1. Section 2 The Physical Layer CSIS 479R Fall 1999 George D. Hickman, CNI, CNE

2. Objectives Identify why rules are needed in computer networks Identify why rules are needed in computer networks Identify the seven layers of the OSI model and how the layers interact Identify the seven layers of the OSI model and how the layers interact Identify the relationship between the OSI reference model and computer network protocols Identify the relationship between the OSI reference model and computer network protocols Identify the basic purpose of the OSI Physical layer Identify the basic purpose of the OSI Physical layer List the characteristics of the two common connection types used in computer networks List the characteristics of the two common connection types used in computer networks Describe the five common physical topologies used in computer networks Describe the five common physical topologies used in computer networks

3. Objectives (con’t) Identify the difference between digital and analog signals Identify the difference between digital and analog signals List the ways a digital signal can be manipulated to represent data List the ways a digital signal can be manipulated to represent data List the ways an analog signal can be manipulated to represent data List the ways an analog signal can be manipulated to represent data Identify the ways that bit synchronization can be achieved Identify the ways that bit synchronization can be achieved Identify the two bandwidth use methods and explain how multiplexing can be used in each. Identify the two bandwidth use methods and explain how multiplexing can be used in each.

4. Why Rules are needed in Computer Networks How is the transmission media physically set up? How is the transmission media physically set up? How are bits transmitted on the media? How are bits transmitted on the media? How do network entities know when to transmit? How do network entities know when to transmit? How do network entities know how much data can be transmitted? How do network entities know how much data can be transmitted? How is a message passed to intended recipients? How do network entities contact one another? How do network entities, with disparate languages, communicate? How are network entities guaranteed that a message has been received correctly?

5. Protocols The communication rules developers use to create network products. The communication rules developers use to create network products. Different needs/vendors developed different rules/protocols. Different needs/vendors developed different rules/protocols. Standards and models were developed to generalize the concepts and provide a common ground for interoperability Standards and models were developed to generalize the concepts and provide a common ground for interoperability

6. OSI Reference Model Application (Layer 7) Application (Layer 7) Presentation( Layer 6) Presentation( Layer 6) Session (Layer 5) Session (Layer 5) Transport (Layer 4) Transport (Layer 4) Network (Layer 3) Network (Layer 3) Data Link (Layer 2) Data Link (Layer 2) Physical (Layer 1) Physical (Layer 1)

7. Memory Aids Order 1 - 7: Order 1 - 7: Please Do Not Throw Sausage Pizza Away Please Do Not Throw Sausage Pizza Away Please Do Not Take Sales Persons’ Advice Please Do Not Take Sales Persons’ Advice Order 7 – 1: Order 7 – 1: All People Seem To Need Data Processing All People Seem To Need Data Processing

8. Layer Interaction between stacks Each layer adds control information (header) containing that layer’s requests or information. Each layer adds control information (header) containing that layer’s requests or information. The corresponding layer on the other machine (same or different stack) processes that layer’s requests The corresponding layer on the other machine (same or different stack) processes that layer’s requests Messages move down the originating stack, across the physical media and up the destination stack, with each layer stripping the header for its layer. Messages move down the originating stack, across the physical media and up the destination stack, with each layer stripping the header for its layer. See Figure 2-2 on page 2-7 See Figure 2-2 on page 2-7

9. Names of Data packages by Layer Physical Layer: bits Physical Layer: bits Data Link Layer: frames Data Link Layer: frames Transport Layer: datagrams and segments Transport Layer: datagrams and segments Application Layer: messages Application Layer: messages All Layers: packets All Layers: packets See Figure 2-3 on page 2-8 See Figure 2-3 on page 2-8

10. Relationship between OSI Model and Computer Network Protocols The Model does NOT cause communication to occur. Protocols based on the Model create processes for communication. The Model does NOT cause communication to occur. Protocols based on the Model create processes for communication. The Model is used to categorize networking technologies and their protocol implementations. The Model is used to categorize networking technologies and their protocol implementations. Book example of a Bookshelf for sorting books. Book example of a Bookshelf for sorting books.

11. Purpose of the Physical Layer The Physical Layer Protocol defines The Physical Layer Protocol defines  Physical network structures  Mechanical and electrical specifications of media  Bit transmission encoding and timing rules Does not describe the media Does not describe the media Various implementations of Physical Layer protocols are “transmission-media-specific” Various implementations of Physical Layer protocols are “transmission-media-specific”

12. Network Connectivity Devices Concentrators, hubs, repeaters Concentrators, hubs, repeaters  (signal regeneration) Transmission media connectors Transmission media connectors  (RJ45, etc.) Modems and Codecs Modems and Codecs  (digital/analog conversions)

13. Processes and Methods Connection types Connection types  Point to point  Multipoint Physical Topology Physical Topology  Bus  Ring  Star  Mesh  Cellular

14. Processes and Methods (con’t) Digital Signaling Digital Signaling  Current State  State Transition Analog Signaling Analog Signaling  Current State  State Transition Bit Synchronization Bit Synchronization  Asynchronous  Synchronous

15. Processes and Methods (con’t) Bandwidth Use Bandwidth Use  Broadband  Baseband Multiplexing Multiplexing  Frequency-Division Multiplex (FDM)  Time-Division Multiplexing (TDM)  Statistical Time-Division Multiplex (StatTDM)

16. Connection Types Point to Point Point to Point  A direct link between two devices  Computer to printer connection  Full bandwidth available Multipoint Multipoint  Three or more devices linked  Cable TV drop  Bandwidth is shared—more devices, less data capacity

17. Bus Topology Co-axe Ethernet media Co-axe Ethernet media Drop cables attached to T-connectors Drop cables attached to T-connectors Both ends of bus must be terminated Both ends of bus must be terminated One end must be grounded One end must be grounded Most bus topologies transmit both directions Most bus topologies transmit both directions

18. Bus Topology Benefits Benefits  Established standards  Easy installation  Less media (cable) needed than other medias Considerations Considerations  Difficult to reconfigure  Difficult to troubleshoot  All stations are affected by media failure

19. Ring Topology Closed loop of point-to-point links Closed loop of point-to-point links Benefits Benefits  Cable faults easy to identify  Dual loop rings have high fault tolerance Considerations Considerations  More difficult than Bus to install and reconfigure  Media failure causes complete network failure (single loop networks)

20. Star Topology Central device or hub used with drop cables Central device or hub used with drop cables Can be nested in hierarchical stars Can be nested in hierarchical stars Benefits Benefits  Easy to reconfigure  Easy to troubleshoot  Media faults are automatically isolated to failed segment Considerations Considerations  More Cable required  Moderately difficult to install

21. Mesh Topology Point-to-point connections between every two devices on the network Point-to-point connections between every two devices on the network Hybrid mesh more common due to realistic cable needs Hybrid mesh more common due to realistic cable needs A single media failure in a true mesh has no loss of communications A single media failure in a true mesh has no loss of communications

22. Mesh Topology (con’t) True Mesh True Mesh  Benefits  Easy to troubleshoot and isolate faults  Considerations  Difficult to install and configure, increases exponentially with number of devices Hybrid Mesh Hybrid Mesh  Benefits  Extremely Fault Tolerant using rerouting of signals  Difficult to install and configure, increases with more devices  Considerations  Can be difficult to troubleshoot and isolate faults

23. Cellular Topology Wireless overlapping “cells” route messages as needed as movement between cells occur. Wireless overlapping “cells” route messages as needed as movement between cells occur. Benefits Benefits  Ease of Installation  No media to reconfigure when adding new devices  Simple fault isolation and troubleshooting Considerations Considerations  All devices attached to a hub are affected by its failure  Property easement or antenna placement potential problem

24. Signaling Types Digital signaling Digital signaling  Finite states  On or off  Time on Digital clock Analog signaling Analog signaling  Constant change  Infinite number of states  Sweep second hand on watch—never stops

25. Signal Encoding--Digital Current State Current State  On or Off State Transition State Transition  Does not matter if on or off  Changing from on to off signals a change

26. Signal Encoding—Analog Analog waves are measured as: Analog waves are measured as:  Amplitude (height)  Frequency (cycles per time unit)  Phase (relative state at time index) Page 2-38 Amplitude-Shift keying (ASK) Amplitude-Shift keying (ASK)  Change in value like voltage Frequency-Shift keying (FSK) Frequency-Shift keying (FSK)  Change in timing of signals Phase-Shift keying (PSK) Phase-Shift keying (PSK)  Change in phase of signal see page 2-40

27. Bit Synchronization Clocks used to time the sampling of bits Clocks used to time the sampling of bits Asynchronous Asynchronous  Start bit is sent to start clocks  Short transmission until stop bit Synchronous Synchronous  Guaranteed State Change (clocks continually adjust)  Separate Clock Signals (separate channel for clock)  Oversampling (increased samples increase accuracy)

28. Bandwidth Schemes Baseband Baseband  Use entire capacity for a single signal  More reliable Broadband Broadband  Multiple “channels” available  More signals possible

29. Multiplexing Allows multiple devices to communicate simultaneously over a single medium Allows multiple devices to communicate simultaneously over a single medium Can transmit several low-traffic channels on a high bandwidth medium Can transmit several low-traffic channels on a high bandwidth medium Multiplexer or Mux is equipment used Multiplexer or Mux is equipment used

30. Frequency Division Multiplexing Various frequencies used to put multiple signals on a single wire Various frequencies used to put multiple signals on a single wire Cable TV Cable TV

31. Time Division Multiplexing Small time slots allocated to various clients Small time slots allocated to various clients Unused slots are wasted, so may not be optimally efficient Unused slots are wasted, so may not be optimally efficient

32. Statistical Time Division Mux TDM with logic to allocate slots TDM with logic to allocate slots Can use first-come first-served or other priority basis Can use first-come first-served or other priority basis Reduces unused time slots for more efficient media utilization Reduces unused time slots for more efficient media utilization