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Outline Block 3: OSI Model and Digital Communications

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Outline Block 3: OSI Model and Digital Communications Engineering Electronic Department Technical University of Catalonia, UPC Campus Terrassa, SPAIN Lecturer: Dr. Luis Romeral 1 Outline Block 3: OSI Model and Digital Communications Subject 1.- OSI Model: definitions - General Concepts - Seven layers OSI - Model - Protocols and Service data Units - Repeaters, Bridges, Routers, and Gateways Subject 2.- Physical Layer - Functions of the Physical Layer - Interfaces RS 232 / RS 485 / Ethernet - Physical media: cables and optical fibers - Modulation - Coding and Synchronization Subject 3.- Aircraft buses - TCP/IP - ARINC - CSDB / FDDI 2 1 OSI Model - Location of the Physical Level All services directly called by the end user 7 Application (Mail, File Transfer,...) Application Presentation Definition and conversion of the protocols 6 data formats (e.g. ASN 1) Session Management of connections 5 (e.g. ISO 8326) End-to-end flow control and error recovery 4 Transport (z.B. TP4, TCP) Network Routing, possibly segmenting 3 (e.g. IP, X25) Transport protocols Link Error detection, Flow control and error recovery, 2 medium access (e.g. HDLC) Coding, Modulation, Electrical and 1 Physical mechanical coupling (e.g. V24) 3 Physical Layer Definition In the Open Systems Interconnection (OSI) communications model, the physical layer supports the electrical or mechanical interface to the physical medium. For example, this layer determines how to put a stream of bits from the upper (data link) layer on to the pins for a parallel printer interface, an optical fiber transmitter, or a radio carrier, i.e., the Physical Layer is responsible for bit-level transmission between network nodes. In copper networks, the Physical Layer is responsible for defining specifications for electrical signals. In fiber optic networks, the Physical Layer is responsible for defining the characteristics of light signals. The physical layer is usually a combination of software and hardware programming and may include electromechanical devices. It does not include the physical media as such. 4 2 Major Functions of Physical Layer The major functions and services performed by the physical layer are related to Bit-by-bit node- to-node delivery: • Providing a standardized interface to physical transmission media, including • Mechanical specification of electrical connectors and cables, for example maximum cable length • Electrical specification of transmission line signal level and impedance • Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc. • Specifications for IR over optical fiber or a wireless IR communication link • Modulation and Line coding • Bit synchronization in synchronous serial communication • Start-stop signalling and flow control in asynchronous serial communication • Carrier sense and collision detection utilized by some level 2 multiple access protocols • Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals 5 Subdivisions of the Physical Layer same for different media medium-independent signalling (e.g. modulation, codification) applies to one media medium-dependent signalling Physical (e.g. optical fibres) Layer electrical / optical applies to one media type specifications (e.g. 200µm optical fibres) mechanical defines the mechanical interface specifications (e.g. connector type and pin-out) The physical layer is also concerned with • Point-to-point, multipoint or point-to-multipoint line configuration • Physical network topology, for example bus, ring, mesh or star network • Serial or parallel communication • Simplex, half duplex or full duplex transmission mode • Autonegotiation 6 3 Concepts relevant to the physical layer Topology Ring, Bus, Point-to-point Mechanical Connector, Pin-out, Cable, Assembly Medium signals, transfer rate, levels Channels Half-duplex, full-duplex, broadcast Control Send, Receive, Collision Modulation Baseband, Carrier band, Broadband Coding/Decoding Binary, NRZ, Manchester,.. Synchronization Bit, Character, Frame Flow Control Handshake Interface Binary bit, Collision detection [multiple access] Signal quality supervision, redundancy control 7 Topologies Link (Point -To-Point) Full-duplex Sender/ Sender/ Examples: Receiver Receiver RS232 Half-duplex Sender/ Sender/ Examples: Receiver Receiver RS485 Master Star Point-to-Point 8 4 Topologies Bus (Half-Duplex, except when using Carrier Frequency over multiple bands) Terminator Examples: (resistor) Ethernet, Profibus Ring (Half-Duplex, except double ring) Examples: SERCOS, Interbus-S consists of point-to-point links Radio Free topology repeater 9 Topologies Ring: a ring consists only of point-to-point links Each node can interrupt the ring and introduce its own frames Classical ring Ring in floor wiring wiring cabinet The wiring amount is the same for a bus with hub or for a ring with wiring cabinet. Since rings use point-to-point links, they are well adapted to fibres 10 5 Electrical: Resistive (direct) coupling Unipolar, unbalanced Bipolar, unbalanced + Us Coax Ru + Us Zw Zw Rd -Us Open Collector Ut Terminator and Ut = 5 V (e.g.) Pull-up resistor (unbalanced) Rt Rt Bus line, characteristic impedance = Zw Wired-OR behaviour (“Low” wins over “High”) Out In Out In Out In device device device 11 Electrical: Balanced Transmission Differential transmitter and receiver: + good rejection of disturbances on the line and common-mode - double number of lines +Ub Differential amplifier Zw symmetrical line (Twisted Wire Pair) Rt (OpAmp) Shield 100 Ω UA UB (Data Ground) Used for twisted wire pairs (e.g. RS422, RS485) Common mode rejection: influence of a voltage which is applied simultaneously on both lines with respect to ground. The shield should not be used as a data ground (inductance of currents into conductors) 12 6 Electrical: Transformer Coupling Provides galvanic separation, freedom of retro-action and impedance matching but no DC-components may be transmitted. cost of the transformer depends on transmitted frequency band (not center frequency) Sender/Receiver Isolation transformer isolation resistors shield Twisted Wire Pair 13 Electrical: MIL 1553 as an example of transformer coupling Double-Transformer (long stub: 0.3 .. 6m) Direct Coupling Sender/Receiver (short stub: 0.3 m) long stub short stub Isolation transformer isolation resistors shield shield Twisted Wire Pair MIL 1553 is the standard field bus used in avionics since the years '60 – it is costly and obsolete 14 7 RS – 232 Interface • Originally developed for modem communication, now serial port in many instruments Unipolar Serial Transmission 25 pines (usually, only 9) Low speed, function of the cable length. Example, for 9600 bauds (bits(sec) , Voltage output:, d < 30m Current output:, d < 500 m Bidirectional transmission, Full - Duplex 15 RS-232 - Mechanical-Electrical Standard Topology: Telephone lines Data DTE DCE DCE DTE Data Terminal 2 2 Terminal Equipment Equipment Terminal Data Communication Modem Computer Cabling rules modem eliminator extension extension Tip: Do not use 2 Modem cables, computer terminal only Extension cable cable cables Electrical: transmitter receiver +12V +3V "0" Space On -12V -3V "1" Mark Off 16 8 RS – 232 Electronics Atention: DB9 DTE Connection ! The MAX232 is a family of line drivers/receivers intended for all EIA/TIA-232E and V.28/V.24 communications interfaces, particularly applications where ±12V is not available. 17 RS – 232 Electronics 18 9 Some other RS- 232 cabling…. Hardware contention control Wihtthreepinsconnection, RxD, TxD, GND, control of transmission have to be made by soft (Protocol) 19 RS – 422 Interface ¾ Bipolar Serial transmission (twisted pair, differential signaling) 37 pines (usually, only 5) Data rates in the Mbps (function of distance): at 19.2 KBPS, 1200 m at 2 MBPS, d = 60 m Bidireccional transmission Differential channel Full – duplex, double cable pair Point to Point Link 20 10 RS – 485 Interface ¾ Bipolar Serial transmission (twisted pair, differential signaling) Simplified RS – 422 interface Enables multi-drop Bidireccional Transmission, Semi-Duplex Additional Link Layer Software is needed (OSI 2 Level) Link Control 21 Differential signaling concept RS – 485 Bus Electrical noise has no effect Bit space Differential signaling 22 11 RS-485 as an example of balanced transmission The most widely used transmission for busses over balanced lines (not point-to-point) TxS RxS TxS RxS TxS RxS •• • B A 100Ω stub Terminator A tap 120Ω 120Ω Data-GND B Zw ≈ 120Ω, C' ≈ 100 pF/m segment length multiple transmitter Short-circuit limitation allowed needed Ishort < 250 mA 23 RS – 485 Electronics Typical RS – 485 Operating Circuit Typical Half-Duplex RS – 485 Network 24 12 Physical Media The Physical Layer defines items such as: connector types, cable types. Physical media defines how is the data transmitted in the real world. • Examples –Wires • Single ended • Differential – Fiber Optics • Quarz • Plastic – Wireless (RF) – Infrared 25 Transmission media: Electrical Wires ¾ To transmit electrical signal (voltage or current) Twisted pair Coax cable Problems: Atenuatons Reflections and radiating energy (connection, end of line) SOLUTION Line Amplifiers (Drives) Terminator resistors, Z0 = √L/C Z0 26 13 Electrical Wires comparison Zw = 50Ω ... 100Ω Coaxial cable core inflexible, costly, dielectric low losses shield screen 10 MHz..100 MHz Shielded twisted Zw = 85Ω..120Ω Wire (Twinax) flexible, cheap, Shield twisting compensates disturbances medium attenuation ~1 MHz..12 MHz very
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