cisco ccna overview

cisco ccna overview.As an example, 1000BASE-T defines an Ethernet standard with a transmission rate of lOOObit/s, a modulation method of baseband (BASE), and a medium of twisted pair (T). The Ethernet physical layer standard also defines the types of cables and connectors used to interconnect two workstations. For example.The 1000BASE-T standard requires the use of Category 6 (CAT6) unshielded twisted pair (UTP) cable and RJ.45 connectors. The Ethernet Media Access Control Ethernet MAC function involves the transfer of information between two Ethernet devices (also known as Ethernet workstations) using .... The method usedIt is independent of the physical layer messaging method. The Ethernet Media Access Control standard defines the following two media access modes. Half-duplex mode: In this mode, two Ethernet devices share the transport medium. Access to the media by Ethernet devices is controlled by implementing Carrier Listen Multiple Access (CSMA/CD) technology with conflict detection. With CSMA/CD technology, the device can detect whether the shared medium is carrying data. If the medium is not carrying data, the device can send data immediately. If two devices send data almost simultaneously, a messaging conflict occurs. When a conflict occurs, the CSMA/CD-enabled device detects the conflict, stops sending data immediately, and waits for a period of time (known as backofftime) before attempting to send data again. The workstation sends out a blocking signal (jamsignal) to inform about the conflict. All workstations that can sense a conflict are considered to be part of the same conflict domain. Early Ethernet implementations used a half-duplex mode. However, because of the shortcomings of this mode (e.g., very low transmission performance), it is rare to see Ethernet operating in half-duplex mode today. A network HUB is a device that allows multiple Ethernet sites to share the same transmission medium for sending and receiving data. See the "LAN HUBs and Bridges" section later in this chapter for more on HUBs. Figure 1-9 shows an example of CSMA/CD-enabled devices communicating with each other. Full-duplex mode: In this mode, a dedicated channel is allocated for data transmission and two devices can send data simultaneously. Therefore, the devices do not have to detect conflicts and wait for them to end before sending data. Full-duplex mode is also known as "conflict-free" mode because no conflicts occur. A switch is a device that does not have a conflict domain and has a dedicated data transmission channel. See the "LAN Switches" section later in this chapter for more information about switches.16 Chapter 1 Network Protocols and Network Device BasicsCarrier ListeningFigure 1-9 Example of CSMA/CD-enabled devices communicating with each other 3. Ethernet Frames Figure 1-10 shows the format of an Ethernet frame.The unit of field length is byte7 1 6 6 2 46-1500Preamble S F D Destination MAC address Source MAC address Type / Length User data FCSEthernet Frame Diagram 1.10 Format of Ethernet Frames