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Reference model
Goal:
message exchange
between application processes
2
What we should do?
message
message
Where is the destination station?
How to reach the station?
What if error happens?
What if congestion happens?
How to make electrical signal?
………..
Electrical signal
Reference model
message
Electrical signal
message
Reference model
• Divide and conquer; divide the whole
work into separate small tasks – layers
• Each layer is independent of other
layers.
• Each layer in a station is communicating
with the same layer in the
corresponding station.
analogy: overseas air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
• Each task is working independently.
Introduction
1-6
Reference model for the travel
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
layers: each layer has its own mission.
– Each layer works independently.
– Lower layers provide services to the
very higher layer.
Introduction
1-7
Reference model
OSI 7 layers
application
presentation
session
transport
network
data link
physical
Internet layers
application
transport
network
link
physical
Application layer
• Application process has its own task to provide some
services.
– Web client/server, email, ftp, DNS etc.
• Then how can processes exchange messages to
provide some services.
Transport Layer
• Transport layer delivers all messages from several
application processes to the corresponding transport
layer.
– Then, how they can differentiate all messages?
AP1
AP2
AP3
Transport
AP1
end-to-end
AP2
AP3
transport
IP
IP
IP
network
access 1
network network
access1 access2
network
Access 2
subnet 1
subnet 2
Network layer
node-to-node delivery
• How to find the path
to reach the
destination.
Link layer
• Link layer takes the
responsibility of
delivering data
between two nodes
which are connected
by the same physical
link.
4.1
2
Link layer
• The data unit to be delivered at the link
layer is called a frame.
• goal
– How to know where the frame begins and
ends?
• In addition,
– Is there any error during transmission?
– Is there any frame loss because of the
processing speed difference between a
sender and a receiver?
– And others.
Physical layer
• How to make a data frame into an
electrical signal? (coding)
Physical layer and link layer
encapsulation
• How is the data unit (PDU) at each layer
delivered to its lower or higher layer at
the boundary?
– encapsulation
• PDU consists of data part and header
part.
• The header has the information with
which each layer can do its own
functions.
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
M
router
Introduction
1-17
encapsulation
Communication protocol
Common protocol
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
2:00
<file>
time
How to specify in protocol
• Procedure to exchange messages
• What kind of information should be
exchanged to do jobs.
• How these information is contained in
the header.