Questions prof Flashcards
What is the OSI model?
- A conceptual framework that divides network communications functions into seven layers: application, presentation, session, transport, network, data link, and physical layer
- It provides a common basis for the coordination of standards development for the purpose of systems interconnection
- Provides a common reference for maintaining consistency of all related standards
What’s meant by a stacked network model?
- It refers to the set of protocols used in a communication network
- The protocol stack is composed of seven layers: application, presentation, session, transport, network, data link, and physical layer
- This stack is present in each client and server
- It implies that each communication must pass through these 7 layers and each layer adds its own header
Differences between OSI and TCP/IP model
- TCP/IP combines some of the layers of the OSI model, it has 4 layers in total
- Combines application, presentation, and session layers into an application layer
- Combines data-link layer and physical layer into a network-access layer
- Network layer is transformed into internet layer
- Same functionality of seven layers is conserved
Two functions of the DLC layer
- Detects and corrects data error occurred at physical layer
- Provides multiple access and channel assignment service
- Constructs data frame using appropriate format for the network
- Maintains quality of service
Difference between session and presentation layers
Presentation layer:
- performs proper data formatting for the receiving node to understand
- performs data encryption and decryption
- performs data compression and decompression
* Primarily concerned with the format of the data (i.e. convert text to ASCII)
Session layer
- Initiates communication link
- Assures reliable communication between transmitting and receiving node
* Primarily concerned with coordinating applications
Define magnetic flux and magnetic field
- Magnetic flux: is the magnetic flux density B which passes through an area A (phi=B*A). It shows the quantity of magnetic line forces produced by the magnet. Magnetic flux is a measure of the total magnetic field which passes through a given surface.
- Magnetic field: is the region around a magnet or a conductor carrying a current where moving charges experience a force. It is a physical field that describes the magnetic influence on moving electric charges.
Write Gauss’s law of electricity in integral form
see word docuement
What are the types of commercials RFIDs?
- Chip-based passive RFID: Near-field (low frequency), far-field (high frequency)
- Chip-based active RFID
- Chip-based battery-assisted (semi-passive) RFID: near-field, far-field
- Chipless RFID: transmission line resonators
Define backscatter modulation
- Communication method in which a passive tag receives energy from a tag reader’s transmission and uses the same energy to send a reply.
- Once the tag receives the energy via electromagnetic waves (interrogating signal+microwave power), the energy travels through the tag’s internal antenna to activate the chip.
- The remaining energy is modulated with the chip’s data and flows back via the tag’s antenna.
Sketch an ASK modulator architecture
see word document
Sketch an PSK modulator architecture
see word document
How is the phase being altered in PSK backscatter modulation?
- By varying the input impedance seen at the antenna reference plane by switching the capacitor ON and OFF
- See word document
Define BER
- Is the energy per bit over the noise level
- total count of error bits to the total count of transmitted bits
- Often is a function of energy per bit to noise power spectral density ratio
Eb/N0 - N0 noise power in a 1Hz bandwidth
- The higher the energy of bits or the difference between the symbols to represent the bits the lower the BER
What are the main design parameters of RFID systems?
- How to power the tag by the reader (interrogation)
- Modulation
- Sensitivity of the reader to detect backscatter signals
- Communication protocol for distinguish tags
- Anti-collision protocols
- Antenna design
- Size and integration
- Frequency bands: LF, HF, UHF
- Range
- Tag type: passive, active
- Tag memory
- Cost
How to decrease BER in NFC systems?
- By increasing the energy of the bits over the noise level
- By increasing the distance between symbols that represent bit 1 and 0
Two commercial applications of NFC
Block diagram of inductively-coupled NFC system
see word document
Define entropy of an information system
- Is the measure of the uncertainty of a discrete random variable X.
- It is measured in bits.
- The higher the entropy the higher the level of information associated with the variable or event. In general, the more certain or deterministic the event is, the less information it will contain.
- The expected number of bits of information contained in each message
- See word document
Sketch entropy vs probability of events
see word document
How to deduce channel capacity from mutual information?
- Channel capacity is the maximum of the mutual information I over all possible input distributions p(x).
- X, Y are discrete random variables
- See word document
How is error correction process taking place? And at which layer of the TCP/IP stack?
- Occurs at the DLC layer that is embedded in the network-access layer for the case of TCP/IP
- Error detection is based on parity checks and cyclic redundancy check CRC techniques.
- Identified erroneous frames are requested again.
- The parity check codes used for error detection are CRC.
- CRC uses a generator polynomial available on both sender and receiver side. Modulo-2 binary division is used to divide binary data by the key, remainder is appended at the end of the data.
- See word document
How to implement long division in hardware?
- By using a linear feedback shift register
- LFSR are composed of shift registers, feedback function, initial state, and clock signal.
- Shift registers are a group of flip-flops arranged in cascade, the output of one flip-flop is linked to the input of the next one.
- Feedback function decides is usually an XOR gate to combine some outputs of the flip-flops and feed those outputs back into the input of one of the flip-flops.
- See word document
Sketch total arrival rate of pure ALOHA process
see word document
Why does slotted ALOHA provides higher throughput than pure ALOHA?
- In pure ALOHA each node transmits packets whenever it receives them (immediate transmission). When collision occurs, and nodes have to wait a random period of time and try again. This randomness increases the chances of collision.
- Because it synchronizes the transmission attempts of all nodes to a certain time point where the channel is slotted. Each node can start transmission at the beginning of these well-defined time slots. Collision might still occur because two nodes can send a frame at the beginning of the time slot.
- Vulnerable time is only Tf increasing the probability of no collision by a factor of two.
Define vulnerable period in ALOHA process
- It’s equal to twice the frame time because even if a bit of a frame collides with a bit of another frame, both frames get discarded.
- Vulnerable time includes: - The frame time: because if transmission of another frame were started in the frame time of the current frame, collision would occur. – time interval (equalt to the frame time) before the frame time because if transmission of another frame were started in this interval time, collision would still occur.
- Is the vulnerable time/window during which any transmission attempt will result in a collision.
- In slotted ALOHA, the vulnerable time period for collision between two frames is equal to the time duration of 1 slot.
