The term encapsulation describes the process of putting headers and trailers around some data. A computer that needs to send data encapsulates the data in headers of the correct format so that the receiving computer will know how to interpret the received data.
You have seen several examples of encapsulation in this chapter already. The web server encapsulated the home page inside an HTTP header in Figure 2-2. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 2-3. IP encapsulated the TCP headers and the data inside an IP header in Figure 2-4. Finally, the network interface layer encapsulated the IP packets inside both a header and a trailer in Figure 2-5.
You can think about the complete process of data encapsulation with TCP/IP as a five-step process. In fact, previous CCNA exams referred to a specific five-step process for encapsulation. This included the typical encapsulation by the application, transport, network, and network interface (referred to as data link) layers as Steps 1 through 4 in the five-step process. The fifth step was the physical layer’s transmission of the bit stream. In case any questions remain in the CCNA question database referring to a five-step encapsulation process, the following list provides the details and explanation. Regardless, the ideas behind the process apply to any networking model and how it encapsulates data:
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The network interface layer defines the protocols and hardware required to deliver data across some physical network. The term network interface refers to the fact that this layer defines how to connect the host computer, which is not part of the network, to the network; it is the interface between the computer and the network. For instance, Ethernet is one example protocol at the TCP/IP network interface layer. Ethernet defines the required cabling, addressing, and protocols used to create an Ethernet LAN. Likewise, the connectors, cables, voltage levels, and protocols used to deliver data across WAN links are defined in a variety of other protocols that also fall into the network interface layer.
Chapter 3, “Data Link Layer Fundamentals: Ethernet LANs,” and Chapter 4, “Fundamentals of WANs,” cover more details about the TCP/IP network interface layer.
Just like every layer in any networking model, the TCP/IP network interface layer provides services to the layer above it in the model. The best way to understand the basics of the TCP/IP network interface layer is to examine the services that it provides to IP.
IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology, things such as which routers are connected to each other, which host computers are connected to which networks, and what the IP addressing scheme looks like. However, the IP protocol purposefully does not include the details about each of the underlying physical networks. Therefore, the Internet layer, as implemented by IP, uses the services of the network interface layer to deliver the packets over each physical network, respectively.
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Continue from previous post (TCP/IP Transport Layer)…
Imagine that you just wrote a letter to your favorite person on the other side of the country and that you also wrote a letter to someone on the other side of town. It’s time to send the letters. Is there much difference in how you treat each letter? Not really. You put different addresses on the envelope for each letter because the letters need to go to two different places. You put stamps on both letters and put them in the same mailbox. The postal service takes care of all the details of figuring out how to get each letter to the right place—whether it is across town or across the country.
Inside the postal service, both letters are processed. One letter gets sent to another post office, then another, and so on, until the letter gets delivered across the country. The local letter might go to the post office in your town and then simply be delivered to your friend across town, without going to another post office.
So what does this all matter to networking? Well, the internetwork layer of the TCP/IP networking model, the Internet Protocol (IP), works much like the postal service. IP defines addresses so that each host computer can have a different IP address, just like the postal service defines addressing that allows unique addresses for each house, apartment, and business. Similarly, IP defines the process of routing so that devices called routers (ingenious name, huh?) can choose where to send packets of data so that they are delivered to the correct destination. Just like the postal service created the necessary post offices, sorting machines, trucks, and personnel to deliver the mail, the internetwork layer defines much of the details needed to implement the necessary networking infrastructure.
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The TCP/IP application layer includes a relatively large number of protocols, with HTTP being only one of those. The TCP/IP transport layer consists of two main protocol options the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). To get a true appreciation for what TCP/IP transport layer protocols do, read post “Fundamentals of TCP and UDP.” However, in this post, you will learn about one of the key features of TCP, which enables us to cover some more general concepts about how networking models behave.
To appreciate what the transport layer protocols do, you must think about the layer above the transport layer, the application layer. Why? Well, each layer provides a service to the layer above it. For example, in Figure 2-2, Bob and Larry used HTTP to transfer the home page from Larry to Bob. But what would have happened if Bob’s HTTP get request was lost in transit through the TCP/IP network? Or, what would have happened if Larry’s response, which includes the contents of the home page, was lost? Well, the page would not show up in Bob’s browser, as you might expect.
So, TCP/IP needs a mechanism to guarantee delivery of data across a network. TCP provides that feature by using acknowledgments. Figure 2-3 outlines the basic acknowledgment logic.
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Arguably, the most popular TCP/IP application today is the web browser. Many major software vendors either have already changed or are changing their software to support access from a web browser. And thankfully, using a web browser is easy—you start a web browser on your computer and select a web site by typing in the name of the web site, and the web page appears.
What really happens to allow that web page to appear on your web browser? These next few sections take a high-level look at what happens behind the scene.
Imagine that Bob opens his browser. His browser has been configured to automatically ask for web server Larry’s default web page, or home page. The general logic looks like that in Figure 2-1.
Figure 2-1 Basic Application Logic to Get a Web Page
So what really happened? Bob’s initial request actually asks Larry to send his home page back to Bob. Larry’s web server software has been configured to know that Larry’s default web page is contained in a file called home.htm. Bob receives the file from Larry and displays the contents of the file in the web browser window.
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TCP/IP Protocol Architecture
TCP/IP defines a large collection of protocols that allow computers to communicate. TCP/IP defines the details of each of these protocols inside document called Requests For Comments (RFCs). By implementing the required protocols defined in TCP/IP RFCs, a computer can be relatively confident that it can communicate with other computers that also implement TCP/IP.
An easy comparison can be made between telephones and computers that use TCP/IP. I can go to the store and buy a phone from one of a dozen different vendors. When I get home, I plug the phone in to the wall socket, and it works. The phone vendors know the standards for phones in their country and build their phones to match those standards. Similarly, a computer that implements the standard networking protocols defined by TCP/IP can communicate with other computers that also use the TCP/IP standards.
Like other networking architectures, TCP/IP classifies the various protocols into different categories. Table 2-2 outlines the main categories in the TCP/IP architectural model.
Table 2-2 TCP/IP Architectural Model and Example Protocols
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The Flintstones are a cartoon family that, according to the cartoon, lived in prehistoric times. Because I want to discuss the thought process behind some imaginary initial networking standards, the Flintstones seem to be the right group of people to put in the example.
Fred is the president of FredCo, where his wife (Wilma), buddy (Barney), and buddy’s wife (Betty) all work. They all have phones and computers, but they have no network because no one has ever made up the idea of a network before. Fred sees all his employees running around giving each other disks with files on them, and it seems inefficient. So, Fred, being a visionary, imagines a world in which people can connect their computers somehow and exchange files, without having to leave their desks. The (imaginary) first network is about to be born.
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So, you are new to networking. You might have seen or heard about different topics relating to networking, but you are only just now getting serious about learning the details. Like many people, your perspective about networks might be that of a user of the network, as opposed to the network engineer who builds networks. For some, that perspective is as a dialup user of the Internet. Others might use a computer at a job or at school; that computer is typically connected to a network via some cable. Figure 1-1 shows the basic end-user perspective of networking.
Figure 1-1 End-User Perspective on Networks
The top part of the figure shows a typical dialup user of the Internet. The user has a PC, and the user plugs in the phone line from the wall into a modem in a PC. By dialing the right phone number, the user connects to the Internet. After connecting, the user can send e-mail, browse web sites, and use other tools and applications as well.
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There are 3 kinds of network cabling diagram that are Crossover wiring diagram, Straight Through wiring diagram and Rollover wiring diagram.
1. Crossover wiring diagram
A crossover cable can be used to:
a) Connect 2 computers directly.
b) Connect a router’s LAN port to a switch/hub’s normal port. (normally used for expanding network)
c) Connect 2 switches/hubs by using normal port in both switches/hubs.
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