OSI Model and TCP/IP: Understanding Network Layers

The Open Systems Interconnection Model

The Open Systems Interconnection (OSI) model is an abstract, layered representation created as a reference for network protocol design. The OSI model divides the process of networking into different logical layers, each of which has unique functionality and to which specific services and protocols are assigned.

Application Layer

The application layer, the seventh layer, is the top layer of both the OSI and TCP/IP models.

Presentation Layer

The presentation layer has three main functions:

• Coding and data conversion of the application layer to ensure that data from the source device can be interpreted by the appropriate application on the target device.
• Compression of the data in a way that can be decompressed by the target device.
• Data encryption for transmission and decryption when they reach their destination.

Session Layer

As indicated by the name, the functions in the session layer create and maintain dialogs between the source and destination applications. The session layer handles the exchange of information to initiate the dialogs, keep them active, and restart sessions that were terminated or deactivated for a period of time.

Most applications, such as web browsers or email clients, incorporate the functionality of layers 5, 6, and 7 of the OSI model.

TCP/IP Protocols

Some of the TCP/IP protocols are:

• The Domain Name Service (DNS) protocol is used to resolve Internet names to IP addresses.
• The Hypertext Transfer Protocol (HTTP) is used to transfer files that make up web pages on the World Wide Web.
• Simple Mail Transfer Protocol (SMTP) is used to transfer mail messages and attachments.
• Telnet, a terminal emulation protocol, is used to provide remote access to servers and network devices.
• The File Transfer Protocol (FTP) is used for interactive file transfer between systems.

Functions of Application Layer Protocols

The protocols of the application layer are used by both the source and destination devices during a communication session. For communication to succeed, the application layer protocols implemented on the source and destination hosts must match.

The Client-Server Model

In the client/server model, the device requesting information is called a client, and the device responding to the request is called a server. The client and server processes are considered to be part of the application layer. The client begins the exchange by requesting data from the server, which responds by sending one or more data streams to the client.

Transferring data from a client to a server is called uploading, and data from a server to a client is called downloading.

Hosts

In a general network context, any device that responds to a request from client applications functions as a server. A server is usually a computer containing information to be shared with many client systems.

The Peer-to-Peer Model

In addition to the client-server model for networking, there is also a peer-to-peer model. Peer-to-peer networks have two distinct forms: network design and peer-to-peer (P2P) applications. Both forms have similar features, but in practice, they are very different.

Peer-to-Peer Networks

In a peer-to-peer network, two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server. Each connected end device (known as a peer) can function as a server or as a client. One computer might assume the server role for one transaction while simultaneously functioning as a client for another. The roles of client and server are set on a per-request basis.