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ERODE ARTS AND SCIENCE COLLEGE (AUTONOMOUS), ERODE
DEPARTMENT OF COMPUTER APPLICATIONS
ACADEMIC YEAR 2019-2020 EVEN SEMESTER
COURSE MATERIALS
SUBJECT: COMPUTER NETWORKS CLASS: II BCA & II B.SC IT
UNIT: I
Introduction: Uses of Computer Networks - Network Hardware - Network Software - Protocol Hierarchies. Design issues for the Layers - Interfaces - Services - Connection-Oriented and Connection Less Services. Service Primitives - The Relationship of Services to Protocols. Reference Models : The OSI Reference Model - the TCP/IP Reference Model - a Comparison of the OSI and TCP Reference Models.
Introduction
- The Historical Perspective
- The 18th century: the great mechanical systems accompanying the Industrial Revolution.
- The 19th century: steam engine.
- The 20th century: information gathering, processing, and distribution.
- The 21th century: Internet, large distributed systems (e.g., Grid), heavy reliance on computers.
- What is Computer Network?
- We will use the term ''computer network'' to mean a collection of autonomous computers interconnected by a single technology.
- Two computers are said to be interconnected if they are able to exchange information.
- Copper wire; fiber optics, microwaves, infrared, and communication satellites, etc.
Uses of Computer Networks
- Business Applications
- Home Applications
- Mobile Users
- Social Issues
Business Applications of Networks
- Resource sharing (hardware, software, information, …)
- Providing communication medium (e-mail, videoconferencing)
- Doing business electronically (B2B, B2C, e-commerce)
A network with two clients and one server Home Network Applications
- Access to remote information
- Person-to-person communication
- Interactive entertainment
- Electronic commerce
In peer- to-peer system there are no fixed clients and servers Mobile Network Users
- Internet - LAN: local area network
- Local area networks, generally called LANs, are privately-owned networks within a single building or campus of up to a few kilometers in size.
- LANs may use a transmission technology consisting of a cable to which all the machines are attached.
- Traditional LANs run at speeds of 10 Mbps to 1000 Mbps, have low delay (microseconds or nanoseconds), and make very few errors.
- Various topologies are possible for broadcast LANs.
(a) Bus Topology (b) Ring Topology
**- MAN: metropolitan area network
- WAN and Internet**
- Hosts, The hosts are owned by the customers (e.g., people's personal computers).
- Subnet typically owned and operated by a telephone company or Internet service provider (ISP).
- The hosts are connected by subnets.
- The subnet consists of two distinct components: transmission lines and switching elements. Transmission lines move bits between machines. Switching elements (routers) are specialized computers that connect three or more transmission lines.
Wireless Networks
- Bluetooth: short range wireless network (<10m).
- Wireless LANs: for moderate range, becomes more and more common today (<100m).
- Network used for cellular telephones: distances involved are much greater and the bit rates much lower (above 1km).
Network Software
- Protocol Hierarchies
- Design Issues for the Layers
- Connection-Oriented and Connectionless Services
- Service Primitives
Packets routing
- Message “M” is transferred from layer 5 to layer 4, with a header containing control information, such as sequence numbers, which helps layer 4 maintain the message order.
- Layer 3 break the message from layer 4 into two pieces to fit the transmission restrictions, while adding another header to tell layer 2 where the dest. is.
- Layer 2 adds the messages from layer 3 with another header, telling the actual (physical) address of the dest, and a trailer, which is the checksum of the message for correction assertion.
- At the receiving machine the message moves upward, from layer to layer, with headers being stripped off as it progresses. Message is sent to the dest. machine.
Design Issues for the Layers
- Every layer needs a mechanism for identifying senders and receivers. (who to talk with)
- The protocol must also determine how many logical channels the connection corresponds to and what their priorities are. (simplex or duplex? single or multiple channel?)
- Error control is an important issue because physical communication circuits are not perfect.
- Message ordering is important ‘cause Not all communication channels preserve the order of messages sent on them.
- An issue that occurs at every level is how to keep a fast sender from swamping a slow receiver with data.
- Inability of all processes to accept arbitrarily long messages. (fragmentation and reassembling the messages)
Connection-Oriented and Connectionless Services
- Connection-Oriented Service: the service user first establishes a connection, uses the connection, and then releases the connection. (e.g., the telephone, tube)
- Connectionless Service: Each message carries the full destination address, and each one is routed through the system independent of all the others. (e.g., the postal system) Usually, connectionless service cannot guarantee the order of messages.
- In order to enhance the reliability of transmission of connection-oriented service, acknowledge each received message is helpful. For example, the file transfers.
- However, some applications prefer fast speed than the reliability. For example, the digitized voice traffic, video conference. Six different types of service
Both reliable and unreliable connection-oriented and connectionless communication coexist
Service Primitives
Common Primitives
Packets sent in a simple client-server interaction on a connection-oriented network Services to Protocols Relationship
- The service defines what operations the layer is prepared to perform on behalf of its users
- A service is a set of primitives that a layer provides to the layer above it.
- A protocol is a set of rules governing the format and meaning of the packets which are exchanged by the peer entities in the same layer.
- Services related to the interfaces between layers;
OSI model
- The Physical Layer - The physical layer is concerned with transmitting raw bits over a communication channel. - Typical questions here are how many volts should be used to represent a 1 and how many for a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established and how it is torn down when both sides are finished, and how many pins the network connector has and what each pin is used for. - The Data Link Layer - The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer. - having the sender break up the input data into data frames and transmit the frames sequentially - The Network Layer - The network layer controls the operation of the subnet (routing).
- Congestion control, QOS (quality of service) - The Transport Layer
- The basic function of the transport layer is to accept data from above, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.
- The transport layer is a true end-to-end layer, all the way from the source to the destination.
- layers 1 through 3 are chained, and layers 4 through 7 are end-to-end, - The Session Layer
- The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control, token management, and synchronization. - The Presentation Layer
- Concerned with the syntax and semantics of the information transmitted, to make it possible for computers with different data representations to communicate. - The Application Layer
- The application layer contains a variety of protocols that are commonly needed by users.
The TCP/IP Reference Model
The TCP/IP reference model
- The Internet Layer - Its job is to permit hosts to inject packets into any network and have them travel independently to the destination (potentially on a different network). They may even arrive in a different order than they were sent, in which case it is the job of higher layers to rearrange them, if in-order delivery is desired.
- The TCP/IP model did not clearly distinguish between services, interface of the layers.
- As a consequence, the protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes.
- OSI model strives to describe the general model of network protocols, while TCP/IP model is specific as it is defined after the real implementations. So TCP/IP model is not useful to describe other protocols.
UNIT - II Transmission Media: Magnetic Media - Twisted Pair -Baseband Coaxial Cable - Broadband Coaxial Cable - Fiber Optics. Wireless Transmission - the Electromagnetic Spectrum - Radio Transmission - Microwave Transmission - Infrared and Millimeter Waves -Lightwave Transmission. Narrow Band ISDN System Architecture. Broadband ISDN and ATM Networks - Cellular Radio - Paging Systems - Cordless Telephones - Analog Cellular Telephones - Digital Cellular Telephones - Personal Communication Services.
Notes: https://inspirit.net.in/books/networking/Introduction%20to%20Computer%20Networks.pdf
https://fmfi-uk.hq.sk/Informatika/Distribuovane%20Systemy/knihy/ICN/index.htm
Transmission Media
The purpose of physical media is to transport a raw bit stream from one machine to another. These media are roughly grouped into guided media, such as copper wire and fiber optics, and unguided
media, such as radio and lasers through the air. Each one has its own characteristic in terms of bandwidthdelay, cost, and ease of installation and maintenance.
Guided Media:
Medium is more important in setting transmission parameters. Guided media includes everything that ‘guides’ the transmission. That usually takes the form of copper wires, optical fiber etc.
Magnetic Media:
The most useful ways to transport data from one computer to another is magnetic tape or floppy disk while this method is not as sophisticated as using a geosynchronous area. It is more cost effective. The bandwidth characteristics of magnetic tape are excellent, the delay characteristics are poor. Transmission time is measured in minutes or hours, not in milliseconds.
- Floppy disk
- CDs
- Magnetic tapes
- Advantage High bandwidth (bulk of data)
- Disadvantage Delay is too long All computer communication involves encoding data in a form of energy, and sending the energy across a transmission medium (i.e., the network links).
- Each pair would consist of a wire used for the positive data signal and a wire used for the negative data signal.
- Any noise that appears on one wire of the pair would occur on the other wire
- The noise appears on both wires; it cancels itself.
- Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP. Cables without a shield are called Unshielded Twisted Pair or UTP. Twisting the wires together results in characteristic impedance for the cable. Typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable.
There are two types of twisted pair cables: Shielded Twisted Pair (STP) Cable Unshielded Twisted Pair (UTP) Cable
STP cables comes with twisted wire pair covered in metal foil. This makes it more indifferent to noise andcrosstalk.
UTP has seven categories, each suitable for specific use. In computer networks, Cat-5, Cat-5e, and Cat- cables are mostly used. UTP cables are connected by RJ45 connectors.
Coaxial cable
- Coaxial Cable consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield.
- An insulating protective coating called a jacket covers the outer conductor. The outer shield protects the inner conductor from outside electrical signals.
- The distance between the outer conductor (shield) and inner conductor plus the insulating material determine the cable properties or impedance.
- Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet.
- A baseband coaxial cable transmits a single signal at a time at very high speed. A baseband cable is mainly used for LANs. Baseband coaxial cable supports frequency range of a-4kHz and are used for digital signaling.
- A broadband coaxial cable can transmit many simultaneous signals using different frequencies.
A coaxial cable
Fiber Optics
- Optical Fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond.
- The typical optical fiber consists of a very narrow strand of glass called the Core. Around the Core is a concentric layer of glass called the Cladding.
- A typical Core diameter is 62.5 microns (1 micron = 10 -6meters).
- Typically Cladding has a diameter of 125 microns.
- Coating the cladding is a protective coating consisting of plastic, it is called the Jacket
Cable type Bandwidth Twisted Pair 100 MHz Coaxial Cable 600 MHz Optical Fiber 1 GHz
Wireless Transmission
- The Electromagnetic Spectrum
- Radio Transmission
- Microwave Transmission
- Infrared and Millimeter Waves
- Light-wave Transmission Wireless transmission is a form of unguided media. Wireless communication involves no physical link established between two or more devices, communicating wirelessly. Wireless signals are spread over in the air and are received and interpreted by appropriate antennas.
When an antenna is attached to electrical circuit of a computer or wireless device, it converts the digital data into wireless signals and spread all over within its frequency range. The receptor on the other end receives these signals and converts them back to digital data.
A little part of electromagnetic spectrum can be used for wireless transmission.
The Electromagnetic Spectrum
The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. The sun, earth, and other bodies radiate electromagnetic energy of varying wavelengths. Electromagnetic energy passes through space at the speed of light in the form of sinusoidal waves.
The electromagnetic spectrum and its uses for communication.
Radio Transmission
Are easy to generate , can travel long distances and can penetrate buildings easily. Are omnidirectional, I.e. they travel in all directions
- Adv : transmitter and receiver do not have to be aligned
- Disadv : interference of signals : less secure : govt license required to use particular frequency band The properties of RW are frequency dependent
- At low frequency: pass thru obstacles well but the power falls off sharply with distance from the source.
