Summary: Computer Netwerken

Read the summary and the most important questions on Computer Netwerken

• 1 Foundation

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• What types of connectivity are there?

  Links
  - point to point
  - multiple access
  
  Nodes
  - hosts and switches
  - circuit switching or packet switching
  
  Clouds
  - routers & gateways. Denotes any type of networks, is not cloud computing
  
  Adressing & routing
  - Unicast
  - Broadcast and multicast

• Explain two types of Switched Networks

  A network can be
  - two or more nodes connected by links
  - or two or more networks connected by nodes. 
  
  Devices that link networks are routers

• What types of Switching Strategies are there?

  Circuit switching
  - carry bit stream over dedicated channel
  - similar to original analog Plain Old Telephone Service (POTS)
  - ideal for bidirectional (call) but horrible for occasional sending like with computers
  
  Packet Switching
  - store-and-forward packets
  - example: Internet
  - The path isn't set before hand. There is sender + address and each switch decides how to send it.

• Explain cost-effective resource sharing

  Multiplexing
  - multiple data flows sharing a physical link
  
  Frequency-Division Multiplexing (FDM)
  - A cable has 4MHz, you can split it in 4KHz so 1000 phonecalls can be made simultaneous
  
  Time Division Multiplexing
  - You can digitalize the call from FDM and turn it into bites. This way 64 MBps can be split in 64 Kbps and have 1000 phone calls
  
  Both arent ideal for computer networks as they're pretty static. The amount of conversations simultaneously is static. What if a conversation doesn't send data for a bit?

• What is statistical multiplexing?

  Is response for static FDM and TDM, and is packet switching
  
  On-demand time division
  -  limited size block of data required: packet
  -  schedule link on a per-packet basis
  - packets from different sources interleaved on link
  
  Buffer required
  - buffer packets that are contending for the link
  - congestion possible (buffer overflow --> packets lost)
  
  Implications
  - packet loss, delay jitter
  - what queuing discipline?

• What are common communication types?

  Request/Reply channels
  - like: remote file acces
  - required: guaranteed delivery, no strict time constraint
  
  Message Stream Channels
  - like: video applications
  - required: guaranteed delivery not so important. Timing constraint important for two-way traffic, less for one. Needs compression

• What are the 4 requirements for a network?

  1: Scalable Connectivity
  2: Cost-effective resource sharing
  3: Support for common services
  4: Manageability
  5: Performance

• What is protocol machinery?

  Protocol graph:
  - most peer to peer communication is indirect
  - peer to peer is direct only at hardware level
  
  Trechter:
  Several applications --> RRP or MSP --> HHP --> sent to next host --> HHP --> RRP or MSP --> Several applications
  
  Multiplexing
  - different applications/protocols use same protocol on lower level
  - demultiplexing requires demux key
  
  Encapsulation
  - header contains control information for peer (including demux key) so for instance [HHP,RRP,Data]
  - standards needed: so Linux, Windows and Max communicate

• What is more important: Bandwidth vs Latency?

  Relative importance
  - 1 byte message: 1ms vs 100 ms latency dominate 1 Mbps vs 100 Mbps: 8 nano sec or 0.08 nano sec
  - 25 MB message: 1Mbps vs 100 Mbps dominates 1 ms or 100 ms: 200s or 2s
  
  Infinite bandwidth
  - RTT dominates:
  Throughput = TransferSize / TransferTime
  TransferTime = RTT + TransferSize / Bandwidth
  - Example: request file 1 MB, 1 GBps, RTT: 100ms
  Transfertime = 100 ms + 8 Mb / 1000 Mbps = 108 ms
  Throughput = 8 Mb / 108 ms = 74 Mbps (<< 1Gbps)    
  
  For small files, don't care about bandwidth. 
  For big files: RTT don't matter, but bandwidth does

• What are application performance requirements?

  Bandwidth requirement
  - as much as possible: not always required
  - fixed, limited rate often sufficient
  - variable rate: compressed video stream
  
  Delay requirements
  - as little as possible: not always required
  - variation in delay (Ritter) is often more important than delay