Layer Data Link Pertemuan 13 Matakuliah: H0484/Jaringan Komputer Tahun: 2007

Bina Nusantara Learning Outcomes Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu : Menjelaskan peran dan fungsi data link layer

Bina Nusantara Outline Materi Flow Control Error Control Link Access

Bina Nusantara Data Link Layer Functions Provide service interface to the network layer Services Provided to the Network Layer Flow Control Regulating data flow Slow receivers not swamped by fast senders Error Control Dealing with transmission errors Framing Character Oriented Bit Oriented Line Management (Medium Access) Polling Contention

Bina Nusantara Frame Transmission

Bina Nusantara Flow Control Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment Ensuring the sending entity does not overwhelm the receiving entity – Preventing buffer overflow Transmission time – Time taken to emit all bits into medium Propagation time – Time for a bit to traverse the link

Bina Nusantara Stop and Wait Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

Bina Nusantara Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k) – Frames are numbered modulo 2 k

Bina Nusantara Sender sliding window

Bina Nusantara Example Sliding Window

Bina Nusantara Automatic Repeat Request (ARQ) Stop and wait Go back N Selective reject (selective retransmission)

Bina Nusantara METODE KOREKSI ERROR FORWARD ERROR CORRECTION (FEC): – DIGUNAKAN SISTEM SANDI YANG SECARA OTOMATIS DAPAT MEMPERBAIKI KESALAHAN YANG DITEMUKAN BACKWARD ERROR CORRECTION – PENGIRIMAN ULANG DATA YANG TERGANGGU LST/FASKD/CINQ

Bina Nusantara STOP AND WAIT

Bina Nusantara STOP AND WAIT- NUMBERING Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost. In Stop-and-Wait ARQ, numbering frames prevents the retaining of duplicate frames.

Bina Nusantara Go Back N If no error, ACK as usual with next frame expected Use window to control number of outstanding frames If error, reply with rejection – Discard that frame and all future frames until error frame received correctly – Transmitter must go back and retransmit that frame and all subsequent frames

Bina Nusantara Go Back N - Normal

Bina Nusantara Selective Reject Also called selective retransmission Only rejected frames are retransmitted Subsequent frames are accepted by the receiver and buffered Minimizes retransmission Receiver must maintain large enough buffer More complex login in transmitter

Bina Nusantara Selective Reject - Lost Frame

Bina Nusantara Selective Reject -Diagram