Understanding the OSI Model

OSI Model Overview

In the late 1970s, the International Organization for Standardization (ISO) started expanding on the DoD model, to develop the Open Systems Interconnection (OSI) reference model for computing. The OSI model defines how hardware and software function at different layers to enable communication between computers. The OSI model is a conceptual framework which can be referenced to better comprehend how devices operate on the network.

Today, the OSI model is the most widely used guide for a networking environment. When manufacturers design new products, they reference the OSI model's concepts on the manner in which network components should function.

The OSI model defines standards for:

The OSI model is made up of seven layers which are presented as a stack. Data which is passed over the network moves through each layer.

The seven layers of the OSI model are:

Each layer of the OSI model has its own unique functions. The process of sending data is typically started at the Application layer, is sent through the stack to the Physical layer, and then over the network to the recipient. Data is received at the Physical layer, and the data packet is then passed up the stack to the Application layer.

Different protocols operate at the different layers of the OSI model. Each layer of the OSI model has its own protocols. TCP and IP are collectively called the protocol stack or the network/transport protocols. This is due to the protocols operating at the Network and Transport layers to make it possible for computers to communicate. A protocol stack, r stack, is a group of protocols which are arranged in layers to enable communication. In the protocol stack, each layer provides services to the layer above it; and each layer also receives services from the layer beneath it. For two computers to partake in communications, each computer has to be running the same protocol stack. They can however have different operating systems.

The Physical Layer (layer 1)

The first layer in the OSI model is the Physical layer which transmits raw bit streams over a physical medium. The Physical layer deals with establishing a physical connection between computers to enable communication. The physical layer is hardware specific and deals with the actual physical connection between the computer and the network medium. All devices that function at the Physical layer handle signalling. Data handled at the layer are in bits (1s and 0s). The 1s and 0s are in represented by pulses of light or electricity.

The details on the actual physical connection defined at this layer include:

The specifications of the Physical layer include:

The issues normally clarified at the Physical Layer include:

The Data-link Layer (layer 2)

The Data-link layer of the OSI model enables the movement of data over a link from one device to another, by defining the interface between the network medium and the software on the computer. The Data-link layer maintains the data link between two computers to enable communications.

The responsibilities of the Data-link layer include:

Ensures that frames are transmitted from one computer to another computer with no errors. It establishes error-free connections between two devices.

Layer 2 manages the ordering of bits, packets, to and from data segments. The ensuing result is called frames. Frames contain data that is already arranged in an orderly manner. The Data-link layer receives packets from the Network layer and structures these packets into frames. The frames are then moved to the Physical layer for sending. A cyclic redundancy check (CRC) is added to the data frame. The CRC detects damaged frames. The computer at the receiving end can request the cyclic redundancy check (CRC) so that it can verify that the frame is not damaged. The Data-link layer can determine when a frame is lost. It also requests any lost frames to be retransmitted. By performing these tasks, the Data-link layer makes it possible for data bits to be transmitted in an organized manner.

The Data-link layer is divided into the following two sublayers:

The Network Layer (layer 3)

The Network layer of the OSI model is responsible for moving packets between devices, by providing end-to-end communications between computers that exist on different network. One of the main functions performed at the Network layer is routing. Routing enables packets to be moved among computers which are more than one link from one another.

The functions performed at the Network layer of the OSI model are listed below:

The Transport Layer (layer 4)

The Transport layer is responsible for transporting data in a sequential manner, and with no data loses. The Transport layer divides large messages into smaller data packets so that it can be transmitted to the destination computer. It also reassembles packets into messages for it to be presented to the Network layer.

The important functions performed at the Transport layer to enable network communication are listed below:

The Transport layer at each computer verifies that the application transmitting the data is actually allowed to access the network. It also verifies that each end of the network connection can start the data transfer process. The transport protocols running on each host partaking in communication monitors the data transfer process, and monitors for errors as well.

The common Transport protocols utilized at this layer are:

The Session Layer (layer 5)

The Session layer of the OSI model enables communication sessions to be established between processes or applications running on two different computers. A process is a specific task that is associated with a particular application. Applications can simultaneously run numerous processes. The Session layer utilizes the virtual circuits created by the Transport layer to establish communication sessions.

The important functions performed at Session layer to establish, maintain and terminate communication sessions are summarized below:

The Presentation Layer (layer 6)

At the Presentation layer of the OSI model, the data being transmitted is translated. The layer is responsible for translating data between the formats which the network requires and the formats which the computer is anticipating. The presentation layer translates the formats of each computer to a common transfer format which can be interpreted by each computer.

The functions performed at the Presentation layer of the OSI are:

Data is translated at the Presentation layer when it is transmitted from the sender to the receiver. The application of the sender moves the data to the Presentation layer. The Presentation layer translates the data to a common format which can be read by both computers. When the data is received, the Presentation layer translates the data to a format which the application can read.

Gateway services also function at the Presentation layer. A gateway can be defined as a connection point between networks which run different systems and applications. Gateways are typically deployed through software. An example is Gateway Services for NetWare (GSNW).

Common gateways include:

The Application Layer (layer 7)

The Application layer is the highest layer of the OSI model, and it provides the interface between the network protocol and the software running on the computer. The Application layer provides the necessary services that support applications. It provides the interface for e-mail, Telnet and File Transfer Protocol (FTP) applications, and files transfers. This is the location where applications interrelate with the network

The common application protocols include:

Understanding Network Protocols and the OSI Model

Specific protocols function at each layer of the OSI model to assist in enabling the particular layer to perform its associated functions. The network protocols enable data to be communicated between computers. When protocols function together to provide layers of the OSI model, they are referred to as a protocol suite or protocol stack.

When network protocols function together to move data between computers, the following process usually occurs:

  1. The data is broken into smaller chunks of data. These chunks of data are called packets.
  2. To identify the destination computer, addressing information is added to the data packets.
  3. The data is then moved to the network card for transmission over the network.
  4. At the receiving computer, the data packets are accepted from the network card.
  5. Any transmission information which was appended to the packet by the sending computer is removed.
  6. The packet is reassembled into the original message.

From the above process, you can see that network protocols assemble, change, and disassemble packets as data is moved through the protocol stack.

The components that make up a packet are listed below:

A packet is divided into three segments, which in turn include those components which are listed above:

A few characteristics of network protocols and protocol stacks are listed below:

Computers communicate using connection-oriented protocols, and connectionless protocols.

With reference to the OSI model, protocols are categorized as follows:

The protocols stacks typically used in networks are listed below:

NWLink IPX/SPX

The Microsoft implementation of Novell's IPX/SPX protocol stack is NWLink IPX/SPX. NWLink IPX/SPX is used in Novell NetWare, and is basically IPX for Windows. Windows Server 2003 includes NWLink IPX/SPX to enable Windows Server 2003 to communicate with legacy Novell NetWare servers and clients. NWLink IPX/SPX could become problematic in large networks because it does have a central IPX addressing scheme which prevents networks from utilizing the same address numbers.

The main advantages of NWLink IPX/SPX are summarized below:

The disadvantages of NWLink IPX/SPX are summarized below:

TCP/IP

TCP/IP is a grouping of protocols which provides a collection of networking services. TCP/IP is the main protocol which Windows Server 2003 utilizes for its network services.

The main protocols in the TCP/IP suite are:

When communication takes place via TCP/IP, IP is used at the Network layer, and either TCP or UDP is used at the Transport layer.

The main advantages of using TCP/IP are summarized below:

The disadvantages of TCP/IP are summarized below:

With TCP/IP, the TCP component of the protocol suite utilizes port numbers to forward messages to the correct application process. Port numbers are assigned by the Internet Assigned Numbers Authority (IANA), and they identify the process to which a particular packet is connected to. Port numbers are found in the packet header.

The main port numbers used are:

NetBEUI

NetBIOS naming is supported in Windows Server 2003. Windows Server 2003 does not though support the NetBEUI protocol. NetBEUI is a single protocol that was initially used in Windows NT 3.1 and Windows for Workgroups operating systems. The protocol provides basic file sharing services for Windows computers, and is designed for small networks. NetBEUI does not perform well on large networks. The protocol can also not support internetwork traffic because it cannot route traffic between networks. NetBEUI cannot address traffic to a computer on a different network.



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