1 Networking A computer network is a collection of computing devices that are connected in various ways in order to communicate and share resources. The generic term node or host refers to any device on a network.

2 Networking One of the guiding principles of network design is “ demand access ” which is implemented as the client/server model

3 Networking Clients Client software runs on a user ’ s computer and sends requests to servers for information. Some example client programs are: –Browsers –Mailers –Fetch, WS-FTP

4 Networking Servers A file server is a computer that stores and manages files for multiple users on a network. A Web server is a computer dedicated to responding to requests for Web pages. A mail server is a computer dedicated to sending/receiving .

Packet Switching To improve the efficiency of transferring information over a shared communication line, messages are divided into fixed-sized, numbered packets. Network devices called routers are used to direct packets between networks. Messages sent by packet switching

6 Types of Networks A local-area network (LAN) connects a relatively small number of machines in a relatively close geographical area. Recently, the term metropolitan-area network (MAN) has been adopted to refer to the communication infrastructures that have been developed in and around large cities.

7 Types of Networks A wide-area network (WAN) connects two or more local-area networks over a potentially large geographic distance. –Often one particular node on a LAN is set up to serve as a gateway to handle all communication going between that LAN and other networks. Communication between networks is called internetworking. –The Internet, as we know it today, is essentially the ultimate wide-area network, spanning the entire globe.

8 Internetworking Local-area networks connected across a distance to create a wide-area network

9 Internet Connections The Internet backbone is a term used to refer to a set of high-speed networks that carry Internet traffic. These networks are provided by companies such as AT&T, GTE, and IBM. An Internet service provider (ISP) is a company that provides other companies or individuals with access to the Internet.

10 Internet Backbone

11 Regional Backbone

12 Internet Connections There are various technologies available that you can use to connect a home computer to the Internet. –A phone modem converts computer data into an analog audio signal for transfer over a telephone line, and then a modem at the destination converts it back again into data. –A digital subscriber line (DSL) uses regular copper phone lines to transfer digital data to and from the phone company ’ s central office. –Cable modem — in this approach, the data is transferred on the same line that carries your cable TV signals.

13 Internet Connections A key issue related to computer networks is the data transfer rate, the speed with which data is moved from one place on a network to another. Both DSL connections and cable modems are broadband connections, which generally mean speeds faster than 128 bits per second.

14 LAN Connections Usually, the connections between computers in a network are made using physical wires or cables. However, some connections are wireless, using radio waves or infrared signals.

15 Open Systems As network technologies grew, the need for interoperability became clear. We needed a way for computing systems made by different vendors to communicate. An open system is one based on a common model of network architecture and a suite of protocols used in its implementation.

16 Network Protocols Network protocols were developed using a layered approach in which each layer relies on the protocols that underlie it. This is called a protocol stack. Layering of key network protocols

17 Protocol Layers Application Layer Transport Layer Network Layer Link Layer

18 Protocol Layers Application Layer Link Layer Network Layer Transport Layer Application Layer Link Layer Network Layer Transport Layer Application Layer Link Layer Network Layer Transport Layer

19 Application Layer Consists of software units that communicate with each other across the Internet. Examples of these “ high-level ” protocols include: –SMTP –POP3 –telnet –FTP –http Passes complete message to the Transport Layer

20 Transport Layer Receives complete messages from the Application layer Divides the message into packets Adds a sequence number to each packet Attaches the destination address to each packet Passes the packets to the Network Layer Examples include: –TCP - Transmission Control Protocol –UDP - User Datagram Protocol

21 Network Layer Receives addressed packets from the Transport Layer Adds an intermediate address –If the destination is within the current net, the intermediate and destination addresses are the same. –Otherwise, the intermediate address is for a router in the current net, which will pass the packet to an adjacent net. Passes the packet to the Link Layer IP - Internet Protocol

22 Link Layer Deals with the details of the particular LAN. –A ring topology connects all nodes in a closed loop on which messages travel in one direction. –A star topology centers around one node to which all others are connected and through which all messages are sent. –In a bus topology, all nodes are connected to a single communication line that carries messages in both directions.

LAN Topologies Various network topologies

24 Transmission Privileges Rings use tokens. Stars use polling. A bus uses CSMA/CD. –Ethernet has become the standard.

25 Network Addresses A hostname is a unique identification that specifies a particular computer on the Internet. For example:

26 Network Addresses The Transport Layer translates a hostname into its corresponding IP address. For example:

27 Domain Name System A hostname consists of several parts. The very last section of the domain is called its top-level domain (TLD) name. Recall the example above: “ ca ” is the TLD

28 Domain Name System Some common examples of TLDs: Top-level domains, including some relatively new ones

29 Domain Name System Organizations based in countries other than the United States use a top- level domain that corresponds to their two-letter country codes. Some of the top-level domain names based on country codes

30 Domain Name System A domain name may be further separated into sections that specify the organization, and possibly a subset of an organization (subdomain) In the example above: “ yorku ” is the domain “ cse ” is the subdomain “ www ” names the computer

31 Domain Name System The domain name system (DNS) is chiefly used to translate hostnames into numeric IP addresses. –DNS is an example of a distributed database. –If a server can resolve the hostname, it does so. –If not, that server asks another domain name server.

32 URLs A hostname is the core part of a Uniform Resource Locator, or URL, which uniquely identifies the page you want out of all of the pages stored anywhere in the world.

33 URLs The four parts of a URL are: –protocol –hostname –path –filename

34 URLs The protocol specified in a URL determines the type of connection that will be made. There are several possible. For example: –ftp –http –pop3 –smtp –telnet

35 The World Wide Web The Web is an infrastructure of distributed information combined with software that uses networks as a vehicle to exchange that information. http is the protocol used to transfer all Web pages.