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.

Problem

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 – g – 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 g and a).

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.

Problem

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 give up when tried 16 times.

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.

Problem

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.

Problem

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.

Problem

TCP Data frame: =242us TCP ACK frame: 20+6 = 26us Each transaction is = 404us And the transferred 1460 * 8 = bits So, 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.

Problem