Telecommunication & Networking An Introduction MIS 524 Winter 2004

Agenda Definitions Communication Model The Telecommunications Problem Networking Internetworking Technical Basics MIS 524 Winter 2004

Definitions Communication: The act of coordinating behavior to some end. Requirements: Source Destination Message Medium Implications MIS 524 Winter 2004

Communication Model Meaning-2 Meaning-1 Expression Interpretation Sender Channel Receiver Encoding Decoding M e s s a g e Challenges: 1. Various processes 2. Will meanings match? 3. Why encode? 4. Purpose? Intention? MIS 524 Winter 2004

Characteristics of Communication Encoding/decoding scheme Speed of transmission (baud) Directionality (one-way, bidirectional, switchable) Noise Equivocation (loss of signal) Ambiguity (loss of meaning) Turntaking (protocol) MIS 524 Winter 2004

The Telecommunications Problem Sender Channel Receiver Encoding Decoding Distance: Sender and Receiver are not in direct contact Equivocation: Message loses power over distance Noise: Channel introduces unwanted message Coordination: It’s not clear what a message event is MIS 524 Winter 2004

Solutions to the problems Sender Channel Receiver Encoding Decoding Distance: Long “wires” of various types Equivocation: Boosting of power (introduces noise) Noise: Special encoding schemes Coordination: Coordination messages (protocols) Notice: Nothing about meaning, intention MIS 524 Winter 2004

Components – 1 Hardware Cabling (or radio or light, etc.) Cards for interfaces Routers Splitters Network servers Multiplexors These may handle some of the challenges MIS 524 Winter 2004

Components – 2 Software Applications Sessions (bundles of connections) Connection (between interactors) Operating Systems (across resource sets) Transport (across physical links) Physical (across physical media) Internetworking (across networks) MIS 524 Winter 2004

Components – 3 Other ISPs (internet service providers) Node services Network services MIS 524 Winter 2004

Technical Basics Complex, electronic Interesting; almost all of the basics are based on human communication Remember the basic problems in communication: Distance Signal Loss Noise Turntaking MIS 524 Winter 2004

Basic Economics Sources aren’t “on” all the time Sources make mistakes; repetition is dangerous and costly Channels are usually relatively expensive Sharing channels is a good use of an expensive resource; sharing is costly All channels are error-prone; the way to compensate is redundancy The more complex the scheme, the higher the cost and the more likely is failure or error. MIS 524 Winter 2004

What Is a Signal? A communication event Has a definite start and stop ANALOG signal: strength is proportional to “content” A communication event Has a definite start and stop Carries information (which is NOT the signal) DIGITAL signal: strength is fixed at either 0 or a constant 1 1 1 1 1 MIS 524 Winter 2004

Inside a Digital Signal The bits that form part of the byte may be ones (at or above a certain level) or zero (below this level). This byte is 1011 0110 (1’s in color) Beginning of byte has special “bit” called a start bit Ending of byte has special “bit” called a stop bit MIS 524 Winter 2004

What Is the Advantage of Digital Signalling? First, simplicity, only two signal levels Second, resistance to noise Third, amplification can work without amplifying noise Fourth, potential to add check bits to reconstruct byte in the event of errors (for example, parity checking). MIS 524 Winter 2004

Amplification Noise intrudes Over distance, signal weakens “On” threashold Noise intrudes Over distance, signal weakens Original 0-1 …and then amplified Signal is “clipped” at threashold level …and sent on its way again MIS 524 Winter 2004

Channel Terminology Directionality Modulation/Keying Bandwidth Simplex, (Half-)Duplex, Full Duplex Modulation/Keying Amplitude Shift, Frequency Shift, Phase Shift Bandwidth Number of signals per second Each signal can carry multiple bits (see next Slide) Multiplexing MIS 524 Winter 2004

Directionality Simplex: In one direction only Half-duplex: Alternating directions (first one way, then the other) Full-duplex: Essentially two simplex signals, one in each direction MIS 524 Winter 2004

Modulation/Keying Value of signal (1 or 0) depends on either Amplitude (above/below a certain level) Frequency (above/below a certain level) Phase (mathematical quality above/below a certain level) These can be combined (or multiplied) to key many bits in a given signal. For example 4 values of amplitude x 4 levels of frequency x 2 levels of phase = 32 combinations or five bits per signal. This increases complexity of hardware, but raises “bandwidth” considerably. MIS 524 Winter 2004

Bandwidth Generally limited by attenuation (equivocation), noise, signal speed Increased by higher frequencies, better amplification, more complex keying schemes, more reliable channels with less noise and less attenuation. Highest bandwidth: fiber optic cables Lowest bandwidth: signal lights, semaphore, string. MIS 524 Winter 2004

Multiplexing Previous slides concentrated on SINGLE communication paths. It is possible to ShARE the path. This is called “multiplexing” Multiplexing may be done through Sharing TIME (time division multiplexing) Sharing FREQUENCIES (frequency division m-xing) Sharing SPACE (space division multiplexing) MIS 524 Winter 2004

What’s Good about Multiplexing Not all sources are maximally operational at all times. This wastes a valuable resource (channel time) Any sharing is complex and comes at a cost, usually equipment Where communication is bursty, multiplexing is good. Where communication is continuous, multiplexing is just an expensive overhead. MIS 524 Winter 2004

Networking Node Node Code Mode MIS 524 Winter 2004

Networking A generalization of the communication model. Each participant can send or receive or both New Challenges: Whose turn is it? Communicating across nodes (transport) Switching Specialized nodes (servers) Sharing resources Common codes MIS 524 Winter 2004

Internetworking Working across networks Challenges:??? Gateway MIS 524 Winter 2004

Networking Challenges Getting a message from one sender to one receiver across a network This requires “addressing” and routing Routing is called “switching” in telecommunications There are many switching schemes; all are additional expenses; but there is a savings in not having to connect all points. They are based on unique identifiers MIS 524 Winter 2004

Switching Problem To avoid switching altogether requires that all points be connected together One solution is to route messages around in a circle or ring Another solution is to have one node (or a new one) be a central “switch” B A A general solution is for each node to know how to route messages to a destination, although it may take several “hops” to get a message through C MIS 524 Winter 2004

Transmission Problems Most nodes are silent most of the time Hence most channels aren’t being used But channels can’t really be hogged by senders and receivers for long periods of time Solution is “packet” switching MIS 524 Winter 2004

Packet Switching Sender’s message is broken into (generally short, fixed-length) packets Each packet is numbered and sent “into” the network The network transmits the packets The node assembles the packets in order (not an easy task) The receiver gets the message from the node. MIS 524 Winter 2004

Example of Packet Switching 456 Message FROM: Node 223 TO: Node 456 Count: 4 This is packet 1 This is packet 2 This is packet 3 This is packet 4 P3 P2 223 P4 P4 P1 P3 P2 Packet reassembly Transmission: each packet has its own path through the network P1 Costs Benefits Packet creation Better use of network Packet handling Smaller units Chance of error More even use of n/w Retransmissions Higher traffic Packet creation MIS 524 Winter 2004