Now click the ⫹ sign next to Ethernet. This is the network architecture used on

Defining Networks with the OSI Model | 39 CERTIFICATION READY How do you define the upper layers of the OSI model? 3.1 In the following exercises, you will do the following: • Define the transport layer by showing connections in the command prompt and describing ports. • Define the session layer by logging into Web sites and other servers, as well as logging on and off of Microsoft networks and email programs. • Define the presentation layer by showing encryption in Windows and within Web sites. • Define the application layer by capturing web server packets and analyzing them. Layer 4 governs the transmission of messages through the communications subnetwork. Two common TCPIP protocols that are utilized on this layer include the Transmission Control Protocol TCP, which is a connection-oriented protocol, and the User Datagram Protocol UDP, which is connectionless. An example of an application that uses TCP is a web brows- er, and an example of an application that uses UDP is streaming media. When you download a web page, you don’t want to lose any packets of information because graphics would appear broken, certain text wouldn’t read correctly, and so on. By using TCP, we ensure that data gets to its final destination. If a packet is lost along the way, it will be resent until the destination computer acknowledges delivery or ends the session. But with streaming media, we are either watching or listening in real time. So, if a packet is lost, we don’t really care, because that time frame of the video or music has already passed. Once the packet is lost, we really don’t want it back. Of course, if the packet loss becomes too severe, the streaming media will become incomprehensible. Connection-oriented also known as CO mode communications require that both devices or computers involved in the communication establish an end-to-end logical connection before data can be sent between the two. These connection-oriented systems are often con- sidered reliable network services. If an individual packet is not delivered in a timely manner, it is resent; this can be done because the sending computer established the connection at the beginning of the session and knows where to resend the packet. In connectionless communications CL mode, no end-to-end connection is necessary before data is sent. Every packet that is sent has the destination address located in its header. This is sufficient to move independent packets, such as in the previously mentioned streaming media. But if a packet is lost, it cannot be resent, because the sending computer never established a logical connection and doesn’t know which logical connection to use to send the failed packet. Layer 4 also takes care of the ports that a computer uses for data transmission. Ports act as logical communications endpoints for computers. There are a total of 65,536 ports, numbering between 0 and 65,535. They are defined by the Internet Assigned Numbers Authority or IANA and divided into categories as shown in Table 2-1. ■ Defining the Upper OSI Layers The upper OSI layers are layers 4 through 7—the transport, session, presentation, and application layers. It is this portion of the OSI model that deals with protocols such as HTTP, FTP, and mail protocols. Compression, encryption, and session creation are also classified by these layers. THE BOTTOM LINE