Television

Principle of Operation of cathode Ray Tube ( CRT)

Thermion Emission

A hot cathode is used to produce a supply of electrons. This may consists of a fine tansand wire or filament which is heated to high temperature .Metals content free electrons if the temperature of the metal is raised, a thermal velocities of the electrons will be increased.

The chance of electrons escaping from the attraction of the positive Irons will then also be raised. Therefore by heating a metal to a high temperature electrons can be boiled off. This is called Thermion Emission.

Once an electron escaped from the cathode it is subjected to a retarding force set up by the negatively charged electrons that have already escaped. An electron is slowed down by this force and may be return to the cathode an emitted electron can also be slowed down by coming in to collision with gas molecules. To minimize this effect cathode is placed within an evacuated glass envelope.

Electrons are continually emitted from the surface of a heated cathode. There are subjected to forces which tend to return them cathode. The cathode is surrounded by a cloud of electrons. The cloud of electrons around the cathode is known as the space charge. Space charge is negative.If another electrode the anode is introduced into the evacuated space and is maintained at positive potential with respect to the cathode it will exert and attractive force on the emitted electrons due to the influence of that attractive force electrons are rapidly accelerated to the anode.A value with just a cathode and anode is known as a diode. A triode has a third electrode known as the control grid inserted between cathode and the anode .the tetrode value has another grid inserted between the control grid and the anode. The pentod value has three control grids.

Network overview

Network overview

What is a network?

A network is a system that transmits any combination of voice, video and or data between users.

A group of connected computers that allow the sharing of information and equipment, such as printers.

Networks can interconnect by other networks and sub networks.A computer network interface card installed.

Computer with network interface card installed.

The network operating system (Ex. windows NT/2000/2003) on the user’s PC client and servers.

The cables connecting all network devices (UTP cables with RJ-45 jacks).

All supporting network components (h..bs, routers and switches, etc)(UTP-Unshielded twisted pair, RJ-Registered jack)

Types of computer networks

Local aria network (LAN).

Metropolitan area network (MAN).

Wide area network (WAN).

Element of a Small LAN

Computers with network interface card installed.

Hub/switch.

Cabling (UTP cables with RJ-45 jacks).

Network operation system.

Local Area Network (LAN)

LAN is the most common type of network found in businesses. LAN’s connect computer and devices located close to each other, usually within the same building. Typically, LAN’s connect no more than 100 computers.

Metropolitan Area Network (MAN)

A collection of local area networks located in the same geographical area, such as city or town.

Wide Area Network (WAN)

Wide area networks generally cover a large geographical area. Connects local and metropolitan area networks that may be located throughout the country or around the world.


The Hardware used on Computer Network

• Hubs

• Switches

•Bridges

• Routers

• Gateways

• Repeaters

• File severs

Hubs
Hubs are the central connectivity point in a star network, work group or subnet. It’s primary function is to receive and send signals along the network between the device connected to it. All communication across the hubs “broadcast” to all connected nodes.

Switches

Switches connect all computer LAN connections, same as the hubs. The difference is that switches can run are able to direct and filter information to and from specific designation.

Bridge
Bridge are used to connect (separate work groups together) two or more network segment together, so that equipment on the network can communicate.

Routers
Routers are able to route and filter information to different network.

Gateways
A gateway is a network point the act as an entrance to another network. The computers that control traffic within the network or at the local internet service provider (ISP) are gateways.
RepeatersA network device that regenerates signals so they can extend the cable length. They do not change any information being transmitted and they can not filter any information.

File severs

A high capacity disk storage computer connected to the network that contains files/applications and shares them as requested the other computers on the network.

Simple Telecommunication System

Network Hierarchy

IC – International Centre
BC – National Centre
TC – Tertiary Centre
SC - Secondary Centre
PC – Primary centre
LE – Local Exchange (Central Office)

The public telephone system consists basically a large number of single telephones unevenly distributed through out of the country, each one being connected to a switching centre called telephone exchange. Each exchanges has lines to other exchanges and also to a suitable positioned main switching centre which in turn has access to all the other main switching centre in the system.Thus, any telephone connected to an exchange can be connected, at will, to any other telephone in the country and by using an international switching centre, to any telephone in the system of any other country.In some countries the Local Exchange is known as the Central Office.

Local Battery (LB)

A local battery powered all the original telephones. This was because the transmitter needed power and the local battery provided the right power irrespective of the length of or bad external wiring.because the batteries were installed in customer premises. This was a major expense for the maintenance team. original batteries xxxere wet cells which could leak and these were letter superseded by dry cells.

Central Battery ( CB)

The ‘LB’ system was superseded by the central Battery System which used a large set of battries at the local exchange and this became possible because of saving made by economics of scale.

‘CB’ Working

Advantages

Easy to maintainInstallation and maintenance cost lost

Disadvantages

Limitations(Distance for line providing)

Mobile Radio and Cellular Telephone Systems

Private Mobile Radio

(PMR) or as it sometimes called professional mobile radio is widely used for businesses as very convenient way of communicating.It typically refers to sophisticated radio trucking services that provided two way mobile radio communications for mobile users to connect to PMR voice and telephony and data networks. There are a wide variety of such systems, for example.PMR offers a two-way communications service. Permits users to talk over a short distance on a simple local system or wide-o complex system, without incurre any costs on the calls made no matter how many or for how long.

Background to private mobile radio (PMR).

PMR is the oldest of mobile communication it has been in used for over 70 years. It is used by many taxi and courier firms, security guards and utility companies. Many rural businesses choose the PMR option, because they find that thy simply do not have mob phone coverage, or they are in an as where the network frequently goes down. Initially the PMR systems consisted of a base station whit a number of mobile stations, communication used a singe frequency, with simplex push to talk transmission.


What determines the range of the radio?

The power of the radio transmitter is one of the main factors in determining range.
Hand-portable radios are limited to 5 watts ERP (radiated power from the aerial), whereas the power allowed from some mobile (vehicle-mounted) radios is 25 watts ERP. Height is another important factor, as the higher the radio waves can travel unobstructed. Under ideal conditions, a PRM radio signal may be received up to 3Km away; a professional handheld up to 5Km; and a vehicle mounted up to several tens of kms. With the addition of repeater systems, the coverage can be extended to an entire country!