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.
