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Switch
Switching office
High-bandwidth fiber trunk
Junction box
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Copper cable TV wire
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Layer 4
Layer 3
Layer 2
Layer 1
Host 1
Layer 4/5 interface
Layer 3/4 interface
Layer 2/3 interface
Layer 1/2 interface
Layer 5 protocolLayer 5
Layer 4
Layer 3
Layer 2
Layer 1
Host 2
Layer 4 protocol
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Layer 2 protocol
Layer 1 protocol
Physical medium
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I like rabbits
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3
2
1
3
2
1
Location B
Message Philosopher
Translator
Secretary
Information for the remote translator
Information for the remote secretary
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Fax #--- L: Dutch Ik hou van konijnen
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lapins
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M M
Layer 2 protocol
2
Layer 3 protocol
Layer 4 protocol
Layer 5 protocol
3
4
5
1
Layer
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Interface
Layer n
ICI SDU
IDU
ICI SDU
SAP
SDU
SAP = Service Access Point IDU = Interface Data Unit SDU = Service Data Unit PDU = Protocol Data Unit ICI = Interface Control Information
Layer n entities exchange n-PDUs in their layer n protocol
Headern-PDU
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Presentation
Application
Session
Transport
Network
Data link
Physical
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6
5
4
3
2
1
Interface
Interface
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PPDU
SPDU
TPDU
Packet
Frame
Bit
Presentation
Application
Session
Transport
Network
Data link
Physical
Host B
Network Network
Data link Data link
Physical Physical
Router Router
Internal subnet protocol
Application protocol
Presentation protocol
Transport protocol
Session protocol
Communication subnet boundary
Network layer host-router protocol
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Session
Transport
Network
Data link
Physical
7
6
5
4
3
2
1
Application
Transport
Internet
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Software
OperatingSystem
UserSpace
Hardware
Physical
NetworkAccess
Internet
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Transport(host−to−host)
TCP/IP
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Presentation
Session
Transport
Network
Data Link
Physical
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FrameRelay
Network
Router
IEEE 802 LAN
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Physical
MAC
LLC
IP
Physical
MAC
LLC
IP
Physical
FrameRelay
FrameRelay
TCP
Application
Physical
IP
TCP
Application
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TCP
IP
Physical Physical
IP
NAP 1 NAP 2
Network AccessProtocol #1
Host A
App XApp Y
TCP
IP
Network AccessProtocol #2
Host B
App YApp X
Network 1 Network 2
Global networkaddress
Subnetwork attachmentpoint address
Logical connection(e.g., virtual circuit)
Logical connection(TCP connection)
Port orservice access point (SAP)
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DataT
Data
Physical
IP
TCP
Application
FrameRelay DataTIF F
1. Preparing the data. Theapplication protocol prepares a blockof data for transmission. For example,an email message (SMTP), a file (FTP),or a block of user input (TELNET).
2. Using a common syntax. If necessary,the data are converted to a form expectedby the destination. This may includea different character code, the use ofencryption, and/or compression.
3. Segmenting the data. TCP may breakthe data block into a number of segments,keeping track of their sequence. Each TCPsegment includes a header containing asequence number and a frame check sequence to detect errors.
5. Fragmenting the segments. IP maybreak a TCP segment into a number of datagrams to meet size requirements ofthe intervening networks. Each datagramincludes a header containing a destinationaddress, a frame check sequence, and othercontrol information.
6. Framing. A frame relay header andtrailer is added to each IP datagram. Theheader contains a connection identifier andthe trailer contains a frame check sequence
7. Transmission. Eachframe is transmitted overthe medium as a sequenceof bits.
4. Duplicating segments. A copy is madeof each TCP segment, in case the lossor damage of a segment necessitatesretransmission. When an acknowledgmentis received from the other TCP entity, asegment is erased.
Peer−to−peer dialogue.Before data are sent, thesending and receivingapplications agree on formatand encoding and agree toexchange data.
Peer−to−peer dialogue.The two TCP entities agreeto open a connection.
Peer−to−peer dialogue.Each IP datagram is forwardedthrough networks and routersto the destination system.
Peer−to−peer dialogue.Each frame is forwarded throughthe frame relay network.
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Physical
MACDataTIL#####M ##### MDataTIF F
LLC
FrameRelay
9. Processing the frame. Theframe relay layer removes theheader and trailer and processesthem. The frame check sequence is used for error detection. Theconnection numberidentifies the source.
13. Transmission. Eachframe is transmitted overthe medium as a sequenceof bits.
12. Framing A MAC header andtrailer is added to each LLC PDU,forming a MAC frame. The headercontains address information and thetrailer contains a frame check sequence.
11. Forming LLC PDU. An LLCheader is added to each IP datagramto form an LLC PDU. The headercontains sequence number andaddress information.
8. Arriving at router. Theincoming signal is receivedover the transmission mediumand interpreted as a frame of bits.
DataTI
IP
10. Routing the packet. IP examinesthe IP header and makes a routingdecision. It determines which outgoinglink is to be used and then passesthe datagram back to the link layerfor transmission on that link.
Peer−to−peer dialogue.The router will pass thisdatagram onto anotherrouter or to the destinationsystem.
Physical
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Physical
LLC
IP
TCP
MAC
Data
Application
DataTI
DataT
14. Arriving at destination. The incomingsignal is received over the transmissionmedium and interpreted as a frame of bits.
DataTIL ####M #### M
15. Processing the frame. The MAC layerremoves the header and trailer and processesthem. The frame check sequenceis used for error detection.
16. Processing the LLC PDU. The LLC layerremoves the header and processes it. Thesequence number is used for flow and errorcontrol.
17. Processing the IP datagram. IPremoves the header. The frame checksequence and other controlinformation are processed.
18. Processing the TCP segment. TCPremoves the header. It checks the framecheck sequence and acknowledges if there isa match and discards for mismatch. Flow controlis also performed.
19. Reassembling user data. If TCPhas broken the user data into multiplesegments, these are reassembled and theblock is passed up to the application.
20. Delivering the data. The applicationperforms any needed transformations, includingdecompression and decryption, and directs the data to the appropriate file or other destination.
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