Komunikasi Data. Data Link Control Protocols

(1)

Komunikasi Data

Data Link

Control

(2)

SAP Komunikasi Data

Pendahuluan Protokol dan Arsitektur

Transmisi Data dan Media

Transmisi

Pengkodean Data Komunikasi Data

Digital Data Link Control

Switchin g

Multiplexing _KomputerJaringan

Spread Spectrum Start Finish UTS UU/UAS Tugas 1 Tugas 2

(3)

(4)

“A conversation forms a two-way communication

link; there is a measure of symmetry between the two

parties, and messages pass to and fro. There is a

continual stimulus-response, cyclic action; remarks

call up other remarks, and the behavior of the two

individuals becomes concerted, co-operative, and

directed toward some goal. This is true

communication.”

—On Human Communication

,

(5)

Data Link Control Protocols

• Pengiriman data melalui

link komunikasi data

yang terlaksana dengan penambahan kontrol

layer dalam tiap device komunikasi,

dinyatakan sebagai

data link control

atau

data link protocol

.

• Data link

adalah medium tramsmisi antara

stasiun-stasiun ketika suatu prosedur data link

control dipakai.

(6)

Data Link Control Protocols

• Kebutuhan dan tujuan untuk komunikasi data secara efektif antara

dua koneksi stasiun transmisi-penerima secara langsung terdiri dari :

Frame synchronization Flow control

Error control Addressing

Control and data Link management

(7)

Frame synchronization

• Data dikirim dalam blok-blok yang disebut

frame. Awal dan akhir tiap frame harus dapat

diidentifikasikan.

(8)

Flow Control

• Flow control

:

stasiun pengirim harus tidak mengirim

frame-frame pada rate/kecepatan yang lebih cepat

daripada stasiun penerima yang dapat

menyerapnya.

• Adalah suatu teknik untuk

memastikan/meyakinkan bahwa suatu stasiun

transmisi tidak menumpuk data pada suatu

stasiun penerima.

• Tanpa flow control, buffer dari receiver akan

penuh sementara sedang memproses data lama.

Karena ketika data diterima, harus dilaksanakan

sejumlah proses sebelum buffer dapat

(9)

Frame 1 Source Destination T im e Frame 1 Frame 2 Frame 2 Frame 3 Frame 3 Frame 4 Frame 4 Frame 5

(a) Error-free transmission

Figure 7.1 Model of Frame Transmission

Frame 5 Frame 1 Source Destination Frame 1 Frame 2 Frame 3 Frame 3 Frame 4 Garbled frame Frame 5 (b) Transmission with losses and errors

Frame 5

tiap tanda panah

menyatakan suatu

perjalanan frame

tunggal. Suatu data

link antara dua

stasiun dan

transmisinya bebas

error. Tetapi

bagaimanapun,

setiap frame yang

ditransmisi

semaunya dan

sejumlah delay

sebelum diterima.

(10)

Stop-and-Wait Flow Control

• Bentuk sederhana dari

flow control

• Cara kerjanya :

Suatu entity sumber mentransmisi suatu frame. Setelah diterima, entity tujuan memberi isyarat untuk menerima frame lainnya dengan mengirim acknowledgment ke frame yang baru diterima. Sumber harus menunggu sampai menerima acknowledgment sebelum mengirim frame berikutnya. Entity tujuan kemudian dapat menghentikan aliran data dengan tidak memberi acknowledgment.

(11)

t₀ _T R T R t₀_{+ 1 T} R T R t₀ + a_T R T R t₀ + 1 + a_T R T R t₀ + 1 + 2a ACK Frame t₀ t₀ + a t₀ + 1 t₀ + 1 + a t₀ + 1 + 2a _T R _T R (b) a > 1 (a) a < 1

(12)

Penjelasan Gambar 7.2

• Menggambarkan efek penggunaan a .

• Gambar 7.2a (a<1) dimana panjang bit lebih kecil daripada frame.

• Pada saat t0, stasiun mulai mentransmisi suatu frame.

• Pada t0+a, leading edge dari frame mencapai stasiun penerima, sementara stasiun pengirim masih melakukan proses transmisi frame.

• Pada t0+1, stasiun pengirim sudah mentransmisi secara lengkap.

• Pada t0+1+a, stasiun penerima sudah menerima seluruh frame dan langsung mentransmisi suatu frame acknowledgment yang pendek. Acknowledgment ini tiba kembali di stasiun pengirim pada t0+1+2a. Jadi total waktu penyebaran : 1 + 2a.

• Total waktu transmisi : 1. Sehingga efisiensi : U = 1 + 2a Hasil yang sama dicapai juga dengan a>1, yang digambarkan pada gambar 7.2b.

(13)

Contoh

• Pada local network dimana transmisi data digital melalui

modem; dimana:

– data rate = 9600 bps

– karena range jarak dari 0,1 – 10 Km

– dengan data rate 0,1 – 10 Mbps

– maka dipakai V = 2x108_m/s;

– ukuran frame yang dipakai 500 bit;

– jika dipakai pada jarak pendek d = 100 m, maka a = 9600 bps x 100 m = 9,6x10-6_{dan pemakaiannya efektif 2x10}8_{m/s x 500 bits} – Jika dipakai pada jarak yang jauh d = 5000 Km, maka a = 9600 x

(14)

Sliding Windows Flow Control

• Mengizinkan beberapa frame bernomor dalam

transit

–

Receiver memiliki buffer W panjang

–

Transmitter mengirim hingga W frame tanpa ACK

–

ACK mencakup jumlah frame berikutnya yang diharapkan

–

Nomor urut dibatasi oleh ukuran bidang (k)

• Frame diberi nomor modulo 2k

• Memberikan ukuran window size 2k _{– 1}

–

Penerima dapat ACK frame tanpa mengizinkan

transmisi lebih lanjut (Receive Not Ready)

• Harus mengirim tanda terima yang normal

untuk melanjutkan

(15)

Figure 7.3 Sliding-Window Depiction

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Window of frames that may be transmitted Frames already transmitted

Frames buffered until acknowledged Last frame transmitted Last frame acknowledged Frame sequence number

Window shrinks from trailing edge as frames are sent

Window expands from leading edge as ACKs are received (a) Sender's perspective

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Window of frames that may be accepted Frames already received

Last frame received Last frame

acknowledged

Window shrinks from trailing edge as frames are received

Window expands from leading edge as ACKs are sent (b) Receiver's perspective

(16)

Penjelasan Gambar 7.3

• Menunjukkan proses sliding-window. Anggap dipakai 3

bit penomoran, maka terdapat 0-7 nomor.

• Pada gambar, pengirim dapat mentransmit 7 buah

frame, yang dimulai dengan frame ke 6.

• _{Setiap kali frame dikirim, daerah dalam kotak akan}

menyusut; setiap kali sebuah acknowledgment

(17)

F0 F1 F2 RR 3 F3 F4 F5 F6 RR 4

Source System A Destination System B

Figure 7.4 Example of a Sliding-Window Protocol

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

(18)

Penjelasan Gambar 7.4

• menunjukkan suatu contoh, dimana dianggap ada 3 bit penomoran dan suatu ukuran window maksimum sebesar 7.

• A dan B mempunyai window yang mengindikasi bahwa A boleh mengirim 7 buah frame, dimulai dengan frame ke 0 (f0).

• Setelah mengirim 3 buah frame (f0,f1,f2) tanpa acknowledgment, A telah menyusutkan windownya menjadi 4 buah frame.

• Window ini menyatakan bahwa A boleh mentransmit 4 buah frame, dimulai dengan frame nomor 3; pada kenyataannya, saya siap

menerima 7 frame, yang dimulai dengan frame nomor 3.

• "Dengan acknowledgment ini, A kembali meminta izin untuk mentransmisi 7 frame masih, diawali dengan frame 3. A mulai

mentransmisi frame 3, 4, 5 dan 6. B mengembalikan ACK 4, dimana mengakui frame 3, dan mengizinkan transmisi frame 4 sampai 2.

• Tetapi, pada waktu acknowledgment mencapai A, A sudah

mentransmisi frame 4, 5 dan 6. Kesimpulannya bahwa A hanya boleh membuka window-nya untuk memperkenankan transmisi dari 4 frame, dimulai dengan frame 7.

(19)

Error Control Techniques

Error

detection _{acknowledgment}Positive

Retransmission after timeout

Negative

acknowledgment and retransmission

Frame hilang : suatu frame gagal mencapai sisi yang lain

Frame rusak : suatu frame tiba tetapi beberapa bit-bit-nya error.

(20)

Automatic Repeat Request (

ARQ

)

• Collective name for

error control

mechanisms

• Effect of ARQ is to

turn an unreliable

data link into a

reliable one

Stop-and-wait

Go-back-N

Selective-reject

(21)

Stop and Wait ARQ

Source transmits single frame

Waits for ACK • No other data can be sent until _{destination’s reply arrives}

If frame received is damaged, discard it • Transmitter has timeout • If no ACK within timeout, retransmit If ACK is damaged, transmitter will not recognize

• Transmitter will retransmit • Receiver gets two copies of

frame

• Use alternate numbering and ACK0 / ACK1

(22)

frame 0 ACK1 frame 1 ACK0 frame 1 ACK 0 frame 0 ACK1 frame 1 Frame trans-mission time ACK trans-mission time Propagation time Time-out interval Frame 0 lost; A retransmits Time-out interval ACK0 lost; A retransmits B discards duplicate frame

Figure 7.5 Stop-and-Wait ARQ

frame 0 ACK 0 T im e A B

Stasiun

sumber

mentransmisi

suatu frame tunggal dan kemudian

harus

menunggu

suatu

acknowledgment

(ACK)

dalam

periode tertentu. Tidak ada data

lain dapat dikirim sampai balasan

dari stasiun tujuan tiba pada

stasiun sumber. Bila tidak ada

balasan maka frame ditransmisi

ulang. Bila error dideteksi oleh

tujuan, maka frame tersebut

dibuang dan mengirim suatu

Negative Acknowledgment (NAK),

yang

menyebabkan

sumber

mentransmisi ulang frame yang

rusak tersebut.

(23)

Go-Back-N ARQ

• Most commonly used error control

• Based on sliding-window

• Use window size to control number of

outstanding frames

• While no errors occur, the destination will

acknowledge incoming frames as usual

–

RR=receive ready, or piggybacked acknowledgment

• If the destination station detects an error in a

frame, it may send a negative acknowledgment

–

REJ=reject

–

Destination will discard that frame and all future

frames until the frame in error is received correctly

–

Transmitter must go back and retransmit that frame

(24)

Selective-Reject (ARQ)

• 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 logic in transmitter

–

Less widely used

(25)

Selective-Reject (ARQ)

• Window Size Limitation

–

For a k-bit sequence number, the maximum window size

is limited to 2

k-1

_.

0 1 2 3 4 5 6 7 0 1 2 3 4 5 ... S R RR7 lost retx

_

0 1 2 3 4 5 6 7 0 1 2 ... S R RR4 lost retx

x

k=3, W=7 k=3, W=4

(26)

RR (P bit = 1) RR 2 RR 4 REJ 4 RR 5 RR 1 frame 0 frame 1 frame 2 frame 3 frame 5 frame 6 frame 4 frame 5 frame 6 frame 7 frame 0 frame 1 frame 2 frame 4 RR 7

(a) Go-back-N ARQ 4, 5, and 6 retransmitted discarded by receiver Timeout RR (P bit = 1) RR 2 RR 4 SREJ 4 RR 7 RR 3 frame 0 frame 1 frame 2 frame 3 frame 5 frame 6 frame 4 frame 7 frame 0 frame 1 frame 2 frame 3 frame 4 frame 4 RR 1 (b) Selective-reject ARQ 4 retransmitted buffered by receiver Timeout

Figure 7.6 Sliding-Window ARQ Protocols

(27)

High Level Data Link Control (HDLC)

Most important data link control protocol

Specified as ISO 3009, ISO 4335

Basis for other data link control protocols

Station types

Primary - controls operation of link

Secondary - under control of primary station

Combined - issues commands and responses

Link configurations

Unbalanced - 1 primary, multiple secondary

(28)

HDLC Data Transfer Modes

Normal Response Mode (NRM)

• Used with an unbalanced configuration

• Primary initiates transfer

Asynchronous Balanced Mode (ABM)

• Used with a balanced configuration

• Either station initiates transmission

• Has no polling overhead

• Most widely used

Asynchronous Response Mode (ARM)

• Used with unbalanced configuration

• Secondary may transmit without permission from primary

(29)

Flag FCS Information Control Address Flag 8 bits 8 extendable 8 or 16 variable 16 or 32 8 (a) Frame format

I: Information S: Supervisory U: Unnumbered 1 2 3 4 5 6 7 8 N(R) N(R) M N(S) S M 0 0 1 1 1 P/F P/F P/F

N(S) = Send sequence number N(R) = Receive sequence number S = Supervisory function bits M = Unnumbered function bits P/F = Poll/final bit

(c) 8-bit control field format

Information 0 N(S) P/F N(R)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Supervisory 1 0 S 0 0 0 0 P/F N(R) (d) 16-bit control field format

Figure 7.7 HDLC Frame Structure

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 8n

0 1

(30)

Original Pattern:

111111111111011111101111110

After bit-stuffing

1111101111101101111101011111010

(31)

Address Field

• Identifies secondary station that transmitted or will

receive frame

• Usually 8 bits long

• May be extended to multiples of 7 bits

–

Leftmost bit indicates if is the last octet (1) or not (0)

• Address 11111111 allows a primary to broadcast a

frame for reception by all secondaries

(32)

Flag FCS Information Control Address Flag 8 bits 8 extendable 8 or 16 variable 16 or 32 8

(a) Frame format

I: Information S: Supervisory U: Unnumbered 1 2 3 4 5 6 7 8 N(R) N(R) M N(S) S M 0 0 1 1 1 P/F P/F P/F

N(S) = Send sequence number N(R) = Receive sequence number S = Supervisory function bits M = Unnumbered function bits P/F = Poll/final bit

(c) 8-bit control field format

Information 0 N(S) P/F N(R)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Supervisory 1 0 S 0 0 0 0 P/F N(R)

(d) 16-bit control field format

Figure 7.7 HDLC Frame Structure

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 8n

0 1

(b) Extended Address Field

• HDLC defines three types of frames, each with a different control field format

– Information frames (I-frames)

 Carry the data to be transmitted for the user

 Flow and error control data, using the ARQ mechanism, are piggybacked on an

information frame

– Supervisory frames (S-frames)

• Provide the ARQ mechanism when piggybacking is not used

– Unnumbered frames (U-frames)

(33)

Control Field

• Use of poll/final (P/F) bit depends on context

• In command frames P bit is set to 1 to solicit (poll) a

response from the peer HDLC entity

• In response frames F bit is set to 1 to indicate the

response frame transmitted as a result of a soliciting

command

• The basic control field for S- and I-frames uses 3 bit

sequence numbers

–

An extended control field can be used that employs 7-bit

sequence numbers

(34)

Information and Frame Check

Sequence (FCS) Fields

Information Field

Present only in I-frames and some U-frames

Must contain an integral number of octets

Variable length

Frame Check

Sequence Field (FCS)

Error detecting code calculated from the remaining bits of the frame, exclusive of

flags

The normal code is the 16 bit CRC-CCITT

Optional 32-bit FCS, using CRC-32, may be employed if the frame length or the

(35)

Name Command/ Response

Description

Information (I) C/R Exchange user data

Supervisory (S)

Receive ready (RR) C/R Positive acknowledgment; ready to receive I-frame

Receive not ready (RNR) C/R Positive acknowledgment; not ready to receive Reject (REJ) C/R Negative acknowledgment; go back N

Selective reject (SREJ) C/R Negative acknowledgment; selective reject

Unnumbered (U)

Set normal response/extended mode (SNRM/SNRME)

C Set mode; extended = 7-bit sequence numbers

Set asynchronous response/extended mode (SARM/SARME)

Set asynchronous balanced/extended mode (SABM, SABME)

Set initialization mode (SIM) C Initialize link control functions in addressed station

Disconnect (DISC) C Terminate logical link connection

Unnumbered Acknowledgment (UA) R Acknowledge acceptance of one of the set-mode commands

Disconnected mode (DM) R Responder is in disconnected mode Request disconnect (RD) R Request for DISC command

Request initialization mode (RIM) R Initialization needed; request for SIM command Unnumbered information (UI) C/R Used to exchange control information

Unnumbered poll (UP) C Used to solicit control information Reset (RSET) C Used for recovery; resets N(R), N(S) Exchange identification (XID) C/R Used to request/report status

Test (TEST) C/R Exchange identical information fields for testing Frame reject (FRMR) R Report receipt of unacceptable frame

Table 7.1

HDLC

Commands

and

Responses

(Table can be found on page 254 in the textbook)

(36)

HDLC Operation

Init ia liz at ion

Signals the other side that initialization is requested Specifies which of the three modes (NRM, ABM, ARM) is requested

Specifies whether 3- or 7-bit sequence numbers are to be used Da ta Tr an sf er

The N(S) and N(R) fields of the I-frame are sequence numbers that support flow control and error control An HDLC module will number them sequentially Receive Ready (RR) is used when there is no reverse

user data traffic _Disc

onn

ect

Either module can initiate

•Either on its own

initiative if there is some sort of fault, or at the request of its higher-layer user

Sends disconnect (DISC) frame

Remote entity replies with a UA

Any outstanding unacknowledged I-frames may be lost

•Recovery is the

responsibility of higher layers



_{Consists of the exchange of I-frames, S-frames and}

U-frames

(37)

A _B I, 0, 0 I, 1, 1 I, 2, 1 I, 3, 2 RR, 4 I, 0, 1 I, 1, 3 I, 3, 4 I, 2, 4

(b) Two-way data exchange

A _B RNR, 4 RNR, 4, F RR, 4, F I, 3, 0 RR, 0, P I, 4, 0 RR, 0, P (c) Busy condition A B I, 3, 0 I, 4, 0 I, 5, 0 I, 4, 0 I, 6, 0 I, 5, 0 REJ, 4 (d) Reject recovery A _B I, 2, 0 RR, 0, P I, 3, 0 RR, 3 RR, 4 RR, 3, F

(e) Timeout recovery

Time-out A _B SABM SABM UA DISC UA

(a) Link setup and disconnect

I, 3, 0

Time-out

Figure 7.9 Examples of HDLC Operation

(38)