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.