7c- video-10-1-24

Class Announcements

• Updated class schedule.

• Midterm 1 covers:

  • Motivation to Slide Set 6a.

  • Questions posted in Quercus pertain to MAC analysis.

• Homework solutions will be posted early for comparison.

• Midterm covers material through Homework 4.

Protocol Overview

• Stop and Wait Protocol:

  • Minimum time for successful transmission:

    • Packet time + Acknowledgment time + 2 * Propagation delay + Processing delay.

  • Link level errors may require retransmissions.

Events and Assumptions

• Independence in Events:

  • Assuming events occur independently leads to:

    • Sequence of failures followed by a success.

    • Geometric random variable used to model transmissions.

  • Goal: Determine average time to successfully send a packet in presence of errors.

Retransmission and Success Probability

• Geometric Random Variable:

  • Number of attempts needed for success:

    • Average of 1/p, where p is the probability of successful transmission of a packet.

• Time out occurs for failed transmissions.

Efficiency of Protocols

• Efficiency Calculation:

  • Efficiency = Packet Time / Average Time for Successful Transmission.

  • Efficiency affected by:

    • Probability of success (p).

    • Propagation delay and acknowledgment lengths.

Selected Repeat ARQ Protocol

• Goal:

  • More efficient than Stop and Wait.

• Buffering Requirements:

  • Both sender and receiver must buffer w packets.

  • Packets sent in order with unique identifiers (0 or 1).

  • Out-of-order packets must be buffered by the receiver; replicas must be discarded.

Buffer Management Policy

• Sender must not send new packets if the sequence number difference exceeds buffer capacity:

  • Difference between highest and lowest sequence number in buffer must be smaller than w.

• Out-of-order packets in the receiver buffer wait until the missing packets are received to pass to network layer.

Problems with Sender Buffer Overflow

• Buffer Overflow Issues:

  • Sender sending packets without regard for receiver buffer capacity.

  • Need to restrict sender by enforcing the buffering policy.

Ensuring Efficiency in Transmission

• When both ends have sufficient buffer space, sender can send without stopping as long as no errors occur.

• Efficiency of Selective Repeat:

  • Efficiency depends on the probability of success.

  • With infinite buffer, efficiency approaches 1.

Disconnection and Link Initialization

• Need to establish an initial sequence number before communication:

  • Sender initiates with an initialization packet.

  • Receiver acknowledges initialization.

  • Disconnection follows similar rules.