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

• Transportation layer – (TCP) sending segments from one end to the other end, making sure that there is no error, etc.

Physical Layer

• The goal – sending bits from one place to another

• Two limiting factors, bandwidth and power.• Shannon’s theorem.

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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.

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.

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).

The basic transmission/reception diagram

• Information bits baseband waveform modulated waveform.

• Received waveform baseband waveform information bits

Problem

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.

Problem

Error Correction and Detection

• By introducing redundancy, the receiver can do some calculation to correct or detect errors.

• 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. Detects error if the remainder is not 0.

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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.

Data Link Layer

• Simplest correct protocol. Stop and Wait.– ACK, Timeout, Sequence number

• 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.

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.– Receiver has window size 1.

• Protocol beyond Go-back-N. – Receiver keeps window greater than 1.

Problem

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.

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.

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]. Wait for the backoff time and then check the channel again. If free, send. If busy again, double CW. Until CW is 1024. give up when tried 16 times.

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.

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Network Layer

• Network Layer – moving packets from one place to the other

• First problem – routing.• Shortest path algorithm.• Link State Algorithm. Distance Vector

Algorithm.

IP

• IP address. A,B, C, D class. CIDR. The allocation of IP addresses.

• Internet Protocols. BGP, OSPF, DHCP, ARP. • NAT.

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Routers

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

algorithm.

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.

TCP

• TCP congestion control. AIMD. Why is it used.• TCP optimizations. TCP Reno. Fast retransmit

and fast recovery.

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TCP Data frame: 20+222=242usTCP ACK frame: 20+6 = 26usEach transaction is 28+242+10+30+28+26+10+30 = 404usAnd the transferred 1460 * 8 = bitsSo, 28.9Mbps

Network Security

• Three tools. Hash function. Block cipher. RSA.• Hash functions. Two properties.• Block Cipher. Pseudo random permutation.

AES. Cipher modes.• RSA.

Network Security

• Applying the tools to realize functions.• Digital Signature. • Certificate.

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