ECE 4371, Fall, 2012 Introduction to Telecommunication Engineering/Telecommunication LaboratoryMultiaccess vs. Point-to-pointInternetworkComparisonISO/OSI Reference ModelSlide 6ISO/OSI Reference Model: Physical LayerModulationISO/OSI Reference Model: Data LinkAutomatic Repeat-reQuest (ARQ)Time-Division MultiplexingISO/OSI Reference Model: NetworkLondon Metro MapISO/OSI Reference Model: TransportParity CheckISO/OSI Reference Model: SessionISO/OSI Reference Model: PresetationSubstitution MethodSlide 19TCP/IP ArchitectureSlide 21Layering & HeadersProtocols and networks in the TCP/IP modelSummaryIEEE 802 StandardsMultiple AccessDuplexingSlide 28Scheduling and Spectrum AllocationRandom AccessPure AlohaContention for AlohaThroughput of AlohaSlotted AlohaReservation ProtocolsCSMA: Carrier Sense Multiple AccessCSMA VariationsMACA – Medium Access with Collision AvoidanceHidden Terminal ProblemExposed Terminal ProblemMACA Solution ConceptMACA ProtocolSolution for Hidden Terminal ProblemSolution for Exposed Terminal ProblemCSMA/CA Collision AvoidanceECE 4371, Fall, 2012Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu HanDepartment of Electrical and Computer EngineeringClass 22Dec. 4th, 2013Multiaccess vs. Point-to-pointMultiaccess vs. Point-to-pointMultiaccess means shared medium.–many end-systems share the same physical communication resources (wire, frequency, ...)–There must be some arbitration mechanism.Point-to-point–only 2 systems involved–no doubt about where data came from !InternetworkInternetworkConnection of 2 or more distinct (possibly dissimilar) networks.Requires some kind of network device to facilitate the connection.Net A Net BComparisonComparisonSpeed and RangeISO/OSI Reference ModelISO/OSI Reference ModelTo address the growing tangle of incompatible proprietary network protocols, in 1984 the ISO formed a committee to devise a unified protocol standard.The result of this effort is the ISO Open Systems Interconnect Reference Model (ISO/OSI RM).The ISO’s work is called a reference model because virtually no commercial system uses all of the features precisely as specified in the model. The ISO/OSI model does, however, lend itself to understanding the concept of a unified communications architecture.ISO/OSI Reference ModelISO/OSI Reference ModelThe OSI RM contains seven protocol layers, starting with physical media interconnections at Layer 1, through applications at Layer 7.OSI model defines only the functions of each of the seven layers and the interfaces between them. Implementation details are not part of the model.ISO/OSI Reference Model: Physical LayerISO/OSI Reference Model: Physical LayerThe Physical layer receives a stream of bits from the Data Link layer above it, encodes them and places them on the communications medium.The Physical layer conveys transmission frames, called Physical Protocol Data Units, or Physical PDUs. Each physical PDU carries an address and has delimiter signal patterns that surround the payload, or contents, of the PDU.Issues:–mechanical and electrical interfaces–time per bit–distancesModulationModulationProcess of varying a carrier signal in order to use that signal to convey information –Carrier signal can transmit far away, but information cannot–Modem: amplitude, phase, and frequency–Analog: AM, amplitude, FM, frequency, Vestigial sideband modulation, TV–Digital: mapping digital information to different constellation: Frequency-shift key (FSK)ISO/OSI Reference Model: Data LinkISO/OSI Reference Model: Data LinkThe Data Link layer negotiates frame sizes and the speed at which they are sent with the Data Link layer at the other end. –The timing of frame transmission is called flow control.Data Link layers at both ends acknowledge packets as they are exchanged. The sender retransmits the packet if no acknowledgement is received within a given time interval. ARQMedium Access Control - needed by mutiaccess networks.Issues:–framing (dividing data into chunks)header & trailer bits–addressing01100010011 10110000001Automatic Repeat-reQuest (ARQ)Automatic Repeat-reQuest (ARQ)Alice and Bob on their cell phones–Both Alice and Bob are talkingWhat if Alice couldn’t understand Bob?–Bob asks Alice to repeat what she saidWhat if Bob hasn’t heard Alice for a while?–Is Alice just being quiet?–Or, have Bob and Alice lost reception?–How long should Bob just keep on talking?–Maybe Alice should periodically say “uh huh”–… or Bob should ask “Can you hear me now?” Time-Division MultiplexingTime-Division MultiplexingFigure Block diagram of TDM system.ISO/OSI Reference Model: NetworkISO/OSI Reference Model: NetworkAt the originating computers, the Network layer adds addressing information to the Transport layer PDUs. The Network layer establishes the route and ensures that the PDU size is compatible with all of the equipment between the source and the destination.Its most important job is in moving PDUs across intermediate nodes.Issues:–packet headers–virtual circuitsLondon Metro MapLondon Metro MapISO/OSI Reference Model: TransportISO/OSI Reference Model: Transportthe OSI Transport layer provides end-to-end acknowledgement and error correction through its handshaking with the Transport layer at the other end of the conversation. –The Transport layer is the lowest layer of the OSI model at which there is any awareness of the network or its protocols. Transport layer assures the Session layer that there are no network-induced errors in the PDU.Issues:–headers–error detection: CRC–reliable communicationParity CheckParity CheckAdd one bit so that xor of all bit is zero–Send, correction, miss–Add vertically or horizontallyApplications: ASCII, Serial port transmissionISO/OSI Reference Model: SessionISO/OSI Reference Model: SessionThe Session layer arbitrates the dialogue between two communicating nodes, opening and closing that dialogue as necessary. It controls the direction and mode (half -duplex or full-duplex). It also supplies recovery checkpoints during file transfers. Checkpoints are issued each time a block of data is acknowledged as being received in good condition.Responsibilities:–establishes, manages, and terminates sessions between applications.–service location lookupISO/OSI Reference Model: PresetationISO/OSI Reference