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Review

Review. Layers. Physical layer – sending bits from one place to another, ensuring an okay BER Data link layer – encapsulate information bits into frames, and send frames from one node to another neighboring node Network layer – send packets from one place to another, may travel multiple hops

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Review

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  1. Review

  2. Layers • Physical layer – sending bits from one place to another, ensuring an okay BER • Data link layer – encapsulate information bits into frames, and send frames from one node to another neighboring node • Network layer – send packets from one place to another, may travel multiple hops • Transportation layer – (TCP) sending segments from one end to the other end, making sure that there is no error, etc.

  3. Physical Layer • The goal – sending bits from one place to another • Two limiting factors, bandwidth and power. • Shannon’s theorem.

  4. Problem d

  5. Physical layer • If bandwidth is abundant, no special things needs to be done. Like 10Mbps Ethernet. Just pulling voltage up and down. • In other cases when bandwidth is not so much, like wireless and telephone lines, have to do something.

  6. Physical Layer • Sine waves. Send a sine wave, the other end will receive a sine wave also. It will be on the same frequency, but the amplitude will be different (attenuation) and phase will be different (propagation delay). • Typically you are allowed to use only a frequency band. Wireless LAN – 802.11g – 2.4G-2.48G. Divided into channels, each channel is 20M. Cellphone – maybe in the 900M band, or the 1.8G band, etc. GSM divides the band (50M) into channels, each channel being 200k. Each channel shared by 8 users in a time division fashion.

  7. Physical Layer • You have to send signals *modulated* on a sine wave on the frequency you are allowed to use. • Modulation techniques. Frequency. Amplitude. Phase. • OFDM (in 802.11g and 802.11a).

  8. The basic transmission/reception diagram • Information bits baseband waveform modulated waveform. • Received waveform baseband waveform information bits

  9. I, Q channels • It is possible to pack two sine waves on the same frequency and send them out, one is sine and the other is cosine, each carrying one baseband waveform. • I channel and Q channel.

  10. Problem a

  11. Error Correction and Detection • By introducing redundancy, the receiver can do some calculation to correct or detect errors. • We talked about Hamming Code in details. • The key is, every error will result in a unique syndrome. You find the syndrome you find the error. If there is a syndrome, there is an error. • Cyclic codes. Any cyclic shift of a codeword is still a codeword. Code generated by doing polynomial calculations.

  12. Problem a.

  13. Data Link Layer • Sending frames from one end of a link to the other end of the link. • The problem is: the receiver may be slow, the link may lose frames, both the data frames and the control frames you want to use.

  14. Data Link Layer • Simplest correct protocol. Stop and Wait. • Transmission delay and propagation delay. • If propagation delay is small, like in a distance of 1km, Stop and Wait is perfectly fine. • Otherwise, not efficient.

  15. Data Link Layer • Go-back-N. • The sender is allowed a window. The frames in this window can be sent without ACK. • Every time a timeout happens, retransmit every thing in the current window. • Selective Repeat. Only retransmit the frame that was time out.

  16. Problem c

  17. MAC • In Ethernet and wireless LAN, nodes share a medium. • How to design a protocol to allow nodes share the medium without causing collisions. • Collision means two frames were sent at the same time.

  18. MAC • You can do polling, but the traffic of computers are random. • Contention-based. Nodes know better when they want to send. • ALOHA – send when want to, no carrier sense.

  19. Ethernet • Ethernet – CSMA/CD. 1-persistent. Exponential backoff. • Sense the medium before sending, if busy, don’t send. When medium is free, send. If noticed a jam signal, stop, backoff. The time to backoff is picked randomly from [0,CW-1]. If collided again, double CW. Until CW is 1024. give up when tried 16 times.

  20. 802.11 • CSMA/CA. non-persistent. Exponential backoff. • Sense the medium before sending, if busy, don’t send. When medium is free, backoff a time picked randomly from [0,CW-1]. If still free, send. If no ACK received, double CW, repeat the process. Give up when tried 1 times.

  21. Problem b

  22. Problem d

  23. Problem b

  24. Network Layer • Network Layer – moving packets from one place to the other • First problem – routing. • Shortest path algorithm. • Link State Algorithm. Distance Vector Algorithm.

  25. IP • IP address. A,B, C, D class. CIDR. The allocation of IP addresses. • Internet Protocols. BGP, OSPF, DHCP, ARP. • NAT.

  26. Problem b

  27. Problem

  28. Problem a

  29. Problem d

  30. Problem d

  31. Routers • Lookup to find next hop link. • Input-buffered switches and scheduling algorithm.

  32. Transportation Layer • TCP – send segments from one end to the other end reliably. • TCP connection set up. Three-way handshake. Initial sequence number. • TCP connection close.

  33. TCP • TCP congestion control. AIMD. Why is it used. • TCP optimizations. TCP Reno. Fast retransmit and fast recovery.

  34. Problem b

  35. Problem TCP Data frame: 20+222=242us TCP ACK frame: 20+6 = 26us Each transaction is 28+242+10+30+28+26+10+30 = 404us And the transferred 1460 * 8 = bits So, 28.9Mbps

  36. Network Security • Three tools. Hash function. Block cipher. RSA. • Hash functions. Two properties. • Block Cipher. Pseudo random permutation. AES. Cipher modes. • RSA.

  37. Network Security • Applying the tools to realize functions. • Digital Signature. • Certificate. • Message authentication. Replay attack.

  38. Problem c

  39. Problem c

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