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Networking H15 Analog and Digital Data Data, Signal, Transmission Encoding and Decoding ( Amplitude Shift, Frequency Shift, PCM etc.) Transmission Simplex, Half - Duplex, Duplex Serial and Parallel Transmission Synchronous + Asynchronous Transmission impairments Attenuation and Attenuation Distortion Networking H15 Media (twisted pair etc.) Switching Techniques Why do we need to switch? Switching Concepts(Crossbar, Multi-Stage) Message, Packet, Circuit Switching Multiplexing Frequency Division, Time Division, Statistical Time Division Networking H15 LANS and WANS Network Topologies Bus, Star, Ring Media Access Control Techniques 802.3, 802.4, 802.5 Protocols Why would you want Protocols in the first place? OSI Model Interconnection Repeaters, Bridges, Routers, Hubs, Gateways Networking H15 TCP/IP Protocol and Addressing +WWW Frame Relay, Cell Relay, FDDI etc X.25 13 Step Approach to Network Design Wireless Networks Distributed Computing Client Server Technologies in Client Server Middleware Groupware Communication considerations 1) AHHH!! What do those electrical signals mean? 2) How can I send a bit, what signal do I use for 0 and which for 1? 3) How do devices make use of the wire? 4) How do I derive meaningful information from all of these bits 5) How are transmission errors discovered and dealt with? 6) How do packets get from one system to another? Communication Considerations 1) How do I send large amounts of data and how do I ensure that I receive all of my data? 2) How do machines keep track of who there are talking to? 3) What language is this, how can I the computer understand different formats? 4) How does a user gain access to the network? 5) How do programmers write programs to use the network? Our solution the OSI model Application Presentation Session Transport Network Data link Physical What is a Protocol? Allows entities (i.e. application programs) from different systems to communicate Shared conventions for communicating information are called protocols Includes syntax, semantics, and timing Why Use Protocol Architecture? Data communications requires complex procedures Sender identifies data path/receiver Systems negotiate preparedness Applications negotiate preparedness Translation of file formats For all tasks to occur, high level of cooperation is required Modular Approach Breaks complex tasks into subtasks Each module handles specific subset of tasks Communication occurs between different modules on the same system between similar modules on different systems OSI Lower Layers Physical Data Link Network OSI Physical Layer Concerned with transmission of unstructured bit stream over physical medium Deals with accessing the physical medium Mechanical characteristics Electrical characteristics Functional characteristics Procedural characteristics OSI Data Link Layer Responsible for error-free, reliable transmission of data Flow control, error correction OSI Network Layer Responsible for routing of messages through network Concerned with type of switching used (circuit v. packet) Handles routing between networks, as well as through packet-switching networks OSI Upper Layers Transport Session Presentation Application OSI Transport Layer Isolates messages from lower and upper layers Breaks down message size Monitors quality of communications channel Selects most efficient communication service necessary for a given transmission OSI Session Layer Establishes logical connections between systems Manages log-ons, password exchange, logoffs Terminates connection at end of session OSI Presentation Layer Provides format and code conversion services Examples File conversion from ASCII to EBDIC Invoking character sequences to generate bold, italics, etc on a printer OSI Application Layer Provides access to network for end-user User’s capabilities are determined by what items are available on this layer OSI in Action: Outgoing File Transfer Program issues command to Application Layer Application passes it to Presentation, which may reformat, passes to Session Session requests a connection, passes to Transport Transport breaks file into chunks, passes to Network Network selects the data’s route, passes to Data Link Data Link adds errorchecking info, passes to Physical Physical transmits data, which includes information added by each layer OSI in Action: Incoming File Transfer Physical receives bits, passes to Presentation may reformat, Data Link Data Link checks for errors, passes to Network Network verifies routing, passes to Transport Transport reassembles data, passes to Session Session determines if transfer is complete, may end session, passes to Presentation perform conversions, pass to Application layer Application presents results to user (e.g. updates FTP program display) Data Communication Terms Data - entities that convey meaning, or information Signals - electric or electromagnetic representations of data Transmission - communication of data by the propagation and processing of signals Examples of Analog and Digital Data Analog Video Audio Digital Text Integers Analog Signals A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency Examples of media: Copper wire media (twisted pair and coaxial cable) Fiber optic cable Atmosphere or space propagation Analog signals can propagate analog and digital data Digital Signals A sequence of voltage pulses that may be transmitted over a copper wire medium Generally cheaper than analog signaling Less susceptible to noise interference Suffer more from attenuation Digital signals can propagate analog and digital data Analog Signaling Digital Signaling Reasons for Choosing Data and Signal Combinations Digital data, digital signal Equipment for encoding is less expensive than digitalto-analog equipment Analog data, digital signal Conversion permits use of modern digital transmission and switching equipment Digital data, analog signal Some transmission media will only propagate analog signals Examples include optical fiber and satellite Analog data, analog signal Analog data easily converted to analog signal Analog Transmission Transmit analog signals without regard to content Attenuation limits length of transmission link Cascaded amplifiers boost signal’s energy for longer distances but cause distortion Analog data can tolerate distortion Introduces errors in digital data Digital Transmission Concerned with the content of the signal Attenuation endangers integrity of data Digital Signal Repeaters achieve greater distance Repeaters recover the signal and retransmit Analog signal carrying digital data Retransmission device recovers the digital data from analog signal Generates new, clean analog signal About Channel Capacity Impairments, such as noise, limit data rate that can be achieved Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions Impairments and Capacity Impairments exist in all forms of data transmission Analog signal impairments result in random modifications that impair signal quality Digital signal impairments result in bit errors (1s and 0s transposed) Transmission Impairments: Guided Media Attenuation loss of signal strength over distance Attenuation Distortion different losses at different frequencies Delay Distortion different speeds for different frequencies Noise distortions of signal caused by interference Transmission Impairments: Unguided (Wireless) Media Free-Space Loss Signals disperse with distance Atmospheric Absorption Water vapor and oxygen contribute to signal loss Multipath Obstacles reflect signal creating multiple copies Refraction Noise Types of Noise Thermal (aka “white noise”) Uniformly distributed, cannot be eliminated Intermodulation When different frequencies collide (creating “harmonics”) Crosstalk Overlap of signals Impulse noise Irregular spikes, less predictable Why Use Analog Transmission? Already in place Significantly less expensive Lower attentuation rates Fully sufficient for transmission of voice signals Analog Encoding of Digital Data Data encoding and decoding technique to represent data using the properties of analog waves Modulation: the conversion of digital signals to analog form Demodulation: the conversion of analog data signals back to digital form Modem An acronym for modulator-demodulator Uses a constant-frequency signal known as a carrier signal Converts a series of binary voltage pulses into an analog signal by modulating the carrier signal The receiving modem translates the analog signal back into digital data Methods of Modulation Amplitude modulation (AM) or amplitude shift keying (ASK) Frequency modulation (FM) or frequency shift keying (FSK) Phase modulation or phase shift keying (PSK) Amplitude Shift Keying (ASK) In radio transmission, known as amplitude modulation (AM) The amplitude (or height) of the sine wave varies to transmit the ones and zeros Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude ASK Illustration 1 0 0 1 Frequency Shift Keying (FSK) In radio transmission, known as frequency modulation (FM) Frequency of the carrier wave varies in accordance with the signal to be sent Signal transmitted at constant amplitude More resistant to noise than ASK Less attractive because it requires more analog bandwidth than ASK FSK Illustration 1 1 0 1 Phase Shift Keying (PSK) Also known as phase modulation (PM) Frequency and amplitude of the carrier signal are kept constant The carrier signal is shifted in phase according to the input data stream Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying) PSK Illustration 0 0 1 1 Differential Phase Shift Keying (DPSK) 0 1 1 0 Analog Channel Capacity: BPS vs. Baud Baud=# of signal changes per second BPS=bits per second In early modems only, baud=BPS Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase Increases information-carrying capacity of a channel without increasing bandwidth Increased combinations also leads to increased likelihood of errors
