Communication Roland Institute of Computer Engineering Vienna University of Technology 29. October 2010
Overview 1. Distributed Motivation 2. OSI Communication Model 3. Topologies 4. Physical Layer 5. Bit Encoding
Definition and Motivation Definition Motivation Part I Distributed Motivation
Definition of a Distributed System Definition and Motivation Definition Motivation A distributed system is a collection of individual computing devices that can communicate with each other - Attiya & Welch 98
Motivation for Distribution Definition and Motivation Definition Motivation Compute a difficult problem together System parts are situated far apart from each other (e.g., automated factory/building/vehicle, mine) Fault Tolerance (e.g., redundant nodes and channels) Modular Design (e.g., PCI bus extensions)
OSI Model Part II OSI Communication Model
OSI Model ISO/OSI Communication Model ISO: International Standard Organization OSI: Open System Interconnection
OSI Model ISO/OSI Communication Model (2) Application: Application Data Itself Presentation Layer: Data encryption, data compression, code translation Session Layer: Managing dialog between en-user application processes Transport Layer: Reliable message transport, preserving message order Network Layer: Switching and routing, creating logical paths, error handling, packet sequencing Data Link Layer: Data packet encoding/decoding, error handling in physical layer, flow control, synchronization (LLC and MAC) Physical Layer: Bit stream, electrical pulses, radio signal
ISO/OSI Communication Model (3) OSI Model Full ISO/OSI model makes sense, if following assumptions are made: Point-to-point communication Event-triggered communication No real-time constraints Full ISO/OSI model does not fit well for embedded systems Distributed embedded systems usually implement Application layer Data Link layer Physical layer
Topologies Bus Star Ring Mesh Part III Topologies
Bus Topology Topologies Bus Star Ring Mesh Each machine connected to a single cable Terminator at each end of the bus required to prevent the signal from bouncing back and forth Inexpensive compared to other topologies When only one cable is used it can be single point of failure
Star Topology Topologies Bus Star Ring Mesh Each machine connected to a central hub via a point-to-point connection Hub acts as a single booster or repeater allowing the signal to travel greater distances Easiest topology to design and implement Adding machines is simple Hub is a single point of failure
Ring Topology Topologies Bus Star Ring Mesh Each computer connected in a ring The signal passes through each machine Typically utilizes a token scheme to control access (i.e., only one machine can transmit at once) The machines act as signal boosters or repeaters which strengthens the signal One machine will cause the entire network to fail
Mesh Topology Topologies Bus Star Ring Mesh Every node connected to every other node (fully connected) No single point of failure. Nodes can route information over other nodes Known latency (if mesh is functional) High wiring costs ( n(n 1) 2 )
Physical Layer Theory Interfaces Part IV Physical Layer
Physical Layer Physical Layer Theory Interfaces Specifies the transmission codes such as coding of the bit patterns in the physical channel transmission speed physical shape of the bit cells Sometimes the protocol is dependent on the physical layer (e.g., CAN)
Baud Rate Physical Layer Theory Interfaces Symbols per second Symbol rate. Number of distinct symbol changes Not the number of bits Repesents number of bits according to encoding Symbol duration time: T s = 1 f s. f s : symbol rate. Example: Baud rate of 1 kbd = 1000 Bd. 1000 symbols per second (e.g., 1000 tones per second in a modem). Symbol duration: 1/1000s = 1 millisecond
Bandwidth/Bit rate Physical Layer Theory Interfaces Digital systems: Bandwidth indicates the number of bits that can traverse a channel in a unit time. Ethernet: 100 Mbit/sec CAN: 1 Mbit/sec Bandwidth is determined by the physical characeristics of the channel EMI constaints MAC
Propagation Delay Physical Layer Theory Interfaces The time interval it takes for a bit to travel from one end of a channel to the other end Depends on: Channel length Transmission speed of the wave (e.g., in cable 2/3 speed of light 0.5µs for 100m cable length)
Physical Layer Theory Interfaces RS 232 Interface Point-to-Point communication Signals with respect to GND 30-60m maximum cable length Specified up to 115. Kbit/s Standard PC interface (e.g., COMx, ttysx) Sub D9 or Sub D25 Inexpensive
RS 232 Interface (2) Physical Layer Theory Interfaces
Physical Layer Theory Interfaces RS 422/485 Interface Only specifies electrical characteristics (e.g., not the pin assignment) Differential signals (twisted pair from sender to receiver and vice versa) Up to 1 km cable length Up to 10 Mbit/s (25 Mbit/s chips available) RS 422 RS 485 Point-to-Point communication Multipoint communication 1200 meters at 200kbps or 50 meters at 10Mbps Up to 32 devices e.g., computer (SCSI-2, SCSI-3), aircraft cabine
Bit Encoding NRZ PWM Manchester MFM Tristate Part V Bit Encoding
Non Return to Zero (NRZ) Bit Encoding NRZ PWM Manchester MFM Tristate 1 is high, 0 is low No self-synchronizing code (i.e., no signal transitions if data contains only 1s or 0s) Therefore bit stuffing is used (e.g., CAN)
(Always) Return to Zero (PWM) Bit Encoding NRZ PWM Manchester MFM Tristate Pulse width modulation 0 is short pulse, 1 is long pulse Synchronization point at every bit cell Range can be extended to multiple or analogue values
Manchester Encoding Bit Encoding NRZ PWM Manchester MFM Tristate Transition in the middle of each bit cell 0 is high low, 1 is low high Synchronization point at every bit cell Required Baud rate is at two times higher than bit rate (Feature size 1/2)
Modified Frequency Modulation Bit Encoding NRZ PWM Manchester MFM Tristate data and clock points data points: 0 no signal change, 1 signal change clock points: change if more than two 0s in a row Enables continuous re-synchronization Feature size of 1
Tristate Encoding Bit Encoding NRZ PWM Manchester MFM Tristate Three possible states (low, zero, high) 1 after 0 is high, 0 after 1 is low 1 after 1 toggles between high and zero 0 after 0 toggles between low and zero Continuous re-synchronization, feature size of 1
Thank you Bit Encoding NRZ PWM Manchester MFM Tristate Thank you for your attention!