Basic Concepts and Principles Chapter 1 Copyright 2001 Panko

2 Network n A Network is an Any-to-Any Communication System – Can connect any station to any other

3 “Connect to GHI” Network n Each Station has a Unique Network Address – To connect, only need to know the receiver’s address – Like telephone number ABC DEF GHI JKL MNO

4 Data Communications n Originally, There was a Sharp Distinction: – Voice and Video Communication versus – Data Communications, in which one or both parties is a computer n Database n Electronic mail n World Wide Web – Distinction is fading because voice and video communication are increasingly computer- based

5 Voice and Video Networks n Telephone Network – Customer premises (home or office) – Local loop (access line) connects customer premises to first switching office Connection Switching Office Local Loop (Access Line) Customer Premises Customer Premises

6 Voice and Video Networks n Hierarchy of switches n Trunk lines connect switches Switch Trunk Line

7 Voice and Video Networks n Circuit – End-to-End Connection between Phones – May pass through multiple switches – And trunk lines Circuit

8 Voice and Video Networks n Reserved Capacity – Circuit capacity is reserved during duration of each call – At each switch – On each trunk line Circuit Reserved Capacity Reserved Capacity

9 Voice and Video Networks n Reserved Circuit Capacity Guarantees Throughput – Never get less than reserved capacity – Nothing like congestion on the Internet n Reserved Circuit Capacity is Expensive – Pay for it whether you use it or not – Good for voice, because conversations are fairly constant – Bad for data, because most data transmission is bursty; e.g., in World Wide Web, download, then stare at screen for a long time until next download

10 Packet-Switched Data Networks n Packet Switching – Circuit switching is expensive due to reserved capacity – Packet switching breaks transmissions into messages – Messages are short (averaging a few hundred bytes) because switches handle short messages efficiently – Messages are called packets (sometimes, frames or other names) Message Packets

11 Packet-Switched Data Networks n Switching Decision – When a packet arrives at a switch, the switch must decide which of several ports (connections) to use to send the packet back out – Complex – Made at each switch B? D? C? Switch A B C D Packet

12 Packet Switched Data Networks n Multiplexing – Packets from many conversations are mixed (multiplexed) over each trunk line – Only pay for the capacity used – Dramatic trunk line cost savings – The reason for packet switching Multiplexing on Trunk Line

13 Analog Transmission n In analog transmission, the state of the line can vary continuously, rising and falling smoothly in intensity among an infinite number of states – State may be voltage, frequency or another line signal characteristic – Voice and video analog – Telephone network assumes analog input signals Time Strength

14 Digital Transmission n In digital transmission, time is divided into periods of fixed length called clock cycles n The line is kept in one of only a few possible states (conditions) during each clock cycle Clock Cycle Time Strength

15 Digital Transmission n At the end of each clock cycle, the line may change abruptly to another of these few states – Can also stay the same Time Strength Abrupt Change Stays Same Clock Cycle

16 Digital Versus Binary Transmission n Digital transmission: a few states (2, 4, 8, 16, etc.) n Binary transmission: exactly two states – One state represents 1, the other 0 DigitalBinary Two StatesFew States 0 1

17 Digital Communication n Modems – Computers have digital output – Telephone network assumes analog input – Modem translates between digital device and analog line for data transmission over the phone system Digital Signal (1101) Modem Analog Signal

18 LANs and WANs n Networks Have Different Geographical Scopes n Local Area Networks (LANs) – Small Office – Office Building – Industrial Park / University Campus n Wide Area Networks (WANs) – Connect corporate sites or – Connect corporate sites with sites of customers and suppliers

19 Elements of a Simple LAN Hub or Switch Wiring Hub or Switch connects all stations Wiring is standard business telephone wiring (4 pairs in a bundle)

20 Elements of a Simple LAN Server Client PC Server Client PCs are used by ordinary managers and professionals; receive service Servers provide services to client PCs Server

21 Elements of a Simple LAN n Client PC – Begin with stand-alone PC – Add a network interface card (NIC) to deal with the network – Networks have many client PCs n Server – Most PC nets have multiple servers