Unit-1 Layers in Computer Networks and Protocols

It is regarding Layering in CNP

1. 2.1
2-2 THE OSI MODEL
Establishedin1947,theInternationalStandards
Organization(ISO)isamultinationalbodydedicatedto
worldwideagreementoninternationalstandards.AnISO
standardthatcoversallaspectsofnetwork
communicationsistheOpenSystemsInterconnection
(OSI)model.Itwasfirstintroducedinthelate1970s.
Layered Architecture
Peer-to-Peer Processes
Encapsulation
Topics discussed in this section:

2. 2.2
ISO is the organization.
OSI is the model.
Note

3. 2.3
Figure 2.2 Seven layers of the OSI model

4. 2.4
Figure 2.3 The interaction between layers in the OSI model

5. 2.5
Figure 2.4 An exchange using the OSI model

6. 2.6
2-3 LAYERS IN THE OSI MODEL
Inthissectionwebrieflydescribethefunctionsofeach
layerintheOSImodel.
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
Topics discussed in this section:

7. 2.7
Figure 2.5 Physical layer

8. 2.8
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

9. 2.9
Figure 2.6 Data link layer

10. 2.10
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

11. 2.11
Figure 2.7 Hop-to-hop delivery

12. 2.12
Figure 2.8 Network layer

13. 2.13
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

14. 2.14
Figure 2.9 Source-to-destination delivery

15. 2.15
Figure 2.10 Transport layer

16. 2.16
The transport layer is responsible for the delivery
of a message from one process to another.
Note

17. 2.17
Figure 2.11 Reliable process-to-process delivery of a message

18. 2.18
Figure 2.12 Session layer

19. 2.19
The session layer is responsible for dialog
control and synchronization.
Note

20. 2.20
Figure 2.13 Presentation layer

21. 2.21
The presentation layer is responsible for translation,
compression, and encryption.
Note

22. 2.22
Figure 2.14 Application layer

23. 2.23
The application layer is responsible for
providing services to the user.
Note

24. 2.24
Figure 2.15 Summary of layers

25. 2.25
2-4 TCP/IP PROTOCOL SUITE
ThelayersintheTCP/IPprotocolsuitedonotexactly
matchthoseintheOSImodel.TheoriginalTCP/IP
protocolsuitewasdefinedashavingfourlayers:host-to-
network,internet,transport,andapplication.However,
whenTCP/IPiscomparedtoOSI,wecansaythatthe
TCP/IPprotocolsuiteismadeoffivelayers:physical,
datalink,network,transport,andapplication.
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
Topics discussed in this section:

26. 2.26
Figure 2.16 TCP/IP and OSI model

27. 2.27
2-5 ADDRESSING
Fourlevelsofaddressesareusedinaninternetemploying
theTCP/IPprotocols:physical,logical,port,andspecific.
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
Topics discussed in this section:

28. 2.28
Figure 2.17 Addresses in TCP/IP

29. 2.29
Figure 2.18 Relationship of layers and addresses in TCP/IP

30. 2.30
InFigure2.19anodewithphysicaladdress10sendsa
frametoanodewithphysicaladdress87.Thetwonodes
areconnectedbyalink(bustopologyLAN).Asthe
figureshows,thecomputerwithphysicaladdress10is
thesender,andthecomputerwithphysicaladdress87is
thereceiver.
Example 2.1

31. 2.31
Figure 2.19 Physical addresses

32. 2.32
AswewillseeinChapter13,mostlocal-areanetworks
usea48-bit(6-byte)physicaladdresswrittenas12
hexadecimaldigits;everybyte(2hexadecimaldigits)is
separatedbyacolon,asshownbelow:
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

33. 2.33
Figure2.20showsapartofaninternetwithtworouters
connectingthreeLANs.Eachdevice(computeror
router)hasapairofaddresses(logicalandphysical)for
eachconnection.Inthiscase,eachcomputeris
connectedtoonlyonelinkandthereforehasonlyone
pairofaddresses.Eachrouter,however,isconnectedto
threenetworks(onlytwoareshowninthefigure).So
eachrouterhasthreepairsofaddresses,oneforeach
connection.
Example 2.3

34. 2.34
Figure 2.20 IP addresses

35. 2.35
Figure2.21showstwocomputerscommunicatingviathe
Internet.Thesendingcomputerisrunningthree
processesatthistimewithportaddressesa,b,andc.The
receivingcomputerisrunningtwoprocessesatthistime
withportaddressesjandk.Processainthesending
computerneedstocommunicatewithprocessjinthe
receivingcomputer.Notethatalthoughphysical
addresseschangefromhoptohop,logicalandport
addressesremainthesamefromthesourceto
destination.
Example 2.4

36. 2.36
Figure 2.21 Port addresses

37. 2.37
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Note

38. 2.38
Example 2.5
AswewillseeinChapter23,aportaddressisa16-bit
addressrepresentedbyonedecimalnumberasshown.
753
A 16-bit port address represented
as one single number.

39. 2.39
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
Note