To ensure that computers in a network are able to communicate, they must share a common language known as a protocol. A protocol is a set of rules or standards that enables communications between computers in a network.

     Protocols are software and must be installed on network components that need them. Computers can communicate with each other only if they use the same protocol. You can understand the function of different protocols by examining the standard model for networks - the Open Systems Interconnection (OSI) reference model. This model is built around seven protocol layers. According to the OSI conceptual model, several protocols must work together to ensure the proper transmission of data. This is achieved with the help of a protocol stack. A protocol stack is a collection of protocols that function together to transmit data across a network of computers.

Open Systems Interconnection (OSI) Reference Model

     Before data is moved through the layers of the OSI model, it must be divided into packets. A packet is a unit of information that is transmitted as a whole from one computer to another on a network. The network passes a packet from layer to layer, and at each layer some additional formatting is added to the packet.

OSI Model
OSI Layer	Function
Appilcation Layer	Defines how applications interact with each other
Presentation Layer	Adds common formatting for data representation
Session Layer	Establishes and maintains communications channels
Transport Layer	Ensures error-free delivery of data
Network Layer	Addresses messages both within and between networks
Data Link Layer	Defines access methods for the physical medium, such as network cable
Physical Layer	Puts the data on the physical medium

Protocol Stacks

     The OSI model defines distinct layers related to packaging, sending, and receiving data transmissions in a network. A layered set of related protocols actually carries out these services. This layered set of protocols running on a network is called a protocol stack. Several protocol stacks are designated as standard protocol models. Some of the common protocol stacks are TCP/IP, IPX/SPX, and AppleTalk. The responsibility for performing specific communications tasks in the network is assigned to protocols working as one of three types: application layer, transport layer, and network protocols.

• Application Protocols
  
  
  Application protocols provide data exchange between applications in a network. Examples of common application protocols include File Tranfer Protocol (FTP) and Simple Mail Transfer Protocol (SMTP).
  
  
  
• Transport Protocols
  
  
  Transport protocols provide for communication sessions between computers and ensure that data moves reliably between computers. A common transport protocol is Transmission Control Protocol (TCP).
  
  
  
• Network Protocols
  
  
  Network protocols provide what are called link services. These protocols define the rules for communicating in a particular network environment. A common protocol that provides network services is Internet Protocol (IP).
  
  
  

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Protocols and Data Transmissions

     To manage large volumes of network traffic, network administrators can divide large networks into network segments. Network segments are smaller networks, which, when combined, form a large network. The transmission of data between network segments is called routing. However, not all protocols support routing. Protocols are categorized as routable or non-routable based on their ability or inability to support routing.

Routable Protocols

     Routable protocols support communications between LANs or network segments that may be spread throughout a building, across a small geographic area, such as a college campus, or across the globe, such as the Internet.

Non-Routable Protocols

     Non-routable protocols, unlike routable protocols, do not support the transmission of data from one network segment to another. NetBEUI and Data Link Control (DLC) are non-routable protocols.

Types of Data Transmissions

     Routable protocols enable the transmission of data between computers in different segments of a network. However, high volumes of certain kind of network traffic, such as the deployment of multimedia applications, can affect network efficiency because it slows down transmission speed. The amount of network traffic generated varies with the three types of data transmissions: unicast, broadcast, or multicast.

• Unicast
  
  
  In a unicast transmission, a separate copy of data is sent from the source to each client computer requesting it. No other computer on the network needs to proces the traffic. Unicast is also referred to as directed transmission. Most traffic on networks today is unicast.
  
  
  
• Broadcast
  
  
  When data is broadcast, a single copy of data is sent to all clients on the same network segment as the sending computer. However, if that data must be sent to only a portion of the network segment, broadcast transmission is not an efficient transmission method because data is sent to the whole segment irrespective of whether it is required.
  
  
  
• Multicast
  
  
  In a multicast transmission, a single copy of the data is sent only to client computers requesting it. Multiple copies of data are not sent across the network. This minimizes the network traffic and enables the deployment of multimedia applications on the network without overburdening the network.