3 Data Buses Flashcards
Name advantages that the star topology has over the bus topology.
- Multiple senders at same time
- Only one single point of failure
- No collisions
- No implicit broadcasts
The sublayers of the Data Link Layer (DLL are:)
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)
What topology uses I2C?
Bus Topology
What does Wired-AND mean?
A zero on the Bus is dominant.
What is the basic MAC concept of I2C? Why are there no collisions when multiple senders start at the same time?
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.
What is the topology of Profibus and how is media access controlled?
Profibus uses a Bus Topology. Media Access is controlled through Master/Slave and Multiple Masters are managed using a Token Bus.
What are properties of the Nyquist-Shannon theorem?
► 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
Name advantages of the Bus topology:
- cheap
- simple
Name disadvantages of the Bus topology:
- multiple acces (Babbling idiot, security)
- Single point of failure
Name advantages of the Star topology:
- No multiple access
- Only central station is single point of failure
Name disadvantages of the star topology:
- expensive central station
- more wiring
Name advantages of the Ring topology:
- High quality of service
- No multiple access
Name disadvantages of the Ring topology:
- Complex (expensive)
- Single Point of failure
What are the properties of the physical layer of the ISO/OSI 7 layer architecture?
► 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
Which methods for bit encoding exist?
► Return to Zero (RZ) ► Non Return to Zero (NRZ) ► Differential NRZ ► Bit stuffing ► Manchester Code ► 4B/5B Code
Explain the synchronization problem and name a solution for it:
► 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
What is Bit Stuffing and for what is it used for?
► 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
What are the properties of the 4B/5B code?
► Uses some form of NRZ
► 80% data rate
► Encoding table prevents long sequences (FDDI with NRZI)
What are the properties of the data link layer of the ISO/OSI 7 layer architecture?
► 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)How can Errors during the data transmission be detected?
► 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 detectionProcedure of succesful transmission:
▪ A sends frame to B
▪ B acknowledges frame
Procedure of unsuccesful transmission (transmission error):
▪ A send frame to B
▪ B detects error
▪ B sends negative acknowledgement (“NACK”)
▪ A resends frame to B
Procedure fo unsuccesful transmission (transmission lost)
▪ A sends frame to B
▪ Timeout occurs
▪ A resends frame to B
What is the purpose of Media Acces Control (MAC)?
Media Acces Control regulates access to a shared medium
What are types of static Media Acces Control (MAC)?
- Frequency (Frequency Devision Multiple Access)
- Time (TDMA)
- Code (CDMA)
- Hybrid
What are types of dynamic MAC?
- with collisions: CSMA/CD (carrier sense multiple access / collision detection)
- without collision: CSMA/CR
Describe CSMA/CD
► Carrier sense multiple access / collision detection
► Wait until medium is free
► Start sending
► If collision is detected
▪ Scramble
▪ Back off
► High data rate / long rangeDescribe CSMA/CR
► 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”)In which areas are the individual bus systems most widely used?
I2C bus: auf Boards ~cm
CAN bus: Automotive, (Automation)
FlexRay: Automotive (sicherheitskritische Anforderungen)
PROFIBUS: for automation in industrial environments
General information about the I2C bus:
► Connects multiple devices on the same board
► Noise-prone (used inside shielded casings)
► Simple and cheap
► Very popular
I²C –Physical Layer
► 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)
CAN –Physical Layer
► 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
General information about the CAN
► 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)
CAN –Data Link Layer
► 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
What is CANopen?
► 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
General information about the FlexRay:
► Automotive network communication protocol
► Deterministic timing →real time capable (X-by-wire)
► Hybrid MAC: TDMA + dynamic part (reservation)
► Integrates parts of ByteFlight protocol (BMW)
FlexRay –Physical Layer
► Shielded twisted pair
► NRZ (non return to zero)
► 8 samples per bit (majority vote of 5 samples)
PROFIBUS-DP –Physical Layer
► 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 repeatersPROFIBUS-DP –Data Link Layer
► 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
