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Embedded Systems > 3 Data Buses > Flashcards

3 Data Buses Flashcards

1
Q

Name advantages that the star topology has over the bus topology.

A
  • Multiple senders at same time
  • Only one single point of failure
  • No collisions
  • No implicit broadcasts
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2
Q

The sublayers of the Data Link Layer (DLL are:)

A

Logic Link Control (LLC)
-Defines the frame format
-Defines Adresses / IDs
-Provides flow control1
-Provides error detection / correction
Media Access Control (MAC)
-Defines which participant may send with which
(Frequency / Code / Time)
-Defines how collisions are handled (if possible)

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3
Q

What topology uses I2C?

A

Bus Topology

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4
Q

What does Wired-AND mean?

A

A zero on the Bus is dominant.

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5
Q

What is the basic MAC concept of I2C? Why are there no collisions when multiple senders start at the same time?

A

The basic concept of Master-Slave is that the Master decides who can send at which time, while Slaves can only answer to read requests by Master. When there are multiple Masters the dominant 0 ”wins”, thus the Masters with a one at the first bit that differs between the senders stops transmitting.

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6
Q

What is the topology of Profibus and how is media access controlled?

A

Profibus uses a Bus Topology. Media Access is controlled through Master/Slave and Multiple Masters are managed using a Token Bus.

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7
Q

What are properties of the Nyquist-Shannon theorem?

A

► Any signal within a band of 0 Hz and fmax Hz must be sampled
with at least 2fmax Hz
► Signal can be reproduced exactly (with infinite effort)
► Signal can be approximated arbitrarily exact
► In practice 5fmax Hz to 10fmax Hz are common

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8
Q

Name advantages of the Bus topology:

A
  • cheap

- simple

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9
Q

Name disadvantages of the Bus topology:

A
  • multiple acces (Babbling idiot, security)

- Single point of failure

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10
Q

Name advantages of the Star topology:

A
  • No multiple access

- Only central station is single point of failure

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11
Q

Name disadvantages of the star topology:

A
  • expensive central station

- more wiring

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12
Q

Name advantages of the Ring topology:

A
  • High quality of service

- No multiple access

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13
Q

Name disadvantages of the Ring topology:

A
  • Complex (expensive)

- Single Point of failure

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14
Q

What are the properties of the physical layer of the ISO/OSI 7 layer architecture?

A

► Defines mechanical properties
▪ Medium: copper, optical fiber, air, EM waves
▪ Connectors: form and pin assignment
► Defines electrical / optical properties
▪ Voltage
▪ Frequencies
▪ Baud rate
▪ Bit encoding
► Hardware
▪ Cable, connector, terminator, antenna, amplifier
▪ Transceiver, repeater, hub

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15
Q

Which methods for bit encoding exist?

A 
► Return to Zero (RZ)
► Non Return to Zero (NRZ)
► Differential NRZ
► Bit stuffing
► Manchester Code
► 4B/5B Code
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16
Q

Explain the synchronization problem and name a solution for it:

A 
► Clocks are never perfectly synchronous
► Clock A ticks every 100μs
► Clock B ticks every 90μs
► Clock A sends nine ones →high level for 900μs
► Clock B interprets this as ten ones

Solution: Differential NRZ

17
Q

What is Bit Stuffing and for what is it used for?

A

► Sender inserts a 0 after a sequence of n 1s (n=6 for USB)
► Receiver (checks and) removes inserted 0s
► Code violations (frame delimiters)
► Prevents long sequences of 1s

18
Q

What are the properties of the 4B/5B code?

A

► Uses some form of NRZ
► 80% data rate
► Encoding table prevents long sequences (FDDI with NRZI)

19
Q

What are the properties of the data link layer of the ISO/OSI 7 layer architecture?

A 
► Encapsulates data (bits) into frames
► Frame formats/synchronization
► Logical link control
► Media access control
► Hardware: Switch, Bridge
► Two sublayers
    ▪ Logical Link Control (LLC)
    ▪ Media Access Control (MAC)
20
Q

How can Errors during the data transmission be detected?

A 
► Parity bit
    ▪ Append parity bit such that the sum off all bits is 
      even / odd
    ▪ One bit error detection
► Cyclic Redundancy Check (CRC)
    ▪ Hash function based on polynomial division
    ▪ Detection of burst errors
► Hamming Code
    ▪ Set of parity bits
    ▪ Single error correction
    ▪ Double error detection
21
Q

Procedure of succesful transmission:

A

▪ A sends frame to B

▪ B acknowledges frame

22
Q

Procedure of unsuccesful transmission (transmission error):

A

▪ A send frame to B
▪ B detects error
▪ B sends negative acknowledgement (“NACK”)
▪ A resends frame to B

23
Q

Procedure fo unsuccesful transmission (transmission lost)

A

▪ A sends frame to B
▪ Timeout occurs
▪ A resends frame to B

24
Q

What is the purpose of Media Acces Control (MAC)?

A

Media Acces Control regulates access to a shared medium

25
Q

What are types of static Media Acces Control (MAC)?

A
  • Frequency (Frequency Devision Multiple Access)
  • Time (TDMA)
  • Code (CDMA)
  • Hybrid
26
Q

What are types of dynamic MAC?

A
  • with collisions: CSMA/CD (carrier sense multiple access / collision detection)
  • without collision: CSMA/CR
27
Q

Describe CSMA/CD

A 
► Carrier sense multiple access / collision detection
► Wait until medium is free
► Start sending
► If collision is detected
    ▪ Scramble
    ▪ Back off
► High data rate / long range
28
Q

Describe CSMA/CR

A 
► Carrier sense multiple access / collision resolution
► Wait until medium is free
► Start sending
► If collision is dominated
    ▪ Stop sending
    ▪ Start receiving
► No Collisions
► Either dominant
    ▪ 1 (“wired or”)  or
    ▪ 0 (“wired and”)
29
Q

In which areas are the individual bus systems most widely used?

A

I2C bus: auf Boards ~cm
CAN bus: Automotive, (Automation)
FlexRay: Automotive (sicherheitskritische Anforderungen)
PROFIBUS: for automation in industrial environments

30
Q

General information about the I2C bus:

A

► Connects multiple devices on the same board
► Noise-prone (used inside shielded casings)
► Simple and cheap
► Very popular

31
Q

I²C –Physical Layer

A

► Two lines connected to pull-up resistors
▪ SCL: serial clock line
▪ SDA: serial data line
► Devices are connected via open connectors
► High level (logical 1): >0.7V (usually 3.3V –5V)
► Low level (logical 0): -0.5V –0.3V
► Maximum Capacity 400pF (few meters)
► Wired-AND (dominant 0)

32
Q

CAN –Physical Layer

A

► Shielded twisted pair (also: optical fiber)
► Comfort bus (low speed) also possible with single line
► Higher data rates use difference signals
▪ Prevents common-mode interference (Gleichtaktstörung)
▪ Can use optional third line CAN_GND
► Simple NRZ with bit stuffing after 5 equal bits
► Wired-AND (dominant 0; see I²C)
► Usually up to 32 participants
► 64, 110, and 128 (with limitations) possible
► More participants with repeaters and bridges
► Bus termination with 120Ω resistor

33
Q

General information about the CAN

A

► Connects multiple controller units in harsh environments
▪ Up to 5km at 10kbit/s
▪ Up to 25m at 1Mbit/s
► Developed by Bosch in 1983
▪ Reduce number and length of cables (weight / cost)
▪ Reduce number and types of connectors (wiring errors)
► Main application as automotive bus
► Also used as industrial field bus (CANopen)

34
Q

CAN –Data Link Layer

A

► Each message type has unique identifier (priority)
► Devices have no address
► 4 frame types
► Arbitration by CSMA/CR: first 0 wins
► Cyclic Redundancy Check (CRC) (15 bit) error detection

35
Q

What is CANopen?

A

► Application layer protocol
► Based on CAN bus
► Four basic services
▪ Request: application requests service
▪ Indication: system notifies application of event
▪ Response: application replies to an indication
▪ Confirmation: system confirms service execution

36
Q

General information about the FlexRay:

A

► Automotive network communication protocol
► Deterministic timing →real time capable (X-by-wire)
► Hybrid MAC: TDMA + dynamic part (reservation)
► Integrates parts of ByteFlight protocol (BMW)

37
Q

FlexRay –Physical Layer

A

► Shielded twisted pair
► NRZ (non return to zero)
► 8 samples per bit (majority vote of 5 samples)

38
Q

PROFIBUS-DP –Physical Layer

A 
► RS-485
    ▪ Shielded twisted pair
    ▪ 9600 bit/s –12Mbit/s
    ▪ Bus topology 
    ▪ 100m –1200m between repeaters (depends on data rate)
    ▪ NRZ
► Optical fiber
    ▪ Star, bus, or (redundant) ring topology
    ▪ Up to 15km between repeaters
39
Q

PROFIBUS-DP –Data Link Layer

A 
► Fieldbus Data Link (FDL)
► Each device has 7 bit address
► 5 frame types
► Master polls / pushes data
► Multi-master by token passing
► CRC (8 bit) error detection
► Hamming distance of 4 for delimiters

Embedded Systems (7 decks)

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