Ethernet

MAC Address

A Media Access Control (MAC) address is a unique identifier burned into the ROM of every Ethernet NIC.

Frames

A frame is a unit of data that is ready to be sent on the network medium. Ethernet frames contain the following components: The preamble is a set of alternating ones and zeros terminated by two ones (11) that mark it as a frame. The destination address identifies the receiving host's MAC address. The source address identifies the sending host's MAC address. The data is the information that needs to be transmitted from one host to the other. Optional bits pad the frame. Ethernet frames are between 64 and 1518 bytes in size. If the frame is smaller than 64 bytes, the sending NIC places junk data in the pad to make it the required minimum of 64 bytes. The cyclic redundancy check (CRC) is the result of a mathematical calculation performed on the frame. The CRC helps verify that the frame contents have arrived uncorrupted.

Ethernet uses Carrier Sense Multiple Access Collision Detection, or CSMA/CD

CSMA/CD stands for carrier sense multiple access/collision detection. It defines the steps network devices take when two devices attempt to use a data channel simultaneously. Ethernet networks use CSMA/CD, including 1000BaseT. CSMA/CD media access method is used to detect collisions. If you're using a switch, you won't have any collisions, so you can have full-duplex communications

Full-Duplex

Collision detection is turned off. The device can send and receive at the same time. Mode requires full-duplex capable NICs. Mode requires switches with dedicated switch ports (a single device per port). Double the rated bandwidth (200 Mbps for 100BaseT and so on.)

Half-Duplex

Collision detection is turned on. The device can send or receive in only one direction at a time. Devices connected to a hub must use half-duplex communication. Up to the rated bandwidth (100 Mbps for 100BaseT and so on.)

Physical Addresses

Ethernet devices are identified by MAC address, which is burned into the network interface card.

Transmission Media

Ethernet supports the following cable types: Unshielded twisted pair (UTP) cables with RJ45 connectors. This is the most common transmission medium used for Ethernet. Fiber optic cables, which are most commonly used for high-speed applications such as server connections or media streaming. Coaxial cables for older Ethernet implementations (often called thinnet or thicknet networks).

Media Access Method

Ethernet uses a contention-based media access method defined by the IEEE 802.3 standards called Carrier Sense, Multiple Access/Collision Detection (CSMA/CD). Devices use the following process to send data: Because all devices have equal access (multiple access) to the transmission media, a device listens to the transmission medium to determine whether it is free before sending data (carrier sense). If it is not free, the device waits a random amount of time and listens to the transmission medium again. If it is free, the device transmits its message. If two devices transmit at the same time, a collision occurs. The sending devices detect the collision (collision detection) and send a jam signal to notify all other hosts that a collision has occurred. Both devices wait a random length of time before attempting to resend the original message (backoff). Devices with collision detection turned on operate in half-duplex mode; devices with collision detection turned off operate in full-duplex mode.

A network is connected following the IEEE 802.3 specifications. Which of the following best describes when a device can transmit messages?

The IEEE 802.3 committee describes the CSMA/CD media access method. Devices listen to the network to determine if the transmission media is free before transmitting.

CRC

The cyclic redundancy check (CRC) is the result of a mathematical calculation performed on the frame. The CRC helps verify that the frame contents have arrived uncorrupted.

UTP

UTP stands for unshielded twisted pair. UTP cables are easy to work with and less expensive than shielded cables.

Collision

two messages are sent at the same time by different hosts, and the signals collide on the network. Computer B receives corrupted information.