1. Physical Layer
B. Konkoth

2. The physical layer is responsible for movements of
individual bits from one node to the next.

3. Physical layer

4. Physical Links How to make computers talk across a wire
How to share the wire
Simple first step ---

5. From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream
Packets
Header/Body
Receiver
Sender
Packet
Transmission

6. Analog
An Analog signal is any continuous signal for which the time varying feature is a representation of some other time varying quantity.
For example, in sound recording, fluctuations in air pressure (sound) strike the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by a condenser microphone. The voltage or the current is said to be an "analog" of the sound.

7. Digital A data technology that uses discrete (discontinuous) values
The word digital is most commonly used in computing and electronics, especially where real-world information is converted to binary numeric form as in digital audio and digital photography.

8. Transmission Media
Transmission medium: the physical path between transmitter and receiver.
Communication of electromagnetic waves is guided or unguided.
Guided media: waves are guided along a physical path (eg, twisted pair, coaxial cable and optical fiber).
Unguided media: means for transmitting but not guiding electromagnetic waves (eg, the atmosphere and outer space).
Repeaters or amplifiers may be used to extend the length of the medium.

9. Transmission Media Choices
Twisted pair
Coaxial cable
Optical fiber
Wireless communications

10. Cables
Coaxial
Twisted pair
Fiber optics

11. Twisted Pair Two insulated wires arranged in a spiral pattern
Copper or steel coated with copper
The signal is transmitted through one wire and a ground reference is transmitted in the other wire.
Typically twisted pair is installed in building telephone wiring.

12. Unshielded Twisted-Pair (UTP)
Typically wrapped inside a plastic cover (for mechanical protection)
A sample UTP cable with 5 unshielded twisted pairs of wires
Metal
Insulator

13. Categories of UTP Cables
UTP cables are classified according to the quality:
Category 1 ― the lowest quality, only good for voice, mainly found in very old buildings, not recommended now
Category 2 ― good for voice and low data rates (up to 4Mbps for low-speed token ring networks)
Category 3 ― at least 3 twists per foot, for up to 10 Mbps (common in phone networks in residential buildings)
Category 4 ― up to 16 Mbps (mainly for token rings)
Category 5 (or 5e) ― up to 100 Mbps (common for networks targeted for high-speed data communications)
Category 6 ― more twists than Cat 5, up to 1 Gbps

14. Shielded Twisted-Pair (STP)
STP cables are similar to UTP cables, except there is a metal foil or braided-metal-mesh cover that encases each pair of insulated wires

15. Twisted Pair
Limited in distance, bandwidth and data rate due to problems with attenuation, interference and noise
Issue: cross-talk due to interference from other signals
“shielding” wire (shielded twisted pair (STP)) with metallic braid or sheathing reduces interference.
“twisting” reduces low-frequency interference and crosstalk.

16. Center conductor Dielectric Braided material Outer outer cover
Coaxial Cable
Center
conductor
Dielectric
material
Braided
outer
Outer
cover

17. Coaxial Cable Divided into two basic categories for coax used in LANs:
50-ohm cable [baseband]
75-ohm cable [broadband or single channel baseband]
In general, coax has better noise immunity for higher frequencies than twisted pair.
Coaxial cable provides much higher bandwidth than twisted pair.
However, cable is ‘bulky’.

18. Communication channel
Baseband – sends 1 signal or 1 channel at any given time
Broadband - enables a single wire to carry multiple signals at the same time

19. Optical Fiber
Optical fiber: a thin flexible medium capable of conducting optical rays.
Optical fiber consists of a very fine cylinder of glass surrounded by concentric layers of glass.
Attenuation in the fiber can be kept low by controlling the impurities in the glass.

20. cladding jacket core Optical Fiber (a) Geometry of optical fiber light
(b) Reflection in optical fiber c

21. Wireless
Infrared - Infrared radiation (IR) is electromagnetic radiation with a wavelength between 0.7 and 300 micrometers
Microwave - Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter
Radio - Radio waves transmit music, conversations, pictures and data invisibly through the air, often over millions of miles - wavelengths in the electromagnetic spectrum longer than microwave

22. Electromagnetic spectrum