Assignment

Signal Networking Communication

A1. Describe, with the aid of diagrams, the advantage of data communication networks.

A2. Compare current national and international communication networks.

B1. Describe, with the aid of diagrams, various techniques in which digital signals can be transmitted.

B2. Explain the process of signal modulation in networking transmission.

C1. Describe and compare, with the aid of diagrams, various transmission mediums e.g. optical, cable, radio

C2. Research the modern trends in transmission mediums. OCN Computer Networking 3570 Milan Bakos

A1. Describe, with the aid of diagrams, the advantage of data communication networks

What is a communication network?

A network is a group of interconnected systems sharing services

and interacting by means of a shared communications link.

A network, therefore, requires two or more individual

systems with something to share (data).

Communication network is a system of computers linked together. Among these computers is possible to transfer information easily and fast. Information is possible to transfer between computers, which are not connected to the network on media such as floppy disc, magneto-optical tape and optical disc.

This kind of transmission is not really effective and takes longer then using network and there is not possible interactive processing of information.

General model of communication networks.

Figure 1.0

Communication: Widely relative interactive information exchange between several objects

Element of traffic infrastructure

Ability to exchange electronic information between several objects

(electronic communication)

Network: Something broadband networking (widely manifold) on a surface

A completion of effectively lied objects in some space

The advantages of communication networks

Only need one main point to transmit data (TV, Radio)

Figure 1.1

Sharing data both ways (Internet)

Figure 1.2

Sharing hardware and data (intranet)

Figure 1.3

At its simplest, a computer network is two or more computers sharing information across a common transmission medium.

Figure 1.4

Computer networks are developing in comparing with stand alone computers many advantages.

Sharing data

Sharing resources

Increased safety of system

Saving space

Data sharing is the main reason for building computers networks. The thing is that necessary data files could use more users at the same time. It is because data files are stored on network servers and connected users can access them. Resources sharing are allowing workstations using resources of network together. Usually sharing hard drives, printers, plotters, scanners, fax modems, cd-roms. Increased safety of system is another advantage of networks. When are all files stored on the server and workstations are loading files from server it is easier to manage security and backups of files. In case of workstation failure it is possible to continue work on another workstation. Saving space is common advantage of networks e. g. printer doesn’t need to be close to one computer ( usb line, lpt line) but we can put printer where it is suitable for us. This is especially using with servers and their backup when jukeboxes are stored in a safe place and it is not near server. A2. Compare current national and international communication networks.

The International Standards Organization (ISO)—whose name is

derived from the Greek prefix iso, meaning “same”—is located in

Geneva, Switzerland. ISO develops and publishes standards and

coordinates the activities of all national standardization bodies. In

1977, the ISO initiated efforts to design a communication standard

based on the open systems architecture theory from which

computer networks would be designed. This model came to be

known as the Open Systems Interconnection (OSI) model.

Network industry uses two types of standards:

De facto standards: arise through widespread commercial and educational use. These standards are often proprietary and usually remain unpublished and unavailable to outside vendors unknown as closed system standards e.g. IBM System Network Architecture

De jure standards: are non-proprietary standards which means that no single company creates them or owns rights to them. These standards are developed with the intend of enhancing connectivity and interoperability by making specifications public e. g. TCP/IP

Why we need standardization?

Standardization is needed because computers are using different operating systems or using different types of networks.

Communications between computers and network are being made on software level by protocols.

Protocols are formal specification that defines procedures to follow when transmitting or receiving data.

Protocols define the format, timing sequence and error checking used on network.

Two computers must run compatible protocol stacks before they can communicate, because each layer in one computer’s protocol stack must interact with a corresponding layer in the other computer’s protocol stack. For example, refer to Figure 2.0. It shows the path of a message that starts in the Transport layer. The message travels down the protocol stack, through the network medium, and up the protocol stack of the receiving computer. If any layer in the receiving computer cannot understand or is not compatible with the corresponding

layer of the sending computer, the message cannot be delivered.

Figure 2.0

To place this concept into perspective, imagine two people wishing

to communicate. If one is blind and the other is deaf, there will be a

communication problem. Both people need to convey the thought

through some form of media. However, the blind person uses voice

to transmit, which requires the receiving person to use hearing, while

the deaf person uses sign language to transmit, which requires the

receiving person to use sight.

B1. Describe, with the aid of diagrams, various techniques in which digital signals can be transmitted.

Standard telephone lines can transmit only analog signal. Computers however store and transmit data digitally. Modems can transmit digital computer signals over telephone lines by converting them to analog form. Converting one signal to another is called modulation. Recovering the original signal is called demodulation. The word modem derives from the terms modulation and demodulation. Modems are classified according to the transmission method they use for sending and receiving data. The two basic types of modems are as follow.

Synchronous (parallel)

synchronous transmission A transmission

method that uses a clock signal to

regulate data flow.

In synchronous transmissions, frames are

separated by equal-sized time intervals.

Timing must be controlled precisely on the

sending and the receiving computers.

Special

characters are embedded in the data

stream to begin synchronization and to

maintain synchronization during the transmission,

allowing both computers to check

for and correct any variations in timing.

Asynchronous (serial)

asynchronous transmission A method

of data transmission that uses start bits and

stop bits to coordinate the flow of data so

that the time intervals between individual

characters do not need to be equal. Parity

also may be used to check the accuracy of

the data received.

See also communications parameters;

data bits; synchronous transmission.

Asynchronous (serial) transmission is simple, inexpensive technology ideally suited for transmitting small frames at irregular intervals such as keyboards or mouses. Because start, stop and parity bits must be added to each character being transmitted, however overhead of asynchronous transmission is high, often in the neighborhood of nearly 20-30 percent. Every bit is following another until is end or start’s bit.

Figure 3.0

Figure 3.1

Synchronous (parallel) transmission has one piece of wire for each transmitted bit. For synchronizing receiver with sender is used a clock pulse. Data are moved at clock pulse order. Data are moving faster than in asynchronous transmission because we don’t need start and stop bits. Synchronous transmission is usually use for printers and scanners. Figure 3.2

B2. Explain the process of signal modulation in networking transmission.

What is signal?

Signal is: a physical bearer which transmit messages by the use of environment,

conventional combination of amplitudes, frequencies, or periods of light, sound or

electric current

Modems transmit digital computer signal over telephone lines by converting them to analog forms.

In networking transmission are usually used two basic types of signals:

Analog signal. Analog information changes continuously and can take on

many different values. An analog clock’s hands move constantly,

displaying time on a continuous scale.

Digital signal. Digital information is characterized by discrete states. A light

bulb, for example, is on or off. A digital clock represents the

time in one-minute intervals and doesn’t change its numbers

again until the next minute. A digital clock can represent

exact minutes but not the seconds that pass in between.

Analog waverforms: usually have shape of sine waves. Constantly vary in one or more values, and these

changes in values can be used to represent data.

Analog waveforms define two

characteristic.

Figure 4.0

Frequency : rate at which the waveform changes. Is measured in Hz which indicates the frequency in cycles per second

Amplitude : measure the strenght of waverform

Digital communication : is method that uses digital (discrete) signals usually binary values to to represent

Informations representing 0 or 1.

Binary values may be encoded as different voltage or current levels.

compared with analog transmissions digital transmissions are generally less

susceptible to noise, are easier to work with for error detection and correction and require somewhat less complex circuity. Figure 4.1

C1. Describe and compare, with the aid of diagrams, various transmission mediums e.g. optical, cable, radio

Cable transmission of signal: are being used for centuries. Data are being send through the cable using electrical signals – electrons. Electron travels through the wire and must compete the resistance to reach the final destination. It is all because V = I. R. We recognize two main groups of cables using copper core:

Figure 5.0

Coaxial cable : gets name because two conductors share a common axis. Cable consist of: A center conductor, although usually solid copper wire, sometimes is made of stranded wire. An outer conductor forms a tube surrounding the center conductor. This conductor can consist of braided wires, metallic foil, or both. The outer conductor, frequently called the shield, serves as a ground and also protects the inner conductor from EMI.

An insulation layer keeps the outer conductor spaced evenly from the inner conductor.

A plastic encasement (jacket) protects the cable from damage.

thinnet – 6mm diameter, grey color, 10 Mbps, inexpensive, falls under RG – 58 family which means that has 50 ohm impedance, can reliably transmit signal for 185 m. Both ends of cable must be terminated by resistor (terminator) and one end must be grounded. Connectors can be installed without expensive tools and bit of practise.

Uses usually in bus topology and Ethernet networks.

Thicknet – (yellow Ethernet) 13 mm, yellow color, harder to work because has thicker copper core. Can be used as a backbone for small thinnet LANs. More expansive than thinnet, but can be safely installed outside. Uses special N- connectors. Can transmit signal to approximately to 500 m. Twisted cable : A basic twisted-pair cable consists of two strands of copper wire twisted together. This twisting reduces the sensitivity of the cable to EMI and controls the tendency of the wires in the pair to cause EMI in each other.

shielded twisted pair (STP) -13 mm diameter, consists of one or more twisted pairs of cables enclosed in a foil wrap and woven copper shielding. The shield in STP cable results in good EMI characteristics for copper cable. Theoretical capacity is 500 Mbps but the most common rate is 16 Mbps. Limit of cable runs to few hundreds meters but 100 m is quite common.

unshielded twisted pair (UTP) - doesn’t incorporate a braided shield into its structure. Used in telephone lines. offers an excellent balance of cost and performance characteristics, easy to install, divided to five perfomance categories. Common bandwith now days is 100 Mbps, and data can be transfered to few hundred metres but usually used 100 m. Used connectors are RJ -45. Fiber optic : gets name because uses as a central conductor – fiber designed to transmit light signals with little loss. The fiber is coated with a cladding or a gel that reflects signals back into the fiber to reduce signal loss. Cable consists of two strands separately enclosed in plastic sheaths. One strand sends and the other receives.In fiber optic transmission we are not using electrons to send signal but we are using photons. Photons travel thru fiber strands and reflecting from strand’s surface. Strand is hollow so there is no resistance for photon to travels so that bandwith for fiber transmission is very high as 200 Gbs for a long distances if light is not leaking from strands and is good refraction of light.

The main disadvantage is that we must have a components to transfer electrical signal to light on both ends of cable. This makes fiber optic transmission very expensive. As light source is used Laser or LEDs. Figure 5.1

Lasers: A laser is a light source that

produces an especially pure light that

is monochromatic (one color) and

coherent (all waves are parallel). The

most commonly used source of laser

light in LAN devices is called an injection

laser diode (ILD).

The purity of

laser light makes lasers ideally suited

to data transmissions because they

can work with long distances and high

bandwidths. Lasers, however, are

expensive light sources used only

when their special characteristics are

required. Fiber optical transmission is using usually as a backbone for a big Lan networks, because the installation of optical cable is very hard and will not be effective or sometimes impossible to connect computers with optical cable between each other, because optical cable has a minimum bend radius. Optical cables are used when we need to transfer big amount of data such as real-time video processing. Another advantage of fiber optic is that there is not possible to eavesdropping because we don’t uses electrical signal.

Wireless transmission of signal.

In wireless transmission we don’t need to use physical medium such as copper or fiber optical cables so we don’t need to spend money for buying cables. In wireless transmission we use as a medium air. But here is one main problem because it is not much free electromagnetic spectrum. As a signal carier we can use a photon ( infra red) or an electric waves (radio, satellite). Four most common wireless transmission methods are :

Gamma Rays

X – Rays

Ultraviolent

--------------------------------- visible light

Terahertz Extremely High frequency

Super High frequency Microwave

Ultra High frequency

Gigahertz Very High frequency

High frequency

Medium frequency

Megahertz Low frequency Radio waves

Very Low frequency

Voice frequency AudioFreq 30Hz25kHZ

Kilohertz Extremely Low frequency

Infrared

Laser

Narrow band radio

Spread spectrum radio

Microwave

Infrared transmission: uses same principle as TV remote control.

The remote control transmits pulse of infrared light that carry coded instruction to a receiver. Exists four methods of infrared transmission

· Broadband optical telepoint

· Line of sight

· Reflective infrared

· Scatter infrared

Infrared transmission is limited to within 100 feet but bandwidth is 10 Mbps. Infrared is insensitive to radio frequency interference, but it is sensitive to rain or fog when bandwidth falls down or is not possible connection. Infrared transmission is commonly used for LAN transmission

Laser transmission: is similar as infrared transmission but laser can transmit signal for several thousand of yards when line of sight is possible.

Narrow band radio transmission: transmissions occur as a single radio frequency. The range of narrow-band radio is greater than that of infrared.It is not neccessary to have a line of sight connection because signal can bounce of walls, buildings or athmosphere. But heavy steel walls can block signal.

Spread-Spectrum Radio Transmission: uses multiple frequencies to transmit messages. Uses two techniques to transmit signal

· frequency hoping 250 kbps – 2Mbps

· direct sequence modulation. 2 – 6 Mbps

Microwave transmission: can take two forms: terrestrial link

Satelitte link

Figure 5.2

Terrestrial transmission : employs earth-based transmitters and receivers. The frequencies used are in the low gigahertz range, 4–6GHz and 21–23GHz, which limits all communications to line-of-sight.

· typically uses a parabolic antenas to transfer signal

· transmitter and receiver must be highly focused!

· terrestrial microwave uses licensed frequencies.

· highly susceptible to atmospheric interference

· transfer rate from 1 Mbps to 10 Mbps

Satellite transmission: relay transmissions through communication satellites that operate in geosynchronous orbits 22,300 miles above the earth.

The time required for a signal to arrive at its destination is called propagation delay.

· Earth stations use parabolic antennas

· Uses for ships and vehicles

· Delays from 0.5 s to 5 s

· Extremely expensive

· operate in the low gigahertz range, typically at 11–14GHz.

· data rates are 1–10Mbps.

· sensitive to EMI and electronic eavesdropping,

Cable type
Cost
Installation
Distance
Other issues

Coaxial thinnet
Less than STP
Inexpensive easy
10 Mps 185m
Less sensitive to EMI than UTP

Coaxial thicknet
Greater than STP, less than fiber
easy
10 Mbps 500m
Less sensitive to EMI than UTP

Shielded twisted pair (STP)
Greater than UTP, less than Thicknet
Fairly easy
16 - 500 Mps 100m
Less sensitive to EMI than UTP

Unshielded twisted pair (UTP)
Lowest
Inexpensive easy
10 – 100 Mps 100m
Most sensitive to EMI

Fiber optic
highest
Expensive difficult
100 Mbs 10s of km
Insensitive to EMI

Infrared
Cheapest of all wireless
Fairly easy, may require line of sight
10 Mbps Under a kilometer
Can attentuate due to fao and rain

Laser
Similar to infrared
Requires line of sight
Can span several kilometres
Can attentuate due to fog and rain

Narrow band radio
More expensive than infrared and laser
Requires trained technician and can involve radio towers
Can span hundreads of kilometres
Low power devices can attentuate to fog, rain and solar flares

Spread spectrum radio
More advanced technology than narrow band radio and more expensive
Requires trained technician and can involve radio towers
Can span hundreads of kilometres
Low power devices can attentuate to fog, rain and solar flares

Microwave
Very expensive as requires link to satellite
Requires trained technician and involve satellite dishes
1 – 10Mbps Can span thousands of kilometres
Can attentuate to ofg, rain and solar flares, long ping

C2. Research the modern trends in transmission mediums.

These days are quite expanding technology which uses as a transmission mediums power lines. It is because electricity is available to everyone and frequency of electric current is different than signal what we use in networking. Power lines can offer quite high bandwidth and there is no need to pay to telephone companies or get a new cable.

For powerline communication to work, power grids must be retrofitted with adapters that change data signals into frequencies that can be carried over electrical lines. At the residential level, computers are outfitted with a special modem that separates electricity from data.

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