Overview of Network Models
Networks
A network is a set of devices (often referred to as nodes)
connected by communication links.
A node can be a computer, printer, or any other device capable
of sending and/or receiving data generated by other nodes on
the network.
A link can be a cable, air, optical fiber, or any medium which
can transport a signal carrying information.
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Computer Network Frameworks
There are some well defined frameworks (i.e., OSI and
TCP/IP) to design computer networks so that communication
between various devices (of any type) can be done.
OSI Model: It is a layered framework which is composed of
seven ordered layers.
Each layer is part of a process, which moves information
across a network from one system to another.
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Seven layers of the OSI model
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Interaction between layers in OSI model
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An exchange using OSI model
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Physical layer
It is responsible for movement of individual bits from one
node to the next node.
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• Representation of bits:
• Data rate:
• Line Configuration:
• Physical Topology:
• Synchronization of bits:
• Transmission mode:
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Data Link layer
The data link layer is responsible for moving frames from one
hop (node) to the next.
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Framing: The data link layer divide the stream of bits received from
network layer into manageable unit called framing.
Physical addressing:
Flow control:
Error control:
Access control:
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Hop-to-hop delivery
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Network layer
The
network
layer
is
responsible
for
the
delivery of individual packets from the source host to the
destination host.
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Logical Addressing:
Routing:
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Source-to-destination delivery
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Transport layer
The transport layer is responsible for
of a message from one process to another.
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the
delivery
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Service point addressing:
Segmentation:
Connection control:
Flow control:
Error control:
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Reliable process-to-process delivery of a message
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Session layer
The
session
layer
is
control and synchronization.
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responsible
for
dialog
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Dialog control: It allow the communication between two processes to take
place either half duplex or full duplex.
Synchronization:
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Presentation layer
The presentation layer is
compression, and encryption.
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responsible
for translation,
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Translation
Encryption
Compression
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Application layer
It is responsible for providing services to the user.
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Network virtual terminal:
File transfer, access and management:
Mail Services:
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Summary of layers
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TCP/IP Protocol Suite
It was developed prior to the OSI model and consists only five
layers.
The layers in the TCP/IP protocol suite do not exactly match
those in the OSI model.
The original TCP/IP protocol suite was defined as having four
layers: host-to-network, internet, transport, and application.
However, when TCP/IP is compared to OSI, we can say that
the TCP/IP protocol suite is made of five layers: physical, data
link, network, transport, and application.
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TCP/IP and OSI model
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Physical and Data Link Layers
At this layer, TCP/IP does not define any specific protocol. It
supports all the standard protocols.
A network in a TCP/IP internetwork can be a LAN or WAN.
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Network Layer
Network layer is responsible for creating a connection between the source
computer and destination computer.
NL is responsible for host-to-host connection and also responsible to
control routers for choosing best routing strategy.
NL in the Internet includes a main protocol called Internet Protocol (IP)
which defines format of packet (called datagram).
IP also perform the routing by passing each datagram through routers
unless it reaches the destination.
NL also includes unicast and multi-cast routing protocols
• A routing protocol does not take part in routing (That is the
responsibility of IP)
• It simply creates forwarding tables for routers to help them in
routing
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IP
It is an unreliable and connectionless protocol.
• No flow control
• No error control
• No congestion control
If any of these services are required by Application, it should
rely on transport layer protocol
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Axillary Protocols which help IP at NL
At Network layer, Some Auxiliary protocols help IP in its
work:
• ICMP
• IGMP
• RARP
• ARP.
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ICMP, IGMP and RARP
Internet Control Message Protocol (ICMP)
• A mechanism used by host and gateways to send notification of
datagram problems back to the sender.
• It sends query and error reporting messages.
Internet Group Management Protocol (IGMP)
• Used to facilitate the simultaneous transmission of a message to a
group of recipients
• It helps IP in multitasking
Reverse Address Resolution Protocol (RARP)
• Protocol allows a host to discover its internet address when it
knows its physical address.
• It is used when computer is connected to a network for first time.
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ARP
Used to associate the logical address with physical address
(link-layer address to a host or a router when network layers
address is given).
Each device on the network is identified by physical or station
address, usually imprinted on the network interface card
(NIC).
ARP is used to find the physical address of the node when its
internet address is known.
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Transport Layer
Logical connection at Transport Layer is end-to-end
It performs the following service:
• To get a message from application program running on
source host
• Deliver it to corresponding application program at
destination host
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Transport Layer
Responsible for delivery of message from a process to another
process
• TCP (Transmission control Protocol)
• UDP (User Datagram Protocol)
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TCP
TCP is a connection oriented protocol
It first establishes a connection is established between both the ends (source
and destination host) before data transmission.
At the sending end TCP divides a stream of data into smaller units called
segments.
Each segment includes a sequence number for reordering after receipt,
together with acknowledgment no. for the segments received.
At the receiving end, TCP collects each datagram as it comes in and
reorder the transmission based on sequence no.
TCP provides:
• flow control
• error control
• congestion control
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UDP
UDP is a connection-less protocol
• Transmits user datagram without first creating a logical
connection
• Each datagram from a user is an independent entity
Does not provide:
• flow control
• error control
• congestion control
Its is a simple protocol which is attractive to an application
program which needs to send short messages and cannot
afford the retransmission of packets involved in TCP if a
packets is corrupted and lost.
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SCTP
Stream Control Transmission Protocol
• provides support for newer applications such as voice
over the internet.
• It combines the best features of UDP & TCP protocol.
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Application Layer
An application is a program running on a computer (which is also termed as a
process)
Logical connection is end-to-end
• Two applications exchange messages between each other via a virtual
bridge between the two.
• Physically, communication is performed via all the layers.
To communicate, one process sends a request message to other process and gets a
reply message
A number of protocols are predefined but a user can also do the same provided
they provide it at both ends.
Some of the protocols are:
• HTTP (Hyper Text Markup Language)
• SMTP (Simple Mail Transfer Protocol)
• FTP (File Transfer Protocol)
• TELNET (TErminaLNETwork)
• SSH (Secure SHell)
• SNMP (Simple Network Managemnet Protocol)
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Parameters
Layers
Usage
OSI Model
TCP/IP Model
It has 7 layers
It has 4 layers
It is low in usage
It is mostly used
Delivery of the package is guaranteed in OSI Model
Delivery of the package is not guaranteed in TCP/IP Model
Replacement of tools and changes can easily be done in
this model
Replacing the tools is not easy as it is in OSI Model
It is less reliable than TCP/IP Model
It is more reliable than OSI Model
Not tied to specific protocols, but examples include HTTP
(Application), SSL/TLS (Presentation), TCP (Transport), IP
(Network), Ethernet (Data Link)
HTTP, FTP, TCP, UDP, IP, Ethernet
Built into Data Link and Transport layers
Built into protocols like TCP
Both connection-oriented (TCP) and connectionless (UDP)
protocols are covered at the Transport layer
TCP (connection-oriented), UDP (connectionless)
Delivery
Replacement
Reliability
Protocol Example
Error Handling
Connection Orientation
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