#4: Digital Transmission Flashcards
Integral of sin(x).dx
-cos(x) + C
What is a LAN? Explain.
Local Area Network.
A network spanning a small physical area where PCs are connected to each other without the need for an intermediate router/switch.
What is the ethernet frame format?
1) Pre-amble (data to tell receiver it’s about to transmit actual data)
2) Destination address (physical MAC address)
3) Source address (also physical MAC address)
4) Type
5) Actual data
6) CRC (error checking)
Draw a diagram of a basic wired bus topology. What are some drawbacks?
Uses coax cable.
What if two devices try and talk at the same time? Packet collision. Therefore requires some packet collision detection,
Draw a diagram of star topology. What are some benefits?
Uses cheaper twisted-pair cable
What’s the difference between a hub, a switch and a router?
- Hub: least expensive + intelligent; when it receives a message, it simply broadcasts it to all devices.
- Switch: learns which ports are connected to which devices, and switches an incoming packet to the respective device.
- Router: the most expensive + complicated. It assigns an IP address to each machine, which can then be used from devices outside the LAN.
What is an ‘MTU’?
Maximum Transmission Unit
Max. packet size (generally 1500 bytes).
What are the 802.11 IEEE standards? What is one fast protocol at the moment?
A set of standards/protocols for wireless LANs.
802.11ac = ~7Gbps, one of the fastest currently
What is CSMA/CA?
CS = Carrier Sense (can only transmit if no-one else is)
MA = Multiple Access (several people share LAN)
CA = Collision Avoidance (makes sure two people don’t transmit at once; otherwise the transmission must re-start)
What is the ‘hidden station’ problem in wireless network transmission?
A common wireless LAN problem. It’s conceivable that two stations can see one station in common, but not each other (i.e. they are out of range of each other, but not of a station in between them). They both try and transmit to the middle station at once, and the middle station receives a garbled mix of station A and B’s transmission. This is why CSMA/CA was invented.
What does ‘ADSL’ stand for?
Asymmetric Digital Subscription Line
How does ADSL work?
- physical phone lines only use about 3kHz for voice data
- this leaves all higher frequency bands unused
- ADSL uses the remaining unused higher frequency bands for up/downlink of digital data
Explain Adaptive Bit Allocation in the context of ADSL.
problem: there’s lots of noise on an ADSL telephone line.
solution:
- up/downlink frequency blocks split up in to ‘sub-channels’
- SNR of each sub-channel monitored
- sub-channels with lower SNR (i.e. less noise) allocated more bits
Draw a block diagram showing how ADSL is modulated. Explain.
- serial bitstream input allocated into ‘blocks’
- ‘blocks’ allocated into ‘sub-channels’ via the constellation map. This defiens the magnitude and phase of these sub-channels to different frequency bands.
- the IDFT (Inverse Discrete Fourer Transform) converts these subchannels into a varying waveform which is the sum of all incoming sub-channel waveforms (uses OFDM principle).
What is the internet?
A bunch of connected LANs.
What is packet switching? Draw a diagram of how a packet might get from one device to another over the internet.
Routing packets to their destination. Each forward from a router to another router = 1 ‘hop’.
What is a 3-way handshake? Draw a diagram and explain.
A method of establishing a connection between a client and a server. The client sends a ‘SYN’ (synchronize); the server responds with a ‘SYN + ACK’ (synchronize and awknowledge), and the client completes the process with an ‘ACK’.
What is the Re-transmission Timeout (RTO)? Draw a diagram.
When a sender misses too many ACKs after sending some packets, it stops sending for a bit (a time governed by the RTO). It then resuming sending, but more cautiously; first one packet (waits until ACK), then two packets, etc…
Explain the concept of TCP congestion avoidance. Draw a diagram.
- If a client (e.g. a PC) sends too many packets to the server (e.g. a router), the servers’ buffer may overflow.
- This causes packets to be lost.
- To prevent such congestion, the sender keeps increasing its packet-sending rate until a packet is lost. TCP interprets this as congestion; stops, reduces its transmission rate, and starts the process again.
