3. Datalink layer Flashcards
(25 cards)
What’s IPv6 Anycast addressing?
- Assigns IP address to nodes on devices in different locations.
- If packet sent to anycast address, it’s forwarded to nearest node with same address
IPv6 applications in load balancing scenarios
- Instead of sending requests to 1 server, system directs each request to closest server.
- no overload & quick responses
IPv6 applications in high availability scenarios
- If node/server stops working, system finds another nearby.
- Ensures user access to service even if system has problems
Time taken to receive ICMP Time Exceeded response for an ICMP Echo Request with network delay
Time = TTL (hops) * Network delay
What’s the Data Link Layer’s purpose?
To provide node-to-node delivery to the Network Layer
Which are the Data Link Layer’s responsibilities?
- Framing
- Error Control
- Control usage of transmission medium
Which are the sublayers of the Data Link layer?
- Logical Link Control:
Handles framing, flow & error control - Medium Access Control (MAC):
Avoids collisions
How does the ALOHA Multiple Access Protocol work?
Pure ALOHA: send data → collision → wait random time → resend
Slotted ALOHA (fewer collisions): Time divided into slots → wait till start of next slot → resend
With CSMA (Carrier Sense Multiple Access) stations listen before sending data.
Which are the different Persistence methods?
One persistent: send when idle.
- Nonpersistent: Wait random time, check if idle, if it’s idle send.
- p-persistent: Send with probability p if idle & wait q seconds for start of next slot (q = 1-p)
How does CSMA/CD (Collision Detection) detect a collision?
If received signal is weaker than signal sent
Give 2 Types of Errors due to interference
- Single-bit Error: 1 bit flipped
- Burst Error: multiple bits flipped
Block coding adds redundant bits to detect errors. Explain how.
- message divided into blocks of k bits (data words)
- r redundant bits added to each block (codewords of size n=k+r)
2^k data words & 2^n codewords.
n>k so 2^n - 2^k codewords not used (invalid).
Explain Two-dimensional Parity Bits
Detect & correct single-bit errors over blocks.
Cyclic Redundancy Check (CRC) creates codewords with polynomials.
a) What does the degree determine? b) Define the pattern.
a) Determines number of bits added to data to create codeword.
b) Has at least 2 bits, first & last bit are 1 & coefficients are 1 or 0.
Explain the CRC procedure for the encoder.
- divide codeword by bit pattern of generating polynomial.
- Append remainder to codeword
Explain the CRC procedure for the decoder.
- Divide codeword received by same divisor that encoder used.
- if remainder is 0, dataword extracted from codeword. Else, discard.
Which Burst errors of length ‘L’ & remainder ‘r’ are detected?
- Errors of size L <= r detected
- Errors of size L = r+1 detected with probability 1-0.5
- Errors of size L > r+1 detected with probability 1- (0.5^r)
If 1 frame has an error, the rest are unaffected.
How do u determine the start & end of a frame?
-
Character-oriented:
- Flags at start & end
- ESC added when Flag found
- If ESC found before Flag, add another ESC before it -
Bit-Oriented:
Flag 01111110 at start & end.
Bit stuffing (after 5 ones insert 0)
What’s ACK (Acknowledgement)?
Sent by receiver after successful frame receipt.
If no ACK received, timeout triggers retransmission.
Define:
a) Simplex Stop-and-Wait Protocol
b) Stop-and-Wait for Noisy Channel
a) send 1 frame, wait ACK
b) timer used, duplicate frames avoided with 1-bit sequence numbers between frames
Define the Sliding window protocol
- sends n frames before any ACK
- network bandwidth not wasted during waiting time.
- requires full duplex links
Explain the Retransmission strategy (if timeout) Go-Back-N procedure.
- Sender sends all frames up to window size N w/o waiting ACK.
- Keeps track of oldest unACK frame.
- If 1 frame times out or gives error, sender goes back & resends that frame & all after it again.
- Receiver only accepts in-order frames, so frames after missing frame r discarded.
Explain the Retransmission strategy (if timeout) Selective Repeat procedure.
- Sender sends frames up to window size N w/o waiting for ACK.
- If frame lost, only that frame is resent.
- Receiver stores out-of-order frames in buffer until missing frames received.
(More efficient but needs larger buffers)
Disadvantages of Go-Back-N
- Wastes bandwidth if only 1 frame is lost
- Correctly sent frames r resent unnecessarily
