Data Link Layer Flashcards
What is a data frame and what components does it have?
A data frame is the data that is being sent over a network.
Components:
- Starting Flag (describes the start of the frame)
- Header (Meta info)
- Payload (Data from the upper layers)
- Checksum
- Ending Flag
What is the flow control and what are 2 protocols?
Helps preventing the flooding of a slower receiver. Sender and receiver use buffer memory to work with. The sender has to respect the size of the receiver memory so no data gets lost transmitting. The receiver tells the sender his size. Sender also has to check that his buffer doesn’t get overloaded by the upper layers.
- Stop and Wait
- Sliding Window
Describe the Sliding Window protocol.
- Uses buffer
- Send number of frames that have space in buffer.
- Sender knows how many frames he sent and how many he can send until buffer is full because of sequence number of each frame
- After the receiver processed his buffer he sends an ACK to the sender
Error correction: Parity: How does it work and what is the problem?
For every char (7bit) an eight bit is added which is the parity bit. With the parity bit number of 1s have to be even. If because of an error one bit has flipped the parity bit is incorrect and therefore the error can be corrected.
- If 2 bits have flipped the parity bit notices nothing.
Error detection: Checksum: How does it work and does detect malicious attacks?
Checksum is sent with every frame. The receiver can calculate the checksum of the frame on his end and compare with the sent checksum. Therefore he knows if all of the message is there.
- No it doesn’t be cause an attacker can easily change the checksum of the frame.
What are the 3 error control protocols?
They base on ARQ (Automatic Repeat Request). It bases on the resending a frame on error by either receiving an NAK or Timeout.
- Stop and wait ARQ
- Go back -n ARQ
- Selective Repeat ARQ
Have a look at the slides in the 100_exam_papers folder.
We have a token-ring network of multiple devices. Each of them wants to send data to another one. How can we give each of them a fair chance to send data? Name 3 access schemes you know
Multiple apps want to transfer data over a link. So we have to be careful and want every device to have the chance to send an that there are no collisions.
- Orthogonal access scheme: Scheduled.
- Controlled access scheme: Round Robin
- Random access scheme: Random.
What is a collision domain?
Where collisions between packets may happen. Connected to a hub everything is a collision domain because he broadcasts every packet
2 Types of switches:
Cut-Through Switch: Reads DST mac and sends frame
Store-and-forward: Reads the whole frame and detects errors and sends the frame
What is the spanning tree protocol?
Protocol to detect loops
Describe the ethernet frame
Look at the slides
How is a mac address constructed?
- 6 Bytes
- First 3 Bytes: Identifies the constructor, Organizationally Unique Identifier
- Last 3 Bytes: Network Interface Controller Specific
What is the meaning of the 2 least significant bits (of the first byte) of a mac address?
1: U/L Bit (Universal / Local): isset -> Working with local addresses
2: I/G Bit (Individual / Group) isset -> Unicast
What is a VLAN and what is a “Trunk port”?
VLANs separate a physical switch into multiple, separate broadcasts domain (vlans). This can be done over multiple switches, so there is the trunk port to communicate between them. The ethernet frame is extended with a VLAN Tag to show to which VLAN the frame belongs.
Describe the process of connection to a WLAN.
Describe with the sequence diagram
