Ethernet• The Origin of Ethernet

– In the late 1960s, the University of Hawaii developed a WAN called ALOHA, that extended LAN technology across a larger geographical area. The university had a large geographical area and they wanted to connect computers that were spread throughout its campus. One of the key features of the network that they designed was the use of CSMA/CD as the access method. This early network was the foundation for today's Ethernet. In 1972, Robert Metcalfe and David Boggs invented a cabling and signaling scheme at the Xerox Palo Alto Research Center (PARC), and in 1975 introduced the first Ethernet product. The original version of Ethernet was designed as a 2.94-Mbps system to connect over 100 computers on a 1 kilometer cable. Xerox Ethernet was so successful that Xerox, Intel Corporation, and Digital Equipment Corporation drew up a standard for a 10-Mbps Ethernet. Today it is a specification describing a method for computers and data systems to connect and share cabling. The Ethernet specification performs the same functions as the OSI Physical and Data Link layers of data communications. This design is the basis for the IEEE's 802.3 specification.

• Ethernet Features– Ethernet is currently the most popular network architecture. This baseband

architecture uses a bus topology, usually transmits at 10 Mbps, and relies on CSMA/CD to regulate traffic on the main cable segment.

– The Ethernet media is passive, which means it draws power from the computer and thus will not fail unless the media is physically cut or improperly terminated.

• Ethernet Basics– The following list summarizes Ethernet features.

• Traditional topology linear bus

• Other topologies star bus

• Type of architecture baseband

• Access method CSMA/CD

• Specifications IEEE 802.3

• Transfer speed 10 Mbps or 100 Mbps

• Cable types thicknet, thinnet, UTP

• The Ethernet Frame Format– Ethernet breaks data down into packages in a format that is different from

the packet used in other networks. Ethernet breaks data down into frames. A frame is a package of information transmitted as a single unit. An Ethernet frame can be between 64 and 1,518 bytes long, but the Ethernet frame itself uses at least 18 bytes; therefore, the data in an Ethernet frame can be between 46 and 1,500 bytes long. Every frame contains control information and follows the same basic organization.

– For example, the Ethernet II frame, used for TCP/IP, that gets transmitted across the network consists of the sections listed in the following table.

• Frame field Description

• Preamble Marks the start of the frame

• Destination and source The origin and destination addresses.

• Type Used to identify the Network layer protocol (IP or IPX).

• Cyclical redundancy check (CRC) Error checking field to determine if the, frame arrived

• without being corrupted

• The 10 Mbps IEEE Standards– Four different 10 Mbps Ethernet topologies:

• 10BaseT

• 10Base2

• 10Base5

• 10BaseFL

– 10BaseT• In 1990, the IEEE committee published the 802.3 specification for running

Ethernet over twisted-pair wiring. 10BaseT (10 Mbps, baseband, over twisted-pair cable) is an Ethernet network that typically uses unshielded twisted-pair (UTP) to connect computers. While 10BaseT normally uses UTP, shielded twisted-pair (STP) will also work without changing any of the 10BaseT parameters. Most networks of this type are configured in a star pattern but internally use a bus signaling system like other Ethernet configurations. Typically, the hub of a 10BaseT network serves as a multiport repeater and often is located in a wiring closet of the building. Each computer is located at the end point of a cable connected to the hub. Each computer has two pairs of wire-one pair is used to receive data and one pair is used to transmit data. The maximum length of a 10BaseT segment is 100 meters (328 feet). Repeaters can be used to extend this maximum cable length. The minimum cable length between computers is 2.5 meters (about 8 feet). A 10BaseT LAN will serve 1,024 computers.

• 10Base2– This topology is called 10Base2 by the IEEE 802.3 specification because it

transmits at 10 Mbps over a baseband wire and can carry a signal roughly two times 100 meters (the actual distance is 185 meters). This type of network uses thin coaxial cable, or thinnet, which has a maximum segment length of 185 meters. There is also a minimum cable length of at least 0.5 meters (20 inches). There is also a 30 computer maximum per 185 meter segment. Thinnet cabling components include:

• BNC barrel connectors

• BNC T connectors

• BNC terminators

– Thinnet networks generally use a local bus topology. IEEE standards for thinnet do not allow a transceiver cable to be used from the bus T connector to a computer. Instead, a T connector fits directly on the network adapter card.

1OBaseT Summary

Category NotesCable Category 3, 4, or 5 UTPConnectors RJ-45 at cable endsTransceiver Each computer needs one; some cards have transceivers built inTransceiver to hub distance 100 meters maximumBackbones for hubs Coaxial or fiber-optic to join a larger LANTotal computers per LAN 1024 by specificationwithout connectivity components

– A BNC barrel connector may be used to connect thinnet cable segments together, thus extending a length of cable. For example, if you need a length of cable that is 30 feet long, but all you have is a 25-foot length and a 5-foot length of thinnet cable, a BNC barrel connector can be used to join the two cable segments together. However, the use of barrel connectors should be kept to a minimum because each connection in the cable reduces the signal quality.

– A thinnet network is an economical way to support a small department or workgroup. The cable used for this type of network is:

• Relatively inexpensive.

• Easy to install.

• Easy to configure.

– A single thinnet network can support a maximum of 30 nodes (computers and repeaters) per cable segment as per the IEEE 802.3 specification.

• The 5-4-3 Rule– A thinnet network can combine as many as five cable segments connected

by four repeaters, but only three segments can have stations attached. Thus, two segments are untapped and are often referred to as inter-repeater links. This is known as the 5-4-3 rule.

– Because normal Ethernet limits would be too confining for a large business, repeaters can be used to join Ethernet segments and extend the network to a total length of 925 meters.

• 10Base5– The IEEE specification for this topology is 10 Mbps, baseband, and 500-

meter (five 100-meter) segments. It is also called standard Ethernet. This topology makes use of thick coaxial, or thicknet. Thicknet generally uses a bus topology and can support as many as 100 nodes (stations, repeaters, and so on) per backbone segment. The backbone, or trunk segment, is the main cable from which transceiver cables are connected to stations and repeaters. A thicknet segment can be 500 meters long for a total network length of 2,500 meters (8,200 feet). The distances and tolerances for thicknet are greater than those for thinnet.

1OBase2 Summary

Category NotesMaximum segment length 185 meters (607 feet)Connection to network adapter card BNC T connectorTrunk segments and repeaters Five segments may be joined using four repeatersComputers per segment 30 computers per segment by specificationSegments that can have computers Three of the five segments may be populatedMaximum total network length 925 meters (3,035 feet)Maximum number of computers per 1,024 by specificationnetwork without connectivity components

– The thicknet cabling components include:• Transceivers.

– Transceivers (transmit and receive) provide communications between the computer and the main LAN cable and are located in the vampire taps attached to the cable.

• Transceiver cables.– The transceiver cable (drop cable) connects the transceiver to the network adapter card.

• DIX or AUI connector.– This is the connector on the transceiver cable.

• N-series connectors including N-series barrel connectors, and N-series terminators.

– The thicknet components work the same way that the thinnet components do.

• The 5-4-3 Rule in Thicknet– One thicknet Ethernet network can have a maximum of five backbone

segments connected using repeaters (based on the IEEE 802.3 specification), of which three can accommodate computers. The length of the transceiver cables is not used to measure the distance supported on the thicknet cable; only the end-to-end length of the thicknet cable segment itself is used.

– Between connections, the minimum thicknet cable segment is 2.5 meters (about 8 feet). This measurement excludes transceiver cables. Thicknet was designed to support a backbone for a large department or an entire building.

• Combining Thicknet and Thinnet– It is common for larger networks to combine thick and thin Ethernet.

Thicknet is good for backbones with thinnet used for branch segments. What this means is that the thicknet cable is the main cable covering the long distances. You may remember that thicknet has a larger copper core and can therefore carry signals for a longer distance than thinnet. The transceiver attaches to the thicknet cable and the transceiver cable's AUI connector plugs into a repeater. The branching segments of thinnet plug into the repeater and connect the computers to the network.

• 10BaseFL– The IEEE committee published a specification for running Ethernet over

fiber-optic cable. 10BaseFL (10 Mbps, baseband, over fiber-optic cable) is an Ethernet network that typically uses fiber-optic cable to connect computers and repeaters. The primary reason for using 10BaseFL is for long cable runs between repeaters, such as between buildings. The maximum distance for a 10BaseFL segment is 2000 meters.

10Base5 Summary

Category NotesMaximum segment length 500 metersTransceivers Connected to the segment (in the tap)Maximum computer-to-transceiver distance 50 meters (164 feet)Minimum distance between transceivers 2.5 meters (8 feet)Trunk segments and repeaters Five segments may be joined using four repeatersSegments that can have computers Three of the five segments may be populatedMaximum total length of joined segments 2,500 meters (8,200 feet)Maximum number of computers per 100 by specificationsegment

• The 100 Mbps IEEE Standard– New Ethernet standards are pushing the traditional Ethernet limits beyond

the original 10 Mbps. These new capabilities are being developed to handle such high-bandwidth applications as:

• CAD (computer aided design)

• CAM (computer aided manufacturing)

• Video

• Imaging and document storage

– Two emerging Ethernet standards that can meet the increased demands are:

• 100BaseVG-AnyLAN Ethernet

• 100BaseX Ethernet (Fast Ethernet)

– Both Fast Ethernet and 100BaseVG-AnyLAN are about five to 10 times faster than standard Ethernet. They are also compatible with existing 10BaseT cabling systems. This means they will allow for Plug and Play upgrades from existing 10BaseT installations.

• 100VG-AnyLAN– 100VG (Voice Grade) AnyLAN is an emerging networking technology that

combines elements of both Ethernet and Token Ring. Originally developed by Hewlett-Packard, it is currently being refined and ratified by the IEEE 802.12 committee. The 802.12 specification is a standard for transmitting 802.3 Ethernet frames and 802.5 Token Ring packets. This technology goes by any of the following names, all of which refer to the same type of network:

• 100VG-AnyLAN

• 100BaseVG

• VG

• AnyLAN

• Specifications– Some of the current 100VG-AnyLAN specifications include:

• A minimum data rate of 100 Mbps.

• Ability to support a cascaded star topology over Category 3, 4, and 5 twisted-pair and fiber-optic cable.

• The demand priority access method which allows for two priority levels (low and high).

• Ability to support an option for filtering individually addressed frames at the hub to enhance privacy.

• Support for both Ethernet frames and Token Ring packets.

• Topology– A 100VG-AnyLAN network is built on a star topology with all computers

attached to a hub. The network can be expanded by adding child hubs to the central hub. The child hubs act as computers to their parent hubs. The parent hubs control transmission of computers attached to their children.

• Considerations– This topology requires its own hubs and cards. Also, the cable distances of

100BaseVG are limited when compared to 10BaseT and other implementations of Ethernet. The two longest cables from the 100BaseT hub to a computer cannot exceed 250 meters. Extending this requires special equipment used to expand the size of a LAN. These cable length limits mean that 100BaseT will require more wiring closets than 10BaseT.

• 100BaseX Ethernet– This standard, sometimes called Fast Ethernet, is an extension to the

existing Ethernet standard. It runs on UTP Category 5 data-grade cable and uses CSMA/CD in a star wired bus, similar to 10BaseT where all cables are attached to a hub. Media Specifications 100BaseX incorporates three media specifications:

• 100BaseT4 (4-pair Category 3, 4, or 5 UTP)

• 100BaseTX (2-pair Category 5 UTP or STP)

• 100BaseFX (2-strand fiber-optic cable)

– These media are described further in the following table.

• Performance Considerations

– Ethernet can use several communication protocols including TCP/IP, which works well in the UNIX environment. This makes Ethernet a favorite in the scientific and academic communities.

• Segmentation– Ethernet performance can be improved by dividing a crowded segment into

two less-populated segments and joining them with either a bridge or a router. This reduces traffic on each segment. Because there are fewer computers attempting to transmit onto the segment, access time improves. Segment division is a good tactic if large numbers of new users are joining the network or new, high-bandwidth applications such as database or video program are being added to the network.

• Network Operating Systems on Ethernet– Ethernet will work with most popular network operating systems.

Value Represents Actual meaning100 Transmission speed 100 Mbps or 100 megabits per secondBase Signal type BasebandT4 Cable type Indicates twisted-pair cable using four telephone-grade pairsTX Cable type Indicates twisted-pair cable using two data-grade pairsFX Cable type Indicates fiber-optic link using two strands of fiber-optic cable