Understanding Data Rate, MAC Protocols, and Network Technologies

Posted on Dec 21, 2023 in Computers

¡Escribe tu texto aData rate =

Shannon: Max bitrate depends on signal/noise ratio   

Nyquist: Max bitrate depends on n. sampling levels

CSMA CD

  • Is a MAC protocol
  • Exponential back-off strategy -> adapts to load on ethernet
  • RTT bounds determines a bound on throughput

IP tunnels

  • Packets traveling in a tunnel carry IP as load
  • Allows for any IP to be sent to any destination (regardless of the IP carried as load)
  • Never carries Ethernet as load
  • Always carry IP packets as load
  • The service offered by the IP protocol is unreliable.

Probability of error scenarios

  • An error can occur if prob>0
  • An error won’t always occur if prob<1

CAN APK

  • Sender must always send a recessive symbol
  • Sender is allowed to send a recessive
  • Receiver is allowed to send a dominant symbol
  • Receiver is allowed to send a recessive symbol

Switched Ethernet

  • Switches carry out Ethernet frames forwardly
  • The max. aggregate throughput of a switch equals the sum of the max. throughput of all input ports.
  • Switches have no knowledge about IP addresses
  • In case a switch is based on the concept of input-buffering, it can suffer from head-of-line blocking problems.

TTE (Time Triggered Ethernet)

  • Multiple senders can send at the same time over the same network
  • A rate-constrained frame can delay other frames in a switch.
  • A best-effort frame can delay other frames in a switch.
  • A time-triggered frame is never delayed by other frames in a switch. (If switches don’t shuffle.)
  • A link is used half duplex.

MACA Ethernet

  • MACA is a MAC protocol.
  • The header of the RequestToSend frame carries information about the size of the data frame to send.
  • If a host receives a ClearToSend frame, it will remain quiet until the corresponding data frame has been received by the destination host.
  • In the MACA protocol, collisions can happen.

CAN, Flexray and Internet

  • NOT possible to resolve contention on the Internet with CSMA/CR technique as used in CAN.
  • The CSMA/CR protocol of CAN CAN’T also be used on a wireless network.
  • NOT possible to resolve contention on the Internet with a static or dynamic segment as is used in Flexray.
  • Answer is: None of the statements is true.

CAN

  • Identifier (priority) is required to compute the minimum bandwidth allocated to the sender of frames with that priority.

CAN BUS

  • Latency  of frame with higher priority.
  • Bandwidth  of frame with higher priority.
  • If bandwidth = 1, best case scenario, 100%

TT CAN

  • Uses bit stuffing.
  • Uses CSMA/CR.
  • When frame is allocated an arbitration window, priority is needed to calculate max. latency.
  • When frame is allocated 1 or more arbitration windows, n⁰ windows is needed to calc. max. lat.

FlexRay (for single and active star topology)

  • Static
    • Min. bandwidth computed with n⁰ slots.
    • Min. bandwidth of sender is independent of other senders.
    • All nodes have same slot.
  • Dynamic
    • Min. bandwidth computed with minislot number.
    • All nodes have same minislot.quí!

PAR (Positive Acknowledgement with Retransmission)

  • Efficiency =  for
  • Throughput(b/s) =  
  • Time out < RTT:  Service is unreliable.
  • Time out > RTT:  Service is reliable.
  • Wire like network: Reliable

Wireless communication in vehicles

  • The Time Slotted Channel Hopping (TSCH) mechanism provides better robustness against interference from WIFI.
  • The quality of low-power wireless links is in general unreliable and time-varying.
  • The beacon-enabled mode of IEEE 802.15.4 standard provides both contention-free and contention-based medium access.
  • Wireless signals with high frequency suffer from attenuation.

Worst case latency

  • Sender does have priority: A is better than E.
  • Sender does NOT have priority: E is better than A.

Polynomial F(x) CRC method

  • Every burst error of length n or less will be detected.
  • Error  won’t be detected.
  • The error  is left undetected.
  • The transmitted code is of form  where  is the remainder when dividing  by  modulo 2.
  • Every error of form  for  will be left undetected because  and this is divisible by
  • The transmitted code is of form
  • All errors with degree n-1 or less will be detected.
  • Burst error of length n won’t be detected.
  •  is a bad choice,  would be better.
  • Only errors that occur in the redundant bit will be detected.
  • Errors that occur in the original bitstream won’t be detected.

Spanning tree

  • Purpose is to eliminate forwarding cycles (loops.)
  • Dynamically changes with the physical topology.
  • Disadvantage: may introduce unnecessary performance bottlenecks.
  • Calculate paths with lower bridge number.

Aethereal TDM (answers to exam (always same exercise))

  • In the same time slot circuits 1 and 2 can exit router R2.
  • In the same time slot circuits 2 and 4 can exit routers R2 and R1 respectively.
  • In the same time slot circuits 1 and 3 can exit routers R1 and R2 respectively.
  • It is possible for circuit 1 to have a higher bandwidth than circuit 2.
  • It is possible for circuit 1 to have a higher bandwidth than circuit 4.

Aethereal Network (components that have to be configured)

  • Master bus (demultiplexor).
  • Network interface and router.

Jacobson/Karels protocol

  • Purpose is to determine dynamically the time-out time of the TCP.
  • Computes estimations of expected values and standard deviations of segment RTT.
  •  and  are used to tune the speed with which the algorithm will adapt to varying RTT.

Dijkstra algorithm (in this case for router F)

  • Just calculate every path asked.
  • For each X (not equal to F), the shortest path between F and X is unique.

Bit Stuffing

  • 4B/5B encoding does not allow more than three consecutive 0’s, and provides higher channel efficiency than the Manchester coding scheme.
  • TTCAN uses bit stuffing.