Introduction Johan Lukkien Computer Networks 2002/2003 Introduction Johan Lukkien 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Overview Motivation, background Terminology Physical organization topology broadcast networks limitations & scaling 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Motivation Connect independent systems Scalability Share resources “Loose” connection communicating partners also operate stand-alone Openness explicit boundaries: interfaces, protocols separate development Scalability size, location (spatial), administration Share resources connect resources & users share the infrastructure; separated from applications 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

What is a network? Physically: physical infrastructure, connecting access points infrastructure: cables, repeaters and hubs access point: physical location where a terminal (a user of the network) attaches Logically: a facility for the exchange of information between disparate applications disparate: unrelated, not sharing memory 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Hardware concepts A network is modeled by a graph Nodes: Processors, Memories, P+M P+M may also be a dedicated network component Connect P with M: tightly coupled Connect P+M’s: loosely coupled Edges represent communication medium broadcast, i.e. multiple access e.g. bus, ethernet point-to-point e.g. “switched’ ethernet 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Broadcast medium All access points see the same physical signals ...though not at the same time introduces dependence on propagation delays need to deal with “collisions” (in fact: interference) arbitration: centralized / distributed try again.... Implementation: Several access points on a single “wire” e.g., bus, coax ethernet, wireless LAN Access points connected by repeaters & hubs hub: repeat and amplify signal (as well as noise) on all lines, except where it came from “layer 1 switch”, e.g. for ethernet repeater: two-party hub (copy signal from one onto other) The above two are functionally identical 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Bus-based Arbitration: master/slave Tightly coupled 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

10Base2 ethernet No arbitration: detect and avoid collision CSMA/CD 10Base2: 10 Mbps, 200meter, coax Loosely coupled 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Network topology issues Maximal distance between any two nodes: diameter Minimum amount of traffic/wires that can go between any division in two halves: bisection bandwidth Number of neighbors of a node: degree Inter-dependence of degree, number of nodes and diameter maximize #nodes for given degree and diameter Moore upperbound roughly logarithmic relation nodes and diameter 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Examples: grid and hypercube (a): degree: 4, diameter: sqrt(N), BW: sqrt(N) (b): degree: log(N), diameter: log(N), BW: N/2 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Examples: tree and star Tree: degree: 3, diameter: log(N), BW: 1 fat tree: scale bandwidth per level Star: degree 1 or N-1, diameter: 2, BW: N/2 or 1 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Terminology Link: a physical medium for communication bi-directional, uni-directional Circuit: sequence of links Channel: a means of communication (physical or logical) Path: sequence of channels The throughput or bandwidth of a channel is the number of bits it can transfer per second The latency or delay of a channel is the time that elapses between sending information and the earliest possible reception of it. Full duplex communication: concurrently, both ways along the channel; vs. half duplex 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Limitations of broadcast medium Capacity shared by connected nodes though average and peak demands may vary ....hence, not too many nodes Propagation delays for collision detection colliding parties need to be able to detect a collision during transmission; ...hence, propagation delay and speed of medium determine minimum packet size arbitration: just long negotiation delays .....hence, limit physical distances Robustness: single point(s) of failure network-wide physical effects of attach/detach/fail ....hence, decouple 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Alternatives Establish a circuit only when needed: circuit switching e.g. (older) telephony, “switched” networks possibly, simultaneous connections guarantees certain quality, makes billing easy resource in-efficient, especially for short-lived interactions vulnerable to errors and failures 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Switching and sharing 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Switching networks For tightly coupled case: “backplane” for a multi-computer Several concurrent connections possible (crossbar) 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Alternatives Establish a circuit only when needed: circuit switching e.g. (older) telephony, “switched” networks possibly, simultaneous connections guarantees certain quality, makes billing easy resource in-efficient, especially for short-lived interactions vulnerable to errors and failures Have physically separated networks and exchange packets rather than signals admits more concurrency, in principle in the limit: networks of size 2 (point-2-point link) no interference anymore no direct link between communicating partners! must know what a packet is can connect heterogeneous networks 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Heterogeneous Connect multi-computers, PC’s, palm-tops using a variety of physical connections Just connect arbitrary networks e.g. wire-less and regular LAN Generally, physical connections do not connect all pairs directly some nodes need to transport information explicitly: store & forward packets these nodes need to interpret (and, hence, understand) the passed information to some extent 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Conclusions Networks based on broadcasting are limited in size (geographical, number of nodes) Collision domain: for collision-based methods, the domain in which physical broadcasting is done Communication based on packets for scaling in size decoupling 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Exercises My laptop’s hard disk is 30 GB. I cycle home in 30 minutes. What are throughput and latency of this “channel”? Exercises 3, 4, 6 on page 81 and 35 on page 83 What are bisection bandwidth, degree and diameter of a line network a ring a torus (the cartesian product of two rings; draw it first) Why are not all computers of the world connected in a single broadcast domain? Mention several reasons. 18-Aug-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking