4 Physical Links How to make computers talk across a wire How to share the wireSimple first step ---
5 From Signals to Packets Analog Signal“Digital” SignalBit StreamPacketsHeader/BodyReceiverSenderPacketTransmission
6 AnalogAn 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 MediaTransmission 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 pairCoaxial cableOptical fiberWireless communications
11 Twisted Pair Two insulated wires arranged in a spiral pattern Copper or steel coated with copperThe 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 wiresMetalInsulator
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 nowCategory 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 PairLimited in distance, bandwidth and data rate due to problems with attenuation, interference and noiseIssue: 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 CableCenterconductorDielectricmaterialBraidedouterOutercover
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 timeBroadband - enables a single wire to carry multiple signals at the same time
19 Optical FiberOptical 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 fiberc
21 WirelessInfrared - Infrared radiation (IR) is electromagnetic radiation with a wavelength between 0.7 and 300 micrometersMicrowave - Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeterRadio - 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