Error Control

• We assume two types of errors
  • A Lost frame error occurs when a frame is never received.
  • A Damaged frame is a frame that arrives but is recognized to be in error.
• Employ the following techniques
  • Error Detection
  • Positive acknowledgment, for frames received which are not in error
  • Retransmission after timeout
  • Negative acknowledgment and retransmission
• These form a protocol called automatic repeat request (ARQ)
• We can use this with
  • Stop and Wait
    • Not much to say.
    • It is simple, but slow.
  • Go-back-N
    • A REJ (reject request) might be sent
    • Or a timeout might occur.
    • If a REJ or timeout occurs, all outstanding frames are retransmitted.
    • Remember, we can lose a data packet or a control packet.
    • What happens if we lose a data packet?
      • The receiver does not know it is on the way
      • So it will never ACK the packet.
      • So the sender will time out and resend the packet.
    • How about a bad packet?
      • The receiver can send a NAK (REJ)
      • The sender can time out and retransmit the data packet.
    • If an ACK is not received
      • A later ACK can acknowledge the packet
      • Or a timeout can occur, and the set of packets can be resent.
    • If a duplicate ACK is received
      • Nothing, just drop it
    • If a duplicate packet is received
      • Nothing, just drop it
  • Selective-reject
    • Much more complicated record keeping
    • Much more chance for error.
• This is the red army/blue army problem.