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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.

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Networking• One of the guiding principles of network

design is “demand access” which is implemented as the client/server model

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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

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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 email.

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

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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.

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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.

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Internetworking

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

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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.

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Internet Backbone

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Regional Backbone

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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.

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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.

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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.

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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.

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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

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Protocol Layers• Application Layer• Transport Layer• Network Layer• Link Layer

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Protocol LayersApplication

Layer

Link Layer

Network Layer

Transport Layer

Application Layer

Link Layer

Network Layer

Transport Layer

Application Layer

Link Layer

Network Layer

Transport Layer

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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

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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

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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

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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

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Transmission Privileges• Rings use tokens.• Stars use polling.• A bus uses CSMA/CD.

– Ethernet has become the standard.

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Network Addresses• A hostname is a unique

identification that specifies a particular computer on the Internet.

• For example:www.cse.yorku.ca

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Network Addresses• The Transport Layer translates a

hostname into its corresponding IP address.

• For example:130.63.236.200

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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:www.cse.yorku.ca

“ca” is the TLD

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Domain Name System• Some common examples of TLDs:

Top-level domains, including some relatively new ones

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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

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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

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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.

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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.

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URLs• The four parts of a URL are:

– protocol– hostname– path– filename

• http://www.yorku.ca/yorkweb/index.htm

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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

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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.