data link control protocol(1)
DESCRIPTION
Lecture Slides on Data Link Protocols from Chapter 23 of the book Electronic Communications Systems by TomasiTRANSCRIPT
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Lecture 01Data-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
Data Link Control Protocol (DLCP)
• A set of rules implementing and governing an orderly exchange of data between OSI Protocol Layer 2 devices.
• Perform certain network functions that ensure a coordinated transfer of data.
ECE @Saint Louis University, Baguio City 3
TYPES OF NETWORK
• PRIMARY/SECONDARY (MASTER/SLAVE )A centrally controlled network where a primary station (master) enacts procedures that determine which station is transmitting and which is receiving at any given instant. All the rest of the stations are secondary stations (slaves) whose data communication actions are controlled by the primary.
• PEER-TO-PEERA type of network where all connected stations are equal (peers), and share equal access to the network. When a station wishes to transmit on this network, it has to contend with all the rest of the stations with an intent to transmit.
ECE @Saint Louis University, Baguio City 4
FUNCTIONS OF DLCP
• LINE DISCIPLINE• FLOW CONTROL• ERROR CONTROL
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LINE DISCIPLINE• The process of coordinating half-duplex
transmission on a data communications network by: – Establishing logical continuity between source and
destination stations before data transfer, and – Ensuring that the destination station is ready and
capable of receiving data
• Fundamental Methods of implementation– Enquiry/ Acknowledgement (ENQ/ACK)– Poll/ Selection (POLL/SELECT)
ECE @Saint Louis University, Baguio City 6
ENQ/ACK• Best suited to simple network environments like two-
point networks
• ENQUIRY– A frame or packet of data which identifies the receiving
station and solicits the receiving station to determine if it is ready to receive data
• ACKNOWLEDGEMENT– ACK. Positive Acknowledgment (Ready to receive)– NAK. Negative Acknowledgement (Not ready to receive,
Message received with error)
ECE @Saint Louis University, Baguio City 7
ENQ/ACK
STATION A STATION BENQ
MESSAGE 1
MESSAGE 2
MESSAGE 2
MESSAGE 3 (EOT)
ACK
ACK
ACK
NAK
ACK
Time
ECE @Saint Louis University, Baguio City 8
POLL/SELECT• Best suited to centrally controlled data communications networks using
multipoint topology
• POLL– A solicitation sent from the primary to the secondary to determine if the
secondary has data to transmit– Sent to one station at a time– Secondary responds with either a message or a negative acknowledgement (NAK)
• SELECTION– How the primary designates a secondary as a destination, a query if the
destination is ready to receive data– Can be broadcast to all secondary stations– Secondary stations respond with either a positive acknowledgement (ACK) or a
negative acknowledgement (NAK)
ECE @Saint Louis University, Baguio City 9
POLL/SELECT
Poll A
PRIMARY
Message
Message
NAK
ACK
SECONDARY A SECONDARY B SECONDARY C
Poll B
Selection B
Selection C
NAK
Time
ECE @Saint Louis University, Baguio City 10
FLOW CONTROL• Defines a set of procedures that tells the
transmitting station how much data it can send before it must stop transmitting and wait for an acknowledgement from the destination
• Fundamental Methods of implementation– Stop-and-wait– Sliding Window
ECE @Saint Louis University, Baguio City 11
STOP-AND-WAIT FLOW CONTROL
• The transmitting station sends one message frame and then waits for an acknowledgment before sending the next frame
• Advantage is simplicity• Disadvantage is its poor network utilization
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STOP-AND-WAIT FLOW CONTROL
STATION A(Source)
STATION B(Destination)
Message Frame 2
Message Frame 3
Message Frame 3
Message Frame 4 (EOT)
ACK
ACK
ACK
NAK
ACK
Time
Message Frame 1
ECE @Saint Louis University, Baguio City 13
SLIDING WINDOW FLOW CONTROL
• The transmitting station sends several message frames in succession before receiving an acknowledgement
• Advantage is faster speed of transmission (better network utilization) compared to Stop-and-wait flow control
• Disadvantage is the complexity of its design
ECE @Saint Louis University, Baguio City 14
SLIDING WINDOW FLOW CONTROL
STATION A(Source)
STATION B(Destination)
Message Frame 1
Message Frame 2
Message Frame (n-1) (EOT)
ACK
0
Time
Message Frame 0
. . . . . . . . .
1
2
2n-1
0 to (2n-1) receptacles
ECE @Saint Louis University, Baguio City 15
ERROR CONTROL• A process encoding overhead codes into a
message frame for the detection of errors when they occur and the provision for the correction of the said errors
• Error detection may be achieved by various error codes like LRC, VRC, or CRC
• Error correction in the OSI Layer 2 is implemented by Automatic Request for Retransmission (ARQ)
• Errors that cannot be resolved by ARQ are referred for resolution to the higher OSI layers
ECE @Saint Louis University, Baguio City
TYPES OF ARQ• Stop-and-Wait ARQ– A message frame is retransmitted if the source
receives a negative acknowledgment from the destination
• Sliding Window ARQ– Go-back-n-frames• The errored frame and all frames transmitted after it
are retransmitted even if the subsequent frames have no error
– Selective Reject (SREJ)• Only the errored frames are retransmitted
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