ccna q&a pdf

ccna q&a pdf.Figure 2.5 Encapsulation and unencapsulation processThe first thing that happens on PC.A is that the user sitting in front of the computer creates some information (this information is called data) and sends it to another place (PC-B) The data includes the actual input from the user (application layer) as well as the formatting information (representation layer). Insession layer, the application or operating system determines whether the destination of the data is local (e.g., hard disk) or remote. In this example, the user sends the information to PC-B (assuming the user is performing a Telnet connection).The session layer determines that the destination is remote and then lets the transport layer transfer the information.The Telnet connection uses TCP/IP and uses a reliable connection (TCP) at the transport layer, and TCP encapsulates the data from higher levels into data segments. As you will see in Chapter 8, TCP only adds headers. The data segment contains information such as the source port number and the destination port number; the source port is a port number greater than 1023 that is not currently in use by PC-A, and the destination port is the well-known port 23, which the target device sees and hands off to the Telnet application.Practical experience suggests that the TCP or UDP source port number should be greater than 49151 according to the RFC standard, but not all operating systems strictly follow this standard, so in many cases the source port number is simply greater than 1023.The transport layer passes the data segment down to the network layer, which encapsulates it in a packet. The packet adds only the header, which contains the Layer 3 logical address information (source and destination addresses), and other information such as the upper layer protocol that created the message. In this example, the message is created by TCP, so the packet header indicates this; in addition, PC-A specifies the source address as its own IP address and the destination address as PC-B's IP address in the packet. This information allows the network layer of the target device to make the following determinations: whether the packet is addressed to itself; and which upper-layer protocol process should be given the encapsulated data segment for processing. In the TCP/IP stack, the terms packet and datagram are synonymous, and they can both be used to describe network layer PDUs o As you will see in Chapter 5, the TCP/IP stack contains many protocols: ARP, TCP, UDP, ICMP, OSPF, EIGRP, and so on.Next, the network layer passes the packet down to the data link layer, which encapsulates the packet into a frame: adding a header and a tail. In this example, the data link layer medium used is Ethernet, which will be discussed in more depth in Chapter 3. The Ethernet frame header contains important fields including the source MAC address and destination MAC address, and a checksum sequence (FCS) value that allows the destination device to determine if the received frame is valid. In this example, PC-A adds its MAC address to the source MAC address field and adds PC-B's MAC address to the destination MAC address field. the use of the FCS is discussed in more detail in Chapter 3.Next, the data link layer frame is passed down to the physical layer. Don't forget at this point that the PDU is a concept we impose on the data to facilitate understanding as well as to help get the information to its destination, whereas to a computer, data is nothing more than a string of binary bits (1s and 0s). The physical layer converts these bits into physical signals depending on the type of cable (connection) used; in this example, the cable is copper, so the physical layer converts the bits into levels: one level for bit value 1, and another level for bit value 0.2.3.2 Passing up the protocol stackFor simplicity, assume that PC-A and PC.B are located on the same copper cable. After the target device receives the physical layer signal, the physical layer converts the level to a binary representation and passes the bit value up to the data link layer.