FL 5 kommunikationssystem
describe ARQ
• ARQ (retransmission) protocols provide a service delivering SDUs error-free, in order, and without duplication
• ARQ protocol key components
– error detection (e.g., parity check codes, Internet checksum)
– sequence numbers
– timeout mechanism
– acknowledgments (ACKs)
stop and wait
Stop-and-wait main principle
– Transmitter
1. Send a frame with a single SDU, sequence number, and error detection bits
2. Retransmit if timeout expires
3. Move on to send next SDU if error-free ACK with correct sequence number is received
– ACKs with errors or wrong sequence numbers are ignored
– Receiver
4. Accept error-free frames with correct sequence number and send ACK to receiver
– Erroneous frames are ignored
– Frames with wrong sequence numbers trigger retransmission of previous ACK
review effective data rate and efficency
• Consider an ARQ-protocol in layer n
• Layer-n service provides error-free transmission of n-SDUs
• The effective data rate of the service, i.e., the data rate provided to layer (n + 1) is R-eff = # of bits in one SDU /
average time to transmit one SDU
• If layer (n − 1) has effective rate R, then the efficiency of the ARQ protocol is η= R-eff /R≤1
go back n
• Main idea: transmit multiple packets without waiting for ACKs • Packets that have been transmitted but not ACKed are called “outstanding” packets
• Outstanding packets are stored in a buffer called a send window