CS 408 Computer Networks Data Transmission Basics Not

CS 408 Computer Networks Data Transmission Basics Not in the text book Excerpts from Chapter 3, 4 and 6 of Stallings, Data and Computer Communications, 6 th ed.

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

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 infinite bandwidth Ø cannot be transmitted over a medium due to medium restrictions Ø Fourier series of a periodic function o (infinite) sum of sines and cosines (terms) o more terms more frequencies (bandwidth) better square-like shape 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 Ethernet

Twisted Pair - Pros and Cons q Cheap q Easy to work with q Short range q Our book says "Low data rate" Ø But nowadays it is possible to go 40 Gbps with Cat 7 cables

Unshielded and Shielded TP q Shielded Twisted Pair (STP) Ø Metal braid or sheathing that reduces interference Ø More expensive Ø Harder to handle (thick, heavy) Ø Not so economical for low rates, but a good alternative for higher rates Ø IBM invention q Unshielded Twisted Pair (UTP) Ø Ordinary telephone wire Ø Cheaper Ø Easier to install Ø Suffers from external EM (Electromagnetic) interference

UTP Categories q Cat 3 Ø Ø up to 16 MHz Voice grade Very old technology, generally in old offices Twist length of 7. 5 cm to 10 cm Ø Ø data grade up to 100 MHz Commonly pre-installed in office buildings Twist length 0. 6 cm to 0. 85 cm q Cat 5 q Cat 6, 6 a Ø Up to 200 MHz and 10 Gbps Ethernet q Cat 7 Ø Up to 600 MHz and 40 Gbps Ethernet (and maybe beyond)

Coaxial Cable For transmission

Coaxial Cable Applications q Most versatile medium q Television distribution Ø Aerial antenna to TV Ø Cable TV q Long distance telephone transmission Ø Can carry 10, 000 voice calls simultaneously Ø Mostly replaced by fiber optic q Cable Internet q Local area networks (old technology)

Coaxial Cable - Transmission Characteristics q Less vulnerable to interference and crosstalk (than twisted pair) Ø due to concentric structure 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 with a lower index of refraction. Thus it acts as a reflector. Overcoat (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 Ø easy installation, less physical space needed in ducts q Lower attenuation Ø less repeaters needed (one in approx. every 50 kms) q Electromagnetic isolation Ø no interference Ø no crosstalk Ø securer

Optical Fiber - Applications q Long distance communication lines 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 o Line-of-sight needed q Omnidirectional Ø Signal spreads in all directions Ø Can be received by many antennas

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 mounted on a tower q Focused beam q Line-of-sight transmission q Long haul telecommunications Ø voice and video Ø what are the advantages/disadvantages of using microwave by a long-distance telephone company? o o o no right-of-way needed No long distance cabling need to buy frequency band needs periodic towers sensitive to atmospheric conditions – e. g. multipath fading Ø alternative: fiber optic – needs right-of-way and cabling

Satellite Microwave q Satellite is a 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 Applications Ø Television Ø Long distance telephone Ø Private business networks

Asynchronous and Synchronous Transmission on Direct Links 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 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 q But parties do not need to agree on starting and stopping time prior to communication (they exchange starting and stopping time info during tranmission) Ø No common clock signal needed Ø That is why this is asynchronous

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, 3 or 4 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 signal Ø clock starts ==> data starts Ø clock stops ==> data stops Ø generally sender-generated Ø data is sampled once per clock cycle Ø no further synchronization needed for short distance and point to point communication