Chapter 3. The Data Link Layer. Data Link Layer Design Issues. Services Provided to the Network Layer Framing Error Control Flow Control

Transcrição

1 Chapter 3 The Data Link Layer Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control 1

2 Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders Functions of the Data Link Layer (2) Relationship between packets and frames. 2

3 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication. Services Provided to Network Layer (2) Placement of the data link protocol. 3

4 Framing A character stream. (a) Without errors. (b) With one error. Framing (2) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing. 4

5 Framing (3) Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver s memory after destuffing. Violação de Códigos Onda de Relógio Bits J K Sinal NRZ Manchester a) Bit 1 - transição positiva (subida) no meio do bit b) Bit 0 - transição negativa (descida) no meio do bit c) Bits J e K - ausência de transição 5

6 Presença e Ausência de Sinal no Meio Tempo Intervalo de silêncio IFG - interframe gap Error Detection and Correction Error-Correcting Codes Error-Detecting Codes 6

7 Error-Detecting Codes Calculation of the polynomial code checksum. Sliding Window Protocols A One-Bit Sliding Window Protocol A Protocol Using Go Back N A Protocol Using Selective Repeat 7

8 Controle de Erro: Protocolo Stop-and-Wait Transmissor Receptor ACK 0 ACK 0 Intervalos de Timeout T Sliding Window Protocols (2) A sliding window of size 1, with a 3-bit sequence number. (a) Initially. (b) After the first frame has been sent. (c) After the first frame has been received. (d) After the first acknowledgement has been received. 8

9 A One-Bit Sliding Window Protocol (2) Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case. The notation is (seq, ack, packet number). An asterisk indicates where a network layer accepts a packet. A Protocol Using Go Back N Pipelining and error recovery. Effect on an error when (a) Receiver s window size is 1. (b) Receiver s window size is large. 9

10 Controle de Erro: Protocolo Selective Repeat Intervalo de TIMEOUT A1 A8 A9 0 1 E Quadros bufferizados Quadros descartados Tempo Controle de Fluxo a) Regula o fluxo de quadros entre transmissor e receptor b) Resolve o problema de diferença entre velocidade de transmissão e recepção c) Não permite que uma estação transmissora mais rápida sobrecarregue uma estação receptora d) Técnicas: Stop-and-Wait Sliding Window 10

11 Protocolo Stop-and-Wait Transmissor ACK 0 ACK 0 Receptor ACK 0 ACK 0 T Protocolo Sliding Window Transmissor Receptor Quadro 2 Quadro 3 ACK 3 Quadro 2 Quadro 3 ACK 3 Quadro 4 Quadro 4 Quadro 2 Quadro 2 T ACK 2 ACK 2 11

12 Protocolo Sliding Window Transmissor Envia um quadro Envia dois quadros 0 Receptor Recebe um quadro Recebe dois quadros Envia um quadro Recebe Ack de quatro quadros Envia um quadro Envia dois quadros Recebe Ack de três quadros Recebe um quadro Envia Ack de quatro quadros Recebe um quadro Recebe dois quadros Envia Ack de três quadros A Sliding Window Protocol Using Selective Repeat (5) (a) Initial situation with a window size seven. (b) After seven frames sent and received, but not acknowledged. (c) Initial situation with a window size of four. (d) After four frames sent and received, but not acknowledged. 12

13 The Data Link Layer in the Internet A home personal computer acting as an internet host. PPP Point to Point Protocol The PPP full frame format for unnumbered mode operation. 13

14 PPP Point to Point Protocol (2) A simplified phase diagram for bring a line up and down. PPP Point to Point Protocol (3) The LCP frame types. 14