Communication Systems

Communication Systems

In case of sending and receiving messages or data from one place to other place, we have many elements working together. All these elements put together to work efficiently is known as system. The communication system has the sole purpose of passing data or information in the most effective manner.

Communication system is the combination of the hardware, software and data transfer links that make up a communication facility of transferring data in a cost effective manner.

A communication system can either be analog or digital or a combination of the two. That is, the information can be transmitted in either analog or digital form with in the communication network. E.g. computer generated data are to be sent over analog telephone lines, the digital signals must be converted to analog form. The technique by which a digital signal is converted to its analog form is known as modulation. The reserve process that is conversion to its analog signal to its digital form at the destination device called demodulation. The process of modulation and demodulation that is the conversion of digital data to analog from and vice versa is carried out by a special device called modem (modulation/demodulation).

The analog form of message is sent via telephone line and then it reaches the destination where they are again transformed into digital pulses by the other modem connected with the receiver PC. Thus the message or data is transmitted and received by the two PC’s.

As the analog signal passes through the telephone lines, its strength starts decreasing with the distance, so amplifiers are used to boost the strength of the signal. These amplifiers add noise to the signal which sometimes causes disruption of the message. But good quality of the modem at receiving end as well as at the transmitting end is able to cut down the effect of the noise.

Data Communication & It's Components

Data Communication

Data Communication means sharing the information. This sharing can be local or remote or with in a building. Local communication usually occurs face to face, while remote communication take place over distance.

This term telecommunication includes telephony, television mean communication at a distance. The world tele is a Greek word which means “far”. The word data refers to the facts. In the context of computer information system, data are represented by binary information unit produced in the form of 0s (zero) and 1s (one).

Data Communication is the exchange of data between the two devices via some of transmission medium. Data communication devices must be the part of the communication system made up of a combination of Hardware and Software. The effective data communication has three fundamental characteristics.

1. Delivery. System must deliver the data to the correct destination.
2. Accuracy. System must deliver accurate data.
3. Timeliness. System must deliver the data in a timely manner.

Components of Data Communication

There are five major components of data communication system: -

1. Message. The information which is to be communicated.
2. Sender. Sending data communication device are the devices that sends data message. It can be computer, workstation, telephone, radio, camera and so on.
3. Receiver. Receiver is the device that receives the message. It can be a computer, workstation, telephone, TV and so on.
4. Medium. It is a physical path by which the message travels from sender to receiver.
5. Protocol. It is the set of rules that governs the data communication. It is agreement between the communication devices. Without a protocol, two devices may be connected but they are not communicating (With each other). Just as a person speaking Tamil cannot understand the language of the person who speaks only Punjabi.

Transmission Technologies

Transmission Technologies

The following are some technologies that apply to communications: -

. Broadband Transmission
. Baseband Transmission

Broadband Transmission

Broadband transmission enables two or more communication channels to share the bandwidth of the transmission media. Broadband networks can simultaneously accommodate video, voice, and data. ISDN is an example of a WAN communication service that can provide broadband transactions.

Bandwidth is basically the difference between the highest and lowest frequencies in a given range. This refers to the capacity of the media. The greater the bandwidth, the faster the data-transfer capabilities will be.

For example: - Ethernet has a bandwidth of 10 megabits per second (Mbps) and Token Ring has a bandwidth of either 4Mbps or 16Mbps.

Baseband Transmission

Baseband transmission uses digital signals over a single frequency. With baseband transmission, the entire communication channel capacity is used to transmit a single data signal. Most LANs use baseband technology.

Bit Rate, Baud Rate & Attenuation

Bit Rate, Baud Rate & Attenuation

Bit Rate

Number of bits transmitted per second or bps rate. The amount of data that a computer network can transmit is called bandwidth of network and is usually measured in kilo bits per second (Kbps) or mega bits per second (Mbps). A bit the smallest unit of information what computer can process can have one of two values, either 0 or 1. A kilo bit in one thousand bit while a mega bit is one million bit the speed at which information can be transmitted across the internet depends on the lowest information transporting capacity along the route and the number of people using that route at any given time. The bit rate is the speed of communication.

Baud Rate

The signaling rate of a line is measured in bauds. It is the switching speed or number of the transition (Voltage or frequency changes) that are made per second. The speed at which data is transmitted referred to as baud. Baud is commonly identified as the number of bits per second that can be transmitted over a communication line.

Attenuation

The loss of signal’s energy due to the resistance of the medium is called attenuation.

Data Communication & Data Transmission Modes

Data Communication

In data communication, four basic terms are used i.e.

→ Data

→ Signal

→ Signaling

→ Transmission

Data refers to information that conveys or contains something of meaning to source/recipient. The information in data may be raw data or contain interpreted result from raw data.

Signal is the data encoding in the form of electric or electromagnetic waves to produce digital or analog signal.

Signaling is the process by which computer interacts with transmission media & sends the signal down the media.

Transmission is the communication of data achieved by propagation or processing of signals.

Data Transmission Modes

There are three ways as modes for transmitting data from one point to another. These are:-

→ Simplex

→ Half-Duplex

→ Full-Duplex

Simplex Mode of Transmission

In Simplex transmission, communication takes place only in one direction. Devices connected to such a circuit are either send-only or receive only device.

Fig-1.1 -- Simplex Mode of Data Transmission

Half-Duplex Mode of Transmission

A Half-Duplex system can transmit data in both directions, but only one direction at a time. This requires two wires. This is a common type of communication because one person can speak at a time. In this case line must be turned around each time the direction is reversed. This involves a switching circuit.

Fig-1.2 -- Half-Duplex Mode of Data Transmission

Full-Duplex Mode of Transmission

A Full-Duplex system allows information to flow simultaneously in both the direction on the transmission path. Use of Full-Duplex line improves efficiency because the line turn around time is removed. It requires four wires.

Fig-1.3 -- Full-Duplex Mode of Data Transmission

Switching Techniques

Switching Techniques

Circuit Switching

When you or your computer places a call (Telephone), the switching equipment within the telephone system search for a physical copper path all the way from your telephone to receiver’s telephone, this technique is called as circuit switching.

Once the call has been setup, a dedicated path between both ends exists & will continue to exist until the call is finished.

An important feature of circuit switching is the need to set-up an end-to-end path before any data can be sent. The time between the end of dialing and start of ringing can easily be 10 seconds and may increase on long-distance or international calls. During this time interval, the telephone system is hunting for a copper path.

As a consequence of copper path between the calling parties, once the set up has been completed, the only delay for data is he propagation time for electromagnetic signal.

Once the call has been put through, you never get busy signals; you might get busy signals before the connection has been established due to lack of switching.

Message Switching

A Message is a logical unit of information and can be of any length. In this method, if a station or a switching office wishes to send a message to another, it first attaches the destination address to message. When the sender has a block of data to be sent, it is stored in the first switching office and then forwarded later. This method is known as store-and-forward. Each block is received entirely, checked for errors and then retransmitted.

In message switching, no physical connection is required between the source and destination.

But one disadvantage is that the message length is unlimited i.e. each switching node must have sufficient storage to buffer message and another one is that a message is delay at each node because of time required to receive the message plus a queuing delay waiting for a chance to retransmit message to next node.

Packet Switching

This method works similar to message switching, however it overcomes the disadvantage of message switching because in this method routing is done on packet basis and not on message basis.

Here, a message is split up into packets of fixed size. Besides the block of data to be sent, a packet has a header that contains source and destination addresses, control information, acknowledgement and error checking bits. With message switching, there was no limit on block size, which means that routers must have disks to buffer long blocks. It also means that a single block may tie up a line for minutes restricting switches for the other traffic. To get around these problems, packet switching was invented.

Packet switching makes sure that no user reserves the transmission line for very long time.

Thus computer networks usually use packet switching, occasionally circuit switching, but never message switching.

Transmission Media

Transmission Media

In OSI Model, the purpose of physical layer is to transfer raw but stream from one computer to another, so various transmission media are used for data or bit transmission. Each one has its own niche in terms of bandwidth, delay, cost & ease of installation & maintenance. They are grouped into guided media i.e. copper wire, fiber optics and unguided media i.e. radio and laser.

Magnetic Media

The most common way to transport data from one computer to another is to write them onto magnetic tapes or floppy disks. This method is often more cost effective and not as sophisticates as geosynchronous satellites. E.g. a video tape can hold 7 gigabytes and a box of 50 x 50 can hold 1000 video tapes i.e. total is 7000 G.B. It can be delivered anywhere in UK/US in 24 hrs by Express Services.

Thus effective bandwidth of this transmission is 56 GB/86400 sec. or 64 Mbps, which is slightly better than the high-speed version of ATM i.e. 622 Mbps. But, if the destination is 1 hr away, then bandwidth increased over to 15 Gbps. Thus, banks used to take back up on Magnetic tapes for performance.

If cost is taken, then a similar picture will be offered i.e. a video tape can reused at least ten times.

Twisted Pair

In Magnetic Tapes, the bandwidth characteristics are excellent, but the delay time is poor. Transmission time is measured in hours/minutes, not in milliseconds, thus the oldest & most common transmission medium is Twisted Pair. It consists of two insulated copper wires, about 1mm thick & they are twisted in helical form like a DNA molecule. The purpose is to reduce electrical interference from similar pair close by. The most common application of twisted pair is telephone system. All telephones are connected to telephone office through twisted pair. They can run several kilometers without amplification, but for longer distances repeaters are needed.

It can be used for either digital or analog transmission. The bandwidth depends upon the thickness of wire and the distance traveled, but several megabits/sec can be achieved for few kilometers. Thus they are offering adequate performance and low cost.

Two types of twisted pairs are used by lot of companies i.e. Category 3 & Category 5.

Category 3 consists of two insulated wires gently twisted together. For such pairs are grouped together in a plastic sheath for protection and to keep eight wires together. It allowed up to four regular telephones or two multiline telephones in each office to connect to telephone company equipment.

Category 5 is similar to Category 3 except they have more twists per centimeter & Teflon insulation, results in less cross talk and better quality signal over longer distances, making suitable for high-speed computer communication.

Co-axial Cable (Coax)

Co-axial cable has better shielding than twisted pairs, so it can spans longer distances at higher speeds. There are two kinds of co-axial cable exists i.e. 50-ohm & 70-ohm cables.

50-ohm is commonly used for digital transmission & 75-ohm is used for analog transmission. 50-ohm cable is also called Baseband Co-axial Cable & 75-ohm is Broadband Co-axial Cable.

A Co-axial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is enclosed by a cylindrical conductor, often as a closely woven braided mesh. The outer conductor is covered in a plastic sheath.

The construction and shielding of co-axial cable gives a better high bandwidth and noise immunity as compared to STP. Bandwidth depends on cable length. They are widely used within telephone system, but now a days replaced by fiber-optics. Coax is widely used in cable television and some LAN’s.

Broadband Coax is another kind of co-axial cable used for analog transmission. Broadband refers to anything wider than 4 KHz. & broadband cable means any cable networks using analog transmission. The cables can be used up to 300 MHz. & can run over 100 Km due to analog signaling. At higher frequency, many bits/Hz can be transmitting using advanced modulation technique.

Broadband Co-axial cables are divided up to multiple channels. For television broadcasting, 6-MHz channels are used. Each channel can be used for analog transmission/television. Television & Data can be mixed on one cable.

The key difference between Broadband & Baseband Co-axial cable, broadband system covers large area & therefore need analog amplifiers to strength the signal periodically. For this two types of broadband cables are used i.e. Dual cable & Signal cable.

But broadband is inferior to baseband for sending digital data. In last, co-axial are relatively cheap as compared to fiber optics and easy to handle.

Fiber Optics

The data is carried by light instead of current or voltage in fiber optics. Light acts as carrier waves. It is used for transmission at very high rate and with very large bandwidth.

Fiber optics has three components:-

→ Light source

→ Transmission Medium

→ Detector

In the center is the glass core through which light propagates. The core can be of 50 ms in diameter, about thickness of human hair.

The core is surrounded by glass cladding to keep the light in the core & thin plastic jacket protect the cladding. They bundled together protected by outer sheath.

There are three different ways to connect fibers i.e. they can terminate in connectors & be plugged into fiber sockets. They can be spliced mechanically & two pieces of fibers can be fused/melted together to form a solid connection, but at each way they lose about 0-20 percent of light at each level. Two kinds of light sourcing used to do signaling i.e. LED’s (Light Emitting Diodes) & semiconductor lasers.

Optical fiber may be multimode or single mode. Multimode use multiple lights paths & make all the parts of signal arrive at same time, appearing to receiver as though they were one pulse, but single mode can allow greater bandwidth & cable run that multimode is more expensive.

Radio Transmission

Radio Waves are easy to generate, can travel longer distances & because they are easily to penetrate, widely used for communication. Radio waves have frequencies between 10 KHz & 1 GHz. Radio waves can travel in all directions from source, so that transmitter & receiver don’t have to be carefully aligned physically. Radio waves are frequency dependent & at low frequency, radio waves pass through obstacles easily & at high frequency, radio waves tend to travel in straight lines & bounce off obstacles. At all frequencies, radio waves interference from motors & other electrical equipment. They include:-

→ Short-wave

→ Very-high frequency (VHF) television & FM radio

→ Ultra-high frequency (UHF) radio & television

Various kinds of antennas can be used to broadcast radio signals. The power of radio-frequency (RF) signal is determined by antennas & trans-receiver.

In HF & VHF bands, the ground waves tend to be absorbed by the earth. The military also communicates in HF & VHF bands. Under certain atmospheric conditions, the signals may bounce several times.

Thus, when radio waves reach ionosphere, a larger of charged particles circling the earth at height of 100-500 km, are refracted by it & sent back to earth.

Microwave Transmission

Microwaves are used for satellites, radios & mobile phones. It covers a part of UHF & SHF band & radio waves also cover the part of UHF & VHF band. Antennas are used to emit & receive microwaves. These antennas are installed at the top of buildings to cover wide areas. Advantage of Microwave is that they do not need so much amplifiers & repeaters. Antennas require line of sight transmission.

In atmospheric conditions like rainfall, increase the interferences. In case of satellite microwaves, the parabolic shaped satellite acts as a relay station between the emitting & receiving antennas. There are two common configurations i.e.

→ Point-to-Point (To connect two specific antennas)

→ Point-to-Multipoint

Satellites can be used by individual business users like (Mobiles, Cable-TVs) with a price as high as the complexity of the technology. But now low-cost system has appeared with VSAT (Very Small Aperture Terminal), where a no. of individuals shares a satellite transmission capacity. Satellite communication allows large bandwidth (i.e. up to 100 Mbps).

Since, Microwaves travel in straight line, if towers are too far apart, the earth will get in the way, and consequently repeaters are needed periodically. The distance between repeaters goes up very roughly with square root of tower height. For 100-m tower, repeaters can be apart/spaced 80 km apart. But unlike radio waves, they do not pass through buildings well. Microwaves communication is so widely used for long distance telephone communication, cellular telephones, telephone distribution & other uses. It is easy to put a microwave tower by buying a plot of ground every 50 km, than to buy optical fiber 50 km long. It also being used for long distance transmission, i.e. in industrial/scientific medicals band.

Infra-red Waves

Infra-red is used for remote control or calculator. In infra-red communication, infrared rays are used between transmitter & receivers. Infra-red communication may be useful to connect the buildings, which cannot be linked by electrical lines. Rain & smoke can reduce the signal. Therefore transmission is only available as tens of meters. They are widely used for short-range communication. The remote control used on Televisions, VCR’s & stereos, all use infrared communication. Thus, they are relatively directional, cheap & easy to build but can’t pass through solid objects. As compared to other system of transmission media, there is no need of government license to operate an infrared system.

Now a days, for indoor wireless LAN’s, computers & offices in a building can be equipped with relatively unfocused infrared transmitter & receivers. Thus portable computers with infrared capability can be on Local LAN without having to physically connect to it. Infrared cannot be used outside because sun shines as brightly in infrared as in visible spectrum.

Network Software

Network Software

Now a days, software plays an important role in computer networks, because without software no meaning. Software is described by structuring few details about that. Functioning of network depends upon its parts or layers, their name and content and function of each layer. For e.g. OSI Reference Model & TCP/IP model are the branches of software network.

Networks are organized in terms of layers or levels. Each layer offers certain services to higher layers ‘n’ layer carries on conversation with ‘n’ layer on another machine. The rules & conventions used in this conversation are collectively called as Protocol or ‘n’ layer protocol. A protocol is an agreement between the communicating parts on how communication is to be proceeding. Violating the protocol will make communication more difficult.

The working of layers in software network is to process data and handle the messages from layer immediately above & below. The processing of data takes place some set of rules. Each layer doesn’t know what is happening above & below to it. The function of each layer is to take data from adjacent layers & process it to other layer under some rules. Each layer on computer can communicate to its Peer i.e. same layer on computer with the help of other layers fall in between them. The method is called Peer Communication. Let us take examples for this on the basis of peer communication using OSI Reference Model.

OSI Model is a open system interconnection model, where it can help to communicate from one system to another using some guidelines or defining protocols. OSI Model defines seven layers/levels for complete communication system i.e. from highest layer (Application Layer) to lowest layer (Physical Layer). But sometimes all seven layers are not needed; only three layers are sufficient. Here in OSI Model, each layer represents a different portion of task.

Network Topologies

Topology

Topology refers to the interconnection of paths between the many users or nodes and how they are arranged or it refers to physical layout of the network in which all devices are connected.

Thus, topology means a topographic study of a specific object, entity, place etc.

Different topologies are used in computer network i.e.

1. Bus Topology
2. Star Topology
3. Ring Topology
4. Intersecting Rings Topology
5. Mesh Or Complete Topology
6. Hybrid Topology
7. Tree Topology

Bus Topology

In this topology, all nodes are connected with single communication medium/channel. One machine act as a transmitter and other act as a receiver, normally co-axial cable acts as a transmitter for BUS. In this, data flow in either direction i.e. if a message goes in channel, it reaches every node and if it matches it accept the message otherwise discards it. All nodes share the co-axial cable.

Thus, Bus is flexible structure because users can put anywhere along its length and at low costs and less difficulty.

But, the biggest limitation with Bus requires more management because needs protocols which decide which node became a transmitter. Second if channel breaks, anywhere then some nodes stops functioning at all. Third, it is very weak in terms of security.

Star Topology

In this topology, there is central node and all PC’s or users communicate via central node. There is no limit in the number of users, which are attached to central node. Thus, reliability is higher.

Suppose, one user wants to sends data message to other in this topology, then the sender makes number of copies according to number of users in Star Topology and reaches to everyone.

This topology is used in telephone system and used when more than one pair of users send data at same time. Thus, central nodes provide multiple path ways.

Thus, reliability is higher because problem in one computer does not affect the other users.

Ring Topology

In Ring Network Topology, each node/station is connected to its upstream neighbour and downstream neighbour. Thus, in this each node receive data from previous node and repeats the information to next node. Thus, information flows from originating node, through all other nodes and back to origin node.

IBM Token-Ring & Fiber Distributed Data Interface (FDDI) is two of ring-type networks. Ring Network has the same efficiency of cabling as the Bus Network.

Intersecting Rings Topology

All routers of one ring are connected with all the other routers of second ring via intersecting router i.e. all computers of a ring are connected to other computers of second ring via intersecting server.

If intersecting server fails, then computers of first ring fails to communicate with computers of second ring.

Mesh or Complete Topology

Any computer is directly linked with all other computers in the network through direct lines.

In a true mesh topology every node has a connection with every other node in the network. The main advantage is that failure of any computer does not effect the functioning of other computers and no node will ever encounter a busy channel when trying to communicate with another node.

`The disadvantage is that when number of nodes goes up. The number of connections increases almost exponentially. This requires more cabling, thus very expensive.

Hybrid Topology

There is no specified links between any two users some users communicate through direct lines and some follows store-and-forward principle. Mixture of all these common network topologies is Hybrid Topology.

This topology can have Ring or Bus or Star on one side and another at other side. It uses point-to-point lines to tie them both together.

Tree Topology

Any user can communicate to any other user through Tree Topology.