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.