Physical Layer B. Konkoth

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

Physical layer

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

From Signals to Packets Analog Signal “Digital” Signal Bit Stream 0 0 1 0 1 1 1 0 0 0 1 Packets 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body Receiver Sender Packet Transmission

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.

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.

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.

Transmission Media Choices Twisted pair Coaxial cable Optical fiber Wireless communications

Cables Coaxial Twisted pair Fiber optics

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.

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

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

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

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.

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

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’.

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

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.

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

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

Electromagnetic spectrum