Internet Technology (AS21)

A network is defined as a group of two or more computer systems linked together. There are many types of computer systems linked together. there are many types of computer networks including

• Local-Area Network (LANs): The computers are geographically close together ( that is, in the same building).
• Wide-Area Networks (WANs): the computers are father apart and are connected by telephone lines or radio waves
• Campus-Area Network (CANs): the computers are withing a limited geographic area, such as a campus or military base

• Metropolitan-Area Networks (MANs): A data network designed for a town or city.

• Home-Area Networks (HANs): A network contained within a user's home that connects a person's digital devices.

• Global Area Network (GAN): a network composed of different interconnected networks that cover an unlimited geographical area.

• protocol: the protocol defines a common set of rules and signals that computers on the network use to communicate. E.g. Internet Protocol (IP)
• topology: The physical and logical arrangements on how different components of a network communicate with each other.
• architecture: Networks can be broadly classified as using either a peer-to-peer or client/server architecture.

Computers on a network are sometimes called nodes. Computers and devices that allocate resources for a network are called servers.

The protocol defines a common set of rules and signals that computers on the network use to communicate. E.g. Internet Protocol (IP)

In Computer Communications:

• Some kind of physical connection is required between communicating parties (e.g. nodes)
• Both parties need to know the shared protocol

These rules exists also in “Human Communication” and answer to three main questions:

•What/How/When is it communicated?

Links in the form of physical connections. The transmission media in networks. three main categories with examples. 

Wired

• Coaxial e.g. TV antennas
• Twisted Pair Cables
• Optical Fiber
• USB cables
• Power lines

Wireless

• Bluetooth
• Wi-Fi

Bus simple and cheap wiring bottleneck on cable

• Common transmission media
• all nodes broadcast signals to all
• Bidirectional. 

Ring network failure possible due to failure of one station

• common media, but in a closed loop
• passes a token
• unidirectional

Star  , a Switch as a single point of failure

• hub or switch in the central node
• pass to stwich
• centralized management. 

others: point-to-point, mesh, three and hybrid. 

The Open System interconnection model is a conceptual model for defining and standardizing network communication mechanisms. there are 7 abstract layers. each layer has its distinct functionality. provides an interface for its predecessor and successor. 

Functional details are hidden within the current layer so that the next layer is not required to have knowledge about its predecessor and successor ( other than the standardized interfaces)

Every layer can add its own header to a data package to transfer management information. these headers are only used by the corresponding layer on the other machine

The TCP/IP Model simplified the classical OSI Model and effectively replaced it. but 

Keep in mind:

•These are not an architecture

•They do not establish which services or protocols to use.

•It suggests what each level should do.

1. Application
   1. Applications, protocols and services that interface with the end user
   2. data is formatted, converted, encrypted decrypted compressed and decompressed and sent or presented to the user
   3. open, close and manage a session between end user application processes
2. Transport
   1. Facilitates end-to-end communications between multiple applications simultaneously (ports)
   2. reliable and unreliable end-to-end data transport and data stream services (TCP, UDP, SCTP)
   3. Connection-oriented, connectionless communications, and data stream services (session establishment and termination)
3. Internet
   1. provide host addressing (IP)
   2. choose the best path to the destination network (Routing)
   3. Switch packets out of the correct interface (Forwarding)
   4. Maintain quality of service (QoS)
   5. Connectionless end-to-end networking
4. Network (Access)
   1. In L2 there are 2 sublayers: logical link control (LLC,802.2) provides services to the upper layers and physical addressing (media access control addresses) 
   2. error checking (CRC)
   3. L1: frames get encoded in sequences of bits, depending on the Links (physical connections) e.g. light pulses, electricity, radio waves..