Data communication protocols in centralised networks (in master:slave environment)

DATA COMMUNICATION PROTOCOLS in Centralised Networks (in Master/slave environment)

Madhumita Tamhane

Madhumita Tamhane

DATA COMMUNICATION PROTOCOLS in Centralised Networks (in Master/slave environment)

✓Set of rules governing orderly exchange of information. ✓LCU controls the flow of data between application programs and

remote terminal. ✓Set of rules LCU follows are called Data Link Protocols. ✓Polling: An invitation from primary to secondary to transmit

message. ✓ A secondary can not poll a primary.

✓Selection: When primary has a data to send to a secondary, it identifies the secondary by ‘selecting’ it.

✓ It is interrogation of secondary to determine its status ✓ - whether ready to receive a message or not. ✓ Secondary can not select a primary.

✓Secondary transmits to primary only, while primary transmits to all.

Asynchronous Protocols

✓Use asynchronous data format and asynchronous modems. ✓Character oriented protocols. ✓Like ‘end of transmission (EOT)’ and ‘start of text (STX). ✓Character occurring anywhere can not be anything but mean the

action designated to them. ✓ e.g. 04H (EOT) if received by secondary , it clears all operations

and comes into line-monitor mode. ✓Data link characters must never occur in data. ✓Secondary station limited to single terminal/printer pair. ✓Commonly used protocols are:

✓ Bell System’s SELECTIVE CALLING SYSTEM(8A1/8B1). ✓ IBM’s ASYNCHRONOUS DATA LINK PROTOCOLS(83B).


✓Polling Sequence: !!!

✓3 character sequence. ✓EOT-

✓ a clearing character, clears all actions of secondary. ✓ Precedes all polling sequences. ✓ Brings all secondaries in line monitor mode.

✓DC3 – ✓ Indicates that next character is the polling address of station. !

✓A , the polled secondary, must respond now.

E O T

D C 3

A ASCII Codes

EOT – End of transmission – 00000100 – 04H

DC3 – Device control 3 -- 00010011 - 13H

Acknowledgement sequences to Polling

✓ 1. !✓ No message to transmit but ready to receive. !!

✓2. !✓ No message to transmit, not ready to receive either. !

!✓3. Formatted data sent

\ \

\A C K


✓Selection Sequence: !!!

✓2 character sequence. ✓EOT-

✓ A clearing character, clears all actions of secondary. ✓ Precedes all selection sequences. ✓ Brings all secondaries in line monitor mode.

✓X , the selected secondary, must respond now.

E O T

X

Acknowledgement sequences to Selection

✓ 1.Ready to receive.

!!

!✓ 2.Not ready to receive. Terminal in local or printer mode

!!!

✓ 3. Not ready to receive but has a formatted data to transmit.

\ \

\A C K

* *

Message format

!!!✓SOH is start of header. ✓Header can include special data e.g. date, time, message number,

message priority till STX is received. ✓STX is start of text.

✓ It terminates header and starts data. ✓STX blinds other stations to the data. Only EOT can bring them to line

monitor mode.

S O H

HeadingS T X

MessageE O T

Message format-

Acknowledgement to message

A C K

1. Acknowledged. No error--

N A K

2. Not Acknowledged. Message has error

C A N

3. Cancel or abort transmission

Asynchronous Protocols – TYPES XMODEM

✓By Ward Christiansen in 1979. ✓FTP for telephone line communication between PC’s. ✓Half Duplex. ✓Stop/wait ARQ. ✓Message divided into characters. ✓Each character contains start bit and stop bits. ✓CRC for data only. ✓CAN sent to abort transmission.

SOH CRC

Header 128 bytes data

Start bit Stop bit

Asynchronous Protocols – TYPES YMODEM

✓Same as XMODEM except… ✓Data 1024 bytes. ✓2 CAN sent to abort transmission. ✓2 Bytes CRC-16. ✓Multiple frames can be sent simultaneously.

!!!

✓Exercise: Find details for other asynchronous modems and submit.

Synchronous Protocols!!

✓Use synchronous data format and synchronous modems. ✓Can be Character oriented protocol or Bit oriented protocol. ✓ Secondary station LCU can serve up to 50 terminal/printer pair. ✓Commonly used protocols are: ✓Character oriented protocol –

✓ e.g. IBM’s 3270 Binary Synchronous Communication Protocol(BSC or Bisync).

✓Bit oriented protocol – ✓ e.g. IBM’s Synchronous Data Link Communication

Protocol(SDLC).

IBM’s Bisync Protocol (BSC)

!!

✓Point to point or multipoint. ✓Half duplex mode using stop and wait ARQ and flow

control. ✓Does not support full duplex mode or sliding window

protocol. ✓Bisync because sync characters are sent in pair.

IBM’s Bisync Protocol (BSC)!!!!!

✓PAD in beginning is called leading pad. ✓PAD can be 55H(01010101) or AAH(10101010). ✓Used for CLK recovery at receiver for bit synchronization.

!✓2 SYNC characters required for CLK recovery at receiver

for character synchronization. ✓SYNC can be 16H in ASCII ( 00010110)

OR 32H in EBCDIC repeated twice.

P A D

S Y N

S P A

S P A

S Y N

E O T

P A D

S Y N

S Y N

P A D

E N Q

“ “General Polling Sequence


✓EOT, a clearing character, brings all secondaries to line monitor mode.

!✓PAD is all 1’s string, gives enough time to secondaries to

clear their assignments and come to line monitor mode.

!✓2 SYNC characters available further to synchronize if PAD

disturbs synchronization.

P A D

S Y N

S P A

S P A

S Y N

E O T

P A D

S Y N

S Y N

P A D

E N Q



✓SPA will hold secondary station’s polling address in ASCII. ✓Transmitted twice to avoid any spurious sequence

initiating a poll. ✓Each secondary has unique SPA. ✓ “ is used as it is twice to indicate that it is a general poll. ✓Any device connected to secondary can transmit the data.

(keyboard, Scanner etc.) ✓If more than one ready, secondary’s LCU decides the

transmitter.

P A D

S Y N

S P A

S P A

S Y N

E O T

P A D

S Y N

S Y N

P A D

E N Q



✓ENQ – request for response – format line turn around character is an enquiry character.

✓Ending polling sequence, primary now wants a response. ✓H/D transmission. ✓ PAD at end is trailing pad. ✓7FH or DEL character. ✓Ensures that RLSD is kept active long enough for the DTE

to receive all character correctly from DCE. ✓Low RLSD will eliminate carrier and RD pin will be inactive.

P A D

S Y N

S P A

S P A

S Y N

E O T

P A D

S Y N

S Y N

P A D

E N Q


IBM’s Bisync Protocol (BSC)!!!!

!

✓It is not general poll, hence “ is not used. ✓A specific device with address mentioned in field DA ,

under station with address mentioned under SPA, is polled.

P A D

S Y N

S P A

S P A

S Y N

E O T

P A D

S Y N

S Y N

P A D

E N Q

D A

D A

Specific Polling Sequence

IBM’s Bisync Protocol (BSC)!!!!

✓Station polling address is replaced by station selection

address. ✓Selection is always specific. ✓It is for the station to divert the incoming data to

appropriate device.

P A D

S Y N

S S A

S S A

S Y N

E O T

P A D

S Y N

S Y N

P A D

E N Q

D A

!D A

Selection Sequence

IBM’s Bisync Protocol (BSC) Acknowledgement to polling!

!!!!

✓Handshake is negative acknowledgement to polling. ✓It has no formatted data to transmit.

P A D

S Y N

S Y N

E O T

P A D

1. Handshake

IBM’s Bisync Protocol (BSC) Acknowledgement to polling

!!!!!

✓Data 128 bytes minimum. ✓BCC is block check sequence. ✓1 byte LRC for ASCII or 2 bytes CRC for EBCDIC. ✓BCC is done between SOH and ETX, excluding the two.

P A D

S Y N

S Y N

S O H

header

2. Formatted Message

S T X

Data

P A D

B C C

E T X


!!!!!!!

✓ Message can be broken into Multiple blocks and accommodated in one frame.

✓ ITB - intermediate text block.

✓ BCC done on smaller blocks is always preferred.

✓ Finally frame will be terminated by ETX.

Multi-block frame

P A D

S Y N

S Y N

S O H

header

S T X

Data

S T X

B C C

I T B

Data

P A D

B C C

E T X


!!!!!!!!

✓If message too long to accommodate in one frame. ✓Multiple frames transmitted. ✓All frames except last, will be terminated in ETB.( More to come) ✓Last frame terminated in ETX.(End of message)

Multi-frame block

P A D

S Y N

S Y N

S O H

header

S T X

Data

S T X

B C C

I T B

Data

P A D

B C C

E T B

P A D

S Y N

S Y N

S O H

header

S T X

Data

S T X

B C C

I T B

Data

P A D

B C C

E T X

ACK More frames and ACK…

IBM’s Bisync Protocol (BSC) Acknowledgement to selection

!!!!!!

✓ Ready to receive first frame using ACK0.

✓ DLE is data link escape character.

✓ DLE 0 will be taken as number 0.

✓ ASCII for ACK0 is DLE and 0

P A D

S Y N

S Y N

D L E

0

1. Positive acknowledgement

P A D

IBM’s Bisync Protocol (BSC) Acknowledgement to selection

✓ 2.Negative acknowledgement- not ready to

receive

P A D

S Y N

S Y N

N A K

P A D

P A D

S Y N

S Y N

D L E

3.Reverse Interrupt (RVI) – Not ready to receive, but has urgent message to transmit.

<P A D

IBM’s Bisync Protocol (BSC) Acknowledgement to DATA

!!!!!!!!!!

✓Stop and wait error control. ✓DLE0 for all even frame correct reception. ✓DLE 1 for all odd frames correct reception.

1.Positive acknowledgement- No error

P A D

S Y N

S Y N

D L E

0P A D

P A D

S Y N

S Y N

D L E

1P A D

or

2. Negative acknowledgement- Error

P A D

S Y N

S Y N

N A K

P A D

IBM’s Bisync Protocol (BSC) Some more on BISYNC

1.Point to point connection asked

S Y N

S Y N

E N Q

2.Wait and ACK- ACK but not ready to receive more.

S Y N

S Y N

W A C K

3.Temporary delay S Y N

S Y N

T T D

4.Connection termination

S Y N

S Y N

E O T

TRANSPARENCY

✓LCU look-alike characters appearing in data force the LCU to go into control mode.

✓Data 03H appearing in data block will be taken as ETX and block will be terminated at receiver.

✓LCU is made transparent to data by DLE(Data link escape). ✓LCU will not search for data link control characters in the block of

data encapsulated by 2 DLE’s. ✓DLE STX places LCU in transparent mode. ✓DLE ETX, DLE ITB, and DLE ETB brings LCU out of transparent

mode. ✓DLE DLE if ASCII of DLE appears in data. ✓DLE SYN used only with transparent messages more than 1second

long. Two SYN characters to ensure character synchronization at receiver.

Engineering

Data communication protocols in centralised networks (in master:slave environment)