Computer Networks

A computer network is a collection of computing devices that are connected in order to communicate and share resources. We use networks to share resources such as printers, data storage or files and databases. E-mail, instant messaging, chat, and Web pages all rely on communication that occurs on a computer network. If two computers are to talk to each other, they need to have a connection with each other.

Types of connections:

Usually, the connection between computers in a network is made using physical wire or cables for example copper wire, fiber optic cable. However, some connections are wireless, using radio waves, or infrared signals to transfer data.

Computer networks contain devices other than computers. Printers, for example can be connected to a network so that anyone on the network can print on them.

Computer networks also contain devices for handling network traffic, such as routers or switches, etc.

Picture of a switch

Wireless Router

We use the term node or host to refer to any device on a network.

Data Transfer Rate: Data Transfer Rate is the speed in which the data is moved from one place on a network to another. The data transfer rate is sometimes referred to as bandwidth of a network.

Protocols:

A protocol is a set of rules describing how two entities interact. In networking, we use protocols to describe how transferred data is formatted and processed.

Computing Models:

Mainframe Oriented Systems

1950s Batch oriented - punch card systems 1. Literally punch holes in cards to feed in programs and data 2. Reports printed on paper 1960s Batch oriented - terminals 1. Use terminals for data entry 2. Reports still done using paper

1970s On-Line, transaction based 1. Some reporting now done using terminals 2. The user interface gains attention. Characteristics: 1. Central system (the mainframe) for storage of programs, and data. 2. Central system for the execution of programs. 3. "Dumb" terminals - Very limited processing capabilities. Just display characters on the screen and get user input.

1980s - The mighty LAN, the birth of client/server

Personal computer revolution of the early 1980s. Cost per unit of processing power falling rapidly 1. 1981: PC 8088 Processor, 256K Memory. 2. 1984: PC/AT 80286 Processor, 512K Memory. 3. 1986: 80386 Processor, 1 MB Memory. 4. 1989: 80486 Processor, 4 MB Memory. 5. 2000: Intel's new 700 MHz Pentium, 2 GB Memory 6. 2013: Intel Core i7-3960X 16 GB Memory User interfaces on PCs - make use of better User Interface design techniques. Eventually, Graphical User Interfaces (GUI)

The "File Server" Model

A file server is a computer that stores and manages files for multiple users on a network. That way every user doesn’t need to have his or her own copy of the files

1. Central system to store programs and data (the file server). 2. Execution of programs done on clients rather than on file server. 3. Done over a local area network (LAN). 4. Clients tend to be homogeneous.

The "Client/Server" Model

1. Central system stores data and part of the programs - The "Server" portion. 2. Client systems execute the user interface and business logic portions - The "Client" portion. 3. Clients send Requests or Transaction to server 4. Server executes transactions then sends results back to client. 5. Done over a local area network. 6. Clients establish a network session with server. Connection- oriented protocols are used.

1990s - Distributed Systems

Many servers spread out over local or wide area networks (LAN or WAN). Clients and servers can be located anywhere in the world. Clients and servers are heterogeneous (dissimilar).

Types of networks

Computer networks can be classified in various ways. A local-area network (LAN) connects a relatively small number of machines in a relatively close geographical area. Various configurations, called topologies, have been used to administer LANs.

Ring Topology

A ring topology connects all nodes in a closed loop on which message travel in one direction. The nodes of a ring network pass along messages until their reach their destination.

Star topology:

A star topology centers around a node to which all others are connected and through which all messages are sent. A star network puts a huge burden on the central node; if it is not working, communication on the network is not possible.

Bus Topology:

All nodes are connected to a single communication line that carries messages in both directions. The nodes on the bus check any messages sent on the bus, but ignore any that are not addressed to them. A bus technology called Ethernet has become the industry standard for local-are network.

Wide Area Network:

A wide-area network (WAN) connects two or more local-area networks over a potentially large geographic distance. A wide-area network permits communication among smaller networks. Often a particular node on a LAN is set up to serve as a gateway. A gateway handles all communication going between that LAN and other networks.

Communication between networks is called internetworking.

The Internet is essentially the ultimate wide-area network, spanning the entire globe.

The Internet is a vast collection of smaller networks that have all agreed to communicate using the same protocols and to pass along messages so that they can reach their final destination.

Internet connections:

Internet backbone is the physical network (usually relying on fiber optic cable) that carries Internet traffic between different networks and is measured in megabits per second. In The US, these networks are provided by telecommunications and computer companies.

The backbone networks all operate using connections that have high data transfer rates ranging from 1.5 megabits per second to over 600 megabits per second.

Internet Service Provider: An Internet service provider or ISP is a company that provides other companies or individuals with access to the Internet.

ISPs connect directly to the Internet backbone or they connect to a larger ISP with a connection to the backbone.

There are various technologies available that you can use to connect a home computer to the Internet. The techniques for a home connection have evolved from phone modem, to digital subscriber line (DSL) or a cable modem.

Phone modem : converts computer data into an analog signal for transfer over a telephone line and then a modem at the destination converts it back again into data. One audio frequency is used to represent binary 0 and another to represent binary 1. To use a phone modem, you must first establish a telephone connection between your home computer and one that is permanently connected to the Internet. You pay your ISP for the right to call one of the several computers that they have set up for this purpose. Once the connection is made, you can transfer data via your phone line to your ISP computer, which sends it on its way through the Internet backbone to its destination.

This way of transferring data is convenient, however it is limited to that of analog voice communication, usually 64 kilobits per second at most.

Digital subscriber Line (DSL): An Internet connection made using a digital signal on a regular phone line. The digital signal is obtained using a different frequency than the voice communication. This way the phone line provides a much higher transfer rate. To offer DSL service, the phone company must set up special computer to handle the data traffic. With DSL, there is no need to dial-up to create a network connection, rather, the DSL line maintains an active line between the home computer and the ISP provider. However, to make use of DSL technology, your home must be within a certain distance from the central office; otherwise, the digital signal degrades while traveling between these two points.

Cable Modem: In this approach, the data is transferred on the same line that your cable TV signals come in. The digital data travels just like another TV channel.

Both DSL connections and cable modems fall under the category of broadband connections, which means a speed of 30 Mbps to 400 Mbps for dedicated business lines

Download/Upload: When you download data, you are getting data from the Internet to your home. Viewing a website, listening to music from a site are all downloading activities. When you send data through the Internet, you are uploading information -sending email, submitting a web based form, or clicking to request access to a website, etc.

Both DSL and cable modems, have different speeds for download and upload, devoting more speed to downloads (52 Mbit/s downstream and 16 Mbit/s upstream over copper wires)