CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [404]
Figure 23-10 NIC built into motherboard
Multispeed Cards All Ethernet networks share the same language, so you can easily have mixed or combined networks. All it takes is a network card capable of running at multiple speeds or even over multiple cables. Most NICs built into motherboards today, for example, are Gigabit auto-sensing cards (Figure 23-10). If you plug into a 100BaseT network, they automatically run at 100 Mbps. If you plug into a 1000 Mbps network, they quickly ramp up and run at 1000 Mbps.
Crossover Cables You can actually hook two network cards together without a hub by using a special UTP cable called a crossover cable. A crossover cable is a standard UTP cable but with one RJ-45 connector using the 568A standard and the other using the 568B. This reverses the signal between sending and receiving wires and thus does the job of a hub or switch. Crossover cables work great as a quick way to network two PCs. You can purchase a crossover cable at any computer store.
Duplex and Half-Duplex All modern NICs can run in full-duplex mode, meaning they can send and receive data at the same time. The vast majority of NICs and switches use a feature called auto-sensing to accommodate very old devices that might attach to the network and need to run in half-duplex mode. Half-duplex means that the device can send and receive, but not at the same time. An obvious example of a half-duplex device is the walkie-talkies you played with as a kid that required you to press and hold the orange button to transmit—at which time you couldn’t hear anything. Half-duplex devices are exceedingly rare in modern computers, but you need to understand this option. Some NICs just can’t handle full-duplex communication when you plug them directly to another NIC by using a crossover cable—that is, no switch. Dropping both NICs down from full-duplex or auto-sensing can sometimes enable these odd NICs to communicate.
Link Lights All NICs made today have some type of light-emitting diode (LED) status indicator that gives information about the state of the NIC’s link to whatever’s on the other end of the connection. Even though you know the lights are actually LEDs, get used to calling them link lights, as that’s the term all network techs use. NICs can have between one and four different link lights, and the LEDs can be any color. These lights give you clues about what’s happening with the link and are one of the first items to check whenever you think a system is disconnected from the network (Figure 23-11).
Figure 23-11 Mmmm, pretty lights!
Hubs and switches also have link lights, enabling you to check the connectivity at both ends of the cable. If a PC can’t access a network, always first check the link lights. Multispeed devices usually have a link light that tells you the speed of the connection. In Figure 23-12, the light for port 2 on the top photo is orange, for example, signifying that the other end of the cable is plugged into either a 10BaseT or 100BaseT NIC. The same port connected to a Gigabit NIC—that’s the lower picture—displays a green LED.
Figure 23-12 Multispeed lights
A properly functioning link light is steady on when the NIC is connected to another device. No flickering, no on and off, just on. A link light that is off or flickering shows a connection problem.
Another light is the activity light. This little guy turns on when the card detects network traffic, so it makes an intermittent flickering when