Detailed encapsulation process All communications on a network originate at a source, and are sent to a destination. The information sent on a network is referred to as data or data packets. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged through a process called encapsulation. Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information. To see how encapsulation occurs, examine the manner in which data travels through the layers as illustrated in Figure . Once the data is sent from the source, it travels through the application layer down through the other layers. The packaging and flow of the data that is exchanged goes through changes as the layers perform their services for end users. As illustrated in Figure , networks must perform the following five conversion steps in order to encapsulate data:

1. Build the data.
   As a user sends an e-mail message, its alphanumeric characters are converted to data that can travel across the internetwork.

1. Package the data for end-to-end transport.
   The data is packaged for internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.

1. Add the network IP address to the header.
   The data is put into a packet or datagram that contains a packet header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.

1. Add the data link layer header and trailer.
   Each network device must put the packet into a frame. The frame allows connection to the next directly-connected network device on the link. Each device in the chosen network path requires framing in order for it to connect to the next device.

1. Convert to bits for transmission.
   The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the medium. A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN.

Lab Activity Lab Exercise: OSI Model Characteristics and DevicesIn this lab, the student will learn the seven layers of the OSI model and the characteristics, functions and keywords relating to each layer. Interactive Media Activity Drag and Drop: Encapsulation Process Flowchart In this lab, the student will complete encapsulation process flowchart. Web Links Data Encapsulation & Decapsulation in the OSI Model http://www.firewall.cx/osi-encap-decap.php

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Content Summary An understanding of the following key points should have been achieved:

• Understanding bandwidth is essential when studying networking
• Bandwidth is finite, costs money, and the demand for it increases daily
• Using analogies like the flow of water and flow of traffic can help explain bandwidth
• Bandwidth is measured in bits per second, bps, kpbs, Mbps, or Gbps
• Limitations on bandwidth include type of media used, LAN and WAN technologies, and network equipment
• Throughput refers to actual measured bandwidth, which is affected by factors that include number of users on network, networking devices, type of data, user’s computer and the server
• The formula T=S/BW (transfer time = size of file / bandwidth) can be used to calculate data transfer time
• Comparison of analog and digital bandwidth
• A layered approach is effective in analyzing problems
• Network communication is described by layered models
• The OSI and TCP/IP are the two most important models of network communication
• The International Organization for Standardization developed the OSI model to address the problems of network incompatibility
• The seven layers of the OSI are application, presentation, session, transport, network, data link, and physical
• The four layers of the TCP/IP are application, transport, internet, and network access
• The TCP/IP application layer is equivalent to the OSI application, presentation, and session layers
• LANs and WANs developed in response to business and government computing needs
• Fundamental networking devices are hubs, bridges, switches, and routers
• The physical topology layouts include the bus, ring, star, extended star, hierarchical, and mesh
• A WAN consists of two or more LANs spanning a common geographic area
• A SAN provides enhanced system performance, is scalable, and has disaster tolerance built in
• A VPN is a private network that is constructed within a public network infrastructure
• Three main types of VPNs are access, Intranet, and Extranet VPNs
• Intranets are designed to be available to users who have access privileges to the internal network of an organization
• Extranets are designed to deliver applications and services that are Intranet based, using extended, secured access to external users or enterprises