Network+ TechNote: 7-layer OSI Model

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Network+ TechNote: 7
layer OSI Model


The OSI (Open System Interconnection) model is developed by ISO in 1984 to provide a
reference model for the c
omplex aspects related to network communication. It divides the
different functions and services provided by network technology in 7 layers. This facilitates
modular engineering, simplifies teaching and learning network technologies, helps to isolate
ems, and allows vendors to focus on just the layer(s) in which their hardware or software
is implemented and enables them to create products that are compatible, standardized, and

The diagram below shows the 7 layers of the OSI Model. To re
member them in the correct
order, a common mnemonic is often used:

Host A

Host B

The Application, Presentation and Ses
sion layers are known as the
Upper Layers

and are
implemented in software. The Transport and Network layer are mainly concerned with
protocols for delivery and routing of packets and are implemented in software as well. The
Data Link is implemented in hard

and software and the Physical layer is implemented in
hardware only, hence its name. These lower two layers define LAN and WAN specifications. A
more detailed description of each layer follows below, but here's what basically happens when
data passes fro
m Host A to Host B:


the Application, Presentation and Session layer take user input and converts it into


the Transport layer adds a segment header converting the data into segments,


the Network layer adds a network header and converts the segments

into packets /


the Data Link layer adds a frame header converting the packets/datagrams into frames,


the MAC sublayer converts the frames into a bits, which the Physical layer can put on
the wire.

The steps are known as the 5 steps of
data e
. When the bits stream arrives at the
destination, the Physical layer takes it of the wire and converts it into frames, each layer will
remove their corresponding header while the data flows up the OSI model until it is converted
back to data a
nd presented to the user. This is also known as


The Application layer provides network services directly to the user's application such as a web
browser or email client. This layer is said to be "closest to the user
". Protocols that operate on
this layer include TELNET, HTTP, FTP, TFTP, SMTP, and NTP.


The Presentation layer 'represents' the data in a particular format to the Application layer. It
defines encryption, compression, conversion

and other coding functions. Examples of
specifications defined at this layer are GIF, JPEG, MPEG, MIME, and ASCII.


The Session layer establishes, maintains, and terminates end
end connect ions (sessions)
bet ween t wo applicat ions o
n t wo net work nodes. It cont rols t he dialogue bet ween t he source
and dest inat ion node, which node can send when and for how long. It also provides error
report ing for t he Applicat ion, Present at ion and Session layer. Examples of prot ocols/API's t hat
operat e

on t his layer are RPC and NETBIOS.


The Transport layer convert s t he dat a received from t he upper layers int o segment s and
prepares t hem for t ransport. The Transport layer is responsible for end
end (source
dest inat ion) deliv
ery of ent ire messages. It allows dat a t o be t ransferred reliably and uses
sequencing t o guarant ee t hat it will be delivered in t he same order t hat it was sent. It also
provides services such as error checking and flow cont rol (in soft ware). Examples of pr
ot ocols
t hat operat e on t his layer are TCP, UDP, NETBEUI, and SPX.

The above Transport layer prot ocols are eit her
connect ionless

connect ion
orient ed

means that a connection (a virtual link) must be established before any
actual d
ata can be exchanged. This guarantees that data will arrive, and in the same order as
it was sent. It guarantees delivery by sending acknowledgements back to the source when
messages are received. TCP is an example of a connection
oriented transport protoc

A common example of connection
oriented communication is a telephone call. You call, the
'destination' picks up the phone and acknowledges, and you start talking (sending data). When
a message or a piece of it doesn't arrive, you say: "What!?" and the

sender will repeat what he
said (retransmit the data).

is the opposite of connection
oriented; the sender does not establish a
connection before it sends data, it just sends it without guaranteeing delivery. UDP is an
example of a connecti
onless transport protocol.


The Network layer converts the segments from the Transport layer into packets (or
datagrams) and is responsible for path determination,

, and the delivery of packets
across internetworks. The network

layer treats these packets independently, without
recognizing any relationship between those individual packets. It relies on higher layers for
reliable delivery and sequencing.

The Network layer is also responsible for
logical addressing

(also known as
network addressing
or Layer 3 addressing) for example IP addressing. Examples of protocols defined at this layer
are IP, IPX, ICMP, RIP, OSPF, and BGP. Examples of devices that operate on this layer are
3 switches and routers. The latter includes WAP
s with built
in routing capabilities
(wireless access routers).


The Data Links provides transparent network services to the Network layer so the Network
layer can be ignorant about the underlying physical network topology. It is re
sponsible for
reassembling bits, taken of the wire by the Physical layer, to frames, and makes sure they are
in the correct order and requests retransmission of frames in case an error occurs. It provides
error checking by adding a CRC to the frame, and fl
ow control. Examples of devices that
operate on this layer are switches, bridges, WAPs, and NICs.

IEEE 802 Data Link sub layers

Around the same time the OSI model was developed, the IEEE developed the 802
such as 802.5 Token Ring and 802.11 for

wireless networks. Both organizations exchanged
information during the development, which resulted in two compatible standards. The IEEE
802 standards define physical network components such as cabling and network interfaces,
and correspond to the Data Li
nk and/or Physical layer of the OSI model. The IEEE refined the
standards and divided the Data Link layer into two sublayers: the

and the



LLC sublayer

LLC is short for
Logical Link Control
. The LLC layer is the upper sublayer of the D
ata Link layer
and is defined in the IEEE 802.2 standard. LLC masks the underlying physical network
technologies by hiding their differences to provide a single interface to the Network layer. The
LLC sublayer uses
Source Service Access Points (SSAPs)

Destination Service Access Points

to help the lower layers communicate with the Network layer protocols, acting as an
intermediate between the different network protocols (IPX, TCP/IP, etc.) and the different
network technologies (Ethernet, Token R
ing, etc.). Additionally, this layer is responsible for
sequencing and acknowledgements of individual frames.


MAC sublayer

The Media Access Control layer takes care of physical addressing and allows upper layers
access to the physical media, handles f
rame addressing, error checking. This layer controls
and communicates directly with the physical network media through the network interface
card. It converts the frames into bits to pass them on to the Physical layer who puts them on
the wire (and vice ve
rsa). IEEE LAN standards such as 802.3, 802.4, 802.5, and 802.11 define
standards for the MAC sublayer as well as the Physical layer.


This layer communicates directly with the physical media. It is responsible for activating,
ining and deactivating the physical link. It handles a raw bits stream and places it on
the wire to be picked up by the Physical layer at the receiving node. It defines electrical and
optical signaling, voltage levels, data transmission rates, as well as m
echanical specifications
such as cable lengths and connectors, the amount of pins and their functions. Examples of
devices that operate on this layer are hubs/concentrators, repeaters, NICs, WAPs, and LAN
and WAN interfaces such as RS
232, OC
3, BRI, and a

Current OSI Model related exam objectives for the Network+ N10
003 exam.

2.2 Identify the seven layers of the OSI model and their functions.

2.4 Identify the OSI layers at which the following network components operate:










NICs (Network Interface Cards)


WAPs (Wireless Access Points)

Date: Wednesday, September 07, 2005


Author: Johan Hiemstra


MCSA 2000/2003 Security+ CWNA