I don't want to spend too much time on theoretical information or to understand the packets sent to
the destination systems and the reply packets as well.
We should talk about the TCP/IP Internet Protocol suite. OSI (open systems interconnection) is a reference
model for how applications can communicate over a network. A reference model is a conceptual framework
for understanding relationships.
The purpose of the OSI reference model is to guide vendors and developers so that the digital communication
products and software programs they create will operate and to facilitate clear comparisons among communications
tools.
Most vendors involved in the telecommunications make an attempt to describe their products and services
in relation to the OSI model and although useful for guiding discussion and evaluation.
OSI is rarely actually implemented. As few network products or standard tools
Keep all related functions together in well-defined layers as related to the model. Developed by representatives
of major computer and telecommunication companies beginning in 1983,
OSI was originally intended to be a detailed specification of actual interfaces.
Instead, the committee decided to establish a common reference model for which others could then develop
detailed interfaces, Which in turn could become standards.
OSI was officially adopted as the international standard by the International Organization of standards.
ISO
So, what about the, OSI Layers
The main concept of OSI is that the process of communication between two endpoints in
a telecommunication network can be divided into seven distinct groups of related functions or layers.
Each communicating user or program is at a computer that can provide those seven layers of function.
So in a given message between users there will be a flow of data down through the layers in the source
computer across the network and then up through the layers in the receiving computer.
The seven layers of function are provided by a combination of applications, operating systems, network
card, device drivers and networking hardware that enable a system to put a signal on a network cable
or out over Wi-Fi or any other wireless protocol.
The seven Open Systems Interconnection layers are: Layer 7.
The application layer.
This is a layer at which communication partners are identified.
Is there someone to talk to?
Network capacity is assessed (Will the network
let me talk to them right now?), and that creates a thing to send or opens the thing received.
Please note that this layer is not the application itself.
It is this set of services an application should be able to make use of directly. Although some applications
may perform application layer functions. Layer 6 - The presentation layer.
This layer is usually part of an operating system and converts incoming and outgoing data from one presentation
format to another.
For example from clear text to encrypted text at one end and back to clear text the other. Layer 5 - The
session layer. This layer sets up coordinates and terminate conversations. Services include authentication
and reconnection after an interruption. On the Internet Transmission Control Protocol.
TCP and User Datagram Protocol UDP provide these services for most applications.
Layer 4. The transport layer.
This layer manages packetization of data then the delivery of the packets. Including checking for errors
in the data once it arrives. On the internet.
TCP and UDP provide these services for most applications as well.
Layer 3. The network layer. This layer handles the addressing and routing of the data. Sending it in the
right direction to the right destination on outgoing transmissions and receiving incoming transmissions
at the packet level.
IP is a network layer for the Internet. Layer 2. The data link layer. This layer sets up links across
the physical network,
putting packets into network frames. This layer has two sub-layers.
The Logical Link Control Layer and the Media Access Control Layer. Ethernet is the main data link layer in use.
Layer 1. The physical layer. This layer conveys the bitstream through the network
at electrical, optical or radio level.
It provides a hardware means of sending and receiving data on a carrier network.
Let's see some of the most famous protocols of the layers.
For ease of use, it's better to talk about physical layer and data link layer together where we have some protocols
and data link layer and some physical media and connection methodologies in physical layer. Ethernet
and 802.11 wireless LAN are the most known protocols of the data link layer either net is the
name of the most commonly used network protocol that controls how data is transmitted over a LAN, which
is a local area network. You need to have Network Interface Cards in the devices that you want to connect
to the network. A wireless local area network (WLAN) is a wireless computer network protocol that links
two or more devices using wireless communication within a limited area such as a home, school, computer
laboratory or office building.
This gives users the ability to move around with a local coverage area and yet still be connected to the
network.
Most modern WLANs are based on
IEEE 802.11 standard and are marketed under the Wi-Fi brand name. IP (internet protocol)
is responsible for addressing host, encapsulating data into transferred packets and routing packets from
a source host to a destination host across one or more IP networks.
The best known transport protocol is the Transmission Control Protocol or TCP.
It is used for connection oriented transmissions whereas the connectionless User Datagram Protocol (UDP) is
used for simpler messaging transmissions.
We're going to talk more in-depth about these protocols in the next lecture.
Again, for ease of use the last three layers session layer presentation layer and application layer are
thought of together as the application layer. Let me put another parenthesis here,
We're talking about the OSI reference model here.
In addition there is another reference model called TCP/IP reference model and instead of OSI model's
last 3 layers. There is only a single application layer in the TCP/IP reference model.
Just keep that in the back of your mind. And back to our subject. The application layer protocols are classified
according to the protocol they are using in the transport layer and these protocols interact with the
end user via applications.
Therefore they are the most known protocols by just about everybody.
Some of the most well known TCP based application layer protocols are HTTP (Hypertext Transfer Protocol)
simply a communications protocol used to send and receive web pages and files on the internet.
Telnet is one of the simplest ways to exchange data between two computers.
It allows two computers anywhere on a computer network including the worldwide internet to exchange
text and that well other data in real time. FTP (file transfer protocol) is a communication protocol for
the rapid simple transmission of files across a network.
SMTP (simple mail transfer protocol) is used to send and relay an e-mail message between email servers.
Note that it is not used to retrieve email messages from a server instead either IMAP or POP is used
to retrieve email messages.
DNS (The domain name system) is the system used to convert a computer's hostname into an IP address on the
Internet.
For example if a computer needs to communicate with a web server nhs.uk. Your computer needs
the IP address of the web server
nhs.uk
.
It is the job of the DNS to convert the hostname to the IP address of the web server. The DNS uses both
the UDP and TCP. SNMP (simple network management protocol) is used in network management systems to
monitor status of devices and also spot problems.