Computer Networks - Data Link Layer

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Questions and Answers

What is the minimum time period a node must wait before transmitting after the previous transmission has ended?

5 bit times

7 bit times

1 bit time

3 bit times (correct)

What is the maximum number of bytes that can be sent in a frame?

6 bytes

9 bytes

7 bytes

8 bytes (correct)

What type of connector is recommended for harsh environments?

Hard-wired (unsealed) connection

Mini (sealed) connector (correct)

Screw type terminations

Barrier strips

What is the purpose of the first byte in a fragmented frame?

To facilitate the reassembly of fragments Signup and view all the answers

What happens when a device needs to send more than 8 bytes of data?

The data is divided into multiple frames Signup and view all the answers

What is the role of the application layer?

To configure and collect data from any network Signup and view all the answers

What is the significance of the first byte being '00' in a fragmented frame?

It indicates the first fragment (number 0) Signup and view all the answers

Why is care required when handling hard-wired connections?

Because the cables are live when removed from a node Signup and view all the answers

What is the maximum allowed current for a power supply on a DeviceNet network using thick or flat wire?

8 A Signup and view all the answers

Which topology does DeviceNet utilize?

Bus topology Signup and view all the answers

What is the maximum size of the data field in a DeviceNet frame?

8 bytes Signup and view all the answers

What is the medium access control method used by DeviceNet?

Carrier sense multiple access with non-destructive bit-wise arbitration Signup and view all the answers

What is the power supply specification for DeviceNet cable power?

Dedicated power supply only Signup and view all the answers

What type of connector does the DeviceNet system utilize?

Unsealed 5-pin connector Signup and view all the answers

What happens if a message larger than the data field size is sent in a DeviceNet frame?

It must be fragmented Signup and view all the answers

What is the termination requirement for DeviceNet's bus topology?

Terminate both ends with 120 Ω resistors Signup and view all the answers

What is the primary disadvantage of Xon-Xoff flow control?

It consumes bandwidth by transmitting control characters within the data stream. Signup and view all the answers

What is the purpose of 'zero insertion' in the Xon-Xoff flow control protocol?

To ensure that the receiver can distinguish between data and control characters. Signup and view all the answers

Which of the following is NOT a characteristic of the RS-232 interface standard?

It typically uses a dedicated control channel for flow control, separate from the data channel. Signup and view all the answers

Which of the following is NOT a valid frame type used in the Fieldbus Control protocol?

Configuration frame Signup and view all the answers

What is the role of the 'Control' field in a Fieldbus Control frame?

To define the function of the frame within the network. Signup and view all the answers

Which of the following network topologies is NOT supported by the Fieldbus Control protocol?

Mesh Signup and view all the answers

Which of the following protocols is standardized as IEC 61158?

Fieldbus Control Signup and view all the answers

What is the primary purpose of the 'Frame Check Sequence' field in a Fieldbus Control frame?

To detect and correct errors that occur during transmission. Signup and view all the answers

Study Notes

Physical Layer (Connectors)

Transmission uses barrier strips or screw type terminations
Hard-wired (unsealed) connection: ends of the cable are 'live' when removed from node and still connected to bus infrastructure, requiring insulation of exposed ends
Mini (sealed) connector: 18 mm round connector recommended for harsh environments (field connections)

Data-Link Layer (Fragmentation)

Any device needing more than 8 bytes of data sent will cause fragmentation
A frame can only contain 8 bytes of data; first byte used for reassembly
Fragmentation: 7 bytes of data sent at a time, with first byte facilitating reassembly
First byte significance:
- 00: First fragment (number 0)
- 41-7F: Intermediate fragment (lower 6 bits of byte is fragment number)
- 80-FF: Last fragment (lower 6 bits of byte is fragment number)

Data-Link Layer (Frame Format)

DeviceNet frame: small data field (8 bytes), messages larger than this need to be fragmented
Frame fields: flag, address, control, information, frame check sequence

Physical Layer (Network Power)

One or more 24 V power supplies can be used to power devices on DeviceNet network
Power supplies should be dedicated to DeviceNet cable power only
8 A current limit on thick/flat wire and 3 A limit on thin wire must not be exceeded

Data-Link Layer (Medium Access)

Medium access control method: carrier sense multiple access with non-destructive bit-wise arbitration (CSMA/NBA)
Node wishing to transmit must defer to any existing transmission

Application Layer

Provides user with standard set of messaging services for all three networks
Allows user to connect to any network and configure/collect data from any network
Saves time and effort during system configuration, no routing tables or added logic required

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Description

Quiz on data link layer concepts, including transmission, connectors, and cable connections.