other data link protocols - · pdf fileother data link protocols: ... character stuffing...

Computer Networks by Prof Kamal K Mehta Associate Professor Dept of Computer Science & Engineering

Other Data Link Protocols:


Data Link Protocol CategoriesA protocol in data communications is the set of rules or

specifications used to implement partially a layer or

one or more layers of the OSI model.

Above protocol is used to implement the data link layer


Asynchronous protocols- treat each character in a bit stream independently

- use start and stop bits to frame the data units

- inexpensive

- used primarily in modems

Synchronous protocols- take the whole bit stream and chop it into characters

of equal size

- faster than asynchronous transmission

Data Link Protocol Categories


Other Data Link Protocols:

Examples

A: Asynchronous Protocols

X-Modem, Y-Modem, Z-Modem, BLAST, KERMIT

B: Synchronous Protocols

Char Oriented Protocols like BSC

Bit Oriented Protocols like SDLC,HDLC, LAPs


Asynchronous Protocol XMODEM Frame

Half-duplex stop-and-wait ARQ protocol

SOH –Start of Header (1 byte)

CRC - Cyclic Redundancy Check

Signal from receiver side

- ACK

- NAK

- CAN (cancel)


Example: Find out the maximum possible channel utilization by X Modem

Asynchronous Protocol, assuming a text file of 100 characters for transmission.

Asynchronous Protocols


Example:

Text document of 100 characters.

Each character is represented

8 bits per character + 1 Bit Start + 1 Bit Stop = 10 bits.

So for 100 character = 100x10 = 1000 bits.

Ideally we must take 800 bits, but practically we need 1000 bits.

Over head bits are

1000-800=200 bits

In terms of character = 200/8=25 character.

Hence we can say that on behalf of 100 character we are transmitting (100+25)

characters extra.

= (100/125) x 100 = 80 % maximum

Efficiency will further decrease , if we add transmission overhead, Retransmission

etc.

Asynchronous Protocols


• YMODEM– similar to XMODEM

– Major differences

• Data unit = 1024 bytes

• CANs = 2

• Follows ITU-T CRC-16 format


What is CRC-16 CRC-12…?

• Example:

– CRC-12 = x12+x11+x3+x2+x+1

CRC-16 = x16+x15+x2+1

CRC-CCITT = x16+x12+x5+1

– CRC-16 and CRC-CCITT catch all

• Single and double errors

• Odd number of bit errors

• Bursts of length 16 or less

• 99.997% of 17-bit error bursts

• 99.998% of 18-bit and longer error bursts

CCITT :(Consultative Committee for International Telephony and

Telegraphy)

CRC-16 calculation link

http://www.piclist.com/techref/method/error/quickcrc16.htm


Features of Blast:

•bit-oriented data encoding

•CRC (cyclic redundancy check) error detection

•a sliding window transmission scheme

•selective retransmission of corrupted blocks

•simultaneous bi-directional data transfer

BLAST (BLocked ASynchronous Transmission), like

XMODEM, it is a communications protocol for file transfer

over asynchronous communication ports and dial-up

modems by ANSI.


Kermit:

It is ftp by Columbia university

It is a computer file transfer/management protocol.

It provides a consistent approach to file transfer, terminal

emulation, and character set conversion across many

different computer hardware and OS platforms.

Emulation:

Hardware and Software that enables one computer to behave like

another computer.

Terminal emulation: Composed of

CLI: Command line interface

TUI : Text User Interface


Z Modem:

Telenet funded a project

Application file transfer protocol.

Throughput problems of XMODEM and KERMIT resolved

Reliability over packet switched networks while

preserving XMODEM's simplicity.


Summary for Asynchronous:

1. Transmission of two extra bits (2 per byte) reduce data

throughput.

2. Synchronization is achieved for each character only.

3. When the sender has no data to transmit (idle line) then

the sender and receiver are NOT synchronized.

4. Asynchronous protocols are suited for low speed data

communications.

Asynchronous transmission, we relied on the start bit to start the motor

and thus begin the preparation to decode the incoming character.


Synchronous Protocols

Character-oriented protocols (or byte-oriented protocols)

- the frame or packet is interpreted as a series of characters

Bit-oriented protocols

- the frame or packet is interpreted as a series of bits


Character Oriented Protocol:

BSC (Binary Synchronous Protocols)

Bit Oriented Protocol:

SDLC (Synchronous Data Link Controll)

HDLC (High Data Link Controll)

LAPs (Link Access Procedure Balance)


Features of Synchronous Protocols:

1. Efficiency is achieved by grouping characters together called BLOCKS

2. No start and stop bits for each character.

3. Send more characters between the start and end sequences.

4. HEDER: start type sequence, prefixes each block of characters.

5. TAIL: A stop type sequence, suffixes each block of characters.

6. An extra ending sequence is added to perform error checking.

7. The tail is expanded to include a check code

8. Group multiple characters into BLOCKS.

9. Developed for High Speed Networks.

No start bit, hence the transmitter sends a special code when it has nothing

to send. To keep the receiver in a state of readiness.


Character-Oriented

Protocols


Binary Synchronous

Communication (BSC)

Supports half-duplex transmission

Uses stop-and-wait ARQ flow control and error correction

- the best-known character-oriented protocol


BSC

• Half duplex, can be used with ASCII, EBCDIC, and

Six Bit Transcode

• Control information is in the form of code words

taken from the character set(Example..?)

• Control information is carried in separate frames

as well as within data frames

– Line control, flow control, error control


BSCData Frames:

S

Y

N

S

Y

N

S

O

H

S

T

X

E

T

X

B

C

C

B

C

CHeader …Data…

SYN: Synchronization Characters, used to alert receiver for

initialization of timing/counter w.r.t the sender.

SOH: Indication of beginning of header.

Header: Includes Address of sender and the Receiver along with

ID of frame number i.e. 0 or 1.


BSC

Data Frames:S

Y

N

S

Y

N

S

O

H

S

T

X

E

T

X

B

C

C

B

C

CHeader …Data…

STX: Indicated that the control information is ending and the data is

starting.

ETX: Indicates end of text and start of control character.

BCC: Block Check Count is same as LRC(1 char), or CRC(2 Char)


Previous Lecture we have seen following topic:

Asynchronous Protocols: X-Modem Y-Modem Z-Modem,

BLAST, KERMIT

Synchronous Protocol:

Character Oriented BSC

Bit Oriented : SDLC HDLC LAP-B

Question 1: While using Character Based Protocol, and using ETX, EOT

Control Characters, do we generate 3 bytes or it is single byte..?

Question 2: How a multi frame transmission is handled in BSC protocol?


BSC

• Multiblock frame:

S

Y

N

S

Y

N

S

O

H

S

T

X

I

T

B

B

C

C

B

C

CHeader …Data…

S

T

X

E

T

X

B

C

C

B

C

C…Data…

Block Block

Multi Block: One frame may have multi blocks(Data Field)

inside.

ITB: Intermediate Text Block: Used to separate data fields.

ETX: Is on behalf of all blocks of a frame.


BSCMulti-frame transmission:

One block may have multiple data field

Each frame is ended with ETB

For every ETB we get ACK

When the last frame is transmitted then we send ETX

ACK is generated on behalf of ETB as well as ETX

S

Y

N

S

Y

N

S

O

H

S

T

X

I

T

B

B

C

C

B

C

CHeader …Data…

S

T

X

E

T

B

B

C

C

B

C

C…Data…

B

C

C

B

C

C

A

C1

K

S

Y

N

S

Y

N

S

Y

N

S

Y

N

S

O

H

S

T

X

I

T

B

B

C

C

B

C

CHeader …Data…

S

T

X

E

T

X

B

C

C

B

C

C…Data…

B

C

C

B

C

C

A

C0

K

S

Y

N

S

Y

N

End of 4 th data segment, end of second Block , end of first frame


BSC• Role of Control Frames:

– Frames used only for signaling

• SYN SYN {Control Characters} BCC

– Connection establishment

– Connection termination (end of transmission)

– Flow and error control(ACK0, ACK1, NAK)


BSC• Data Transparency

– When sending binary data, certain information

may be identical to control characters

– Solution: Define transparent blocks and use

character stuffing

– Start transparent text block with DLE STX

– End transparent text block with DLE ETX, DLE

ITB, DLE ETB

– To send DLE character, send DLE DLE


Bit Oriented Protocol

SDLC HDLC LAP B

Synchronous Data link Control High Data Link Control Link Access Protocol (Balanced);

ISO 3309 ISO 4335


What is the role of HDLC …?

General Features:

Provides Frame Format (Standard)

Classifies Systems

Deals with Link configuration(How link is used..?)

Defines mode of transmission (Who will initiate..?)

Flow / Error control

Synchronous Transmission

Bit oriented system

Developed by ISO , examples are ISO 3309 & ISO 4335.

HDLC (High Data Link Protocol)


HDLC

3:Supports Three station type 4: HDLC Link Configurations

5: Supports Three Modes of transmission

1:Transmission Details

2: Supports three frame types

Half Duplex and Full Duplex transmission.

Switched and non Switched Channels.

Point to Point and Multipoint Lines.

I frame: Information frame Six fields

S frame: Supervisory frame Five fields

U frame:Unnumbered frame Six fields

Primary

Secondary

Combine

Balance

Unbalance

Symmetric

NRM (Normal Response Mode)

ARM (Asynchronous Response Mode)

ABM (Asynchronous Balance Mode)


Supports Three station type

Combine Station:

May issue commands and responses

Primary Station:

Controls operation of link

Frames issued are called commands

Maintains separate logical link to each secondary station.

Secondary Station:

Under control of primary station

Frames issued called responses


HDLC Link Configuration

One primary and one or more secondary stations

Supports full duplex and half duplex

Two combined stations

Supports full duplex and half duplex


HDLC Link Configuration


Mode of Transmission

(1)Normal Response Mode

•Normal Response Mode (NRM)

Unbalanced configuration

Primary initiates transfer to secondary

Secondary may only transmit data in response to command from primary

Used on multi-drop lines

Host computer as primary

Terminals as secondary



(2)Asynchronous Balance Mode

•Asynchronous Balanced Mode (ABM)

Balanced configuration

Either station may initiate transmission without receiving permission

Most widely used


(3) Asynchronous Response Mode (ARM)

Unbalanced configuration

Secondary may initiate transmission without

permission from primary

Primary responsible for line

Rarely used



HDLC Frame Types

I frame: Information frame Six fields

S frame: Supervisory frame Five fields

U frame: Unnumbered frame Six fields.


Details of HDLC Fields


Details of HDLC Fields

• Delimit frame at both ends 01111110

• May close one frame and open another

• Receiver hunts for flag sequence to synchronize

• Bit stuffing used to avoid confusion with data containing 01111110

– 0 inserted after every sequence of five 1s

– If receiver detects five 1s it checks next bit

– If 0, it is deleted

– If 1 and seventh bit is 0, accept as flag

HDLC Flag Field


Flag field is also used for following purposeBreaks a long single frame into multiple frames.

Bit stuffing is used to avoid confusion.


Bit stuffing and removal process


• Identifies secondary stations that sent or will receive frame

• Addressing is done irrespective of , who is sending (Primary or Secondary)

• Usually 8 bits long

• May be extended to multiples of 7 bits

– LSB of each octet indicates that it is the last octet (1) or not (0)

• All ones (11111111) is broadcast

HDLC Address Field


HDLC Address Field

Bit streams are;.

00010110 01001010 01101101 00101101 00100100 10110101It is difficult to identify end of last byte of address frame

00010110 01001010 01101100 00101100 00100100 10110101

Address bits


• First one or two bits of control filed identify frame type

HDLC Control Field (Used to identify Frame Type)

P/F Poll / Final bit “POLL : To prompt receiver to know what is happening”

N(S) Sequence no of frame sent

N(R) Sequence no of frame expecting

CODE Code for supervisory or unnumbered frame


Poll/Final

•Application depends on the context

•Command frame (P bit is set)

•Response frame (F bit is set)

Poll

Final


HDLC Information Field

•Only in I frame and some U frames

•Variable length


HDLC FCS Field

•Error detection

•16 bit CRC

•Optional 32 bit CRC


I 0,0

I 1,0

I 0,2

I 2,1

I-frame control field in HDLC

Explanation of N(S) and N(R)


S-frame control field in HDLC

I 1,0

REJ , 1

I 1,0

RR, 2

Explanation of CODE field of S-frame


11.21 U-frame control field in HDLC


Table 11.1 U-frame control command and response

Command/response Meaning

SNRM Set normal response mode

SNRME Set normal response mode (extended)

SABM Set asynchronous balanced mode

SABME Set asynchronous balanced mode (extended)

UP Unnumbered poll

UI Unnumbered information

UA Unnumbered acknowledgment

RD Request disconnect

DISC Disconnect

DM Disconnect mode

RIM Request information mode

SIM Set initialization mode

RSET Reset

XID Exchange ID

FRMR Frame reject


HDLC Operation

Exchange of

Information frame

Supervisory frames and

Unnumbered frames

Three phase

Initialization

Data transfer

Disconnect

Timing diagram

Link setup & disc

Two way data Exchange

Busy Condition

Reject Recovery

Time out recovery


Timing Diagram to Explain

1. Link Set up & Disconnect

2. Two Way Exchange of Data

3. Busy Condition

4. Reject Recovery

5. Time out Recovery


Examples of Operation1: Link Setup and Disconnect

SABM

“T”

SABM

UA

DISC

UA

A B

Identify the frame type

All are U frames


Examples of Operation2:Two way Data Exchange

I 0,0

I 1,1

I 0,1

I 2,1

I 1,3

I 3,2

I 2,4

I 3,4

RR 4

A B


All are I frames

except RR4(S Frame)


Examples of Operation3:Busy Condition (M/c A is not sending next frame and not receiving further frames from

B

RNR 4 ( Busy)

I 3,0

RR 0,P(Command)

RNR 4,F(Response)

RR 0,P ( Command)

RR 4,F ( Response)

A B

I4,0

Busy


I frame as well as S Frame


Examples of Operation4: Reject Recovery

I 3,0

REJ 4

A B

I 5,0

I 4,0

I 4,0

I 5,0


Examples of Operation5:Time out Recovery with P/F bits

I 2,0A B

I 3,0RR 3

T

RR 0,P

RR 3,F

I 3,0

RR 4

Computer Networks by Prof Kamal K Mehta Associate Professor Dept of Computer Science & Engineering Computer Networks by Prof Kamal K Mehta Associate Professor Department of Computer Science & Engineering SSCET Bhilai (C.G.)

IEEE Protocols


IEEE Network Protocols

Computer Networks by Prof Kamal K Mehta Associate Professor Department of Computer Science & Engineering SSCET Bhilai (C.G.)


Number Topic

802.1 Overview & Architecture of LAN’s

802.2 Logical Link Control

802.3 Ethernet

802.4 Token Bus

802.5 Token Ring ( IBM entry into LAN world)

802.6 Dual queue dual bus( Early metropolitan area Network)

802.7 Technical advise on broadband tech

802.8 Technical advise on fiber optic technology

802.9 Isochroous LAN

802.10 Virtual LAN’s and security

802.11 Wireless LAN’s

802.12 Demand Priority ( Hewlett-Packard’s LAN)

802.13 Unlucky no, No body wanted it.

802.14 Cable modems

802.15 Personal Area Net ( BLUE TOOTH)

802.16 Broad band networks

802.17 Resilience Packet Ring.


Introduction to the following MAC (Media Access Control) Technique

CSMA /CD: Carrier Sense Multiple Access/Collision Detect

As soon as collision is detected, then protocol will send JAM signal to all other station.

Token Bus:

Token Ring:

DQDB(Dual Queue Dual Bus):


Two Unwanted situations:

Collision

Congestion


Function of Media Access Control Sub Layer

1. Provides Access to Shared Media

2. Allow Multiple device to uniquely identify each other in DLL

3. Control of Access to media

4. Detection of Error


Goal of Media Access Control Sub Layer

1. Initialization: When power on, it will enable all station of enter into

state of readiness.

2. Fairness: Treat each station equally, as per the specification

3. Priority: To assign priority as per the application need.

4. Limitation to one station: Must allow only station at one time

5. Receipt: Ensure reception of packet in only one copy to correct

destination, in correct order.


Goal of Media Access Control Sub Layer

6. Error Limitation: Protocol must be able to implement suitable Error

handling mechanism

7. Recovery: If 2 packets collides , then it must be able to recover (able to

halt the transmission and select one station to retransmit).

8. Re configurability: Must support addition/deletion of nodes

9. Compatibility: Must support the devices supplied by all vendors.

10. Reliability: Must ensure network operation in spite of failure of one or

more stations.

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