1 Pertemuan 15 Layer Data Link Matakuliah: H0174/Jaringan Komputer Tahun: 2006 Versi: 1/0

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

3 Outline Materi Flow Control Error Control Link Access

4 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

5 Frame Transmission

6 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

7 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

8 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

9 Sender sliding window

10 Example Sliding Window

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

12 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

13 STOP AND WAIT

14 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.

15 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

16 Go Back N - Normal

17 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

18 Selective Reject - Lost Frame

19 Selective Reject -Diagram

20 Pertemuan 16 Layer Data Link - Lanjutan Matakuliah: H0174/Jaringan Komputer Tahun: 2006 Versi: 1/0

21 Learning Outcomes Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu : Menjelaskan ketentuan baku dan format frame pada data link layer

22 Outline Materi Standards Frame format

23 Frame Structure In synchronous transmission all transmissions are in frames Single frame format for all data and control exchanges Basic Structures: –Character Oriented Protocol (COP) - BSC (Bisynchronous) –Bit Oriented Protocol (BOP) - HDLC, SDLC

24 Generic Frame Structure (BOP)

25 Bit stuffing and removal Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag.

26 Data Link Protocols HDLC - High-Level Data Link Control SDLC - Synchronous Data Link Control PPP - Point to Point Protocol Ethernet HDLC can be considered as the base protocol format

27 High-Level Data Link Control Frame format for bit-oriented protocols.

28 PPP – Point to Point Protocol The PPP full frame format for unnumbered mode operation.

29 Protocol stack Link Control Protocol (LCP) Authentication Protocols Network Control Protocol (NCP) Bit Padding Digital Signals (DS) Services

30 Encapsulated LCP packet

31 PPP – Point to Point Protocol

32 An example