Download Ethernet

Ethernet
This lesson introduces the Ethernet network architecture. Over the years, Ethernet has
become the most popular media access method to the desktop computer and is used in
both small and large network environments. Ethernet is a nonproprietary industry
standard that has found wide acceptance by network hardware manufacturers.
Problems related to using Ethernet hardware products from different hardware
manufacturers in a single network are nearly nonexistent. This lesson presents an
overview of the major Ethernet components, features, and functions.
After this lesson, you will be able to:
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Identify the standard Ethernet components.
Describe the features of each IEEE Ethernet standard topology.
Identify the cabling for a given IEEE Ethernet standard topology.
Determine which Ethernet topology would be appropriate for a given
site.
Estimated lesson time: 50 minutes
The Origin of Ethernet
In the late 1960s, the University of Hawaii developed a WAN called ALOHA. (A
WAN extends LAN technology across a larger geographical area. For more
information on WANs, see Chapter 1, "Introduction to Networking.") The university
occupied a wide area and sought to connect computers that were spread throughout
the campus. One of the key features of the university's network was its use of
CSMA/CD as the access method.
This early network was the foundation for today's Ethernet architecture. 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 system of 2.94 megabits
per second (Mbps) to connect over 100 computers on a 1-kilometer (.62 miles) cable.
Xerox Ethernet was so successful that Xerox, Intel Corporation, and Digital
Equipment Corporation drew up a standard for a 10-Mbps Ethernet. Today, the 10Mbps Ethernet is one of several specifications describing methods for computers and
data systems to connect and share cabling.
Ethernet Specifications
Although networking standards are not discussed in detail until Chapter 5, it is
important for you to be aware of them at this point. In 1978, the International
Organization for Standardization (ISO) released a set of specifications for connecting
dissimilar devices. This set of standards is referred to as the OSI reference model (OSI
stands for Open Systems Interconnection). The Ethernet specification performs the
same functions as the OSI physical and data-link layers of this model. As you will see
later, these specifications affect how hardware links, or passes information to and
from, ISO standards. In the 1980s the IEEE published Project 802. This project
generated standards for design and compatibility for hardware components that
operated within the OSI physical and data-link layers. The standard that pertains to
Ethernet is the IEEE 802.3 specification.
Ethernet Features
Ethernet is currently the most popular network architecture. Figure 3.13 shows a
simple Ethernet bus network. Notice that the cable is terminated at both ends. 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 requires no power source of its own
and thus will not fail unless the media is physically cut or improperly terminated.
Figure 3.13 Simple Ethernet bus network terminated at both ends
Ethernet Basics
Table 3.2 summarizes Ethernet features:
Table 3.2 Summary of Ethernet
Feature
Description
Traditional topology
Linear bus
Other topologies
Star bus
Type of architecture
Baseband
Access method
CSMA/CD
Specification
IEEE 802.3
Transfer speed
10 Mbps or 100 Mbps
Cable type
Thicknet, thinnet, UTP
The Ethernet Frame Format
Ethernet breaks data down into packages in a format that is different from the packets
used in other networks: Ethernet breaks data down into frames. (Remember that the
terms "packet" and "frame" can be used interchangeably; in the context of Ethernet,
the term "frame" is used.) A frame is a package of information transmitted as a single
unit. An Ethernet frame can be between 64 and 1518 bytes long, but the Ethernet
frame itself uses at least 18 bytes; therefore, the data in an Ethernet frame can be
between 46 and 1500 bytes long. Every frame contains control information and
follows the same basic organization.
For example, the Ethernet II frame, used for Transmission Control Protocol/Internet
Protocol (TCP/IP), which gets transmitted across the network, consists of the sections
listed in Table 3.3 (TCP/IP has become the de facto standard for data transmission
over networks, including the Internet):
Table 3.3 Components of an Ethernet II Frame
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, usually either IP or
IPX (Novell's Internetwork Packet Exchange)
Cyclical redundancy
check (CRC)
Error-checking field to determine if the frame arrived without
being corrupted
An illustration of an Ethernet frame is shown in Figure 3.14.
Figure 3.14 Sample Ethernet II frame
Ethernet networks include a variety of cabling and topology alternatives. The
remaining sections of this lesson present these alternatives based on their IEEE
specification.
The 10-Mbps IEEE Standards
This section looks at four 10 Mbps Ethernet topologies:
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10BaseT
10Base2
10Base5
10BaseFL
10BaseT Standard
In 1990, the IEEE committee published the 802.3 specification for running Ethernet
over twisted-pair wiring. The result, 10BaseT (10 Mbps, baseband, over twisted-pair
cable), is an Ethernet network that typically uses unshielded twisted-pair (UTP) cable
to connect computers. Usually, 10BaseT employs UTP, but shielded twisted-pair
(STP) cabling will also work without changing any of the 10BaseT parameters.
Most networks of this type are configured in a star pattern, but internally they use a
bus signaling system like other Ethernet configurations. Figure 3.15 shows a multiport
hub used to extend an Ethernet LAN. 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 endpoint of a cable that is 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 1024 computers.
Figure 3.15 A multiport repeater (hub) can be used to extend an Ethernet LAN
Figure 3.16 shows how a 10BaseT solution provides the advantages of a star-wired
topology. The UTP cable features data transmission at 10 Mbps. It is easy to make
changes by moving a modular patch cord on the patch panel. A change at the patch
panel will not affect other devices on the network; this differs from a traditional
Ethernet bus network.
Figure 3.16 A patch panel makes moving computers easy
Patch panels should be tested for rates higher than 10 Mbps. The latest hubs can
provide connections for both thick and thin Ethernet cable segments. In this
implementation, it is also easy to convert thick Ethernet cable to 10BaseT cable by
attaching a mini 10BaseT transceiver to the AUI port of any network interface card.
Table 3.4 summarizes 10BaseT specifications:
Table 3.4 10BaseT Specifications Summary
Category
Notes
Cable
Category 3, 4, or 5 UTP.
Connectors
RJ-45 at cable ends.
Transceiver
Each computer needs one; some cards have built
in transceivers.
Transceiver to hub distance
100 meters (328 feet) maximum.
Backbones for hubs
Coaxial or fiber-optic cable to join a larger LAN
or to carry major traffic between smaller
networks.
Total number of computers per LAN 1024 by specification.
without connectivity components
10Base2 Standard
Another topology is 10Base2, given this name in the IEEE 802.3 specification
because it transmits at 10 Mbps over a baseband wire and can carry a signal about two
times 100 meters (the actual distance is 185 meters, or 607 feet).
This type of network uses thin coaxial cable, or thinnet, which has a maximum
segment length of 185 meters (607 feet) and a minimum cable length of at least 0.5
meters (20 inches) between workstations. There is also a 30-computer maximum per
185-meter segment.
Thinnet cabling components include:
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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 NIC.
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 nine
meters (30 feet) long, but all you have is a 7.5-meter (25-foot) length and a 1.5-meter
( 5-foot) length of thinnet cable, you can join the two cable segments together using a
BNC barrel connector. However, the use of barrel connectors should be kept to a
minimum because each connection in the cable reduces the signal quality and adds to
the risk of cable separation and disconnection.
A thinnet network is an economical way to support a small department or workgroup.
The cable used for this type of network is:
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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.
In Figure 3.17, there are five segments, four repeaters, and trunk segments 1, 2, and 5
are populated (have computers attached to them). Trunk segments 3 and 4 exist only
to increase the total length of the network and to allow the computers on trunk
segments 1 and 5 to be on the same network.
Figure 3.17 The thinnet 5-4-3 rule: 5 segments, 4 repeaters, and 3 populated
segments
Because normal Ethernet limits are 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
(3035 feet). The following table summarizes 10Base2 specifications:
Table 3.5 10Base2 Specifications Summary
Category
Maximum segment length
Notes
185 meters (607 feet).
Connection to network interface card BNC T connector.
Trunk segments and repeaters
Five segments can be joined using four repeaters.
Computers per segment
30 computers per segment by specification.
Segments that can have computers
Three of the five segments can be populated.
Maximum total network length
925 meters (3035 feet).
10Base5 Standard
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 cable (see Figure 3.18), also known as
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. The distances and tolerances for thicknet are greater than those for thinnet:
a thicknet segment can be 500 meters (1640 feet) long for a total network length of
2500 meters (8200 feet).
Figure 3.18 Thicknet cable composition
The thicknet cabling components include:
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Transceivers These are devices that can both 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 NIC.
DIX (or AUI) connectors These are the connectors on the transceiver cable.
N-series connectors, including N-series barrel connectors, and N-series
terminators The thicknet components work in the same way as the thinnet
components. Figure 3.19 shows a thicknet cable with a transceiver attached and
a transceiver cable. It also shows the DIX or AUI connector on the transceiver
cable.
NOTE
"AUI," an acronym for attachment unit interface, is a 15-pin (DB-15) connector
commonly used to connect a NIC to an Ethernet cable; AUIs and DIXs are discussed
in Chapter 2, "Basic Network Media."
Figure 3.19 Thicknet backbone with attached transceiver and cable
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 up to
three can accommodate computers. Figure 3.20 shows how the 5-4-3 rules are applied
to thicknet. 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.
Figure 3.20 Thicknet 5-4-3 rule; 5 backbone segments, 4 repeaters, and 3 segments
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. Table 3.6 summarizes
10Base5 specifications:
Table 3.6 10Base5 Specifications Summary
Category
Notes
Maximum segment length
500 meters (1640 feet).
Transceivers
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 can be joined using four
repeaters.
Segments that can have computers
Three of the five segments can be populated.
Maximum total length of joined segments
2500 meters (8200 feet).
Maximum number of computers per
segment
100 by specification.
Combining Thicknet and Thinnet Cable
It is common for larger networks to combine thick and thin Ethernet cable. Thicknet
cable is good for backbones, while thinnet cable is used for branch segments. What
this means is that the thicknet cable is the main cable covering the long distances. As
described in Chapter 2, "Basic Network Media," thicknet cable 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 Standard
The IEEE committee published a specification for running Ethernet over fiber-optic
cable. The result, 10BaseFL (10Mbps, 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 to accommodate long cable runs between
repeaters, such as between buildings. The maximum distance for a 10BaseFL segment
is 2000 meters (about 6500 feet).
The 100-Mbps IEEE Standards
New Ethernet standards are pushing the traditional Ethernet limits beyond the original
10 Mbps. These new capabilities are being developed to handle such highbandwidth
applications as:
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Computer-aided design (CAD).
Computer-aided manufacturing (CAM).
Video.
Imaging and document storage.
Two Ethernet standards that can meet the increased demands are:
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100BaseVG-AnyLAN Ethernet.
100BaseX Ethernet (Fast Ethernet).
Both 100BaseVG-AnyLAN and Fast Ethernet are about 5 to 10 times faster than
standard Ethernet. They are also compatible with existing 10BaseT cabling systems.
This means they allow for Plug and Play upgrades from existing 10BaseT
installations.
100VG-AnyLAN Standard
The 100VG (Voice Grade) AnyLAN is an emerging networking technology that
combines elements of both Ethernet and Token Ring architectures. 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:
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100VG-AnyLAN
100BaseVG
VG
AnyLAN
Specifications
Some of the current 100VG-AnyLAN specifications include:
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A minimum data rate of 100 Mbps.
The ability to support a cascaded star topology over Category 3, 4, and 5
twisted-pair and fiber-optic cable.
The demand-priority access method that allows for two priority levels (low and
high).
The 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 in which all computers are
attached to a hub. Figure 3.21 shows a parent hub with five child hubs. Adding child
hubs to the central hub can expand the network. The child hubs act as computers to
their parent hubs. The parent hubs control transmission of computers attached to their
children.
Figure 3.21 Parent hub with five attached child hubs
Considerations
This topology requires its own hubs and cards. Also, the cable distances of
100BaseVG are limited when compared to 10BaseVG and other implementations of
Ethernet. The longest cable from the 100BaseVG hub to a computer cannot exceed
250 meters (about 820 feet). Extending this limit requires special equipment used to
expand the size of a LAN. These cable-length limits mean that 100BaseVG will
require more wiring closets than 10BaseVG.
100BaseX Ethernet Standard
This standard, sometimes called Fast Ethernet, is an extension of the existing Ethernet
standard. It runs on UTP Category 5 data-grade cable and uses CSMA/CD in a starwired bus topology, similar to 10BaseT where all cables are attached to a hub.
Media Specifications
100BaseX incorporates three media specifications:
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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 Table 3.7:
Table 3.7 100BaseX Media Specifications
Value
Represents
Actual meaning
100
Transmission speed 100 Mbps
Base
Signal type
Baseband
T4
Cable type
Indicates twisted-pair cable using four telephone-grade pairs
TX
Cable type
Indicates twisted-pair cable using two data-grade pairs
FX
Cable type
Indicates fiber-optic link using two strands of fiber-optic cable
Performance Considerations
Ethernet architecture can use multiple communication protocols and can connect
mixed computing environments such as Netware, UNIX, Windows, and Macintosh.
Segmentation
Ethernet performance can be improved by dividing a crowded segment into two lesspopulated segments and joining them with either a bridge or a router. Bridges and
routers are discussed later in more detail in Chapter 7, "Elements of Network
Connectivity." Figure 3.22 shows how a bridge is used to extend a network. This
reduces traffic on each segment. Because fewer computers are attempting to transmit
onto the segment, access time improves.
Figure 3.22 Using a bridge to segment a network and reduce network traffic
Consider dividing segments if large numbers of new users are joining the network or
if new, high-bandwidth applications, such as database or video programs, are being
added to the network.
Network Operating Systems on Ethernet
Ethernet will work with most popular network operating systems including:
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Microsoft Windows 95, Windows 98, and Windows 2000.
Microsoft Windows NT Workstation and Windows NT Server.
Microsoft Windows 2000 Professional and Windows 2000 Server.
Microsoft LAN Manager.
Microsoft Windows for Workgroups.
Novell NetWare.
IBM LAN Server.
AppleShare.
UNIX.
Lesson Summary
The following points summarize the main elements of this lesson:
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Ethernet is one of the most popular network architectures.
Ethernet is governed by the specifications found in the OSI reference model
physical layer and data-link layer, as well as IEEE 802.3.
Table 3.8 summarizes the specifications for Ethernet architecture discussed in this
lesson. It outlines the minimum set of standards required to conform to IEEE
specifications. A particular implementation of the network architecture may differ
from the information in the table.
Table 3.8 Ethernet Specifications (IEEE 802.3)
10Base2
10Base5
10BaseT
Topology
Bus
Bus
Cable type
RG-58 (thinnet
coaxial cable)
Thicknet; oneCategory 3, 4, or 5 unshielded
centimeter (3/8-inch) twisted-pair cable
shielded transceiver
cable
Connection to BNC T
connector
NIC
Terminator
resistance, ?
(ohms)
50
Star bus
DIX or AUI
connector
RJ-45
50
Not applicable
Impedance, ?
50 ± 2
50 ± 2
85-115 unshielded twisted-pair;
135-165 shielded twisted-pair
Distance
0.5 meters
between
computers (23
inches)
2.5 meters (8 feet)
between taps and
maximum of 50
meters (164 feet)
between the tap and
the computer
100 meters (328 feet) between
the transceiver (the computer)
and the hub
500 meters (1640
feet)
100 meters (328 feet)
185 meters (607
Maximum
cable segment feet)
length
Maximum
connected
segments
5 (using 4
repeaters); Only
3 segments can
have computers
connected.
5 (using 4 repeaters). Not applicable
Only 3 segments can
have computers
connected.
Maximum
total network
length
925 meters (3035 2460 meters (8000
feet)
feet)
Not applicable
Maximum
computers
per segment
30 (There can be
a maximum of
1024 computers
per network.)
1 (Each station has its own
cable to the hub. There can be a
maximum of 12 computers per
hub and a maximum of 1024
transceivers per LAN without
some type of connectivity.)
100