tailieunhanh - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 12

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 12 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | I Page 79 Tuesday May 20 2003 2 53 PM Networking Models 79 These devices allow users to share create and obtain information. Host devices can exist without a network but without a network host capabilities are greatly reduced. Host devices are physically connected to the network media using a network interface card NIC . They use this connection to perform the tasks of sending e-mails printing reports scanning pictures or accessing databases. A NIC is a printed circuit board that fits into the expansion slot of a bus on a computer motherboard or it can be a peripheral device. It is also called a network adapter. Laptop or notebook computer NICs are usually the size of a PCMCIA card. Each NIC carries a unique code called a MAC address. MAC addresses are covered more in a moment. As the name implies the NIC controls host access to the medium. There are no standardized symbols for end-user devices in the networking industry. They bear a resemblance to the real device to allow for quick recognition. Network devices provide transport for the data that needs to be transferred between end-user devices. Network devices extend cable connections concentrate connections convert data formats and manage data transfers. Examples of devices that perform these functions are repeaters hubs bridges switches and routers. The following sections provide an overview of some common networking devices. Repeaters Repeaters are networking devices that exist at Layer 1 the physical layer of the OSI reference model. To understand how a repeater works it is important to understand that as data leaves a source and goes out over the network it is transformed into either electrical or light pulses that pass along the networking medium. These pulses are called signals. When signals leave a transmitting station they are clean and easily recognizable. However the longer the cable length the weaker and more deteriorated the signals become as they pass along the networking medium. The .