Download Lecture 3-3

Introduction to Computer
Networks
Ethernet
This lesson introduces the Ethernet network
architecture.
The Origin of Ethernet (1)
• In the late 1960s, the University of Hawaii
developed a WAN called ALOHA.
• 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.
• In 1972, Robert Metcalfe invented a cabling and
signaling scheme at the Xerox Palo Alto Research
Center (PARC), and introduced the first Ethernet
product in 1975.
The Origin of Ethernet (2)
• 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 10/100-Mbps Ethernet is one of several
specifications describing methods for computers
and data systems to connect and share cabling.
Ethernet Specifications
• 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.
• In the 1980s the IEEE published Project 802. The
standard that pertains to Ethernet is the IEEE
802.3 specification.
Ethernet Features
• Ethernet is currently the most popular network
architecture.
• This baseband
architecture uses a
bus topology;
transmits at 10
Mbps; and relies on
CSMA/CD to
regulate traffic on
the main cable
segment.
Ethernet Basics
• The following is the Summary of Ethernet Basics
• Traditional topology
Other topologies
Linear bus
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
• An Ethernet frame can be between 64 and 1518
bytes long,
• Components of an Ethernet II Frame :
• Preamble: Marks the
start of the frame.
• Type: Used to
identify the network
layer protocol, usually
either IP or IPX
The 10/100-Mbps IEEE Standards
• There are four 10 Mbps Ethernet topologies:
•
10BaseT,
10Base2,
10Base5,
10BaseFL
• Two Ethernet standards that can meet the
increased demands are:
•
100BaseVG-AnyLAN Ethernet.
100BaseX Ethernet (Fast Ethernet).
10BaseT Standard
• In the IEEE 802.3 specification, the 10BaseT (10
Mbps, baseband, over twisted-pair cable), is an
Ethernet network that typically uses unshielded
twisted-pair (UTP) cable to connect computers.
• Most networks of this type are configured in a star
pattern, but internally they use a bus signaling
system like other Ethernet configurations.
Patch Panel
• 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.
10BaseT Specifications Summary
• The following is the Summary of Ethernet Basics
• Cable
Connectors
Transceiver
Tansceiver to hub
distance
Category 3, 4, or 5 UTP.
RJ-45 at cable ends.
Each computer needs one;
some cards have built in.
100 meters
Total
computers per
number of
LAN 1024 by specification.
Backbones for hubs
Coaxial or fiber-optic cable
10Base2 Standard
• 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.
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.
10Base5 Standard
• The IEEE
specification
for this
topology is 10
Mbps,
baseband, and
500-meter
(five 100meter)
segments. It is
also called
standard
Ethernet.
The 5-4-3 Rule in Thicknet
• Thicknet was designed to support a backbone for
a large department or an entire building.
10BaseFL Standard
• 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.
• The maximum distance for a 10BaseFL segment is
2000 meters (about 6500 feet).
100VG-AnyLAN Standard
• The 100VG (Voice Grade) AnyLAN combines
elements of both Ethernet and Token Ring
architectures.
• The 802.12 specification is a standard for
transmitting 802.3 Ethernet frames and 802.5
Token Ring packets.
• A minimum data rate of 100 Mbps.
• 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).
Topology of 100VG-AnyLAN
• A 100VG-AnyLAN network is built on a star
topology. Adding child hubs to the central hub can
expand the network.
• The longest
cable from
the hub to
a computer
cannot
exceed 250
meters.
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 star-wired bus topology, similar
to 10BaseT where all cables are attached to a hub.
• 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)
Ethernet Performance
Considerations: 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.
• This
reduces
traffic on
each
segment.