hm_6_flow
TRANSCRIPT
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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
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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