Department of Engineering Science
ES465/CES 440, Intro. to Networking & Network Management
Modulation & Modems (Ethernet & 802.3)
http://www.sonoma.edu/users/k/kujoory
References
• “Computer Networks & Internet,” Douglas Comer, 6th ed, Pearson, 2014, Ch15,
Textbook, 5th ed, slides by Lami Kaya (LKaya@ieee.org) with some changes.
• “Computer Networks,” A. Tanenbaum, 5th ed., Prentice Hall, 2011, ISBN:
13:978013212695-3.
• “Computer & Communication Networks,” Nader F. Mir, 2nd ed, Prentice Hall, 2015, ISBN:
13: 9780133814743.
• “Data Communications Networking,” Behrouz A. Forouzan, 4th ed, Mc-Graw Hill, 2007
• “Data & Computer Communications,” W. Stallings, 7th ed., Prentice Hall, 2004.
• “Computer Networks: A Systems Approach," L. Peterson, B. Davie, 4th Ed., Morgan
Kaufmann 2007.
Ali Kujoory
6/30/2016
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Topics Covered
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15.1 Introduction
15.2 The Venerable Ethernet
15.3 Ethernet Frame Format
15.4 Ethernet Type Field & Demultiplexing
15.5 IEEE's Version of Ethernet (802.3)
15.6 LAN Connections & Network Interface Cards
15.7 Ethernet Evolution & Thicknet Wiring
15.8 Thinnet Ethernet Wiring
15.9 Twisted Pair Ethernet Wiring & Hubs
15.10 Physical & Logical Ethernet Topology
15.11 Wiring in an Office Building
15.12 Variants of Twisted Pair Ethernet & Speeds
15.13 Twisted Pair Connectors & Cables
Ali Kujoory
6/30/2016
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15.1 Introduction
• This chapter
– continues the discussion of LANs by focusing on wired LAN
technologies
– explains how concepts from previous chapters form the basis of
Ethernet
– shows the wired LAN technology that has proceeded to dominate
all others
References:
• Ethernet frame https://en.wikipedia.org/wiki/Ethernet_frame
• IEEE 802.3as – Ethernet Frame Expansion
http://www.ieee802.org/3/as/public/0607/802.3as_overview.pdf
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15.2 The Venerable Ethernet
• Although the hardware devices, cabling, & media used
with Ethernet have changed dramatically
– many of the fundamentals remain constant
• One of the most interesting aspects of Ethernet evolution
concerns the way newer versions of Ethernet remain
backward compatible
– a new version can sense an older form & automatically adapt to
accommodate the older technology
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15.3 Ethernet Frame Format
• The term frame format refers to the way a packet is
organized
– including details such as the size & meaning of individual fields
• The main reason that older versions of Ethernet have
remained compatible with newer versions
– arises from the frame format, which has remained constant
since the DIX standard was created in the 1970s
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15.3 Ethernet Frame Format
• Fig. 15.1 illustrates the basic (original) format & the
details of the frame header
• As the figure shows
– an Ethernet frame consists of a fixed-length header
– a variable-length payload, &
– a fixed-length CRC
Fig. 15.1 Illustration of the Ethernet frame format & header details.
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15.4 Ethernet Type Field & Demultiplexing
– When transmitting a
datagram in an Ethernet
frame, the sender assigns a
type 0800
• The type field in an
Ethernet frame provides
multiplexing &
demultiplexing that
– allows a given computer to
have multiple protocols
operating simultaneously
• The protocols used on the
Internet send IP datagrams
& ARP messages over
Ethernet
– Each is assigned a unique
Ethernet type
• When a frame arrives at its
destination, the receiver
– examines the type field, &
– uses the value to determine
which software module
should process the frame
• Fig. 15.2 illustrates the
demultiplexing
• Hex 0800 for IP datagrams &
• Hex 0806 for ARP messages
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15.4 Ethernet Type Field & Demultiplexing
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15.5 IEEE's Version of Ethernet (802.3)
• IEEE developed a standard
for Ethernet (1983) &
– attempted to redefine the
Ethernet frame format
• IEEE working group that
produced the standard is
numbered 802.3
– professionals often refer to it
as 802.3 Ethernet
• Major difference between
conventional Ethernet &
802.3 Ethernet arises from
– 802.3 standard
• interprets the original type
field as a packet length, &
• adds 8-byte header that
contains the packet type
– The extra header is known as
a Logical Link Control /
Sub-Network Attachment
Point (LLC/SNAP) header;
• most professionals simply call
it a SNAP header
• Fig. 15.3 illustrates the
format
– interpretation of the type field
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15.5 IEEE's Version of Ethernet (802.3)
4 bytes
802.1Q tag
(optional)
Ali Kujoory
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2 bytes
Ethertype
or length
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15.5 IEEE's Version of Ethernet (802.3)
• The overall frame size in
802.3 Ethernet remains the
same as conventional
Ethernet: 1518 bytes
– IEEE reduced the maximum
payload from 1500 bytes to
1492 bytes
– SNAP header occupies the
first 8 bytes of the payload
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• To keep the two versions of
Ethernet compatible, a
convention is used:
– If bytes 13-14 of a frame
contain a numeric value less
than 1500
• the field is interpreted as a
packet length & the 802.3
standard applies, otherwise
• the field is interpreted as a
type field & the original
Ethernet standard applies
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15.6 LAN Connections & Network Interface Cards
• NIC appears to be an I/O
device
– it is connected in the same
manner as a disk or video
device
• A NIC handles
–
–
–
–
address recognition
CRC computation
frame recognition
sending/receiving frames
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• A NIC consists of a circuit
board with a plug on one
side
• Most computers come with
a NIC already installed
• The NIC is independent from
the rest of the computer, &
– a user can choose to replace
the NIC without making other
changes
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15.7 Ethernet Evolution & Thicknet Wiring
• Ethernet has undergone
several major changes
– with the most significant
changes in media & wiring
• Hardware used with
Thicknet was divided into
two major parts
• The original Ethernet wiring
scheme was informally
called thick wire Ethernet or
Thicknet
– because the medium
consisted of a heavy coaxial
cable
– the formal term for the wiring
is 10Base5
The 10 refers to its transmission speed of 10 Mbit/s.
The BASE is short for baseband signalling as
opposed to broadband,& the 5 stands for the
maximum segment length of 500 metres.
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– A NIC handled the digital
aspects of communication
– A separate electronic device
called a transceiver
connected to the Ethernet
cable that handles
• carrier detection
• conversion of bits into
appropriate voltages for
transmission, &
• conversion of incoming signals
to bits
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15.7 Ethernet Evolution & Thicknet Wiring
• A physical cable known as an Attachment Unit Interface
(AUI) connected a transceiver to a NIC in a computer
• A transceiver was usually remote from a computer
– E.g., in an office building, transceivers might attach to an Ethernet
in a hallway ceiling
• Fig. 15.4 illustrates how the original Thicknet wiring
used an AUI cable to connect a computer to a transceiver
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15.7 Ethernet Evolution & Thicknet Wiring
https://en.wikipedia.org/wiki/10BASE5
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15.8 Thinnet Ethernet Wiring
• A 2nd generation of Ethernet used a thinner coaxial
cable that was more flexible than Thicknet
– Formally named 10Base2 & informally known as Thinwire
Ethernet or Thinnet
• Thinnet integrates a transceiver directly on the NIC
– runs a coaxial cable from one computer to another
• Fig. 15.5 illustrates Thinnet wiring
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https://en.wikipedia.org/wiki/10BASE2
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15.8 Thinnet Ethernet Wiring
• Thinnet had advantages & disadvantages
• The primary advantages
– Lower overall cost & ease of installation
– No external transceivers were needed
– Thinnet cable could be installed in a convenient path
• The chief disadvantage
– because the entire network was vulnerable
• If a user unplugged a segment of the network to relocate wires or move
a computer, the entire network would stop working
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15.9 Twisted Pair Ethernet Wiring & Hubs
• A 3rd generation of Ethernet
wiring made a dramatic shift:
– In place of coaxial cable
• it uses a central electronic
device separate from the
computers attached to the
network
– Thus, when someone now
refers to Ethernet, they are
referring to twisted pair
Ethernet
• The electronic device that
served as the central
interconnection was known
as a hub
– Instead of heavy, shielded
cabling
• it uses twisted pair wiring
• The technology is informally
known as twisted pair
Ethernet, &
– Hubs were available in a
variety of sizes,
– has replaced other versions
• with the cost proportional to
size, but
• recently replaced with
switches
https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair
Originallly by StarLAN in AT&T
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15.9 Twisted Pair Ethernet Wiring & Hubs
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15.9 Twisted Pair Ethernet Wiring & Hubs
• A hub emulates a physical
cable
– making the entire system
operate like a conventional
Ethernet
• A system that uses a hub in
CSMA/CD receives a copy
of each frame &
– uses the address to
determine
• Twisted pair Ethernet retains
the same frame format as
the previous versions
• In fact, software on a
computer cannot
distinguish between thick
Ethernet, thin Ethernet, &
twisted pair Ethernet
– The NIC handles the details
& hides any differences
• whether to process or ignore
the frame
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6/30/2016
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15.10 Physical & Logical Ethernet Topology
• Twisted pair Ethernet
appears to follow a star
topology
• To understand Ethernet
topology
– In fact, the term hub arose to
clarify the concept of a
central interconnection
point
– However, because a hub
emulates a physical cable
• the system appears to perform
as if computers attach to a
cable
– we must distinguish between
logical & physical topologies
– Logically
• twisted pair Ethernet employs
a bus topology
– Physically
• twisted pair Ethernet forms a
star-shaped topology
– In fact, professionals joked
that a hub really provided a
bus in a box
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6/30/2016
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15.11 Wiring in an Office Building
• The styles of wiring used for
LANs make little difference
in a machine room or
laboratory
• The type of wiring makes a
major difference in terms of
–
–
–
–
type
number of wires needed
distance spanned
cost
• Fig. 15.7 depicts wiring on a
floor of an office building
– Note that twisted pair
Ethernet requires many
individual cables to go
between offices & a central
point (wiring closet)
– Thus, twisted pair Ethernet
requires careful labeling of
cables
• The three versions of
Ethernet wiring illustrate the
three principal forms that
LANs use
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Fig. 15.7 Illustration of various LAN wiring schemes that
have been used in an office building (Part I)
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Fig. 15.7 Illustration of various LAN wiring schemes that
have been used in an office building (Part II)
Compare the ThinNet topology with the twisted pair using a hub,
considering material, equipment, & person-power costs, &
maintenance & repairs efficiency.
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6/30/2016
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15.12 Variants of Twisted Pair Ethernet & Speeds
• Significant improvements
have been made in the
quality & shielding available
in twisted pair cables
– the data rate used on twisted
pair Ethernet has increased
• Fig. 15.8 summarizes the
three types of twisted pair
Ethernet & the cable used
with each
• Higher-speed Ethernet
technologies use an
electronic device known as a
switch rather than a hub
Ali Kujoory
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• To remain backward
compatible
– standards for the higherspeed versions specify that
interfaces automatically
sense the speed at which a
connection can operate, &
– slow down to accommodate
older devices
– E.g., if one plugs an Ethernet
cable between an old device
that uses 10BaseT & a new
device that uses 1000BaseT
• the new device will autosense
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15.12 Variants of Twisted Pair Ethernet & Speeds
1000BaseT up to 100 meters, https://en.wikipedia.org/wiki/Gigabit_Ethernet
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15.13 Twisted Pair Connectors & Cables
• Twisted pair Ethernet uses
RJ45 connectors
– which are larger versions of
the RJ11 connectors used to
connect telephones
• What is straight & crossed
Ethernet cable?
• A straight cable, used
between a computer & a
switch
– connects each pin of the
RJ45 attached to one end of
the cable directly to the
corresponding pin on the
RJ45 at the other end
Ali Kujoory
6/30/2016
• A crossed cable, which is
used to connect two
switches
– connects a pin on one end to
a different pin on the other
end
• To help technicians make
the correct connections
– individual wires in a Category
5 or Category 6 cable are
coated with colored plastic
• Fig. 15.9 lists the color
codes used with a straight
cable
Straight-through & Cross-over cables
http://www.home-network-help.com/straight.html
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15.13 Twisted Pair Connectors & Cables
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