data link control protocol(2)

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Lecture 02Data-Link Protocols

Chapter 23Electronic Communications Sytems , Fifth Editon

By: Wayne Tomasi

ECE @Saint Louis University, Baguio City

Prepared by: Engr. Jeffrey Des B. Binwag

ECE @Saint Louis University, Baguio City 2

CLASSIFICATONS OF DLCP

• CHARACTER ORIENTED PROTOCOLS– Interpret a frame of data as a group of successive

bits combined into predefined patterns of fixed length bits each representing a unique character

– Data frames for these protocols include control characters that convey important information pertaining to DLCP functions

– Also called BYTE ORIENTED Protocols– Examples: X-Modem, Y-Modem, Z-Modem, BLAST,

IBM 83B, IBM BSC


CLASSIFICATONS OF DLCP

• BIT-ORIENTED PROTOCOLS– A discipline for serial bit-by-bit information

transfer over a data communications channel– Information is transferred as a series of successive

bits that may be interpreted individually on a bit-by-bit basis or in groups of several bits as opposed to a fixed length group of bits

– Examples: SDLC, HDLC


CLASSIFICATONS OF DLCP• ASYNCHRONOUS PROTOCOLS– Simple, character oriented protocols generally

used on two-point networks using asynchronous data and asynchronous modems

– Examples: X-MODEM, Y-MODEM, IBM 83B• SYNCHRONOUS PROTOCOLS– Protocols used on point to multi-point networks

using synchronous data and synchronous modems– Examples: IBM BSC, SDLC, HDLC


ASYNCHRONOUS DLCP

• XMODEM– Developed by Ward Christiansen in 1979– The first file transfer protocol (FTP) designed as an

asynchronous, character-oriented protocol to facilitate transferring data between two personal computers

– Specifies a half-duplex stop-and-wait protocol using a data frame comprised of four fields


ASYNCHRONOUS DLCP• XMODEM PROCEDURAL SEQUENCE

– Destination station sends a negative acknowledgement (NAK) to the source station

– The source sends the first data frame and waits for an acknowledgement

– The destination receives and evaluates the data frame from the source. It responds with an ACK if there is no error or a NAK if an error is detected

– If the source receives an ACK it sends the next frame. If it receives a NAK it retransmits the previous frame. If it does not receive an ACK or a NAK after a predetermined length of time, it times out and retransmits the previous frame

– A single cancel (CAN) character may be used to abort an ongoing transmission


ASYNCHRONOUS DLCP

• XMODEM FRAME FORMAT

SOH

CRC

8Bits

8Bits

8Bits

8Bits

Header Field Data Field128 bytes

Fixed length

Error Detection

(8 bits)

(Sequence Number with Complementary Redundancy)

Start of Heading


ASYNCHRONOUS DLCP• YMODEM– Similar to XMODEM except for the following:

1. The information field has a capacity of 1024 bytes.2. Two CAN characters are required to abort a

transmission.3. ITU-T-CRC 16 is used to calculate the frame check

sequence.4. Multiple frames can be sent in succession and then

acknowledged with a single ACK or NAK characters (Sliding window flow control)


SYNCHRONOUS DLCP• BINARY SYNCHRONOUS COMMUNICATION (BSC)– A synchronous character-oriented data-link protocol

developed by IBM– Also called bisync or bisynchronous communication– Each data transmission is preceded by a unique

synchronization character sent twice


SYNCHRONOUS DLCP• BSC FRAME FORMAT

SYN Message

Bisync FieldSynchronization Characters

Transmitted in Pairs

SYN

• Poll• Selection• Message• Acknowledgement

• ASCII : 16H (00010110)• EBCDIC : 32H (00110010)


BSC POLLING SEQUENCES• GENERAL POLLING SEQUENCE

PAD

• PAD. Pad• Leading Pad (55H or AAH)• Trailing Pad (FFH)

• EOT. End of Transmission• ENQ. Inquiry (Line Turnaround)

SYN

SYN

EOT

PAD

SYN

ENQ

PAD

SPA

SYN

SPA

“ “

• SYN. Synchronization Character• SPA. Station Polling Address• “. General Poll


BSC POLLING SEQUENCES• SPECIFIC POLLING SEQUENCE

PAD



SYN

SYN

EOT

PAD

SYN

ENQ

PAD

SPA

SYN

SPA

DA DA

• SYN. Synchronization Character• SPA. Station Polling Address• DA. Device Address


BSC POLLING SEQUENCES• POLLING SEQUENCE NEGATIVE ACKNOWLEDGEMENT


• EOT. End of Transmission• SYN. Synchronization Character

PAD

SYN

SYN

EOT

PAD


BSC SELECTION SEQUENCES• SELECTION SEQUENCE

PAD



SYN

SYN

EOT

PAD

SYN

ENQ

PAD

SSA

SYN

SSA

DA DA

• SYN. Synchronization Character• SSA. Station Selection Address• DA. Device Address


BSC SELECTION SEQUENCES• SELECTION SEQUENCE POSITIVE ACK


• SYN. Synchronization Character• DLE. Data Link Escape

PAD

SYN

SYN

DLE

PAD

0


BSC SELECTION SEQUENCES• SELECTION SEQUENCE NAK (RVI. Reverse Interrupt)


• SYN. Synchronization Character• DLE. Data Link Escape

PAD

SYN

SYN

DLE

PAD

6


BSC MESSAGE SEQUENCES• MESSAGE SEQUENCE


• SYN. Synchronization Character• SOH. Start of Heading • STX. Start of Text

• ETX. End of Text • ETB. End of Text Block• May be used in place of ETX

• BCC. Block Check Character• LRC for ASCII• CRC-16 for EBCDIC

PAD

SYN

SYN

SOH Heading

BCC

PAD

STX

ETXMessage


BSC MESSAGE SEQUENCES• MESSAGE SEQUENCE POSITIVE ACKNOWLEDGEMENT

PAD

SYN

SYN

DLE

PAD

0

PAD

SYN

SYN

DLE

PAD

1

FOR EVEN-NUMBERED BLOCKS

FOR ODD-NUMBERED BLOCKS


BSC MESSAGE SEQUENCES• MESSAGE SEQUENCE NEGATIVE ACKNOWLEDGEMENT

PAD

SYN

SYN

NAK

PAD


• SYN. Synchronization Character• NAK. Negative Acknowledgement


BSC TRANSPARENCY

• A process that disables the BSC controller’s capability to identify data link control characters

• Used for communication with devices that use non-ASCII and non-EBCDIC codes such as those in microprocessor controlled security alarm systems or environmental monitoring systems

• Transparent mode is activated when the STX character is preceded by a DLE


OTHER DLE APPLICATIONS1. DLE ETB. Used to terminate all blocks of data

except the final block2. DLE ITB. Used to terminate blocks of transparent

text other than the final block when ITB (end of intermittent block) is used for block-terminating character

3. DLE SYN. With bisync, two SYN characters are inserted in the text in messages lasting more than one second to ensure that the receive controller maintains character synchronization


BSC CHARACTER CODESCharacter Hex UseACK0 / ACK1 (Alternating positive acknowledgements)

1070 1061

Indicates that text was received without errors. Also is a positive response (acknowledgement) to an ENQ (bid) on a point-to-point line. In 3780 Bisynchronous Communications, this positive acknowledgement alternates between 1070 (even) and 1061(odd).

DISC (Disconnect) 1037 Indicates a mandatory disconnect on a switched line connection. The station sending this message will disconnect without having to receive confirmation from the receiving station.

DLE (Data Link Escape) 10 Changes the meaning of the control character immediately following the DLE. For example, the sequence DLE EOT indicates that both stations on a switched connection should disconnect from the line.

ENQ (Enquiry) 2D Has the following uses:-- Initiates (bids for) control of transmission on a point-to-point connection-- Indicates the end of a polling or address sequence on a multi-point connection-- Indicates a request for a station to repeat its response (received from target station)-- Indicates that a block of text should be ignored (if the blocks ends with a ENQ)-- Indicates the end of an identification sequence on a switched connection


BSC CHARACTER CODESEOT (End Of Transmission) 37 Indicates the end of a transmission for the sending system. This is also sent

when the sending system exhausts its retry limit of sending an ENQ to a remote system without a response.

ETB (End Of Text Block) 26 Indicates the end of a block of data, but not the last block transmitted (more data will be coming from the station that transmits this).

ETX (End Of Text) 03 Indicates the end of a message if ETBs are used (text is transmitted in blocks). Also can indicate the end of the last block of text transmitted (completed text transmission)

ITB (Intermediate Text Block)

1F Divides a block of text into smaller groups of text (Intermediate Text Blocks) for error checking.

NAK 3D Indicates a Not Ready condition (most common), or indicates that an error occurred. Also is sent as a negative response to a line bid or switched line security identification.

Pad (Leading Pad Character)

55 or AA

At least 2 leading pad characters are transmitted to synchronize the clocks at the transmitting and receiving stations.

Pad (Trailing Pad Character)

FF A trailing pad character ensures that all bits of the last character of a message are transmitted.

ECE @Saint Louis University, Baguio City

BSC CHARACTER CODES

24 (END)

RVI (Reverse Interrupt) 107C Transmitted by the receiving station rather than an ACK to indicate a positive acknowledgement and to request that the transmitting station stop transmitting so that the receiving station can transmit. (some people may refer to this as a "turnaround request")

SOH (Start Of Header) 01 Indicates the start of heading information.

STX (Start Of Text) 02 Indicates the start of text.

SYN (Sync) 32 Establishes and maintains character synchronization.

TTD (Temporary Text Delay)

022D Indicates to the receiving station that the transmitting station is temporarily not ready to transmit. (station that transmits this is having an issue)

WACK (Wait Before Transmitting)

106B Transmitted by the receiving station to indicate to the transmitting station a positive acknowledgement and a temporary not-ready-to-receive condition (this is usually recovered by the application at which point the transmission continues).

data link control protocol(2)

Education

pad leading pad

saint louis university

ds y n pad

ds y ns y nd

frame of data

ds y ns y ne o tp

asynchronous data

data transmission