Matakuliah : H0484/Jaringan Komputer
Tahun
: 2007
Layer Data Link
Pertemuan 13
Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa
akan mampu :
• Menjelaskan peran dan fungsi data link layer
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Outline Materi
• Flow Control
• Error Control
• Link Access
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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
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Frame Transmission
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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
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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
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Sliding Windows Flow Control
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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 2k
Sender sliding window
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Example Sliding Window
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Automatic Repeat Request (ARQ)
• Stop and wait
• Go back N
• Selective reject (selective retransmission)
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METODE KOREKSI ERROR
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
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LST/FASKD/CINQ
STOP AND WAIT
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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.
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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
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Go Back N - Normal
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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
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Selective Reject - Lost Frame
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Selective Reject -Diagram
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