Data Transmission The basics of media, signals, bits, carries, and modems (Part I)

Physical Layer The lowest layer in any network architecture model It is concerned with the transparent transmission of “raw” bits across a communications medium It deals with the physical characteristics (mechanical, electrical, procedural) of data transmission and communication. It is responsible for: –providing basic signaling (control, data) –signal modulation –encoding/decoding –activate/deactivate physical medium (PM) –bit-timing (clocking) –mapping between different formats

Physical Layer Sender Transmitted Signal Transmission Medium Received Signal (Attenuated & Distorted) Because of Propagation Effects Receiver Propagation

Data Communication Source initiates the communication. Destination address (identifier) is required for the network to establish a communication path between the source and the destination Destination must be prepared to receive data Source and destination hosts may belong to different types of networks. Line speed and/or packet formats mismatches should be taken care of (i.e., via fragmentation, conversion, etc.) Example: consider a file transfer (e.g., FTP)

Analog and Binary Data Binary Data Analog Data Smoothly changing among an infinite number of states (loudness levels, etc.) Two states: One state represents 1 The other state represents 0

Binary Data and Binary Signal 15 Volts (0) 0 Volts -15 Volts (1) Transmitted Signal 00 1 There are two states (in this case, voltage levels). One, (high) represents a 0. The other (low) represents a 1.

Binary Data and Binary Signal 15 Volts (0) Clock Cycle 0 Volts -15 Volts (1) Transmitted Signal 00 1 Time is divided into clock cycles The State is held constant within each clock cycle. It can jump abruptly at the end of each cycle. One bit is sent per clock cycle.

Binary Data and Digital Signal Client PC Server In binary transmission, there are two states. In digital transmission, there are few states (in this case, four). With four states, two information bits can be sent per clock cycle. 00, 01, 10, and 11 Binary transmission is a special case of digital transmission.

Baud Rates for Digital Signals Client PC Server Suppose that the clock cycle is 1/10,000 second. Then the baud rate is 10,000 baud (10 kbaud). The bit rate will be 20 kbps (two bits/clock cycle times 10,000 clock cycles per second). (The bit rate gives the number of information bits per second.) Baud Rate = # of Clock Cycles/Second

In Summary- Bit Rate and Baud Rate Two terms frequently used in data communication Bit rate: the number of bits sent in one second, usually expressed in bits per second (bps) More important to know how long it takes to process each piece of information Duration of a bit (bit interval): the time required to send one single bit. bit interval = 1/bit rate Example: bit rate = 55.6 kbps, duration of a bit = 1/55600 = second = 18 microseconds

Baud Rate Refers to the number of signal units per second that are required to represent those bits. Related to the bandwidth -- the fewer signal units required, the more efficient the system and the less bandwidth required to transmit more bits More important to know how efficiently we can move those data from place to place Bit rate = Baud rate * the number of bits represented by each signal unit Example: An analog signal carries 4 bits in each signal element. If 1000 signal elements are sent per second, then baud rate = 1000 bauds per second, bit rate = 1000 * 4 = 4000 bps

Perspective Analog Data –Smooth changes among an infinite number of states— like hands going around an analog clock Digital Data –Few states –In a digital clock, each position can be in one of ten states (the digits 0 through 9) Binary Data –Two states (a special case of digital)

Basic Idea For Transmission Media Encode data as energy and transmit energy Decode energy at destination back into data Energy can be electrical, light, radio, sound,... Each form of energy has different properties and requirements for transmission

Transmission Media Transmitted energy is carried through some sort of medium Transmitter encodes data as energy and transmits energy through medium –Requires special hardware for data encoding –Requires hardware connection to transmission medium –Media can be copper, glass, air,...

Copper Wires Twisted pair: a pair of insulated copper wires. Can run for a few kms (oldest and most common). Several standards: STP and UTP (Unshielded Twisted Pair).

4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector Single Twisted Pair Jacket Four pairs (each pair is twisted) There is insulation around each wire.

4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector A length of UTP is called a cord. There is no metal shielding around The individual pairs or around the entire Cord. Hence the name unshielded UTP UTP Cord

Copper Wires (continued) Coaxial cable: copper wire surrounded by an insulator encased by another copper conductor (mesh) covered by a protective plastic sheath

Glass Fibers Thin glass fiber carries light with encoded data Plastic jacket allows fiber to bend (some!) without breaking Fiber is very clear and designed to reflect light internally for efficient transmission Light emitting diode (LED) or laser injects light into fiber Light sensitive receiver at other end translates light back into data

Glass Fibers (continued) Very reliable and high capacity Can run up to tens of kms Not affected by electromagnetic inference Easy to install but a costly technology

Multimode & Single-Mode Fiber Cladding Light Source Core Multimode Fiber Modes Light only travels in one of several allowed modes Multimode fiber must keep its distance short or limit modal distortion Multimode fiber goes a few hundred meters and is inexpensive to lay It is dominant in LANs

Multimode & Single-Mode Fiber Light Source Single Mode Fiber Cladding Core Single Mode Core is so thin that only one mode can propagate. No modal dispersion, so can span long distances without distortion. Expensive, so not widely used in LANs. Popular in WANs

Multimode and Single-Mode Fiber Multimode –Limited distance (a few hundred meters) –Inexpensive to install –Dominates fiber use in LANs Single-Mode Fiber –Longer distances: tens of kilometers –Expensive to install –Commonly used by WANs and telecoms carriers

Wireless Air is the medium Radio –Data transmitted using radio waves –Energy travels through the air rather than copper or glass –Conceptually similar to radio, TV, cellular phones –Can travel through walls and through an entire building –omnidirectional (broadcast)

Radio Wave Amplitude Wavelength Frequency Measured in Hertz (Cycles per Second) 2 Cycles in one Second, so 2 Hz Wavelength * Frequency = Speed of Propagation

Wireless Satellite: unidirectional, costly, propagation is a consideration

Wireless Microwave –High frequency radio waves –Unidirectional, for point-to-point communication –Antennas mounted on towers relay transmitted data Infrared –Infrared light transmits data through the air –Similar to technology used in TV remote control –Can propagate throughout a room (bouncing off surfaces), but will not penetrate walls

Wireless Laser –Unidirectional, like microwave –Higher speed than microwave –Uses laser transmitter and photo-sensitive receiver at each end –Point-to-point, typically between buildings –Can be adversely affected by weather

Wireless Propagation Problems Laptop Comm. Tower Inverse Square Law Attenuation Very Rapid Attenuation with Distance Compared to Wires and Fiber

Wireless Propagation Problems Laptop Comm. Tower Shadow Zone: No Signal Multipath Interference Signals Arriving at Slightly Different Times Can Interfere

Choosing A Medium Copper wire is mature technology, rugged and inexpensive; maximum transmission speed is limited Glass fiber: –Higher speed –More resistant to electromagnetic interference –Spans longer distances –Requires only single fiber –More expensive; less rugged

Choosing A Medium Radio and microwave don't require physical connection Radio and infrared can be used for mobile connections Laser also does not need physical connection and supports higher speeds

Data Transmission Data transmission requires:  Encoding bits as energy  Transmitting energy through medium  Decoding energy back into bits Energy can be electric current, radio, infrared, light Transmitter and receiver must agree on encoding scheme and transmission timing

Encoding--Using Electric Current To Send Bits  Simple idea - use varying voltages to represent 1s and 0s  One common encoding use negative voltage for 1 and positive voltage for 0  In following figure, transmitter puts positive voltage on line for 0 and negative voltage on line for 1

All details specified by a standard Several organizations produce networking standards –IEEE Institute for Electrical and Electronics Engineers –ITU (International Telecommunications Union) –EIA (Electronic Industries Association) Hardware adheres to standard interoperable Encoding Details

Asynchronous and synchronous communications Asynchronous communication: data are transmitted one character at a time –transmitter and receiver do not explicitly coordinate each data transmission –transmitter can wait arbitrarily long between transmissions –receiver does not know when a character will arrive. May wait forever Transmission Modes

Asynchronous Communication To ensure meaningful exchange –start bit before character –one or more stop bits after character –1s when idle Used, for example, when transmitter such as a keyboard, may not always have data ready to send

Synchronous Communication No start/stop bits. Sends bytes contiguously Periodically transmit clocking information Uses flags (special bit sequences) as delimiters for frames More efficient transmission

Example use –Connection to keyboard/mouse –Serial port on PC (as opposed to parallel transmission) Specified by EIA Voltage is +15 or –15 Cable limited to ~50 feet Use asynchronous communication The RS-232C Standard

Start bit –Same as 0 –Not part of data Stop bit –Same as 1 –Follows data Illustration Of RS-232

Duration Of A Bit In RS-232 Determined by baud rate –Typical baud rate: 9.6 kbaud, 14.4 kbaud, 28.8 kbaud –bit_rate = baud_rate –Duration of a bit is 1/baud_rate Sender and receiver must agree a priori Receiver samples signal Disagreement results in framing error

Two-Way Communication Desirable in practice Requires each side to have transmitter and receiver Called full duplex

Illustration Of Full-Duplex Communication Transmitter on one side connected to receiver on other Separate wires needed to carry current in each direction Common ground wire

Electric Transmission In real world –Electric energy dissipates as it travels along –Wires have resistance, capacitance, and inductance which distort signals –Magnetic or electrical interference distorts signals –Distortion can result in loss or misinterpretation

Illustration Of Distorted Signal For A Single Bit In practice –Distortion can be much worse than illustrated

Consequences RS-232 hardware must handle minor distortions –Take multiple samples per bit –Tolerate less than full voltage Can not use electrical current for long- distance transmission

Reading Materials Chapter 4 Chapter 5: Sections