Circuits, Multiplexing, Media Slides

• Learning Objectives: 


  •Summarize the characteristics of circuits in the OSI physical layer•Explain why analog and digital signals are translated•Describe the benefits of digital transmission•Explain how modems work•Outline how modulation is used to send multiple bits simultaneously•Compare point-to-point and multipoint communication circuits
• Physical Layer: 


  •Layer 1 in the OSI model•Network hardware, with a focus on circuits•Two meanings of the term circuit◦Physical circuit: connects devices & include wires◦Logical circuit: describes transmission characteristics of the connection•Two types of data flow through circuits◦Analog (wave-based)◦Digital (binary: 0 or 1)
• Circuits: Data translation and transmission:

  •Data can be translated from analog to digital, or vice versa, for part or all of a transmission•Digital computer data can be transmitted over an analog telephone circuit by using a modem◦Translates the computer’s digital data into analog data that can be transmitted through voice communication circuits, and translates it back at the receiver’s end ◦Translate analog voice data into digital form for transmission using a codec•Sender and receiver must agree on the symbols used for transmission and on the symbol rate.
• Circuit Data Types:

  Circuits carry two fundamental types of data:

  •Analog – Continuous, can be any value within a range•Used to carry voice data over POTS•Digital – Discrete, can only have certain specific values•Binary computer data has 2 levels (1 or 0)

  
• Signal Transmission (basics)

  At a very basic level, a sender conveys a message to a receiver by varying the voltage on the circuit over time.Digital (binary) data might be conveyed by changing the voltage from +15V (representing one) to -15V (representing zero)Analog data might be conveyed by changing the voltage to any value between +15V and -15V

  
• Benefits of Digital Transmission


  •Produces fewer errors than analog transmission◦Easier to detect and correct errors with binary values•Permits higher maximum transmission rates ◦e.g., fiber-optic cable•More efficient ◦Possible to send more data through a given circuit•More secure ◦Easier to encrypt•Far simpler to integrate voice, video, and data on the same circuit
• Circuit Configuration Point-to-point:

  •Point-to-point (dedicated) circuits include most wired connections today.

  
• Circuit Configurations: Multipoint

  •••Multipoint (shared) circuits are most commonly used in wireless today.
• Data Flow: 


  Simplex •Data flows in one direction. Half-duplex •Data flows in both directions, but one direction at a time. Full-duplex •Data flows simultaneously in both directions

  
• Data Flow: Examples

  
• Multiplexing 


  •Divides high-speed circuit into several slower (logical) circuits• Main advantage: cost efficiency by reducing the amount of equipment• Categories of multiplexing◦ Frequency/wavelength◦ Time

  
• Multiplexing

  •Frequency Division Multiplexing (FDM)◦ Divides the circuit bandwidth “horizontally”◦ Creates separate channels within frequency band◦ Guardbands separate channels to prevent interference

  
• Multiplexing

  • Wavelength Division Multiplexing (WDM)◦ A variant of FDM used in fiber optic circuits◦ Divide circuit capacity into channels by wavelengths◦ Wavelength is inversely proportional to frequency ( w=1/f∙c )◦ Associated with colors in visible spectrum◦ Dense WDM can divide circuit into more than 100 channels per fiber each transmitting at 10 Gbps

  
• Multiplexing

  
• Time Division Multiplexing (TDM)– Circuit is divided “vertically” by taking turns– In traditional TDM, all have equal turns– More efficient than FDM, but may have idle time slots

  
• Multiplexing

  • Statistical Time Division Multiplexing (STDM)◦ Variant of TDM which allocates slots based on statistical network usage◦ Goal is to reduce idle times◦ Adds computation overhead; statistical predictor not always right

  
• Multiplexing Inverse

  •Combines many low-speed circuits into one high-speed circuit•Example: two T-1 lines multiplexed◦Creates a capacity of 2 x 1.544 Mbps = 3.088 Mbps
• Media 


  • Physical matter used to carry voice or data transmissions◦ Guided media – transmission flows along physical medium◦ Wireless (radiated) media – transmission flows through the air
• Guided Media - Twisted Pair (TP) Cable

  • Insulated pairs of wires bundled together• Wires twisted to reduce electromagnetic interference• Some use additional shielding (STP)• Commonly used for telephones, LANs• Characteristics◦Price: inexpensive◦ Distance: typically up to 100m◦ Use: telephones, LANs

  
• Guided Media – Coax Cable  


  • Consists of 

  (A) outer plastic sheath 

  (B) woven copper shield 

  (C) inner dielectric insulator, and 

  (D) copper core

  • Less prone to interference•Characteristics◦Price: inexpensive (but more costly than TP)◦ Distance: up to 2 km (1.2 miles)Use: cable TV / Internet

  
• Guided Media – Fiber Optic Cable

  • Optical core made of glass or plastic• Data transmitted using light from lasers or LEDs• Resistant to interference and  corrosion•Extremely fast data rates• Characteristics◦ Price: expensive◦ Distance: 500m – 100km◦ Uses: trunk line / backbone,

                long distance circuits 

                (e.g., undersea cables)

  
• Guided Media - Fiber Optic Cable cont

  •Types:◦Multimode (about 50 micron core)◦Graded index multimode◦Single mode (about 5 micron core)•If you want to know more about differences between single and multimode fiber optics, check out this article:

     multimode versus single mode (Milne, rfvenue, 2016) 
• Wireless Media - Radio 


  • Wireless transmission of electrical waves through air• Includes each device on network with a radio transceiver operating at a specific frequency range•Enables mobile network communication• Characteristics◦Distance: depends on frequency and power◦ Use: wireless LANs, cellular and cordless phones, baby monitors
• Wireless Media – Microwave

  Microwave◦ High-frequency radio communication◦ Requires line of sight (large antennas and towers)◦ Affected by weather◦ Characteristics◦Distance: ~60 km (due to curvature of earth◦Use: Trunk line / Backbone, long distance
• Wireless Media – Satellite


  • Satellite transmission◦ Special means of microwave communication◦ Long distance leads to propagation delays

  
• Which Media Are Best? 


  • Factors to consider in media selection◦ Type of network◦ Cost◦ Transmission distance◦ Security◦ Error rates◦ Transmission speeds