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Chapter 1 Communication Networks and Services Chapter 1 Communication Networks and Services Network Architecture and Services Telegraph Networks & Message Switching Telephone Networks and Circuit Switching Computer Networks & Packet Switching Future Network Architectures and Services Key Factors in Network Evolution Chapter 1 Communication Networks and Services Network Architecture and Services Communication Services & Applications z A communication service enables the exchange of information between users at different locations. z Communication services & applications are everywhere. E-mail E-mail server Exchange of text messages via servers Communication Services & Applications z A communication service enables the exchange of information between users at different locations. z Communication services & applications are everywhere. Web Browsing Web server Retrieval of information from web servers Communication Services & Applications z A communication service enables the exchange of information between users at different locations. z Communication services & applications are everywhere. Instant Messaging Direct exchange of text messages Communication Services & Applications z A communication service enables the exchange of information between users at different locations. z Communication services & applications are everywhere. Telephone Real-time bidirectional voice exchange Communication Services & Applications z A communication service enables the exchange of information between users at different locations. z Communication services & applications are everywhere. Cell phone Real-time voice exchange with mobile users Communication Services & Applications z A communication service enables the exchange of information between users at different locations. z Communication services & applications are everywhere. Short Message Service Fast delivery of short text messages Many other examples! z Peer-to-peer applications z Napster, Gnutella, Kazaa file exchange z Searching for ExtraTerrestrial Intelligence (SETI) z Audio & video streaming z Network games z On-line purchasing z Text messaging in PDAs, cell phones (SMS) z Voice-over-Internet Services & Applications z Service: Basic information transfer capability z Internet transfer of individual block of information z Internet reliable transfer of a stream of bytes z Real-time transfer of a voice signal z Applications build on communication services z E-mail & web build on reliable stream service z Fax and modems build on basic telephone service z New applications build on multiple networks z SMS builds on Internet reliable stream service and cellular telephone text messaging What is a communication network? Communication Network z The equipment (hardware & software) and facilities that provide the basic communication service z Virtually invisible to the user; Usually represented by a cloud z Equipment z Facilities z Routers, servers, z Copper wires, coaxial switches, multiplexers, cables, optical fiber hubs, modems, … z Ducts, conduits, telephone poles … How are communication networks designed and operated? Communication Network Architecture z Network architecture: the plan that specifies how the network is built and operated z Architecture is driven by the network services z Overall communication process is complex z Network architecture partitions overall communication process into separate functional areas called layers Next we will trace evolution of three network architectures: telegraph, telephone, and computer networks Network Architecture Evolution 1.0E+14 ? 1.0E+12 1.0E+10 1.0E+08 1.0E+06 per second 1.0E+04 Information transfer 1.0E+02 1.0E+00 1850 1875 1900 1925 1950 1975 2000 Next Telegraph Telephone Internet, Optical Generation networks networks & Wireless networks Internet Network Architecture Evolution z Telegraph Networks z Message switching & digital transmission z Telephone Networks z Circuit Switching z Analog transmission → digital transmission z Mobile communications z Internet z Packet switching & computer applications z Next-Generation Internet z Multiservice packet switching network Chapter 1 Communication Networks and Services Telegraph Networks & Message Switching Telegraphs & Long-Distance Communications Approaches to long-distance communications z Courier: physical transport of the message z Messenger pigeons, pony express, FedEx z Telegraph: message is transmitted across a network using signals z Drums, beacons, mirrors, smoke, flags, semaphores… z Electricity, light z Telegraph delivers message much sooner Optical (Visual) Telegraph z Claude Chappe invented optical telegraph in the 1790’s z Semaphore mimicked a person with outstretched arms with flags in each hand z Different angle combinations of arms & hands generated hundreds of possible signals z Code for enciphering messages kept secret z Signal could propagate 800 km in 3 minutes! Message Switching z Network nodes were created where several optical telegraph Network Node North lines met (Paris and other sites) line z Store-and-Forward Operation: West z Messages arriving on each line line were decoded East z Next-hop in route determined by line destination address of a message South z Each message was carried by hand line to next line, and stored until operator became available for next transmission Electric Telegraph z William Sturgeon Electro-magnet (1825) z Electric current in a wire wrapped around a piece of iron generates a magnetic force z Joseph Henry (1830) z Current over 1 mile of wire to ring a bell z Samuel Morse (1835) z Pulses of current deflect electromagnet to generate dots & dashes z Experimental telegraph line over 40 miles (1840) z Signal propagates at the speed of light!!! z Approximately 2 x 108 meters/second in cable Digital Communications z Morse code converts text message into sequence of dots and dashes z Use transmission system designed to convey dots and dashes Morse Morse Morse Morse Code Code Code Code A ·— J ·——— S ··· 2 ··——— B —··· K —·— T — 3 ···—— C —·—· L ·—·· U ··— 4 ····— D —·· M —— V ···— 5 ····· E · N —· W ·—— 6 —···· F ··—· O ——— X —··— 7 ——··· G ——· P ·——· Y —·—— 8 ———·· H ···· Q ——·— Z ——·· 9 ————· I ·· R ·—· 1 ·———— 0 ————— Electric Telegraph Networks z Electric telegraph networks exploded z Message switching & Store-and-Forward operation z Key elements: Addressing, Routing, Forwarding z Optical telegraph networks disappeared Message Message Message Source Message Switches Destination Baudot Telegraph Multiplexer z Operator 25-30 words/minute z but a wire can carry much more z Baudot multiplexer: Combine 4 signals in 1 wire z Binary block code (ancestor of ASCII code) z A character represented by 5 bits z Time division multiplexing z Binary codes for characters are interleaved z Framing is required to recover characters from the binary sequence in the multiplexed signal z Keyboard converts characters to bits Baudot Telegraph Multiplexer Keyboard … Paper A Tape 3 A Baudot Printer 2 A Baudot 1 Multiplexer Demultiplexer …B Paper 2 B 1 Tape Printer C 1 C 2 … …A2D1C1B1A1 Paper Tape D 1 Printer D 2 3 D … 5 bits / character Paper Tape Printer Elements of Telegraph Network Architecture z Digital transmission z Text messages converted into symbols (dots/dashes, zeros/ones) z Transmission system designed to convey symbols z Multiplexing z Framing needed to recover text characters z Message Switching z Messages contain source & destination addresses z Store-and-Forward: Messages forwarded hop-by-hop across network z Routing according to destination address Chapter 1 Communication Networks and Services Telephone Networks and Circuit Switching Bell’s Telephone z Alexander Graham Bell (1875) working on harmonic telegraph to multiplex telegraph signals z Discovered voice signals can be transmitted directly z Microphone converts voice pressure variation (sound) into analogous electrical signal z Loudspeaker converts electrical signal back into sound z Telephone patent granted in 1876 z Bell Telephone Company founded in 1877 Signal for “ae” as in cat Microphone Loudspeaker analog electrical soundsignal sound Bell’s Sketch of Telephone Signaling z Signaling required to establish a call z Flashing light and ringing devices to alert the called party of incoming call z Called party information to operator to establish calls Signaling + voice signal transfer The N2 Problem z For N users to be fully connected directly z Requires N(N – 1)/2 connections z Requires too much space for cables z Inefficient & costly since connections not always on 1 N = 1000 N(N – 1)/2 = 499500 N 2 . 4 3 Telephone Pole Congestion Circuit Switching z Patchcord panel switch invented in 1877 z Operators connect users on demand z Establish circuit to allow electrical current to flow from inlet to outlet z Only N connections required to central office 1 N N –1 2 3 Manual Switching Strowger Switch z Human operators intelligent & flexible z But expensive and not always discreet z Strowger invented automated switch in 1888 z Each current pulse advances wiper by 1 position z User dialing controls connection setup z Decimal telephone numbering system z Hierarchical network structure simplifies routing z Area code, exchange (CO), station number 1st digit 2nd digit . 0 0 0 . 9 . 0 9 9 9 Strowger Switch Hierarchical Network Structure Toll CO = central office Tandem Tandem CO CO CO CO CO Telephone subscribers connected to local CO (central office) Tandem & Toll switches connect CO’s Three Phases of a Connection 1. Telephone Pick up phone network Dial tone. 2. Telephone network Connection set up Dial number 3. Telephone network Network selects route; 4.
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