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UNIT 1
Introduction to Networks, Data and signals-analog and digital, periodic analog signals, digital signals, bit rate, baud rate, bandwidth. Transmission impairments- attenuation, distortion and noise. Data communication protocols and standards, Network models - OSI model-layers and their functions. TCP/IP protocol suite.
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A computer network is a set of data processing nodes that are interconnected for the purpose of data communication. It Change the way we do business and the way we live. Business decisions have to be made more quickly and Decision depends on immediate access to accurate information. Businesses today rely on computer networks and internetworks. Development of the PC changes a lot in business, industry, science and education. Similar revolution is occurring in data communication and networking. Technologies advances are making it possible for communications links to carry more and faster signals. Services are evolving to allow the use of this expanded capacity. For example telephone services extended to have: Conference calling Call waiting Voice mail Caller ID The aim of computer networks are Data sharing and resource sharing Communication: Means sharing information
- Local (face to face) or remote (over distance)
- Telecommunication
- Telephone, telegraph and television
- Means communication at a distance
- Tele is Greek for far Data: Refers to information
- Presented in any form
- Agreed upon by the parties ( creating & using) Data communication is the exchange of data between two devices via some form of transmission medium (wire cable). Communication system made up of a combination of hardware and software. Effectiveness of data communication system depends on:
- Delivery : The system must deliver data to correct destination. Data received by the indented user only
- Accuracy: The system must deliver data accurately (no change).
- Data changed & uncorrected is unusable
- Timeliness: The system must deliver data in timely manner
- Data arrived late are useless
- In the same order (video and audio) & without delay (Real time transmission)
- Jitter: Variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packet.. (uneven quality in the video is the result)
Components:- A data communication system is made up of five components
- Message: the information (data) to be communicated. It consist of text, numbers, pictures, audio, or video
- Sender: the device that sends the data message
- Computer, workstation, telephone handset, video camera, …
- Receiver: the device that receives the message
- Computer, workstation, telephone handset, television, ….
- Medium: The physical path by which a message travels from sender to receiver - Twisted pair, coaxial cable, fibre optics, radio waves etc
- Protocol: a set of rules that govern data communications. An agreement between the communicating devices - Devices may be connected but not communicating (no protocol) - Arabic speaker with Japanese speaker twisted pair, coaxial cable, fiber-optic, radio waves Data Representation Text: Sequence of bits (0s or 1s) Different sets of patterns to represent text symbols (each set is called: code ) ASCII: 7 bits (128 symbols) common coding system today is: Unicode uses: 32 bits to represent a symbol or character in any language Numbers:
Half-Duplex (one-lane with two-directional traffic) :Each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. The entire capacity of a channel is taken over by the transmitting device. It is also known as two way alternate. Example: Walkie-talkies
Full-Duplex (Duplex) (two-way street) : Both stations can transmit and receive at same time. Signals going in either direction sharing the capacity of the link. It is also known as two way simultaneous. Sharing can occur in two ways: Link has two physically separate transmission paths:One for sending and the other for receiving The capacity of the channel is divided between signals travelling in both directions Example: Telephone network
Network : A set of devices (nodes) connected by communication links Node : computer, printer, … Distributed Processing : .Most networks used it.Task is divided among multiple computers instead of one single large computer Network Criteria
- Network must meet a certain number of criteria
- The most important of the network criterions are: Performance Reliability Security Performance :-Performance can be measured in many ways including transmit time and respond time.
- Transit time: Amount of time required for a message to travel from one device to another
- Response time: Elapsed time between an inquiry and a response
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Performance depends on : 1- Number of users: large number slow response time. 2- Type of transmission medium: fiber-optic cabling faster than others cables. 3- Capabilities of the connected hardware: affect both the speed and capacity of transmission. 4- Efficiency of the software: process data at the sender and receiver and intermediate affects network performance. Performance is evaluated by two contradictory networking metrics:
- Throughput (high): a measure of how fast we can actually send data through a network
- Delay (low) Reliability:- Reliability is measured by:
- Frequency of failure
- Recovery time of a network after a failure
- Network’s robustness in a catastrophe: protect by good back up network system Security
- Protecting data from unauthorized access
- Protecting data from damage and development
- Implementing policies and procedures for recovery from breaches and data losses (Recovery plan)
Physical Structures: Type of connection Network: Two or more devices connected through links Link: Communication pathway that transfers data from one device two another Two devices must be connected in some way to the same link at the same time. Two possible types: Point-to-Point Multipoint Point-to-Point Dedicated link between two devices Entire capacity of the link is reserved for transmission between those two devices Use an actual length of wire or cable
To link n devices fully connected mesh has n ( n - 1) / 2 physical channels (Full-Duplex) Every Device on the network must have n – 1(input/output) ports Example: 8 devices in mesh has links: n(n-1) / 2 number of links = 8 (8-1)/2 = 28 number of ports per device = n – 1 = 8 – 1 = 7
Advantages Each connection carry its own data load (no traffic problems) A mesh topology is robust Privacy or security Fault identification and fault isolation Disadvantages Big amount of cabling Big number of I/O ports Installation and reconnection are difficult Sheer bulk of the wiring can be greater than the available space Hardware connect to each I/O could be expensive Mesh topology is implemented in a limited fashion; e.g., as backbone of hybrid network
Star: Each device has a dedicated point-to-point link only to a central controller called Hub No direct traffic between devices The control acts as an exchange
Advantages Less expensive than mesh (1 Link + 1 port per device) Easy to install and reconfigure Less cabling Additions, moves, and deletions required one connection Robustness : one fail does not affect others Easy fault identification and fault isolation
Disadvantages Single point to failure, the dependency of the whole topology on one single point (hub), so if the hub goes down the whole system is dead. More cabling than other topologies ( ring or bus) Used in LAN
Bus It is multipoint One long cable acts as a backbone Used in the design of early LANS, and Ethernet LANs Nodes connect to cable by drop lines and taps Signal travels along the backbone and some of its energy is transformed to heat Limit of number of taps and the distance between taps
Drop line:- a connection running between the device and the main cable.
Move two connections for any moving (Add/Delete) Easy of fault isolation Disadvantage Unidirectional One broken device can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break Hybrid Topology(Tree):- it integrates multiple topologies together.
Example: having a main star topology with each branch connecting several stations in a bus topology Advantages :
Point to point wiring for individual segments. Supported by several hardware and software venders. Disadvantages Overall length of each segment is limited by the type of cabling used. If the back bone link breaks, the entire segment goes down. More difficult to configure and wire than other topologies.
Categories of Networks
Network Category depends on its size Two primary categories LAN : Covers area < 2miles WAN : Can be worldwide MAN : Between LAN & WAN, span 10s of miles Local Area Network (LAN):- A LAN is usually privately owned and links the devices in the same office, building, or campus. Depending on the needs of an organization and the type of technology used a LAN can be simple as 2 PCs & 1 printer in home or office, or it can extend throughout a company and include audio and video peripherals. Currently LAN size is limited to a few kilometers.
LANs are designed to allow resources to be shared between personal computers or work stations. The resource to be shared can include hardware (printers), software( application programs), or data.
LAN is distinguished by: Size (# users of OS, or licensing restrictions) Transmission medium (only one type) Topology (bus, ring, star) Data Rates (speed): Early: 4 to 16 Mbps Today: 100 to 1000 Mbps Wide Area Networks (WAN) Provides long-distance transmission of data, image,audio and video information over large geographic areas that may comprise a country, a continent or even a whole world
Interconnection of Networks: Internetworks (The Internet)
Two or more networks connected together
Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use An internet is 2 or more networks that can communicate with each other The Internet is a collaboration of more than hundreds of thousands of interconnected networks This communication system came in 1969 Mid-1960 (ARPA) Advanced Research Projects Agency in (DOD) was interested to connect mainframes in research organizations 1967, ARPA presented its ideas for ARPANET Host computer connecting to (IMP) interface message processor. Each IMP communicate with other IMP 1969, four nodes (universities) connected via IMPs to form a network
Software (NCP) Network Control Protocol provided communication between the hosts. 1972, Vint Cerf and Bob Kahn invented (TCP) Transmission Control Protocol Later TCP was split to (TCP) Transmission Control Protocol and (IP) Internetworking Protocol Made of many LANs and WANs Every day new networks area added and removed Internet services Providers (ISPs) offer services to the end users International service providers National service providers Regional service providers Local service providers
Protocols and Standards Protocol synonymous with rule Standards: agreed-upon rules
Upon recommendation from the Internet authorities draft published as Request for Comment (RFC) RFC is edited, assigned a number, and made available to all interested parties
Data and Signals To be transmitted data must be transformed to electromagnetic signals. Both data and signal that represent them can be either analog or digital. The term analog data refers to information that is continuous; Eg:- Sound made by human voice Digital data refers to information that has discrete states. Eg:- Data stored in computer memory (0’s And 1’s). Analog data take on continuous values. Digital data take on discrete values. Analog signals can have an infinite number of values in a range. Digital signals can have only a limited number of values.
Both analog and digital signal can take one of two form. Periodic :- A periodic signal complete a pattern with in a measurable time frame is called a period (cycle) and repeat that pattern over subsequent identical period. Non periodic:- It changes without exhibiting a pattern or cycle that repeats over time. The completion of one full pattern is called cycle. PERIODIC ANALOG SIGNALS In data communications, we commonly use periodic analog signals and non periodic digital signals. Periodic analog signals can be classified as simple composite.
A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals.
All Signals are composed of 3 properties
Amplitude Frequency Phase Amplitude :- Amplitude is the maximum displacement of points on a wave, which you can think of as the degree or intensity of change. Two signals with the same phase and frequency, but different amplitudes
Figure 3.2 A sine wave
Frequency is the rate of change with respect to time. Change in a short span of time means high frequency.Change over a long span of time means low frequency.If a signal does not change at all, its frequency is zero.If a signal changes instantaneously, its frequency is infinite. Phase :- Phase is the position of a point in time (an instant) on a waveform cycle. Phase describes the position of the waveform relative to time 0. A complete cycle is defined as the interval required for the waveform to return to its arbitrary initial value. Phase is measured in degree or radius (360° is 2π radius). A phase shift of 360° correspond to a shift of a complete period. Three sine waves with the same amplitude and frequency, but different phases
A phase shift of of 180° corresponds to the shift to ½ of a period. A phase shift at a of 90° corresponds to a shift of ¼ quarter of a period. A composite periodic signal A composite periodic analog signal is composed of multiple sine waves.A complete sine wave in the time domain can be represented by one single spike in the frequency domain.A single-frequency sine wave is not useful in data communications. We need to send a composite signal, a signal made of many simple sine waves. According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases.If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies. If the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies.
Wavelength :-which is the distance between two successive like points on a wave. Wavelength is like the distance between two adjacent peaks or two adjacent valleys. Stated another way, wavelength is the time required to complete one full cycle of the wave.
