Introduction to Networking

Unit 5

Lesson 2 - Ethernet

Ethernet Configurations

The most straightforward Ethernet configuration is shown on the Ethernet Hub Configuration Diagram. This star configuration uses unshielded twisted pair (UTP) to connect an Ethernet hub to workstations, peripherals, and servers. All nodes attached to the hub share the same 10-Mbps bandwidth.

Ethernet Hub Configuration

However, 10-Mbps Ethernet networks may not provide enough bandwidth for an organization. As network applications are implemented using high-resolution graphics, video, and other rich media data types, pressure is growing for increased bandwidth. Two additional Ethernet standards, Fast Ethernet and Gigabit Ethernet, can allow an organization to provide higher data rates while continuing to use familiar and proven Ethernet technology.

Fast Ethernet

The Fast Ethernet standard (IEEE 802.3u) can send Ethernet frames at 100 Mbps over Category 5 UTP (100BaseTx) or fiber optic cabling (100BaseT). The frame format of Fast Ethernet is identical to that of 10-Mbps Ethernet, thus a network can move up to the higher data rate if it can meet the following requirements:

	If UTP cabling is used, it must be Category 5 or higher.
	All NICs and hubs must support the 100-Mbps data rate.

In addition, a network may need to adjust its UTP wiring layout to meet the shorter maximum distances allowed in the 100BaseTx standard. To provide such a high data rate, the longest length of UTP may only be 100 meters (m).

Smart network administrators plan for future upgrades to Fast Ethernet by using Category 5 cabling in all new installations, and by equipping all computers with NICs that support both 10- and 100-Mbps data rates. The slight extra cost of these components is more than offset by a simpler network conversion.

Gigabit Ethernet

Gigabit Ethernet (IEEE 802.3z) provides throughput of 1,000 Mbps (1 Gbps). Like earlier versions, Gigabit Ethernet uses the frame format specified in the IEEE 802.3 standard, as well as the familiar CSMA/CD medium access control method.

The Gigabit Ethernet standard includes three specifications for optical fiber cabling and two specifications for copper:

	1000BaseSX ("S" for short wavelength) defines optical transceivers or Physical Layer devices for multimode fiber cabling using laser light at a wavelength of 770 to 860 nanometers (nm). This wavelength range is commonly referred to as 850 nm. 1000BaseSX transceivers are less costly than those found in products implementing the long wavelength specification described below.
	1000BaseLX ("L" for long wavelength) defines optical transceivers or Physical Layer devices for either multimode or single-mode fiber, using laser wavelengths of 1,270 to 1,355 nm (commonly referred to as 1,350 nm).
	1000BaseLH ("LH" for long haul) is a multivendor specification that defines optical transceivers for Physical Layer devices that support distances greater than the 1000BaseLX specification. Each vendor has a set of transceivers designed to cover different distances. Although it is not an IEEE standard, many vendors are working to interoperate with IEEE 1000BaseLX equipment.
	1000BaseCX ("C" for copper) defines transceivers or Physical Layer devices for shielded copper cabling. 1000BaseCX is intended for short-haul copper connections (25 m or less) within wiring closets.
	1000BaseTX ("T" for twisted pair) will specify Gigabit Ethernet over four-pair Category 5 UTP copper cabling for distances up to 100 m. This standards development effort is still in progress.

Activities

See the Activities and Extended Activities section in Unit 5 Lesson 2 in your textbook Introduction to Networking to test what you have learned so far.

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