Download (Seven Layers of Open Systems Interconnection (OSI

In Seven Layers of Open Systems Interconnection (OSI) Model lesson, you will
learn about the seven layers of OSI model and their functions.
Student A read introduction and Layers 1,2 and 3
Student B read about layers 4,5,6, and 7
Student C read about Layers 4 and 3 in the Four layers of TCP/IP model text below
Student D read about layers 2 and 1 in the Four layers of TCP/IP model text below
Student A:
If network communications need to happen without any trouble, many problems must be
solved. Coordinating all these problems is complex and not easy to manage. To make these tasks
smooth, in 1977 the International Standards Organization (ISO) proposed the Open Systems
Interconnection (OSI) network model. The Open Systems Interconnection (OSI) model breaks
down the problems involved in moving data from one computer to another computer. This
model categorizes these hundreds of problems into seven layers. A layer in Open Systems
Interconnection (OSI) model is a portion that is used to categorize specific problems.
Open Systems Interconnection (OSI) Seven Layered reference model is only a reference model.
All the problems which are related to the communications are answered by specific protocols
operating at different layers. The following image shows the seven layers described.
Seven Layers of Open Systems Interconnection (OSI) Model
Layer 1. Physical Layer
The first layer of the seven layers of Open Systems Interconnection (OSI) network model is
called the Physical layer. Physical circuits are created on the physical layer of Open Systems
Interconnection (OSI) model. Physical layers describe the electrical or optical signals used for
communication. Physical layer of the Open Systems Interconnection (OSI) model is only
concerned with the physical characteristics of electrical or optical signalling techniques which
includes the voltage of the electrical current used to transport the signal, the media type
(Twisted Pair, Coaxial Cable, Optical Fibre, etc.), impedance characteristics, physical shape of the
connector, Synchronization etc. The Physical Layer is limited to the processes needed to place
the communication signals over the media, and to receive signals coming from that media. The
lower boundary of the physical layer of the OSI model is the physical connector attached to the
transmission media. The physical layer of the does not include the transmission media.
Transmission media stays outside the scope of the Physical Layer and are also referred to as
Layer 0.
Layer 2. Datalink Layer
The second layer of the seven is called the Datalink layer. The Data Link layer resides above the
Physical layer and below the Network layer. Datalink layer is responsible for providing end-toend validity of the data being transmitted. The Data Link Layer is logically divided into two
sublayers, The Media Access Control (MAC) Sublayer and the Logical Link Control (LLC)
Sublayer.
Media Access Control (MAC) Sublayer determines the physical addressing of the hosts. The MAC
sub-layer maintains MAC addresses (physical device addresses) for communicating with other
devices on the network. MAC addresses are burned into the network cards and constitute the
low-level address used to determine the source and destination of network traffic. MAC
Addresses are also known as Physical addresses, Layer 2 addresses, or Hardware addresses.
The Logical Link Control sublayer is responsible for synchronizing frames, error checking, and
flow control.
Layer 3. Network Layer
The third layer is the Network layer. The Network layer is responsible for managing logical
addressing information in the packets and the delivery of those packets to the correct
destination. Routers, which are special computers used to build the network, direct the data
packet generated by Network Layer using information stored in a table known as a routing table.
The routing table is a list of available destinations that are stored in memory on the routers. The
network layer is responsible for working with logical addresses. The logical addresses are used
to uniquely identify a computer on the network, but at the same time identify the network that
system resides on. The logical address is used by network layer protocols to deliver the packets
to the correct network. The Logical addressing system used in Network Layer is known as IP
address.
IP addresses are also known as Logical addresses or Layer 3 addresses.
Student B
Layer 4. Transport Layer
The fourth layer is the Transport layer. The Transport layer handles transport functions such as
reliable or unreliable delivery of the data to the destination. On the sending computer, the
transport layer is responsible for breaking the data into smaller packets, so that if any packet is
lost during transmission, the missing packets will be sent again. Missing packets are determined
by acknowledgments (ACKs) from the remote device, when the remote device receives the
packets. At the receiving system, the transport layer will be responsible for opening all of the
packets and reconstructing the original message.
Another function of the transport layer is TCP segment sequencing. Sequencing is a connectionoriented service that takes TCP segments that are received out of order and place them in the
right order.
The transport layer also enables the option of specifying a "service address" for the services or
application on the source and the destination computer to specify what application the request
came from and what application the request is going to.
Many network applications can run on a computer simultaneously and there should be some
mechanism to identify which application should receive the incoming data. To make this work
correctly, incoming data from different applications are multiplexed at the Transport layer and
sent to the bottom layers. On the other side of the communication, the data received from the
bottom layers are de-multiplexed at the Transport layer and delivered to the correct application.
This is achieved by using "Port Numbers".
The protocols operating at the Transport Layer, TCP (Transmission Control Protocol) and UDP
(User Datagram Protocol) uses a mechanism known as "Port Number" to enable multiplexing
and de-multiplexing. Port numbers identify the originating network application on the source
computer and destination network application on the receiving computer.
Layer 5. Session Layer
The position of Session Layer is between Transport Layer and the Presentation Layer. Session
layer is the fifth layer. It is responsible for establishing, managing, and terminating connections
between applications at each end of the communication.
In the connection establishment phase, the service and the rules (who transmits and when, how
much data can be sent at a time etc.) for communication between the two devices are proposed.
The participating devices must agree on the rules. Once the rules are established, the data
transfer phase begins. Connection termination occurs when the session is complete, and
communication ends gracefully.
In practice, Session Layer is often combined with the Transport Layer.
Layer 6. Presentation Layer
The position of Presentation Layer is just below the Application Layer. When the presentation
layer receives data from the application layer, to be sent over the network, it makes sure that the
data is in the proper format. If it is not, the presentation layer converts the data. On the other
side of communication, when the presentation layer receives network data from the session
layer, it makes sure that the data is in the proper format and once again converts it if it is not.
Formatting functions at the presentation layer may include compression, encryption, and
ensuring that the character code set (ASCII, Unicode, EBCDIC (Extended Binary Coded Decimal
Interchange Code, which is used in IBM servers) etc.) can be interpreted on the other side.
For example, if we select to compress the data from a network application that we are using, the
Application Layer will pass that request to the Presentation Layer, but it will be the Presentation
Layer that does the compression.
Layer 7. Application Layer
The Application Layer is the seventh layer in the OSI network model. This is the top-most layer.
Real traffic data will be often generated from the Application Layer. This may be a web request
generated from HTTP protocol, a command from telnet protocol, a file download request from
FTP protocol etc.
In this lesson (Seven Layers of Open Systems Interconnection (OSI) Model), you have learned
what are the Seven Layers of Open Systems Interconnection (OSI) Model and the functions of
these seven layers. The top-most layer of the Seven Layers of Open Systems Interconnection
(OSI) Model is the Application Layer and the bottom-most layer of the Seven Layers of Open
Systems Interconnection (OSI) Model is Physical Layer.
Four Layers of TCP/IP model, Comparison and Difference between TCP/IP and OSI models
Student C:
Like OSI network model, TCP/IP also has a network model. TCP/IP was on the path of
development when the OSI standard was published and there was interaction between the
designers of OSI and TCP/IP standards. The TCP/IP model is not the same as the OSI model. OSI
is a seven-layered standard, but TCP/IP is a four layered standard. The OSI model has been very
influential in the growth and development of TCP/IP standard, and that is why much OSI
terminology is applied to TCP/IP. The following figure compares the TCP/IP and OSI network
models.
As we can see from the above figure, presentation and session layers are not there in TCP/IP model.
Also note that the Network Access Layer in TCP/IP model combines the functions of Datalink Layer
and Physical Layer.
Layer 4. Application Layer
The Application layer is the top most layer. It is present on the top of the Transport layer. It defines
TCP/IP application protocols and how host programs interface with Transport layer services to use
the network.
Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP
(Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer
Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) ,
DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.
Layer 3. Transport Layer
Transport Layer is the third layer. The position of the Transport layer is between Application layer
and Internet layer. The purpose of the Transport layer is to permit devices on the source and
destination hosts to carry on a conversation. It defines the level of service and status of the
connection used when transporting data.
The main protocols included in the Transport layer are TCP (Transmission Control Protocol) and UDP
(User Datagram Protocol).
Student D:
Layer 2. Internet Layer
The internet Layer is the second layer. Its position is between the Network Access Layer and
Transport layer. The Internet layer packs data into data packets known as IP datagrams, which
contain a source and destination address (logical address or IP address) information that is used to
forward the datagrams between hosts and across networks. The Internet layer is also responsible for
the routing of IP datagrams.
A packet switching network depends upon a connectionless internetwork layer. This layer is known
as the Internet layer. Its job is to allow hosts to insert packets into any network and have them
deliver independently to the destination. At the destination side data packets may appear in a
different order than they were sent. It is the job of the higher layers to rearrange them in order to
deliver them to the proper network applications operating at the Application layer.
The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control
Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol)
and IGMP (Internet Group Management Protocol).
Layer 1. Network Access Layer
The Network Access Layer is the first layer. It defines details of how data is physically sent through
the network, including how bits are electrically or optically signaled by hardware devices that
interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper
wire.
The protocols included in Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay
etc.
The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access
Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media
when Ethernet operates in a shared media. An Access Method determines how a host will place data
on the medium.
IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the
wire when the wire is free from network traffic. When a host wants to place data on the wire, it will
check the wire to find whether another host is already using the medium. If there is traffic already in
the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if
two systems place data on the medium at the same instance, they will collide with each other,
destroying the data. If the data is destroyed during transmission, the data will need to be
retransmitted. After collision, each host will wait for a small interval of time and again the data will
be retransmitted.
Text Swapping:
1. Now get together with another student who has read the same text as you and discuss what
is was about.
2. Now students A and B get together and tell each other about your text
Students C and D do the same
3. Now C’s and A’s make pairs and tell each other about both your text and the one your
partner told you about
Students B and D do the same
Vocabulary
The Seven Layer Model
Find an underlined word in the text that means
a. When many signals are put together into a single complex one.
b. How well a concept or function works, how much it does what it is supposed to do.
c. The act of sending something
d. A small piece of something
e. A set of rules that specify how different things should interact or communicate together.
f. At the same time
g. The resistance of an electric circuit or a component in one.
The Four Layer Model
a. when two systems or programmes interact
b. a pause
c. two wires wrapped around each other
d. bump together
e. Allow or let
f. a network in which data is broken down into small units so it can be sent along many different
paths.