Information Technologies Dersi 3. Ünite Özet

A Brief History of Computer Communication

Computer communication traces its beginning back to the early 1940s. Since that time, there have been many advances in the field, and today the whole world is connected through networking facilities. While the early researchers had worked on sending commands from one machine to another, today’s modern technology offers many different services such as the World Wide Web, wireless communication, the Internet of Things, and cloud computing.

Computer Networks and the Internet

The computer networks enable communication devices to exchange data among themselves. Users can share data, files, and resources, communicate with each other, store and access data centrally by the help of computer networks.

A computer network is built upon the networking hardware. The basic hardware components are as follows:

• A network interface controller (NIC) is a circuit board that enables a computer to connect to a network.
• A repeater regenerates incoming signals so that they can travel long distances.
• A hub is a multiport repeater.
• A bridge connects multiple network segments to create a single aggregate network.
• A switch centralizes communications among multiple devices within a local area network.
• A router connects multiple computer networks by using wired or wireless connections.
• A modem serves as a bridge between a local network and the Internet.
• A firewall is a protective system that lies between a computer network and the Internet.

The Internet and the World Wide Web: The Internet is the network of networks that connects millions of computers and people around the world. The World Wide Web (WWW), also known as the Web, is the most commonly used and growing application of the Internet. The key technologies of the Web are HTML (Hypertext Markup Language), HTTP (Hypertext Transfer Protocol), web servers, and web browsers.

Transmission Media

In computer communication, transmission media (medium) is the pathway that carries information from sender to receiver. In general, transmission media is classified into two groups as guided media and unguided media:

1. Guided Media: In this category, the type of transmission media is physically tangible. The signals propagate over a solid substance such as copper wire. Guided media contains unshielded twisted pairs, shielded twisted pairs, coaxial cables, and optical fibers.
2. Unguided Media: In this category, the data is sent through the air. During transmission, the data is available to any device that is capable of receiving wireless signals. Unguided media contains radio waves and microwaves.

Network Switching

Switching is the process of forwarding packets from one port to another leading towards the destination. Switching is required to deliver information through the links between the sender and the receiver.

Switching is usually done with three techniques:

1. Circuit Switching: This technique establishes a path between the sender and the receiver with guaranteed isolation from the other parts. The circuit is established before the communication starts. A telephone network is a typical example of a circuit switched network.
2. Message Switching: In this technique, the sender treats the whole message as a data unit and transfers it in its entirety, one hop at a time. There is no need to establish a dedicated path between the sender and the receiver. An e-mail system is an instance of message switched network.
3. Packet Switching: This technique divides the message into smaller parts called packets. The destination information is included in the header of a packet. The main advantage of packet switching is that many devices can share the same channel at the same time. However, it is hard to implement these protocols.

Virtual Circuits and Datagrams: The primary approaches in packet switching are virtual circuits and datagrams. In virtual circuit approach, a single route is set up between the sender and the receiver. Since all the packets follow this route, the delivery of packets in correct order is guaranteed. In datagram approach, each packet is transmitted independently based on the source and destination addresses. Thus, the packets may be lost or arrive out of order.

Classification of Computer Networks

The computer networks are generally classified according to their geographical scale. Ranging from smallest to largest, there exist four primary groups of computer networks:

• A personal area network (PAN) connects electronic devices to a person within a very limited range. In this network type, the devices can communicate very fast since the data is transferred in an internal network.
• A local area network (LAN) interconnects devices within a small area such as an office, home, building, or factory. The main purpose of building a LAN is to provide an environment for workstations and personal computers to exchange information and share resources.
• A metropolitan area network (MAN) is a computer network that usually covers a campus or a city. A typical example of a MAN is the cable television network of a city.
• A wide area network (WAN) connects multiple LANs that are located in different geographical regions. These networks are widely used to share, exchange, and manage data among organizations and individuals.

Local Area Networking

Network topologies show the arrangement of the elements in the network. There are five common topologies in local area networking:

1. Bus Topology: The devices share a single communication line that is known as bus. A device sends a message by broadcasting it to the bus, and the intended receiver accepts the message and processes it while others ignore it.
2. Star Topology: All the devices connect to a central device that is called network hub. In case of the failure of this hub, the whole network stops working.
3. Ring Topology: All the devices are connected in a closed loop that forms a ring. Data transmission occurs around this ring. A failure in a single device disturbs the whole network.
4. Mesh Topology: Each device has a point-to-point connection with a single or multiple devices. This topology can be wither full mesh or partial mesh. In full mesh, every device has a connection to every other appliance inside the network. However, in partial mesh, not all the devices have direct communication with every other device in the network.
5. Tree Topology: This topology brings star and bus topologies together. Multiple hub devices are connected to the bus, and each of these hubs acts as the root of a single star topology. This topology is efficient for large networks since it divides the network into smaller parts.

Wireless LANs: A wireless LAN (WLAN) connects two or more devices via a wireless connection within a limited area such as a campus or building. WLANs use radio frequency to transmit data through air. Due to this fact, it does not need wired connections. The ease of installation makes WLANs very popular. Most of the modern devices like tablets and smart phones support WLAN connections.

IEEE 802.11 defines media access control and physical layer specifications for WLANs. The term “Wi-Fi” represents a WLAN protocol based on this network standard.

Reference Models

The OSI Reference Model : The OSI is a general-purpose paradigm for describing how computers communicate with each other over a network. In this model, the computer network architecture is divided into seven layers:

1. Physical layer deals with the transmission and reception of the raw bit stream over a physical medium.
2. Data link layer is responsible for providing error-free transfer of data frames from one node to another one.
3. Network layer makes the traffic routing decisions and provides traffic control, fragmentation, and logical addressing.
4. Transport layer delivers data across network connections.
5. Session layer is responsible for session establishment, maintenance, and termination.
6. Presentation layer translates data to a standard format.
7. Application layer provides a network interface for applications.

The TCP/IP Reference Model: The TCP/IP is a reference model that is used by the majority of the current Internet architecture. While TCP defines how applications can create communication channels across a network, IP defines how computers can get from each other over a set of routed networks. The TCP/IP model contains four layers:

1. Network access layer combines the functions of physical and data link layers of the OSI model.
2. Internet layer is responsible for addressing, packaging, and routing functions.
3. Transport layer provides application layer with session and datagram communication services.
4. Application layer provides applications with the ability to access the services of other layers.

Comparison of the OSI and the TCP/IP: Although these models have similar properties, they also have following differences:

• While the OSI is theoretical, the TCP/IP is practically used for data transmission over the Internet.
• While the OSI has seven layers, the TCP/IP has four layers.
• Replacing a protocol is easy in the OSI, harder in the TCP/IP.
• The delivery of packets is guaranteed in the transport layer of the OSI. However, TCP/IP does not ensure the delivery.
• The TCP/IP does not clearly distinguish between services, interfaces, and implementations, whereas, the OSI does.

Security in Computer Networks

The Internet makes computer systems more vulnerable than ever. In order provide a secure networking environment; these three primary goals should be satisfied:

• Confidentiality is the protection of valuable information from unauthorized access.
• Integrity is keeping the information accurate and consistent over its entire life cycle.
• Availability is the ability of a network system to respond information access requests.

Confidentiality, integrity, and availability are usually referred as CIA triad.

Although security architects and administrators work hard to protect their systems, it may not possible to develop 100% secure systems. Security attacks try to change data and system resources, gain information, and discover vulnerabilities. Most common security attack types are eavesdropping, Man-in-the-Middle, Man-in-the-Browser, Denial-of-Service, and spoofing.

Network Management

Network management is the process of administering the computer networks. The requirements of network management are described with FCAPS model, which stands for:

• Fault: A network system should ensure that all of its components work properly. When a fault occurs, the system should detect, isolate, resolve, and log the event.
• Configuration: A network system should be capable of managing and monitoring system configuration information.
• Accounting: In order to provide a fair distribution of network resources, a network system should track the usage of resources.
• Performance: The network performance should be kept at acceptable levels.
• Security: Security policies control the access to the resources and protect the system against cyber or physical attacks.

Simple Network Management Protocol (SNMP): SNMP is a protocol that facilitates the exchange of information between network devices. The four essential components of this protocol are managed devices, SNMP manager, SNMP agents, and Management Information Base (MIB).