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Data Communication Essentials

Data Communication Essentials. Excerpted from Joe Conron’s data communications course. A Communications Model. Source generates data to be transmitted Transmitter Converts data into transmittable signals Transmission System Carries data Receiver Converts received signal into data

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Data Communication Essentials

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  1. Data Communication Essentials Excerpted from Joe Conron’s data communications course

  2. A Communications Model • Source • generates data to be transmitted • Transmitter • Converts data into transmittable signals • Transmission System • Carries data • Receiver • Converts received signal into data • Destination • Takes incoming data

  3. Simplified Communications Model - Diagram

  4. Key Communications Tasks • Transmission System Utilization • Interfacing • Signal Generation • Synchronization • Error detection and correction • Addressing and routing • Recovery • Message formatting • Security • Network Management

  5. Networking • Point to point communication not usually practical • Devices are too far apart • Large set of devices would need impractical number of connections • Solution is a communications network

  6. Simplified Network Model

  7. mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” The Network Core

  8. End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required Network Core: Circuit Switching

  9. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM

  10. each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed Network Core: Packet Switching resource contention: • aggregate resource demand can exceed amount available • congestion: packets queue, wait for link use • store and forward: packets move one hop at a time • Node receives complete packet before forwarding

  11. Sequence of A & B packets does not have fixed pattern  statistical multiplexing. In TDM each host gets same slot in revolving TDM frame. D E Packet Switching: Statistical Multiplexing 10 Mb/s Ethernet C A statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link

  12. Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 6) Is packet switching a “slam dunk winner?” Packet switching versus circuit switching

  13. Local Area Networks • Smaller scope • Building or small campus • Usually owned by same organization as attached devices • Data rates much higher • Usually broadcast systems

  14. Protocols • Used for communications between entities in a system • Must speak the same language • Entities • User applications • e-mail facilities • terminals • Systems • Computer • Terminal • Remote sensor

  15. Key Elements of a Protocol • Syntax • Data formats • Signal levels • Semantics • Control information • Error handling • Timing • Speed matching • Sequencing

  16. human protocols: “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols What’s a protocol? protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

  17. a human protocol and a computer network protocol: TCP connection reply. Get http://gaia.cs.umass.edu/index.htm Got the time? 2:00 <file> time What’s a protocol? Hi TCP connection req. Hi

  18. In Summary, a protocol is .... • An agreement about communication between two or more entities • It specifies – Format of messages – Meaning of messages – Rules for exchange – Procedures for handling problems

  19. Protocol Specification • As designers, we can specify a protocol using • Event-Time Diagrams • Transition Diagrams • We can implement a protocol with a Finite State Machine (FSM) • Internet Protocols are formalized by RFCs which are administered by IETF • You can find any RFChere

  20. Event -Time Diagrams • Define causal ordering • Define indication/request/response actions

  21. Transition Diagram • Illustrates • States • Input (the Event that causes transition) • Transitions (to new states)

  22. Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Protocol “Layers”

  23. Why layering? Dealing with complex systems: • explicit structure allows identification, relationship of complex system’s pieces • layered reference model for discussion • modularization eases maintenance, updating of system • change of implementation of layer’s service transparent to rest of system • e.g., change in gate procedure doesn’t affect rest of system • Can layering sometimes be undesirable?

  24. application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” application transport network link physical Internet protocol stack

  25. network link physical link physical M M M Ht M Hn Hn Hn Hn Ht Ht Ht Ht M M M M Hn Ht Ht Hl Hl Hl Hn Hn Hn Ht Ht Ht M M M source Encapsulation message application transport network link physical segment datagram frame switch destination application transport network link physical router

  26. OSI • Open Systems Interconnection • Developed by the International Organization for Standardization (ISO) • Seven layers • A theoretical system delivered too late! • TCP/IP is the de facto standard

  27. packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers How do loss and delay occur? A B

  28. 1. nodal processing: check bit errors determine output link transmission A propagation B nodal processing queueing Four sources of packet delay • 2. queueing • time waiting at output link for transmission • depends on congestion level of router

  29. 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s transmission A propagation B nodal processing queueing Delay in packet-switched networks Note: s and R are very different quantities!

  30. Data Communication and Networks Lecture 2 Data Transmission and Encoding Concepts September 14, 2006

  31. Simplified Data Communications Model

  32. S(t) = A sin(2ft + Φ)

  33. Terminology (1) • Transmitter • Receiver • Medium • Guided medium • e.g. twisted pair, optical fiber • Unguided medium • e.g. air, water, vacuum

  34. Terminology (2) • Direct link • No intermediate devices • Point-to-point • Direct link • Only 2 devices share link • Multi-point • More than two devices share the link

  35. Terminology (3) • Simplex • One direction • e.g. Television • Half duplex • Either direction, but only one way at a time • e.g. police radio • Full duplex • Both directions at the same time • e.g. telephone

  36. Analog and Digital Data Transmission • Data • Entities that convey meaning • Signals • Electric or electromagnetic representations of data • Transmission • Communication of data by propagation and processing of signals

  37. Data • Analog • Continuous values within some interval • e.g. sound, video • Digital • Discrete values • e.g. text, integers

  38. Signals • Means by which data are propagated • Analog • Continuously variable • Various media • wire, fiber optic, space • Speech bandwidth 100Hz to 7kHz • Telephone bandwidth 300Hz to 3400Hz • Video bandwidth 4MHz • Digital • Use two DC components

  39. Data and Signals • Usually use digital signals for digital data and analog signals for analog data • Can use analog signal to carry digital data • Modem • Can use digital signal to carry analog data • Compact Disc audio

  40. Analog Transmission • Analog signal transmitted without regard to content • May be analog or digital data • Attenuated over distance • Use amplifiers to boost signal • Also amplifies noise

  41. Digital Transmission • Concerned with content • Integrity endangered by noise, attenuation etc. • Repeaters used • Repeater receives signal • Extracts bit pattern • Retransmits • Attenuation is overcome • Noise is not amplified

  42. Advantages & Disadvantages of Digital • Cheaper • Less susceptible to noise • Greater attenuation • Pulses become rounded and smaller • Leads to loss of information

  43. Attenuation of Digital Signals

  44. Interpreting Signals • Need to know • Timing of bits - when they start and end • Signal levels • Factors affecting successful interpreting of signals • Signal to noise ratio • Data rate • Bandwidth

  45. Encoding Schemes • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3

  46. Nonreturn to Zero-Level (NRZ-L) • Two different voltages for 0 and 1 bits • Voltage constant during bit interval • no transition I.e. no return to zero voltage • e.g. Absence of voltage for zero, constant positive voltage for one • More often, negative voltage for one value and positive for the other • This is NRZ-L

  47. Nonreturn to Zero Inverted • Nonreturn to zero inverted on ones • Constant voltage pulse for duration of bit • Data encoded as presence or absence of signal transition at beginning of bit time • Transition (low to high or high to low) denotes a binary 1 • No transition denotes binary 0 • An example of differential encoding

  48. NRZ

  49. Differential Encoding • Data represented by changes rather than levels • More reliable detection of transition rather than level • However: in complex transmission layouts it is easy to lose sense of polarity

  50. Biphase • Manchester • Transition in middle of each bit period • Transition serves as clock and data • Low to high represents one • High to low represents zero • Used by IEEE 802.3 • Differential Manchester • Midbit transition is clocking only • Transition at start of a bit period represents zero • No transition at start of a bit period represents one • Note: this is a differential encoding scheme • Used by IEEE 802.5

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