1. Internet: A Network of Networks

2. Spring 2002Computer Networks Applications Disparate Network Technologies Different goals: Speed, cost, distance; Different standards for: Expected carrier; Coding messages, e.g. how many volts denote a “1”; Detecting and recovering from errors;  Difficult to interconnect different networks

3. Spring 2002Computer Networks Applications Coping with Incompatibility Two possible solutions: Design a network technology that encompasses all needs; Drawbacks: hard to design, expensive, difficult to change. Find a way to interconnect all types of networks  Internetworking

4. Spring 2002Computer Networks Applications Connecting disparate networks Idea: a computer can be connected to two (or more) different networks Each connection requires a different circuit board; Network 1 Network 2 C: a computer with two network connection A B

5. Spring 2002Computer Networks Applications Routers Special purpose computers used for interconnecting networks; Essentially a router receives messages originating from one network and sends (routes) them to the other network; The process of selecting a network over which to send a message is called routing; Ex: computers A and B can communicate via router C;

6. Spring 2002Computer Networks Applications Routers (cont.) Consists of a central processor, memory and network interfaces; Run special purpose software; An internet consists of a set of networks interconnected by routers. An internet formed by using three routers to interconnect four physical networks.

7. Spring 2002Computer Networks Applications Internet The global Internet consists of thousands of computer networks interconnected by routers. Internet appears as a single, seamless communication system to which many computers can attach. Achieves universal service: each computer is assigned an address; Any computer can send a message to any other computer;

8. Spring 2002Computer Networks Applications Internet (cont.) Internet connects both WANs and LANs; WAN---provides efficient and fast long distance communication; National WAN or backbone network; Wide Area Backbone LAN 1LAN 3 LAN 2

9. Spring 2002Computer Networks Applications Connecting to Internet Internet service providers (ISP): in exchange for a fee an ISP forwards data from the customer’s computer to destinations on the Internet and from various computers back to the customer’s computer; Two types of fees: A fee for using the Internet  usually flat A charge for a physical connection  only when one has a dedicated connection with the ISP

10. Spring 2002Computer Networks Applications Connecting to the ISP Dial-up access Leased circuits Cable modem DSL technologies Wireless access

11. Spring 2002Computer Networks Applications Connecting to the ISP (cont.) Dial-up access The computer has to have a modem & the software that uses the modem; The software instructs the modem to place a telephone call to the telephone # of the ISP At the ISP another modem answers the call and accepts Internet packages Advantages: inexpensive Drawbacks: slow

12. Spring 2002Computer Networks Applications Connecting to the ISP (cont.) Leased Circuits Advantages: High-speed data delivery Availability Control Drawbacks: Cost

13. Spring 2002Computer Networks Applications Connecting to the ISP (cont.) Cable Modem: Uses coaxial cable for transmission; Requires a cable modem to connect the client computer with the cable system (and of course a cable modem at the ISP); Advantages: Faster than dial-up; They use existing wiring; Not so expensive as leased circuits; Disadvantage: Performance may degrade: many client modems take turns to send/receive packages

14. Spring 2002Computer Networks Applications Connecting to the ISP (cont.) DSL (Digital Subscriber Line) Uses telephone wires However, instead of encoding data in sounds, it transfers data as electrical signals. DSL signals and telephone signals do not interfere Advantages: High data rates (similar to cable) Less expensive than leased circuits Uses existing wiring No sharing

15. Spring 2002Computer Networks Applications Connecting to the ISP (cont.) Wireless Used in remote areas Similar rate of transfer as cable and DSL.