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Layers of the OSI model
Shelby Hoke
University of Phoenix
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Layers of the OSI model
The OSI model is a conceptual framework for describing the functions of a networking system.
The International Organization for Standardization (ISO) designed it in 1984 to show how different
devices receive and send data. It divides the upper layers into seven functional groups: application,
presentation, session, transport, network, and data-link layers. A key goal of the OSI model is to guide
technology vendors and software developers in creating digital communication products and software
programs that can interoperate, and to provide a framework for describing the functions of networks and
telecommunications systems that is clear and concise. The OSI model layers display the functions
required for a device to successfully send data to another device or server through a cloud resource, such
as Blackboard or Outlook. The seven-layered OSI model specifies how network devices communicate
with each other. Protocols are defined in the network layer, while functions are specified in the
application layer. Transport layer security (TLS) is a standard way web browsers and other network
devices implement encryption. The TLS protocol is enabled in the transport layer, while IPv4/IPv6
protocols are executed in the network layer. This paper will explain the OSI model and its seven layers,
what they do, and how they interact with each other.
Application Layer
The application layer is the topmost layer of the OSI model. This is where users send and receive
data, including e-mails. Protocols in the application layer include defining message types, defining
processes for both parties' communication, defining syntax of messages and interaction with other layers
such as physical devices or databases. The application layer is also part of the TCP/IP suite; it consists
of protocols that focus on process-to-process communication across an IP network and enable sending
and receiving emails, graphics, accessing files from cloud-based storage or from a database and remote
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job entry. The Hypertext Transfer Protocol (HTTP) is another protocol widely used on the internet for
sending and receiving data, such as receiving and sending webpages, accessing them, and plain text and
audio. An example of this layer would be a person accessing a web browser such as Firefox or Chrome.
The application layer communicates with the next layer by sending a request in forms of packets to it.
The Hypertext Transfer Protocol (HTTP) is an application layer protocol that allows users to access
webpages, send and receive files, and listen to audio. An example of this layer would be a person
accessing a web browser such as Firefox or Chrome. The transport layer handles the actual data transfers
by sending packets to the next layer.
Presentation Layer
T The next layer of the OSI model is the Presentation layer, which converts data from one format
to another. This layer also maintains the syntax of the data and translates it into a form that can be
understood by other parts of the system. The Presentation layer generates serialized data that can be sent
across diverse types of networks or transported over digital networks such as cable or satellite television.
Its primary responsibility is to ensure that complex data structures are converted into flat byte strings
(through TLC or XML mechanics), which simplifies communication between different systems. This
layer helps eliminate concerns in other layers over syntactical differences in how data is represented
between them. For example, when someone sends a text file encoded in EBCDIC across a network (e.g.,
a computer network, an intranet, the Internet) and wants it transformed into ASC11 so that it can be read
by computers all over the world, this conversion is done by presenters at this layer. Because of
responding to a service request from the application layer, the presentation layer issues a service request
to the session layer.
Session Layer
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The Session layer of the OSI model is the fifth layer, and it deals with dialogue between
applications and computers. This layer manages sessions between Presentation layer entities. It creates,
oversees, and then deconstructs these sessions. Protocols in this suite include X.225 and ISO 8327. If a
connection is lost, these protocols will attempt to recover it. If a connection has not been accessed for a
long time, these protocols will terminate it and try to renew it. As the "gatekeeper" between Presentation
and Transport layers, this layer is concerned with security. A circuit level gateway (firewall) is present
in this layer too. Session hijacking is quite common in this layer; techniques include cross-site scripting
(XSS), side-jacking (XJS), fixation (XFF), cookie theft (XTC), brute force attempts. There are several
methods to prevent session hijacking. One way is to force the use of HTTPS or some other protocol that
ensures encryption. Cookies can also be prevented from being accessed by client-side scripts. Session
key regeneration can be configured so that after it has established automation, it will regenerate the
session key. Another way is to use a VPN (virtual private networks) to prevent attackers from
intercepting traffic and breaking apart the session layer data into segments before transmitting them back
to the receiving end.
Transport Layer
The transport layer performs functions such as converting messages into Protocol Data Units
(PDUs), controlling reliability through flow control, segmentation, and error detection. When data from
the Session layer is received by the transport layer, it splits the bytes into smaller units. It then passes
these pieces of data to the Network layer for confirmation that all pieces have arrived securely. There are
also several security protocols in this layer, including session hijacking, SQL injection cookie poisoning
and many more. In addition to this, TCP (Transmission Control Protocol) allows computing devices
across a network to communicate with one another via application programs. The Transport layer
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provides end-to-end communication between two processes on remote hosts. It receives data from the
Application layer, breaks it up into smaller sections and gives it to the Network layer.
Network Layer
Next, we have the Network layer, which is commonly referred to as the layer that routing takes
place in. The router's job is to ensure packets move from one network to another. The protocols at this
layer enable peer-to-peer and end-to-end communication. Moreover, each interface on a router has its
own IP address since it's on a different network. In this layer, more widely used protocols are employed,
including Internet Protocol (IPv4), Internet Protocol (IPv6), Address Resolution Protocol (ARP), and
Internet Control Message Protocol (ICMP). Ip addresses are implemented by IPv4 and IPv6 for more
than 4 billion addresses worldwide. To track down the source of a corrupted message, Internet Control
Message Protocol (ICMP) gives communication information in reference to network connectivity
difficulties. The Network layer communicates with the Data Link layer by handling routing and sending
data between different networks, as well as receiving packets from layers above and below it.
Data Link layer
The Data Link layer is primarily responsible for the transfer of data between two nodes on the
same local area network segment (LAN) or between adjacent network nodes in a wide area network
(WAN). It also performs services including framing and link access, reliable delivery, flow control and
error detection and correction. Media Access Control (MAC) controls devices interaction while Logical
Link Control (LLC) addresses and multiplexes. The Data Link layer adds addressing data and control
information to the packet or datagram which creates a frame. Data is transferred between nodes on the
same local area network segment (LAN) or between adjacent network nodes in a wide area network
(WAN). Data link protocols include Ethernet for local area networks (multi-node), PPP for point-to-
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point connections, HDLC for point-to-point connections, and ADCCP for point-to-point (dual node)
communication. These protocol data units, called data-link frames, do not cross local networks. They
focus on local delivery, addressing and media arbitration. By focusing on these higher-layer functions of
inter-network routing and global addressing, data link protocols can focus on lower-layer functions like
frame generation, transmission, and reception.
Physical Layer
The last layer is the Physical Layer. A network consists of various components, such as power
plugs, receivers, connectors, and cables. The physical layer sends data bits from one device to another
(like a computer). Encoding types are defined by the Physical Layer (by how 0's and 1's are encoded in a
signal). It is the physical layer that handles the communication of unstructured raw data streams over a
physical medium. It also maintains the data rate (how many bits can be sent per second). Furthermore,
this layer performs synchronization, provides an interface between devices and transmission mediums,
and provides two configurations: point-to-point and multi-point. Data packets can be forwarded from
one port (sender port) to the leading destination port by means of this layer, which also assists in the
selection of transmission medium, physical topology, and routing mechanisms. Moreover, the Physical
layer possesses additional vulnerabilities that are only found there; environmental vulnerabilities. An
example of these vulnerabilities would be natural disasters, fire, or dust. Other vulnerabilities in this
layer include physical damage, obstruction, or malfunction. These vulnerabilities can cause loss of
physical assets, loss of power, theft of data or hardware, and could open a door for a hacker to get inside
a person’s computer. Some threats in this layer are eavesdropping, encryption attacks, decryption
downgrade attacks, and encoding attacks. For large organizations, it may seem slightly more
complicated to ensure the security of the physical layer. Many of these organizations implement policies
that can help prevent common threats and vulnerabilities. A few of these policies include; require multi-
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factor authentication, use highly secure credentials that are difficult to clone or crack, eliminate
redundancies across teams and processes for faster incident response, and restrict access to anywhere
laptops and computers are left unattended.
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References
Bagurdes, R. (n.d.). Networking Concepts and Protocols. Pluralsight.
Retrieved November 24, 2022, from https://app.pluralsight.com/course-
player?clipId=b88640a8-b61d-40cb-8c26-02fb9d9a99ce.
YouTube. (2021). Communications and Network Security Part 2 -Osi and
Tcp/Ip Models. YouTube. Retrieved November 24, 2022, from
Communications and Network Security Part 2 - OSI and TCP/IP Models
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.
