Published on January 7, 2008
EE 551/451, Fall, 2006Communication Systems: EE 551/451, Fall, 2006 Communication Systems Zhu Han Department of Electrical and Computer Engineering Class 18 Oct. 24th, 2006 Digital Communication System: Digital Communication System Project 1 7.2 (part) and 7.4 Spectrum of line coding: Scrambling Cipher basics, not required Basic steps for spectrum analysis: Basic steps for spectrum analysis Figure 7.3, 7.4 Basic pulse function and its spectrum P(w) For example, rect. function is sinc Input x is the pulse function with different amplitude as figure 7.3c Carry different information with sign and amplitude Auto correlation is the spectrum of Sx(w) Overall spectrum Line coding schemes: Line coding schemes 10 points in the finals NRZ: NRZ R0=1, Rn=0, n>0 Figure 7.5 pulse width Tb/2 P(w)=Tb sinc(wTb/2) Bandwidth Rb for pulse width Tb RZ scheme: RZ scheme DC Nulling Split phase Figure 7.6(a) Polar biphase: Manchester and differential Manchester schemes: Polar biphase: Manchester and differential Manchester schemes In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. 802.3 token bus and 802.4 Ethernet Bipolar schemes: AMI and pseudoternary: Bipolar schemes: AMI and pseudoternary R0=1/2, R1=-1/4, Rn=0,n>1, page 307 for reasons Figure 7.8 Reason: the phase changes slower Multilevel: 2B1Q scheme: Multilevel: 2B1Q scheme NRZ with amplitude representing more bits On-Off Signaling: On-Off Signaling R0=1/2, Rn=1/4, page 304 Sx(w), 7.18a Sy(w), 7.18b Spectrum figure 7.7 4 Points in the Final: 4 Points in the Final Draw the spectrums of three different line codes and describe why the spectrums have such shapes. Scrambling: Scrambling Make the data more random by removing long strings of 1’s or 0’s. Improve timing The simplest form of scrambling is to add a long pseudo-noise (PN) sequence to the data sequence and subtract it at the receiver (via modulo 2 addition); a PN sequence is produced by a Linear Shift Feedback Register (LSFR). In receiver, descrambling using the same PN. Secure: what is the PN and what is the initial Scrambling: Scrambling Example 7.2 Exercise: 100000000000 Scrambling Example: Scrambling Example Scrambler Descrambler Encryption: Encryption Encryption is a translation of data into a secret code. Encryption is the most effective way to achieve data security. To read an encrypted file, you must have access to a secret key that enables you to decrypt it. Unencrypted data is called plain text; encrypted data is referred to as cipher (text). Encryption can be used to ensure secrecy, but other techniques are still needed to make communications secure: authentication, authorization, and message integrity. Message integrity - both parties will always wish to be confident that a message has not been altered during transmission. The encryption makes it difficult for a third party to read a message, but that third party may still be able to alter it in a useful way. Authentication is a way to ensure users are who they say they are - that the user who attempts to perform functions in a system is in fact the user who is authorized to do so. Authorization protects computer resources (data, files, programs, devices) by allowing those resources to be used by resource consumers having been granted authority to use them. Digital rights management etc. Encryption – cipher taxonomy: Encryption – cipher taxonomy CIPHERS MODERN CIPHERS CLASSICAL CIPHERS PUBLIC KEY PRIVATE KEY SUPERPOSITION TRANSPOSITION ROTOR MACHINES Quantum CIPHERS Transposition Method: Transposition Method Da Vinci’s code Ex. I am a student I m s u e t a a t d n Substitution Method: Substitution Method Shift Cipher (Caesar’s Cipher) I CAME I SAW I CONQUERED H BZLD H TZV H BNMPTDSDC Julius Caesar to communicate with his army Language, wind talker Rotor Machine: Rotor Machine The primary component is a set of rotors, also termed wheels or drums, which are rotating disks with an array of electrical contacts on either side. The wiring between the contacts implements a fixed substitution of letters, scrambling them in some complex fashion. On its own, this would offer little security; however, after encrypting each letter, the rotors advance positions, changing the substitution. By this means, a rotor machine produces a complex polyalphabetic substitution cipher. German Enigma machine used during World War II for submarine. Movie U571, Italian Job Key: Key Public Key System - RSA: Public Key System - RSA Named after its inventors Ron Rivest, Adi Shamir and Len Adleman Base on Number Theory y=ex (mod N) => x=?? If the size of N is 100, it takes 100 billion years to decipher with 1GHz computer. Applications Digital Signatures Digital Cash: Movie, swordfish Timestamping Services: Movie, entrapment Election Movie, mercury rising Encryption – cipher taxonomy: Encryption – cipher taxonomy Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include substitution ciphers and transposition ciphers. During the early 20th century, more sophisticated machines for encryption were used, rotor machines, which were more complex than previous schemes. Encryption methods can be divided into symmetric key algorithms and asymmetric key algorithms. In a symmetric key algorithm (DES, AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables him to perform decryption. Quantum Cryptography: Quantum Cryptography Use physics law, if the signal is measured (eavesdropped), the receiver can always detected. Mission is really impossible: Mission is really impossible When you see it, the information has been already changed Questions?: Questions?