Week 6 Flashcards
What does the physical layer do?
Directly connects devices on the same circuit using a tangbile medium such as cables/wires/radiowaves etc.
How are our bits represented for transmission?
Represented a bits/bytes of strings converted into SIGNALS
What are codes?
They are used to convert data into numbers and then into sequences of bits
What are types of media that signals can travel through?
Electrical - cables
Radio waves - ‘air’/space
Light - ‘air’/fibre optics
How do we ensure that two computers can understand each other?
Standardisation of protocols
We require BOTH codes and signals to be standardised
What are signals?
Signals are eletrical of optical wave forms that transmit sequences of bits across media
What are the two types of data?
Digital
Analog
What are the features of digital data?
- Discrete values
e. g. 0 or 1, characters in the alphabet, ASCII etc. - Discrete step from one symbol to the next
What are the features of analog data?
- Continous data / Range of possible values
- Continous variation over time
What are digital signals?
They are waveforms with a limnted number of DISCRETE STATES
What are analog signals?
They are continous, often sinusoidal waves
Examples include sound/light/electromagnetic waves
What are the transmision types of data?
- Digital signal used to transmit digital data
- Analog signal used to transmit digital data
- Digital signal used to transmit analog data
NOTE: Analog to analog is NOT related to computers as there is NO digitialization of data (e.c. talking to another person)
What is bandwidth?
It is the difference between highest and lowest frequencies in a range(band) of frequencies
Features of POTS bandwidth?
Need ONE wave cycle per symbol
Therefore, x symbol per x hz/s
What is a baud?
Symbols per second
What does it mean when ADSL has a high cost in the last mile?
The greatest cost in implementing ADSL is when it is within a mile from homes as each customer needs a seperate cable connection.
Why is ADSL Asymmetric? Why make is asymmetric?
The band allocated to downstream data is much greater than the bands given for upstream data.
It is asymmetric because it ensures that the uploaded data is minimal as increasing it increases the potential for crosstalk. Downloaded data is also greater to further minimise effects of crosstalk on the signals.
What is crosstalk?
This is when there is interference between connections.
Depends on the distance of the signal from the senter and distance between wires.
What are the two sublayers of the datalink layer?
Logical Link Control (LLC)
- Handle PDU header and trailer
- Error control
- Defines interface with the network layer
Media Access Control (MAC)
- Encodes/decodes between physical layer symbols and frames (bits)
- Error detection
- Controls when the device transmits
What are the two approches MAC uses to determine when to transmit data?
Contention
- Device waits until the circuit is free before transmitting
Controlled Access
- Device waits until given permission before transmitting
What is MAC contention?
- Any device can transmit at any time (first come first served)
- Used in ethernet
What are collisions and how does contention manage collisions?
Collisions occur when two devices transmit data at the same time
- Packets in a collision are damaged
- Collisions avoided by carrier sensing (listening on network for transmission)
- Detect collusions and retransmit
What is original ethernet topology?
Shared bus between all devices.
Any message that comes to the bus arrives to all ports
What is the toplogy of ethernet?
There is a hub that devices connect to (point-to-point, looks like a star)
The hub acts like a shared bus.
What tells devices following a controlled access MAC approach whose turn it is to transmit?
A controller provides permission to devices.
What are the types of controlled access approches?
- Roll poll calling (controller tells devices if it is their turn to transmit)
- Hub polling/token passing (a token is passed between devices, device with token can transmit)
What are the types of transmission techniques?
Asyncrhonous transmission
- Each character (byte) sent independently
- One 7-bit ASCII character at a time
- Used for dumb/dummy terminals (server-based architecture)
NOTE: Server-based is when ALL logic and data storage is handled by server. Client has dummy terminal.
Synchronous transmission
- Several bytes sent together in a frame
- Frame is approx 1500 bytes
- Includes addressing information (for multi-point circuits)
- Includes syncrhonisation characters (SYN) acts as start bit
What are the bits used in asyncrhonous transmission?
- Start bit (marks start of data)
- Parity bit (for error detection)
- Stop bit (returns circuit to idle
What are MAC addresses?
A unique id for a network interface. Only used to address computers in the same LAN
- 6 Bytes, stored in hardware
- Manufacter MUST make sure its unique
- Used in ethernet and wifi
- First 3 bytes identify manufacterer
What error detection?
General idea is to calcualte error-detection value (EDV) and transmit with message
A larger EDV means better error detection BUT less efficient detection
What are the three error detection techniques?
- Parity check
- Checksum
- Cyclic Redundancy Check (CRC)
How can we correct errors? What are teh two approaches?
Backward error correction
- Reciever asks sender to re-transmit the frame
- Most common error correction in modern networks
Forward error correction
- Reciever repairs frame (does not ask to retransmit)
- Used if re-transmission is not feasable
What is automatic repeat request (ARQ)?
Sender will send a frame, waits for reciever to return ACK(nowledgement)
If sendter recieved a Negative ACK (NAK) then it will retransmit the frame. This is done until an ACK is recieved.
What is continous ARQ?
Same as ARQ but instead of stopping and waiting for ACK after each frame, continously sends frames and recievs ACKs/NAKs between sent frames.
What is forward error correction (FEC)?
Transmit additional info:
- allows reciever to detect error
- Alows reciever to locate erroneous bits
- Invert erroneous bits to repair message
Where is error correction completed?
Transport layer (for ARQ)
What makes a network efficient?
An efficient network allows us to transmit the highest possible volume of accurate information.
The higher the volume of accurate information transmitted, the lower the cost due to less error correction, management, overhead etc.
What factors affect efficiency?
Base transmission rate of the circuit
Errors
- Error rate of circuit
- Error control in protocol (FEC introduces overhead, less efficient)
Protocol overhead
- How many bit are added for error checking, frame delimiters etc.
What is efficiency? How is it determined?
Efficiency = Number of information bits transmitted / total number of bits transmitted
Why are the optimal frame size to efficiency graphs concave?
Start less efficient with small frame sizes due to the high protol overhead comapred to smaller frames.
Large frames have increased error rate.
What layer do hubs belong to?
Physical layer
What type of MAC does HDLC use? (NOTE HDLC does not have a source address)
Roll call polling
