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    Kontrol Komunikasi

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    Latar Belakang

    Kemungkinan terjadi kesalahan pada transmisiserta receiver data perlu mengatur rate terhadapdata yang diterima

    Lapisan kontrol pada setiap perangkat

    komunikasi yang menyediakan fungsi Flowkontrol, pendeteksian kesalahan dan kontrol

    kesalahanDATA LINK CONTROL PROTOCOL

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    Persyaratan dan Tujuankomunikasi Data Efektif

    Sinkronisasi Frame Flow Control

    Pengalamatan

    Kontrol dan data pada jalur yang sama

    Manajemen jalur

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    Flow Kontrol

    Teknik yang memastikan bahwa entitas pentransmisitidak membanjiri entitas penerima dengan data buffer overflow

    a tu pentransm s an waktu mengirimkan bit dari frame ke media

    Waktu propagasi

    waktu yang digunakan bit melintasi jalur diantarasumber dan tujuan.

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    Mode Transmisi Frame

    Stop and wait flow control

    (kontrol flow berhenti dan tunggu)

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    Stop and Wait

    Sumber mengirimkan frame

    Tujuan menerima frame dan memberi balasan dan siap

    Sumber Tujuanmedia

    data frame

    mener ma rame Sumber menunggu balasan diterima dan mengirimkan

    frame

    Tujuan menghentikan data dengan cara tidak memberi

    balasan (ACK) Prosedur ini bekerja baik jika frame yang dikirim sedikit.

    Blok data besar dipecah-pecah menjadi blok yangkecil dalam beberapa frame

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    Pengiriman Blok Kecil Ukuran penyangga terbatas

    Blok data yang besar menyebabkan transmisi lebih lama sehinggaterjadi kesalahan

    pengiriman ulang frame

    Media bersama (LAN), satu station tidak boleh menempati mediadalam waktu lama karena dapat terjadi penundaan pada stationpengirim yang lain.

    Prosedure stop and wait tidak cukup digunakan untuk frame multiple

    pada pesan tunggal satu frame dilintaskan dalam satu waktu

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    Penggunaan Jalur Stop and Wait

    Transmisi Frame

    Frame balasan

    Waktu transmisi =1, waktu propagasi=a

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    Sliding Windows Flow Control

    Permasalahan tidak hanya satu frame sekaligus yang dapat dikirimPanjang bit lebih besar dari panjang frame tidak efisien

    Frame multiple diangkut dalam waktu yang sama

    Penerima mempunyai panjang buffer W

    Transmitter (A) mengirimkan frame W tanpa balasan (ACK) Setiap frame diberi nomor

    Penerima (B) mengirimkan ACK dengan urutan nomor frameberikutnya

    Urutan nomorbidang k-bit (k) kisaran urutan nomor 0 sampai 2k-1 Frame diberi nomor modulo 2k

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    Diagram Sliding Window

    23=80 sampai 7

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    Contoh Sliding Window

    Ditransmisikan tanpa balasanA menyusutkan 4 frame

    Transmisi 4 frame

    Dibuang Dikirimkan 7 frame

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    Sliding Window Enhancements

    Protocol dapat memotong aliran frame dari sisi yang lain denganmengirimkan (Receive Not Ready)

    Receiver dapat meminta ACK frame tanpa pemberitahuan

    Jika komunikasi duplex menggunakan piggybacking

    Data dikirimkan dengan urutan nomor frame plus bagian yangmemuat urutan nomor yang digunakan untuk balasan.

    Data yang dikirimkan mememuat urutan nomor balasan tanpadata maka transmiter tersebut mengirimkan frame ACK yangterpisah RR atau RNR

    Jika ada data yang terkirim tetapi tanpa balasan untuk dikirimmaka mengirimkan ulang urutan balasan terakhir atau ACK validflag (TCP)

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    Pendeteksian Kesalahan

    Menambahkan bit oleh transmitter untuk pendeteksiankesalahan bit

    Parit

    Nilai dari bit parity yaitu transmisi karakterparitas ganjil (ood parity) atau parity genap (even parity)

    Parity genap menyebabkan kesalahan bit bisa tidak terdeteksi.

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    Cyclic Redundancy Check

    For a block of kbits transmitter generates nbit sequence Transmit k+nbits which is exactly divisible by some

    number

    If no remainder, assume no error

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    Error Control

    Pengkontrolan kesalahn berkaitan denganmekanisme mendeteksi dan memperbaikikesalahan an ter adi ada

    petransmisian frame. Frame Hilang

    Frame Rusak

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    Teknik Mengkontrol Kesalahan

    Pendeteksian Kesalahan Balasan positif

    Mengembalikan balasan positif untuk frame bebas-kesalahan diterima dengan baik

    Sumber melakukan retransmisi frame yang belumdibalas setelah beberapa saat tertentu

    Balasan Negatif dan RetransmisiTujuan mengembalikan balasan negatif kepada frameyang dideteksi mengalami kesalahan dan sumbermelakukan retransmisi.

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    Automatic Repeat Request(ARQ)

    Stop and wait Go back N

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    Stop and Wait

    Sumber mengirimkan frame tunggal, menunggu ACK tidak ada frame dikirim sampai jawaban dr tujuan tiba sumber.

    Jika frame yang diterima rusak, membuang frame

    Transmitter dilen ka i encatat waktu

    Jika tidak ada ACK sampai waktu tertentu, pengiriman ulangframe dengan frame tiruan

    Jika ACK rusak, damaged, transmitter tidak mengenali frame

    Transmitter pengiriman ulang frame

    Penerima menerima 2 frame yang sama sehingga membuangframe

    Pemberian label 0 atau satu, ACK0 dan ACK1

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    Stop and Wait -Diagram

    ACK0 membalas penerimaan frame bernomor 1danmenunjukkan bahwa receiver siap untuk frame bernomor 0

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    Kelebihan Stop and Wait

    Sederhana Tidak efisien

    diadaptasikan agar penggunaan jalur lebihefisien.

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    Go Back N (1)

    Berdasarkan sliding window Jika tidak ada error, tujuan mengirimkan ACK untuk

    frame selanjutnya (RRReceive Ready)

    Men unakan window untuk kontrol umlah frame

    Jiak error, mengembalikan REJrejectMembuang frame sampai frame yang diterima benar

    Transmitter menerima REJ akan mengirimkan ulangsemua frame yang mengalami kesalahan

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    Go Back N Frame Rusak

    Receiver mendeteksi kesalahan frame Imembuang frame

    -

    Transmitter gets rejection-i

    Transmitter retransmits frame iand all

    subsequent

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    Go Back N - Lost Frame (1)

    Frame i lost Transmitter sends i+1

    Receiver send reject i

    Transmitter goes back to frame iand

    retransmits

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    Go Back N - Lost Frame (2)

    Framei

    lost and no additional frame sent Receiver gets nothing and returns neither

    acknowledgement nor rejection

    frame with P bit set to 1

    Receiver interprets this as command which itacknowledges with the number of the next frame it

    expects (frame i) Transmitter then retransmits frame i

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    Go Back N - DamagedAcknowledgement

    Receiver gets frameiand send acknowledgement (

    i+1)which is lost

    Acknowledgements are cumulative, so next

    times out on frame i If transmitter times out, it sends acknowledgement with P

    bit set as before

    This can be repeated a number of times before a resetprocedure is initiated

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    Go Back N - DamagedRejection

    As for lost frame (2)

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    Go Back N -Diagram

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    Selective Reject Disebut juga selective retransmission

    Retransmisi frame mempunyai balasan negatif (rejectedframes)

    Menerima frame yang datang dan menahan framesampa rame va ter ma

    Minimalkan retransmission

    Receiver harus mempertahankan buffer yang besar

    Lebih komplek (transmitter) karena memuat logik untuk

    diselipkkan pd frame shg urutan tepat

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    Selective Reject -Diagram

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    Figure 11.4

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    Error-Free Stop and Wait

    T = Tframe + Tprop + Tproc + Tack + Tprop + Tproc

    T = time to transmit frame

    Tprop = propagation time

    Tproc = processing time at station

    Tack = time to transmit ack

    Assume Tproc

    andTack

    relatively small

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    T Tframe + 2Tprop

    Throughput = 1/T = 1/(Tframe + 2Tprop)frames/sec

    Error-Free Stop and Wait (2)

    Tframe and the total time to send the dataand get the response Tframe + 2Tprop

    U = Tframe = 1Tframe + 2Tprop 1 + 2a

    where a = Tprop/ Tframe

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    The Parameter aa = propagation time = d/V = Rd

    transmission time L/R VLwhere

    d = distance between stations

    = v y

    L = length of frame in bits

    R = data rate on link in bits per sec

    Rd/V ::= bit length of the linka ::= ratio of link bit length to the length of frame

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    Stop-and-Wait Link Utilization

    If Tprop large relative to Tframe thenthroughput reduced

    transmission time, line is mostly idle

    Problem is only one frame in transit at a

    time Stop-and-Wait rarely used because of

    inefficiency

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    Error-Free Sliding Window ARQ

    Case 1: W 2a + 1Ack for frame 1 reaches A before A has

    exhausted its window

    Case 2: W < 2a +1A exhausts its window at t = W and cannot send

    additional frames until t = 2a + 1

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    Figure 11.10

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    Normalized Throughput

    1 for W 2a + 1

    U =

    W for W < 2a +1

    2a + 1

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    Stop-and-Wait ARQ with Errors

    P = probability a single frame is in error

    Nx = 1

    -

    = average number of times each frame must betransmitted due to errors

    U = 1 = 1 - P

    Nx (1 + 2a) (1 + 2a)

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    Selective Reject ARQ

    1 - P for W 2a + 1

    =

    W(1 - P) for W < 2a +1

    2a + 1

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    Go-Back-N ARQ

    1 - P for W 2a + 1

    U = 1 + 2aP

    W(1 - P) for W < 2a +1(2a + 1)(1 P + WP)

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    High Level Data Link Control

    HDLC ISO 33009, ISO 4335

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    HDLC Station Types

    Primary stationControls operation of link

    Maintains separate logical link to eachsecondary station

    Secondary station

    Under control of primary station

    Frames issued called responses

    Combined station

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    HDLC Link Configurations

    UnbalancedOne primary and one or more secondary

    stations

    Supports full duplex and half duplex Balanced

    Two combined stations

    Supports full duplex and half duplex

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    HDLC Transfer Modes (1)

    Normal Response Mode (NRM)Unbalanced configuration

    Secondary may only transmit data inresponse to command from primary

    Used on multi-drop lines

    Host computer as primary

    Terminals as secondary

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    HDLC Transfer Modes (2)

    Asynchronous Balanced Mode (ABM)Balanced configuration

    without receiving permissionMost widely used

    No polling overhead

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    HDLC Transfer Modes (3)

    Asynchronous Response Mode (ARM)Unbalanced configuration

    permission form primaryPrimary responsible for line

    rarely used

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    Frame Structure

    Synchronous transmission All transmissions in frames

    exchanges

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    Frame Structure Diagram

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    Flag Fields

    Delimit frame at both ends 01111110

    Receiver hunts for flag sequence tosynchronize

    Bit stuffing used to avoid confusion withdata containing 01111110

    0 inserted after every sequence of five 1s

    If receiver detects five 1s it checks next bit

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    Bit Stuffing

    Example withpossible errors

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    Address Field

    Identifies secondary station that sent orwill receive frame

    May be extended to multiples of 7 bitsLSB of each octet indicates that it is the last

    octet (1) or not (0)

    All ones (11111111) is broadcast

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    Control Field Different for different frame type

    Information - data to be transmitted to user (next layerup)

    Flow and error control piggybacked on informationrames

    Supervisory - ARQ when piggyback not used

    Unnumbered - supplementary link control

    First one or two bits of control filed identify frame type

    Remaining bits explained later

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    Control Field Diagram

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    Poll/Final Bit

    Use depends on context Command frame

    1 to solicit (poll) response from peer

    Response frame

    F bit1 indicates response to soliciting command

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    Information Field

    Only in information and some unnumberedframes

    Variable length

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    Frame Check Sequence Field

    FCS Error detection

    Optional 32 bit CRC

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    HDLC Operation

    Exchange of information, supervisory andunnumbered frames

    Initialization

    Data transfer

    Disconnect

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    Examples of Operation (1)

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    Examples of Operation (2)

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    Other DLC Protocols(LAPB,LAPD) Link Access Procedure, Balanced (LAPB)

    Part of X.25 (ITU-T) Subset of HDLC - ABM

    switching network node Link Access Procedure, D-Channel

    ISDN (ITU-D)

    ABM Always 7-bit sequence numbers (no 3-bit)

    16 bit address field contains two sub-addresses

    One for device and one for user (next layer up)

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    Other DLC Protocols (LLC)

    Logical Link Control (LLC)

    IEEE 802

    Different frame format

    n contro sp t etween me um access ayer

    (MAC) and LLC (on top of MAC)

    No primary and secondary - all stations are peers

    Two addresses needed

    Sender and receiver Error detection at MAC layer

    32 bit CRC

    Destination and source access points (DSAP, SSAP)

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    Other DLC Protocols(Frame Relay) (1)

    Streamlined capability over high speedpacket witched networks

    .

    Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF)

    Two protocolsControl - similar to HDLC

    Core - subset of control

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    Other DLC Protocols(Frame Relay) (2)

    ABM 7-bit sequence numbers

    2, 3 or 4 octet address field

    Data link connection identifier (DLCI)

    Identifies logical connection More on frame relay later

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    Other DLC Protocols (ATM)

    Asynchronous Transfer Mode Streamlined capability across high speed

    Not HDLC based

    Frame format called cell

    Fixed 53 octet (424 bit) Details later

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    Required Reading

    Stallings chapter 7 Web sites on HDLC, frame relay, Ethernet