Definition
of Computer Network
A computer network is a set of computers connected together via a data
communication sub network for the purpose of sharing resources
From this definition, we conclude that the main components of
computer network are:
1- Computers: These are the objective devices of the network as they contain the main resources
to be shared between them. The resources include hardware and software components
such as: CPU, Memory, Storage, Files, Programs, Data bases, I/O
devices, etc.
It should be noted that the computers are usually called "hosts" and some of them can be replaced by other
DTE (Data Terminal Equipment) devices such as: Printers, Scanners, Plotters,
disk storage, etc. Also, the host itself is considered to be a DTE device.
2- Data Communication Subnetwork: This is
responsible of transferring data between the different computers of the
network. The
structure of the subnet can be simple as in the
case of small networks or can be
complicated in case of large networks. The subnet, generally, contains; communication media, switches, routers, modems, etc.
complicated in case of large networks. The subnet, generally, contains; communication media, switches, routers, modems, etc.
3- Network Operating Software(NOS): This is a set of programs that controls all the
components of the network. In each computer, there is a NOS and also in the
switches and routers. The NOS can be an integral part of the device OS or as an
independent part that can be added to the OS. the NOS is the most important part of a computer network as it is responsible of safe and secure transmission of data between
computers and also of sharing the different resources
so that the objectives of the computer network are realized.
Classifications of Computer Networks:
The classification can be done in different ways according to the chosen criteria which can be: occupied area, topology, and transmission media.
Classifications Based on Occupied Area:
The area occupied by the network may vary from a small room, to the whole world; therefore, we have the following classes:
1-Local Area Network (LAN): This network has small dimensions as it occupies a room, floor, building, campus (i.e. several buildings). An example of this is the networks available in Al-ISRA University
Note: To appreciate the speed, let us calculate how many books can be transferred between two computers in one second. Suppose that the book consists of 500 pages of text. Suppose each page includes:
40 lines * 80 characters = 3200 characters.
If the character is coded as one byte, then the whole book size will be coded as:
500 * 3200 * 8= 12.800.000 bits
This mean that the book is about 13 Mbits and hence it is possible to transfer (1000 / 13) books per second i.e. about 80 book/second.
2-Wide Area Network (WAN): The occupied area ranges from one county to the whole world. The main features are: low speed, high cost, use of routers, and use of complicated communication systems. The best example of WAN is the "Internet".
3-Metropolitan Area Network (MAN): The occupied area is one city and the features are between LAN and MAN.
Classifications Based on Topology:
This has been explained earlier.
Classifications Based on Transmission Media:
The transmission media used in building the network can be wires or wireless. The wires can be coaxial cables, UTP, Optical fibers.
The wired networks have been shown earlier when studying the topologies. The wireless networks may use infrared wave or radio waves
Topologies of Computer Networks
There are many topologies depending on the occupied area by the network, type of transmission medium, etc. The main configurations are as follows:
Linear (BUS) Network:
· The occupied area is small and hence it is considered to be of type LAN i.e. Local Area Network.
· The data Communication subnet is very simple as it consists of TV cables and BNC connectors.
· Any signal transmitted from one host is received by all and hence it is necessary to prevent two hosts or more from transmitting simultaneously.
Star Network:
§ Occupied area is small and hence it is a LAN.
§ Any host can be disconnected at any time without stopping the system (network).
§ Subnet consists of electronic circuit and telephone cables of type UTP (Unshielded Twisted Pair).
WAN Network:
The Wide Area Network occupies large area. The main component of this network is the "Router"
In this figure, we should note the following:
1. The Router is usually a special computer i.e. dedicated computer for certain application.
2. Any router can be connected to other routers and host computers.
3. The data from host to host can travel in different paths which are determined by the routers according to "routing algorithms".
Each Router has proper NOS but does not have application programs or data.
we should note the following:
1. The Router is usually a special computer i.e. dedicated computer for certain application.
2. Any router can be connected to other routers and host computers.
3. The data from host to host can travel in different paths which are determined by the routers according to "routing algorithms".
4. Each Router has proper NOS but does not have application programs or data.
Concept of Signal
Concept of Signal
As we have seen earlier, the data communication subnet is the core of computer network; therefore, it is necessary to study some communication basics as shown below.
Concept of Signal
A signal is a parameter that may change its value with time e.g. voice, sound, current, voltage, scanned picture. The signal is represented as a function of time as shown below:
1- DC signal: It is characterized by a constant value for its amplitude as shown in figure 2.1. A good example of this signal is a battery voltage.
2- AC signal: The signal amplitude changes with time in a periodic fashion and can be represented by a sinusoidal function as shown in figure 2.2.
The main parameter of this signal is its frequency F which represents the number of cycles per unit time. If the unit time is a "second" then F is measured in hertz "HZ". The AC signal is also called "sinusoidal signal". Example: AC mains volt.
3-Binary Signal: This signal has two values only as shown in figure 2.3. To make this signal carry data, it is necessary to specify a certain duration called "bit interval". If the bit interval is T then the data rate is 1/T which represents the number of bits per unit time (second). The "data rate" is also called a" bit rate". A good example is the signal sent by computer to a "Modem".
4- General Analog Signal: This signal changes its amplitude in non-defined manner A good example of this is the voice signal.
As we have seen earlier, the data communication subnet is the core of computer network; therefore, it is necessary to study some communication basics as shown below.
Concept of Signal
A signal is a parameter that may change its value with time e.g. voice, sound, current, voltage, scanned picture. The signal is represented as a function of time as shown below:
1- DC signal: It is characterized by a constant value for its amplitude as shown in figure 2.1. A good example of this signal is a battery voltage.
2- AC signal: The signal amplitude changes with time in a periodic fashion and can be represented by a sinusoidal function as shown in figure 2.2.
The main parameter of this signal is its frequency F which represents the number of cycles per unit time. If the unit time is a "second" then F is measured in hertz "HZ". The AC signal is also called "sinusoidal signal". Example: AC mains volt.
3-Binary Signal: This signal has two values only as shown in figure 2.3. To make this signal carry data, it is necessary to specify a certain duration called "bit interval". If the bit interval is T then the data rate is 1/T which represents the number of bits per unit time (second). The "data rate" is also called a" bit rate". A good example is the signal sent by computer to a "Modem".
4- General Analog Signal: This signal changes its amplitude in non-defined manner A good example of this is the voice signal.
A communication medium is any material (or space) that allows certain type of signals to pass through it e.g. telephone wires allows electrical signal (voltage, current); air allows voice signal.
Usually, the signal carries information (data) and hence the communication medium is used for transferring data from a location to anther location.
The signal when passing through a communication medium suffers from:
1- Noise: this is unwanted signal, causes interference to the original signal, and may destroy some or all the carried data. The noise may be generated internally by the medium due to environment changes or may be caused by other external signals passing around the medium e.g. the noise we hear in telephone line when a mobile device rings near it.
2- Attenuation Distortion: The signal is a kind of energy e.g. voice signal is a mechanical energy, electrical signal is electrical energy. When a signal passes through a medium then part of its energy is wasted e.g. the voice signal becomes very weak after traveling some distance. This means that the signal is attenuated by the medium and has to be amplified or regenerated (repeated) after traveling some distance. The most important fact is that the attenuation rate differs with frequency i.e. each frequency component of a signal suffers from attenuation in a different rate and hence the original signal may get distorted.
Delay Distortion: Each frequency component takes time to travel certain distance in a communication medium and this time is called "delay". The main problem is that the delay varies with frequency and hence the frequency components will not travel in the same speed and therefore will not arrive at a certain location together and this causes a signal distortion called "Delay Distortion".
The main communication media that can be used to transfer (transmit) signals are:
- Twisted pair used to transfer electrical signals.
- Coaxial cable used to transfer electrical signals.
- Optical fiber used to transfer optical signals.
- Radio waves used to transfer electromagnetic signals.
- Infrared waves used to transfer Infra red signals.
- Many others.
For these media, we are going to study some of their properties such as bandwidth, bit rate, error rate, construction and others. It is worth saying that these properties are affected by length of media, diameter of media, type of media material, type of environment and surroundings, shape and construction of media.
2-9-1 Twisted pair (TP)
TP is the oldest and still the most common communication media. A TP consists of two insulated copper wires twisted together in a helical form as shown in figure 2.17. The purpose of twisting is to decrease the effect of surrounding electromagnetic field and also to decrease the effective radiation from the TP itself so that it dose not make crosstalk to other neighboring twisted pairs. Twisted pairs are usually grouped together and protected by a plastic sheath (case of unshielded twisted pair UTP) or by a plastic sheath and braided wire shield (case of shielded twisted pair STP). The properties of TP depends on number of twists per unit length, therefore
(C)
(a) Category 3 UTP.
(b) Category 5 UTP.
The main advantages of TP are:
1- Easy to handle (cut, weld, connect, tap, twist, carry, etc.).
2- Low cost but it should be noted that Cat. 5 cables are more expensive than Cat. 3, also, STP is more expensive than UTP.
The main disadvantages of TP are:
1) Comparatively, narrow band width and hence low bit rate. For example, it is possible to transfer 10 Mbps for maximum distance of 100 meter on Cat. 3 UTP cable (on Cat. 5, it is possible to transfer up to 100 Mbps for the same distance of 100 meter) when used in LANs.
2) Comparatively, more affected by surrounding electromagnetic field which means more noise and hence less bit rate according to Shannon's theorem.
3) Comparatively, higher attenuation and hence "Repeater" devices are necessary on short distances of UTP cables.
The main application of UTP cables are in telephone networks where Cat. 3 is used and, also, in LANs where Cat. 3 and Cat. 5 are used.
Coaxial Cable (Coax)
This type of construction makes some of its properties better than TP and some worse.
The main advantages compared to TP are:
1) Larger BW and hence higher bit rate, fore example, the thin coaxial cable can be used to transfer 10 Mbps for up to 200 meters in LANs.
2) Less affected by electromagnetic field and hence lower noise which means higher bit rate.
3) Less electromagnetic radiation and hence does not cause crosstalk in neighboring cables.
4) Less attenuation and hence repeaters are not needed on longer distances.
The main disadvantages compared to TP are:
1) More difficult to handle as it needs special connectors (BNC connectors) and has larger weight.
2) Higher cost.
It should be noted that there are two types of coax:
- Thin Coax of 0.5 inch diameter
- Thick Coax of 0.75 inch diameter
The thin Coax is widely used in:
- TV systems (cable TV, connect antenna to TV).
- MANs computer networks.
- Long distance telephone line (now is being replaced by fibers).
It should be noted that the use of coax in MANs are due to:
- The spread of cable TV systems in western cities and hence can be used for MANs.
- The coax has low attenuation and hence can be used without repeaters on city level.
2-9-3 Optical Fiber (Fiber)
Here, we are using optical light to carry data signal instead of electrical current used in copper wires. The light itself is carried by a special transparent material (glass) surrounded by other materials . The light passes through the core via continuous reflections or in other forms depending on fiber type.
The main advantages of optical fibers compared to copper wires are:
1) Very high BW and hence very high bit rate (e.g. 1000 Mbps for more than 1000 meter).
2) Low attenuation and hence repeaters are less needed unless for very long distances (e.g. 200 km).
3) Not affected by electromagnetic interference which means less noise and higher bit rate.
4) Low error rate of order 10-5 bit i.e. 1 error bit every 105 transmitted bit.
5) Light weight.
However, there are some disadvantages such as:
1) Difficult to handle (no tapping, small angle twist, difficult to connect) and hence requires special tools and expertise.
2) The need for electric-optic transducers
3) More expensive.
The specifications of fibers are getting better with the advance of technology and the need for them are getting higher due to the need for high bit rate, therefore, the application of fibers are growing all the time so that, in the future, the fibers will reach at our homes.
It is worth noting that one fiber only is needed to transfer the signal but in copper it is necessary to use two wires.
ليست هناك تعليقات:
إرسال تعليق