Computer Networks 20022003 Introduction Johan Lukkien 3122021 Johan

Computer Networks 2002/2003 Introduction Johan Lukkien 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 1

Overview • Motivation, background • Terminology • Physical organization – topology – broadcast networks • limitations & scaling 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 2

Motivation • Connect independent systems – “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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 3

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 4

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 5

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 6

Bus-based • Arbitration: master/slave • Tightly coupled 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 7

10 Base 2 ethernet • No arbitration: detect and avoid collision CSMA/CD – 10 Base 2: 10 Mbps, 200 meter, coax • Loosely coupled 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 8

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 9

Examples: grid and hypercube • (a): degree: 4, diameter: sqrt(N), BW: sqrt(N) • (b): degree: log(N), diameter: log(N), BW: N/2 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 10

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 11

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 12

Limitations of broadcast medium 1. Capacity shared by connected nodes – though average and peak demands may vary –. . hence, not too many nodes 2. 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 3. Robustness: single point(s) of failure – network-wide physical effects of attach/detach/fail –. . hence, decouple 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 13

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 14

Switching and sharing 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 15

Switching networks • For tightly coupled case: “backplane” for a multi-computer • Several concurrent connections possible (crossbar) 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 16

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 17

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 18

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 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 19

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. 3/12/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 20