William Stallings Data and Computer Communications Excerpts from

William Stallings Data and Computer Communications Excerpts from Chapter 3, 4 and 6 Data Transmission Basics

Data Transmission q Converting into Electromagnetic (EM) signals q Transmitting those signals through medium q Medium Ø Guided medium o e. g. twisted pair, optical fiber Ø Unguided medium o e. g. air, water, vacuum

Spectrum & Bandwidth q Spectrum Ø range of frequencies contained in signal q bandwidth Ø width of spectrum

Data Rate and Bandwidth q A perfect square wave has inifinite bandwidth Ø Fourier analysis Ø cannot be transmitted over a medium due to medium restrictions Ø more bandwidth o less distortions o expensive Ø less bandwidth o more distortions ==> more errors o cheap q Higher bandwidth = higher data rate

Transmission Media q Guided Ø Twisted pair Ø Coaxial cable Ø Optical fibers q Unguided Ø radio Ø microwave Ø infrared

Electromagnetic Spectrum

Magnetic Media q Can give good data rate q Sometimes the best way : ) Ø especially for large volume of data transfer

Twisted Pair

Twisted Pair - Applications q Most common medium q Telephone network Ø Between house and local exchange (subscriber loop) q Within buildings Ø To private branch exchange (PBX) q For local area networks (LAN) Ø 10 Mbps or 100 Mbps

Twisted Pair - Pros and Cons q Cheap q Easy to work with q Low data rate q Short range

Twisted Pair - Transmission Characteristics q Analog Ø Amplifiers every 5 km to 6 km q Digital Ø Use either analog or digital signals Ø repeater every 2 km or 3 km

Unshielded and Shielded TP q Unshielded Twisted Pair (UTP) Ø Ordinary telephone wire Ø Cheapest Ø Easiest to install Ø Suffers from external EM interference q Shielded Twisted Pair (STP) Ø Metal braid or sheathing that reduces interference Ø More expensive Ø Harder to handle (thick, heavy) Ø IBM invention

UTP Categories q Cat 3 Ø up to 16 MHz Ø Voice grade found in most offices Ø Twist length of 7. 5 cm to 10 cm q Cat 5 Ø data grade Ø up to 100 MHz Ø Commonly pre-installed in new office buildings Ø Twist length 0. 6 cm to 0. 85 cm

Coaxial Cable

Coaxial Cable Applications q Most versatile medium q Television distribution Ø Ariel to TV Ø Cable TV q Long distance telephone transmission Ø Can carry 10, 000 voice calls simultaneously Ø Being replaced by fiber optic q Cable Internet q Local area networks

Coaxial Cable - Transmission Characteristics q Less susceptible to interference q Periodic amplifiers/repeaters are needed

Optical Fiber Core: thin fiber (8 - 100 micrometers), plastic or glass Cladding: Glass or plastic coating of fiber. Specially designed. Interface is a reflector. Jacket: plastic layer to protect against environmental dangers

Optical Fiber - Benefits q Greater capacity Ø Data rates of hundreds of Gbps q Smaller size & weight q Lower attenuation q Electromagnetic isolation Ø no interference Ø no crosstalk Ø more secure q Greater repeater spacing Ø 10 s of km at least

Optical Fiber - Applications q Long-haul trunks q Metropolitan trunks q Rural exchange trunks q Subscriber loops q LANs

Wireless Transmission q Unguided media q Transmission and reception via antenna q Directional Ø Focused beam Ø Careful alignment required q Omnidirectional Ø Signal spreads in all directions Ø Can be received by many antennae

Frequencies q 1 GHz to 40 GHz Ø referred as microwave frequencies Ø Highly directional Ø Point to point Ø Satellite q 30 MHz to 1 GHz Ø Omnidirectional Ø Broadcast radio

Terrestrial Microwave q Typical antenna is a parabolic dish q Focused beam q Line of sight transmission q Long haul telecommunications Ø voice and video (TV) Ø what are the advantages/disadvantages of using microwave by a long-distance telephone company? o no right-of-way needed o needs periodic towers o sensitive to atmospheric conditions – e. g. multipath fading Ø alternative: fiber optic – needs right-of-way

Satellite Microwave q Satellite is relay station q Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Ø transponder = frequency channel q may also broadcast Ø TV q Requires geo-stationary orbit q Television q Long distance telephone q Private business networks

Asynchronous and Synchronous Transmission q Problem: SYNCHRONIZATION Ø Sender and receiver must cooperate Ø must know when to start and stop sampling Ø must know the rate of data q Two solutions Ø Asynchronous Ø Synchronous

Asynchronous Transmission q No long bit streams q Data transmitted one character at a time Ø generally 7 - 8 bits per character q Prior communication, both parties must Ø agree on the data rate Ø agree on the character length in bits

Asynchronous Transmission

Asynchronous Transmission Behavior q In idle state, receiver looks for 1 to 0 transition q Then samples next “character length” intervals q Then looks for next 1 to 0 for next char q Stop bit is used to make sure a 1 to 0 transition for the next character q Overhead is 2 or 3 bits per char (start, stop and/or parity bits)

Synchronous Transmission q Block of data transmitted without start or stop bits q No overhead (except error detection/correction codes) q Common clock Ø generally sender-generated Ø data is sampled once per clock cycle Ø clock starts ==> data starts Ø clock stops ==> data stops Ø no further synchronization needed for short distance and point to point communication

Synchronous Transmission q Synchronous communication concept in Computer Networking is different q Need to indicate start and end of block q Use preamble and postamble flags q More efficient (lower overhead) than async