Chap 02 osi model

CChhaapptteerr 22
TThhee OOSSII MMooddeell aanndd
tthhee TTCCPP//IIPP PPrroottooccooll SSuuiittee
Objectives
Upon completion you will be able to:
• Understand the architecture of the OSI model
• Understand the layers of the OSI model and their functions
• Understand the architecture of the TCP/IP Protocol Suite
• Differentiate between the OSI model and the TCP/IP Suite
• Differentiate between the three types of Internet addresses
TCP/IP Protocol Suite 1

2.1 The OSI Model
EEssttaabblliisshheedd iinn 11994477,, tthhee IInntteerrnnaattiioonnaall SSttaannddaarrddss OOrrggaanniizzaattiioonn (IISSOO) iiss aa
mmuullttiinnaattiioonnaall bbooddyy ddeeddiiccaatteedd ttoo wwoorrllddwwiiddee aaggrreeeemmeenntt oonn iinntteerrnnaattiioonnaall
ssttaannddaarrddss.. AAnn IISSOO ssttaannddaarrdd tthhaatt ccoovveerrss aallll aassppeeccttss ooff nneettwwoorrkk
ccoommmmuunniiccaattiioonnss iiss tthhee OOppeenn SSyysstteemmss IInntteerrccoonnnneeccttiioonn (OOSSII) mmooddeell.. IItt
wwaass ffiirrsstt iinnttrroodduucceedd iinn tthhee llaattee 11997700ss..
TThhee ttooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn iinncclluuddee::
LLaayyeerreedd AArrcchhiitteeccttuurree
PPeeeerr--ttoo--PPeeeerr PPrroocceesssseess
EEnnccaappssuullaattiioonn

NNoottee::
ISO is the organization.
OSI is the model

Figure 2.1 The OSI model

Figure 2.2 OSI layers

Figure 2.3 An exchange using the OSI model

2.2 Layers in the OSI Model
The functions of each layer in the OSI model iiss bbrriieeffllyy ddeessccrriibbeedd..
PPhhyyssiiccaall LLaayyeerr
DDaattaa LLiinnkk LLaayyeerr
NNeettwwoorrkk LLaayyeerr
TTrraannssppoorrtt LLaayyeerr
SSeessssiioonn LLaayyeerr
PPrreesseennttaattiioonn LLaayyeerr
AApppplliiccaattiioonn LLaayyeerr
SSuummmmaarryy ooff LLaayyeerrss

Figure 2.4 Physical layer

NNoottee::
The physical layer is responsible
for the movement of individual bits
from one hop (node) to the next.

Figure 2.5 Data link layer

NNoottee::
The data link layer is responsible for
moving frames from one hop (node) to
the next.

Figure 2.6 Hop-to-hop delivery

Figure 2.7 Network layer

NNoottee::
The network layer is responsible for
the delivery of individual packets from
the source host to the destination host.

Figure 2.8 Source-to-destination delivery

Figure 2.9 Transport layer

NNoottee::
The transport layer is responsible for
the delivery of a message from one
process to another.

Figure 2.10 Reliable process-to-process delivery of a message

Figure 2.11 Session layer

Figure 2.12 Presentation layer

Figure 2.13 Application layer

Figure 2.14 Summary of layers

2.3 TCP/IP Protocol Suite
TThhee TTCCPP//IIPP pprroottooccooll ssuuiittee iiss mmaaddee ooff ffiivvee llaayyeerrss:: pphhyyssiiccaall,, ddaattaa lliinnkk,,
nneettwwoorrkk,, ttrraannssppoorrtt,, aanndd aapppplliiccaattiioonn.. TThhee ffiirrsstt ffoouurr llaayyeerrss pprroovviiddee
pphhyyssiiccaall ssttaannddaarrddss,, nneettwwoorrkk iinntteerrffaaccee,, iinntteerrnneettwwoorrkkiinngg,, aanndd ttrraannssppoorrtt
ffuunnccttiioonnss tthhaatt ccoorrrreessppoonndd ttoo tthhee ffiirrsstt ffoouurr llaayyeerrss ooff tthhee OOSSII mmooddeell.. TThhee
tthhrreeee ttooppmmoosstt llaayyeerrss iinn tthhee OOSSII mmooddeell,, hhoowweevveerr,, aarree rreepprreesseenntteedd iinn
TTCCPP//IIPP bbyy aa ssiinnggllee llaayyeerr ccaalllleedd tthhee aapppplliiccaattiioonn llaayyeerr..
PPhhyyssiiccaall aanndd DDaattaa LLiinnkk LLaayyeerrss
NNeettwwoorrkk LLaayyeerr
TTrraannssppoorrtt LLaayyeerr
AApppplliiccaattiioonn LLaayyeerr

Figure 2.15 TCP/IP and OSI model

2.4 Addressing
Three different levels of addresses are used iinn aann iinntteerrnneett uussiinngg tthhee
TTCCPP//IIPP pprroottooccoollss:: pphhyyssiiccaall ((lliinnkk)) aaddddrreessss,, llooggiiccaall ((IIPP)) aaddddrreessss,, aanndd
ppoorrtt aaddddrreessss..
PPhhyyssiiccaall AAddddrreessss
LLooggiiccaall AAddddrreessss
PPoorrtt AAddddrreessss

Figure 2.16 Addresses in TCP/IP

Figure 2.17 Relationship of layers and addresses in TCP/IP

ExamplE 1
In Figure 2.18 a node with physical address 10 sends
a frame to a node with physical address 87. The two
nodes are connected by a link. At the data link level
this frame contains physical (link) addresses in the
header. These are the only addresses needed. The rest
of the header contains other information needed at
this level. The trailer usually contains extra bits
needed for error detection.
See Next Slide

Figure 2.18 Physical addresses

ExamplE 2
As we will see in Chapter 3, most local area networks
use a 48-bit (6 bytes) physical address written as 12
hexadecimal digits, with every 2 digits separated by a
colon as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

ExamplE 3
In Figure 2.19 we want to send data from a node with network
address A and physical address 10, located on one LAN, to a
node with a network address P and physical address 95, located
on another LAN. Because the two devices are located on
different networks, we cannot use link addresses only; the link
addresses have only local jurisdiction. What we need here are
universal addresses that can pass through the LAN
boundaries. The network (logical) addresses have this
characteristic.
See Next Slide

ExamplE 3
(Continued)
The packet at the network layer contains the logical addresses,
which remain the same from the original source to the final
destination (A and P, respectively, in the figure). They will not
change when we go from network to network. However, the
physical addresses will change as the packet moves from one
network to another. The boxes labeled routers are
internetworking devices, which we will discuss in Chapter 3.
See Next Slide

Figure 2.19 IP addresses

ExamplE 4
As we will see in Chapter 4, an Internet address (in
IPv4) is 32 bits in length, normally written as four
decimal numbers, with each number representing 1
byte. The numbers are separated by a dot. Below is an
example of such an address.
132.24.75.9
An internet address in IPv4 in decimal numbers

ExamplE 5
Figure 2.20 shows an example of transport layer
communication. Data coming from the upperlayers
have port addresses j and k ( j is the address of the
sending process, and k is the address of the receiving
process). Since the data size is larger than the network
layer can handle, the data are split into two packets,
each packet retaining the service-point addresses ( j
and k). Then in the network layer, network addresses
(A and P) are added to each packet.
See Next Slide

ExamplE 5 (Continued)
The packets can travel on different paths and arrive at
the destination either in order or out of order. The two
packets are delivered to the destination transport
layer, which is responsible for removing the network
layer headers and combining the two pieces of data
for delivery to the upper layers.
See Next Slide

Figure 2.20 Port addresses

ExamplE 6
As we will see in Chapters 11, 12, and 13, a port
address is a 16-bit address represented by one decimal
number as shown below.
753
A 16-bit port address represented as one single number.

2.5 IP Versions
IP became the official protocol for the Internet iinn 11998833.. AAss tthhee IInntteerrnneett
hhaass eevvoollvveedd,, ssoo hhaass IIPP.. TThheerree hhaavvee bbeeeenn ssiixx vveerrssiioonnss ssiinnccee iittss iinncceeppttiioonn..
WWee llooookk aatt tthhee llaatttteerr tthhrreeee vveerrssiioonnss hheerree..
VVeerrssiioonn 44
VVeerrssiioonn 55
VVeerrssiioonn 6

Chap 02 osi model

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Chap 02 osi model