8/28/2015  A. Orda, R. Rom, A. Segall, Design of Computer Networks Prof. Ariel Orda Room 914, ext 4646

8/28/2015  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 )

8/28/2015  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

8/28/2015  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

8/28/2015  A. Orda, R. Rom, A. Segall, 5 Network types, by distance between switches Note: Distance between switches normally determines the data transmission speed

8/28/2015  A. Orda, R. Rom, A. Segall, 6 Network topology types Point-to-Point Topologies Broadcast Topologies B A S E F H J D C G I K Z M N L Wireless Ad-Hoc

8/28/2015  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

8/28/2015  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

8/28/2015  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

8/28/2015  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

8/28/2015  A. Orda, R. Rom, A. Segall, 11

8/28/2015  A. Orda, R. Rom, A. Segall, 12 Switching Methods (continued)

8/28/2015  A. Orda, R. Rom, A. Segall, 13 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

8/28/2015  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

8/28/2015  A. Orda, R. Rom, A. Segall, 15 General queue specification In this course we shall treat only M/M/n queues. service input output M / M /n Poisson arrivals Exponential service Number of servers

8/28/2015  A. Orda, R. Rom, A. Segall, 16 Follows that: –During a small time interval holds: Prob( one arrival during (t, t+ t)) = Exponential arrivals Definition 1: Numbers of arrivals in non-overlapping intervals are independent and probability of k arrivals during time interval t : is the average arrival rate Prob ( no arrival during namely o(x) goes down to 0 faster than x

8/28/2015  A. Orda, R. Rom, A. Segall, 17 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 Exponential service time (ST)

8/28/2015  A. Orda, R. Rom, A. Segall, 18 System State Probability that there are k users in the system none has arrived none has left For k=0: In the limit:

8/28/2015  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

8/28/2015  A. Orda, R. Rom, A. Segall, 20 Steady State ( ) Notation: assuming the limit exists In steady state holds Then Solution Calculation of P 0 and P k The solution is valid if. For the system has no steady state. In general, condition for existence of steady state is.

8/28/2015  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

8/28/2015  A. Orda, R. Rom, A. Segall, 22 Little’s formula 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 the system Average delay Average arrival rate