching + Packet Switching combination • Packetized Data is being switched • Path is established upon call setup and is fixed throughout the call • No reserved Bandwidth • Properties: • Need for preparation phase • Packets arrive in order • There may be gaps because of losses if there is no DLC on links • Fixed Path • Congestion Problem can still arise VC Switching is very popular in modern high-speed networks  A. Orda, R. Rom, A. Segall,

11.  A. Orda, R. Rom, A. Segall,

12. Switching Methods (continued)  A. Orda, R. Rom, A. Segall,

13. Design Problems • Design Problems • Switch design • Communication means type • Switching method • Use of communication means • Topological Design • Routing method • Flow and Congestion Control • Design Criteria • Performance: • Delay • maximal or average • per user or for entire network • Throughput • Cost • Reliability and Survivability • Adaptivity and Scaling • Simplicity of Protocols  A. Orda, R. Rom, A. Segall,

14. Queues • Packets arrive randomly • Wait in line to be transmitted • Service time is the transmission time • Random elements: • packet arrival time • service time, if packets are not of fixed length • Need for statistical specification Communication link as a queue  A. Orda, R. Rom, A. Segall,

15. Poisson arrivals M / M /n Exponential service Number of servers General queue specification • In this course we shall treat only M/M/n queues. output service input  A. Orda, R. Rom, A. Segall,

16. is the average arrival rate Exponential arrivals • Definition 1: Numbers of arrivals in non-overlapping intervals are independent and probability of k arrivals during time interval t : • Follows that: • During a small time interval holds: • Prob( one arrival during (t, t+ t)) = Prob ( no arrival during namely o(x) goes down to 0 faster than x  A. Orda, R. Rom, A. Segall,

17. Exponential service time (ST) • Probability that a user requires service time < t (service time cdf): • Probability that a user in service at time t is still in service at time • Probability that a user in service at time t completes it by time  A. Orda, R. Rom, A. Segall,

18. System State Probability that there are k users in the system none has arrived none has left In the limit: For k=0:  A. Orda, R. Rom, A. Segall,

19. Example Now we can calculate This is a differential equation for whose solution is : We can continue this way for every k  A. Orda, R. Rom, A. Segall,

20. Steady State ( ) Notation: assuming the limit exists In steady state holds Then Solution Calculation of P0and Pk The solution is valid if . For the system has no steady state. In general, condition for existence of steady state is .  A. Orda, R. Rom, A. Segall,

21. State Transition Diagram • Based on transition rates • State “ flow” conservation • Example: dashed circles. • Example : ellipse: • Steady state equations can be written directly from the state diagram • Can also write diagram for : • as a function of the state • as a function of the state  A. Orda, R. Rom, A. Segall,

22. Little’s formula Average delay • Explanation: • average user arrives to system and finds users • when he leaves, there are users, therefore while he was in the system users arrived • the period he was in the system is and during this period arrived • Little’s theorem holds also for more complicated systems • Use for M/M/1 Average number of users in thesystem Average arrival rate  A. Orda, R. Rom, A. Segall,