cs441-lecture-20240910

Lecture Notes - CS441

Tuesday, September 10th 2024

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• On the speakers

• You have to be able to explain yourself and why you did the things you did for your project.
• What’s your experience as a student: your projects.
• Colleges generally use the same textbooks, therefore what makes your experience different? Why should I hire you?
  • How you present yourself. How you solve the problem. Your experience. The foundations and classes you learned them from.
  • Soft skills, don’t be a jerk.
• A recruiter thinks of three things when considering you:
  • Are you an asset? Are you valuable or a liability?
  • Are you a team player?
  • Can you fit with the company culture?
  • Team or by yourself? Both.

• Multiplexing

• Why we have it
• Bandwidth is the most important part of computer networks
  • It’s like highways
  • Engineers have to optimize the highways
• Two ways to preserve the bandwidth
  • Multiplexing
  • Compression
• Highway analogy: compression is like the 2+ or more lane on the highway
• Multiplexing is when you’re trying to get on the highway, the red/green light before you enter
  • This is to ensure the highway runs smoothly, no traffic etc.

• Introduction

• Under simplest conditions, medium can carry only one signal at any moment in time
• Medium: wire, wireless, Bluetooth, etc.
• Two types of multiplexing:
  • Frequency division multiplexing
  • Time division multiplexing
  • (Also code division multiplexing)

• Frequency Division Multiplexing

• Assignment of nonoverlapping frequency ranges to each “user” or signal on a medium
• For satellite sending channels 1-n to earth:
  • Freq 0-10kHz
  • Channel 1: 0-0.9kHz
  • Channel 2: 1-1.9kHz
  • Thus, all signals are transmitted at the same time
• Each channel is assigned a set of frequencies and is transmitted over the medium
• .
• AM - Amplitude modulation
  • Change amplitude, depends on the power
• FM - Frequency modulation
• PM - Phase modulation
  • Used by the army, uses the start of the sine wave, more difficult
• XM - Satellite modulation

• FDM Continued

• Oldest multiplexing technique

• Time Division Multiplexing

• Sharing of the signal is accomplished by dividing available transmission time on a medium among users
• Satellite:
  • Channel 1: 1s
  • Channel 2: 2s
  • Channel 3: 3s
• Divided into two subdivision basic forms:
  • Synchronous time division multiplexing
  • Statistical time division multiplexing
• Digital signaling is used exclusively

• Synchronous Time Division Multiplexing

• Original time division multiplexing
• Generated data faster than other devices
• Back to satellite, what if channel 2 is off the air? We waste the bandwidth in this case
  • This is where you see test photos (the colors, circles, bars) to fill in the space
• Buffering as a result of too much info being sent from one channel, the circle becomes slower

• Statistical Time Division Multiplexing

• This solved both above problems, it distributes the time, instead of 1s it’s 1.3s or 1.5s.
• A statistical multiplexor transmits the data from active workstations only.
• If workstation is not active, no space is wasted in the multiplexed system.

• Wavelength Division Multiplexing

• Multiplexes multiple data streams into a single fiber-optic line
• Different wavelength lasers (called lambdas) transmit the multiple signals

• Discrete Multitone

• A multiplexing technique commonly found in digital subscriber (DSL) systems
  • DSL not around anymore
• DMT combines hundreds of different signals, or subchannels, into one stream
• Interestingly, all of these subchannels belong to a single user, unlike the previous multiplexing techniques
• In your house, you have a parking spot
  • You don’t use it all the time, work, going out, etc.
  • What if you came home and someone else was parked in your spot
  • What the hell are you doing in my spot? It’s my spot.
  • You have the right to kick the person out of the spot
  • DSL the whole bandwidth connected to your house belongs to you
  • This is why companies keep calling to combine internet, landline, phone to one line, because of this

• Code Division Multiplexing

• What’s inside your cellphone
• Also known as code division multiple access
• An advanced technique that allows multiple devices to transmit on the same frequencies at the same time
• Each mobile device is assigned a unique 64-bit code
• We call cellphones cellphones because they are a part of a cell of transmission
• 
• Fiber optics:
  • First mile problem, when ISP send electrical signals, they have to convert to light for fiber optics
  • Last mile problem, turning the light back into electrical signals