TCP/IP Physical layer, Thesis of Computer Networks

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Unit 8

Physical layer

Transmission Media (Wired )

 Magnetic media (offline)

 (^) Tapes, diskettes  (^) High bandwidth  (^) A 8 mm tape = 7 GB  A 505050 Cm box = 1000 tapes =7000 GB 7000GB/24 Hrs= 648 Mbps 7000GB/1Hr=15Gbps  (^) Sometimes it's cheaper and faster to load a box of tapes in your car!  (^) Problem: Delay!

 Twisted pair (Online)

 (^) Simply two wires twisted together – thickness=1mm The twisting cuts down on electrical interference.  (^) Heavily used in the phone system Typical office has four pairs for phones.  (^) Until some Kilometers/ Some Mbps  (^) For Analog and Digital

Transmission Media (3)

 Baseband Coaxial cable

 (^) Used for digital transmissions (called baseband.)  (^) Good noise immunity.  (^) Data rates as high as 2 Gbps for 1 Km distance.  (^) Now being replaced by fiber.

 Broadband Coaxial cable

 (^) Used for analog transmissions (called broadband.)

 Can run 300 MHz for long distances.

 (^) Analog signaling has better S/N than digital signaling.  (^) Interfaces must convert digital signals to analog and vice versa.  (^) Designed for long distances - can use amplifiers.

Transmission Media (4)

 Fiber Optic (1)

 Transmission of light through fiber

 Bandwidth more than 50,000 Gbps!

But now restricted to 1Gbps!

Reason: Electrical and optical signal conversion

 Including 3 components:

1. Light source: Pulse of light=1, absence of light=
2. Transition medium: an ultra-thin fiber of glass
3. detector: generate an electrical pulse when light falls on it

 (^) Similar to coax (without braid)

Transmission Media (6)

 Fiber Optic (3)

 Properties include total internal reflection and

attenuation of particular frequencies.

 Fiber Optic Networks - can be used for LANs

and long-haul.

 A fiber-optic LAN

Transmission Media (7)

 Comparison of fiber optic and copper wire

Fiber Copper

Bandwidth Higher^ Lower

Distance between repeaters 30 KM^ 5 Km

Interference Low^ High

Physical Smaller/Lighter^ -

Flow Uni-directional^ Bi-directional

Electromagnetic Spectrum

Wave Properties

Radio, Microwaves, Infrared, and Visible Light

 can all be used for transmitting information

 AM, FM

UV, X-rays, and Gamma Rays

 would be even better due to their higher

frequencies

 hard to produce and modulate

 do not propagate well through buildings

 dangerous to living things

Propagation of Radio Waves

VLF, LF, and MF bands

Ionosphere

HF and VHF bands

Earth Earth

Radio waves follow the ground

Microwave Transmission

Microwaves

 travel in straight lines (over 100 MHz)

 can be narrowly focused (by a dish)

 the transmitting and receiving antennas must be

accurately aligned with each other.

 do not pass through buildings well.

 can be absorbed by water/rain

 widely used for long-distance telephone

communication, cellular telephones, TV

distribution

Lightwave Transmission

Each side needs its own laser and its own

photodetector.

The laser’s strength, a very narrow beam, its

weakness.

 Difficult aiming at far distance

offers high bandwidth

easy to install