link layer protocols
DESCRIPTION
Link Layer Protocols. Martin Weiss. Objectives of this Meeting. Explain what a protocol is Compare connection-oriented with connectionless protocol types Describe some protocol mechanisms Describe different link layer protocols. Review of the Last Meeting. Error detection Flow control - PowerPoint PPT PresentationTRANSCRIPT
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Link Layer Protocols
Martin Weiss
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Slide 2
Objectives of this Meeting
Explain what a protocol is Compare connection-oriented with
connectionless protocol types Describe some protocol
mechanisms Describe different link layer
protocols
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Slide 3
Review of the Last Meeting
Error detection Flow control Performance of flow control
techniques
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Slide 4
Data Link Layer Issues
Type Error detection and control Flow control
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Slide 5
OSI Reference ModelEnd User Application Process
Application Services: FTAM, MHS, EDI
Data Representation, Transfer Syntax, Data TransformationDialog Synchronization and Control
End-to-End Message TransferNetwork Routing and Addressing
Data Link Control, Error Checking, FramingMechanical and Electrical Interface
Data Communications NetworkPhysical Connection
Network Independent Message Interchange
Syntax-Independent Message Interchange
Distributed Information ServicesApplication
PresentationSession
TransportNetworkLinkPhysical
OSI Layer
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Slide 6
Structure of Protocols
Source AP Desintation AP
Application Process (AP) to AP Messages
Source LinkLayer
Desintation LinkLayer
L_DATA.request L_DATA.indicationLink Layer PDU’s
Physical Transmission Medium
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Slide 7
Connectionless PrimitivesDTE A DTE B
Link Layer
DATA.request
DATA.indication
DATA.indication
DATA.request
Link LayerApp. Process App. Process
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Slide 8
Connection-Oriented Primitives
DTE A DTE B
Link Layer
CON.request
CON.confirm
CON.indication
Link LayerApp. Process App. Process
DATA.request
DATA.indication
DATA.indication
DATA.request
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Slide 9
Kermit Protocol Structure
SOH LEN SEQ TYPE data BCC CR
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Slide 10
Kermit Details SOH = Start of Header LEN = Number of Bytes in the
Frame SEQ = Sequence Number of the
Current Frame
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Slide 11
Kermit Details TYPE = Type of Frame
– S: Send Initiation Parameters– F: Filename– D: File Data– Z: End of File– B: End of Transaction– Y: ACK– N: NAK– E: Error (Fatal)
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Slide 12
Kermit Details Data = Contents of Frame BCC = Block Check Character CR = End of Block Marker (ASCII
CR Character)
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Slide 13
Operation of Kermit
Kermit
Time
KermitUser User
Kermit KermitConnect Connect
Receive
Send (File)
Data Block [1]Sent
Call Setup
V(S) = 0 V(R) = 0
V(S) = 1I[0,S,Init]
ACK[0,Y,Init]
I[1,F,Filename]ACK[1,Y]
V(R) = 1
V(R) = 2V(S) = 2
I[2,D,data]
ACK[2,Y]V(R) = 3
V(S) = 3 Data Block [1]Received
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Slide 14
Operation of Kermit (Error)
Kermit
Time
KermitUser User
Data Block [1]Sent
V(R) = 2V(S) = 2 I[2,D,data]
ACK[2,Y]V(R) = 3
V(S) = 3
Data Block [2]Sent
I[3,D,data]
NACK[3,N]V(R) = 3
V(S) = 4
I[3,D,data]ACK[3,Y]
V(R) = 4
Data Block [1]Received
Data Block [2]Received
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Slide 15
Operation of Kermit (Lost Frame)
Kermit
Time
KermitUser User
Data Block [1]Sent
V(R) = 2V(S) = 2 I[2,D,data]
ACK[2,Y]V(R) = 3
V(S) = 3
Data Block [2]Sent
I[3,D,data]V(S) = 4
I[3,D,data]ACK[3,Y]
V(R) = 4
Timeout
Data Block [1]Received
Data Block [2]Received
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Slide 16
Operation of Kermit (Lost ACK)
Kermit
Time
KermitUser User
Data Block [1]Sent
V(R) = 2V(S) = 2 I[2,D,data]
ACK[2,Y]V(R) = 3
V(S) = 3
Data Block [2]Sent
I[3,D,data]V(S) = 4
I[3,D,data]
ACK[3,Y] V(R) = 4
Timeout
ACK[3,Y]V(R) = 4
Data Block [1]Received
Data Block [2]Received
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Slide 17
Operation of Kermit
Destination Computer
Kermit
Time
KermitUser User
Data Block [n*]Sent
EOFExit
Data Block [n*]Received
EOF
EOTExit
V(S) = n* I[n*,D,data]ACK[n*,Y]
V(R) = n*I[(n+1)*,Z]
ACK[(n+1)*,Y]
I[(n+2)*,B]
ACK[(n+2)*,Y]
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Slide 18
Structure of Bisync Information Frames
SYN SYN SOH Identifier Address STX Text ETX BCCSingle Block Message
SYN SYN SOH Identifier STX Text ETB BCCIntermediate of Multiblock Message
SYN = SynchronizationSOH = Start of HeaderSTX = Start of TextETX = End of TextETB = End of Text BlockIdentifier = Sequence Number of Block
First of Multiblock MessageSYN SYN SOH Identifier Address STX Text ETB BCC
End of Multiblock MessageSYN SYN SOH Identifier STX Text ETX BCC
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Slide 20
HDLC Frame Format
Flag Address Control Information FCS Flag8 8/16 8/16 0 to N 16/32 8
Frame Format
Control Field for Normal Information Frame
0 N(S) N(R)P/F
Control Field for Normal Supervisory Frame
0 N(R)P/F1 S
Control Field for Normal Information Frame
MP/FM1 0
P/F = Poll/Final BitN(S) = Send Sequence NumberN(R) = Receive Sequence Number
S: RR RNR REJ SREJ
M: Defines 9 Commands and 4 Unnumbered Responses
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Slide 21
Unidirectional Data (ACK and NAK)
Sender ReceiverUnACK’edList
V(S) V(R) UnACK’edList
V(S)I(0,0)
0 1 00 0 0
N(S) = V(R) => Frame OKRR(1)-
I(1,0)
I(2,0)
2 0
3 0N(S) = V(R) => Frame OK
N(S) = V(R) => Frame OK
1
1,2
RR(2)
RR(3)
2
-
V(R)0
0 1
0 2
0 3
(N(S), N(R))
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Slide 22
Unidirectional Data (With Error)
Sender ReceiverUnACK’ed V(S) V(R) UnACK’ed
ListV(S)
I(3,0)3 4 0
3 0 0
I(4,0)5 0
N(S) = V(R) => Frame OK
3,4
3,4 REJ(3)
RR(5)-
V(R)3
0 4
(N(S), N(R))
N(S) = 4 V(R) = 3 ERROR!
I(3,0)
I(4,0)N(S) = V(R) => Frame OK
0 5
-
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Slide 23
Example 2: Bidirectional Data
Sender ReceiverUnACK’edList
V(S) V(R) UnACK’edList
V(S)I(0,0)
0 1 00 0 0
I(0,1)-I(1,1)
I(2,1)2 1
3 1
1
1,2
I(1,3)-
V(R)0
0 1
1 2
2 3
(N(S), N(R))
1 1 01 1
-
1 3 -
13 2
RR(2) 2 3 -
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Slide 24
Modems Error Correction (LAPM)
DTE
Modem (DCE)
User InterfacePart(UIP)
ErrorCorr-ectingPart(ECP) Public
TelephoneNetwork
Modem (DCE)
DTE
User InterfacePart(UIP)
ErrorCorr-ectingPart(ECP)