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www.ciscopress.com Networking Basics CCNA 1 Chapter 6
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Networking Basics CCNA 1Chapter 6
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Ethernet Fundamentals
The History of the Internet• Created by Robert Metcalfe and others at Xerox
in early-to-mid 1970s• They later teamed with Digital Equipment
Corporation (DEC) and Intel; published a set of proprietary standards
• These standards became known as the DIX (DEC, Intel, Xerox) standards
• Version 2 of the standards, DIXv2, was published in 1980 and became 10BASE5
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Ethernet Fundamentals
The History of the Internet• IEEE took over the development of these
standards in the early 1980s• The first committee (committee 802) was given
the task of developing IEEE standards for LANs• The 802.3 subcommittee worked on Ethernet
standards• The 802.2 subcommittee worked on several
types of LANS, including Ethernet and Token Ring; both 802.2 and 802.3 relate to Ethernet
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Ethernet Fundamentals
The History of the Internet• Ethernet standards include:
– 10BASE5 (1980)– 10BASE2 (1985)– 10BASE-T (1990)– Fast Ethernet – 100 Mbps (1995)– Gigabit Ethernet – 1 Gbps (1998)– 10 Gigabit Ethernet – 10 Gbps (2002)
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Ethernet Fundamentals
The History of the Internet• Each new type of Ethernet supports the same
basic Ethernet frame• By using common framing, Ethernet has
remained simple and successful• Reasons for Ethernet’s continued success:
– Relatively simple– Adding a new type of Ethernet is easy, because other
types are already understood
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Ethernet Fundamentals
The History of the Internet• Reasons for Ethernet’s continued success
(continued):– Ethernet is reliable, with well-tested
components and protocols– Ethernet is inexpensive; new types generally
experience rapid price reductions within a few years of introduction
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Ethernet Fundamentals
The Names of Different IEEE Ethernet Types
• Ethernet standards differ in two main respects:– Speed– Type of cabling supported
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Ethernet Fundamentals
The Names of Different IEEE Ethernet Types • Commonly used Ethernet names list the basic
differentiating features:– Speed – the speed is listed in Mbps before the word
“BASE”– Baseband transmission – all current Ethernet
standards use baseband transmission– Cabling – the text after the word “BASE” indicates the
type of cabling; e.g. “T” means “twisted-pair”
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Ethernet Fundamentals
The Names of Different IEEE Ethernet Types
• Commonly used Ethernet names list the basic differentiating features:– Example: 10BASE-T
• 10 Mbps• Baseband transmission• Twisted-pair cabling
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Ethernet Fundamentals
The Names of Different IEEE Ethernet Types
• Baseband versus broadband– Baseband means that a single frequency is
used to encode bits – Some earlier (now obsolete) Ethernet
technologies used broadband to send signals• 10BROAD36• Broadband is a range of frequencies
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Ethernet Fundamentals
IEEE Ethernet Standards and the OSI Model
• IEEE 802.3 Standards– Committee began in 1980– Goal was to standardize 10BASE5 Ethernet– Standard matched the DIXv2 standard in most
ways– One big difference was in the way 802.3 does
framing
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Ethernet Fundamentals
IEEE Ethernet Standards and the OSI Model • IEEE 802.3 Standards concern several details:
– Physical transmission details (cables, connectors, encoding, speeds)
– Media access issues (carrier sense multiple access with collision detection – CSMA/CD)
– Errors during transmission– MAC address (location and format)– Ability of NICs to synchronize to the incoming signal
by using a preamble and Start Frame Delimiter (SFD)
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Ethernet Fundamentals
The IEEE 802.2 Logical Link Control Standard
Original IEEE 802 Subcommittees
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Ethernet Fundamentals
The IEEE 802.2 Logical Link Control Standard• 802.1 and 802.2 subcommittees had responsibility for
features than could be used by all three types of LANs• Original focus of 802.2 subcommittee was to standardize
any feature that spanned all three types of LANs• This subcommittee determined all three LAN types needed
to somehow control the LAN, using a set of logic and rules• The standard was named Logical Link Control (LLC)
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Ethernet Fundamentals
Comparing Ethernet Standards to the OSI Model
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Ethernet Fundamentals
Comparing Ethernet Standards to the OSI Model
• Each new type of Ethernet standard defines some physical layer standards and data link layer details
• E.G., all Ethernet standards define the details of physical transmission over some medium, so they match the OSI physical layer
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Ethernet Fundamentals
Comparing Ethernet Standards to the OSI Model
• The IEEE 802.3 standard defines many physical details, as well as the lower half of the data link layer - Media Access Control (MAC)
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Ethernet Fundamentals
Comparing Ethernet Standards to the OSI Model
• MAC protocol performs several functions the physical layer (Layer 1) standards cannot
• Layer 1 standards cannot communicate with upper-layer protocols, but the MAC protocol can
• Layer 1 standards cannot identify other computers, but MAC addressing can
• Layer 1 standards define how to send bits but cannot interpret their meaning, but MAC can by defining framing
• Layer 1 standards cannot manage the process of which device can send at what time, so MAC protocol defines CSMA/CD
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Ethernet Fundamentals
Ethernet Framing• Framing refers to two things:
– The process of encapsulating data inside a header and possible a trailer
– The meaning given to the bits inside those headers and trailers
• Framing defines the meaning of transmitted bits• Mailing a letter
– Write the letter– Put in envelope– Write address on envelope in a particular place– Put the stamp in a particular place
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Ethernet Fundamentals
Ethernet Framing
• Framing is similar to mailing a letter– The NIC adds a header and trailer around the
Layer 3 protocol data unit (PDU) to encapsulate the data
– The header and trailer must have properly formatted information to be delivered correctly
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Ethernet Fundamentals
Encapsulating Packets Inside Ethernet Frames
• The process of adding a header and trailer around the Layer 3 PDU is called “encapsulation”
• Sometimes called “framing”• The term “frame” refers to the resulting
bits that include the Ethernet header and trailer
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Ethernet Fundamentals
Ethernet Encapsulation and the IEEE 802.3 Frame
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Ethernet Fundamentals
Encapsulating Packets Inside Ethernet Frames
• The encapsulation process:– The IP software gives the IP packet to the
Ethernet software– The Ethernet software encapsulates the IP
packet between and Ethernet header and trailer– The Ethernet NIC physically transmits the bits
that comprise the frame over an Ethernet LAN
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Ethernet Fundamentals
Three Styles of Ethernet Framing
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Ethernet Fundamentals
The Fields in the IEEE 802.3 Frame• Header and trailer contain a defined number of
bytes• Different sets of bytes have different meanings• E.G., the destination field contains the six-byte
MAC address
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Ethernet Fundamentals
IEEE 802.3 Header
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Ethernet Fundamentals
IEEE 802.3 Header
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Ethernet Fundamentals
The Fields in the IEEE 802.3 Header• Destination MAC address – allows a switch to
forward the frame, allows a PC to determine if the frame is meant for it
• Source MAC address – identifies the sending NIC or interface, helps a switch build a switching table
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Ethernet Fundamentals
DIX Framing and IEEE Framing• IEEE 802.3 framing for Ethernet is slightly
different than the DIXv2 framing specification– DIXv2 did not use the term SFD, called the first eight
bytes the Preamble– DIXv2 Type field identifies the contents in the frame– IEEE 802.3 was planned to be backwardly compatible
with DIXv2– IEEE 802.3 can tell if the frame is DIXv2 or IEEE
802.3 by looking at the value of the Length/Type field
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Ethernet Fundamentals
The Format of MAC Addresses• Media Access Control address characteristics:
– 6-byte hexadecimal (hex) numbers– Each NIC has a burned-in (permanent) MAC address– First 3 bytes are called an Organizationally Unique
Identifier (OUI) and refer to the manufacturer– Second 3 bytes are unique numbers to the
manufacturer (a serial number)
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Ethernet Fundamentals
Structure of a MAC Address
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Ethernet Fundamentals
Sending Frames over Different Types of Ethernet
• Each type of Ethernet uses the exact same framing• An Ethernet frame can be sent by a device on one
type of Ethernet and can go over links using different types of Ethernet with no problems
• By using the same type of framing, the IEEE has enabled companies to slowly migrate to newer types of Ethernet
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Ethernet Fundamentals
Sample Campus LAN with Different
Types of Ethernet
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Ethernet Fundamentals
LLC, SNAP and Determining the Type of Protocol
• Additional Background on IEEE 802.2 Logical Link Control (LLC)– “Link” in LLC refers to the concept of how two devices
communicate over a transmission medium – the two devices must be “linked”
– Two base features of the IEEE 802.2 header• Identify type of data inside frame’s data field• Control the transmission between two devices on a LAN,
specifically to perform error recover when frames are lost
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Ethernet Fundamentals
LLC, SNAP and Determining the Type of Protocol
• Additional Background on IEEE 802.2 Logical Link Control (LLC) (continued)– Two base features of the IEEE 802.2 header
(continued)• The error recovery function is not used today, but the first
function is
• The de-encapsulation process needs to know the type of data in the data field; however, many networks use only TCP/IP as a Layer 3 protocol
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Ethernet Fundamentals
LLC, SNAP and Determining the Type of Protocol
• The 802.3 subcommittee defined a field to identify the type of data, the Destination Service Access Point (DSAP)– This field holds a number that identifies the
contents of the data
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Ethernet Fundamentals
IEEE 802.3 and 802.2 Headers
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Ethernet Fundamentals
The IEEE SNAP Header• The DSAP field (1 byte long) did not allow
enough protocol values – ran out before one could be assigned to IP!
• Original solution was to use an optional header called the Subnetwork Access Protocol (SNAP)
• Header includes an OUI field (mostly unused) and a 2-byte protocol Type field
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Ethernet Fundamentals
The DIX and IEEE Type Field• Three Type fields
– 1970s – original DIX frame format (2-byte Type field)
– mid 1980s – IEEE standard for framing, with SNAP header (802.2 and 802.3), and 1988 (SNAP header part)
– 1997 – IEEE revised standard for framing with Length/Type field
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Ethernet Fundamentals
Ethernet Framing
with Variations
on the Type Field
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Ethernet Operation
Rules Governing When a Device Can Transmit: CSMA/CD
• CSMA/CD algorithm defines rules that NICs must follow• In the original Ethernet physical bus topology, only one
device can send at a time• If multiple devices try to send at the same time, their
electrical signals overlap and are added together• The resulting signal does not represent the original
signal of either device• Such an occurrence is called a “collision”
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Ethernet Operation
Rules Governing When a Device Can Transmit: CSMA/CD - the CSMA/CD
Algorithm• The original MAC standard suggests:
– Wait until the LAN is unused, then send the frame– Listen to detect if the frame collided with another
frame– If no collision occurred, the frame must have made it
across the LAN– If a collision did occur, wait and then try to send the
original frame again
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Ethernet Operation
Rules Governing When a Device Can Transmit: CSMA/CD - the CSMA/CD Algorithm
• The CSMA/CD algorithm in detail:– Wait until the LAN is silent– Send a frame– Transmitting device listens for collisions while transmitting– If no collisions occur, process is complete– All devices whose transmitted frames collided send a jamming
signal (32 bits of 1s and 0s)– All devices whose frames collided set semi-random collision
back-off timers.– Once the timer expires, a device can begin the process again
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Ethernet Operation
Origins of the Name CSMA/CD
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Ethernet Operation
CSMA/CD and Collisions on 10BASE-T LANs with Hubs
• Hubs operate at Layer 1, simply repeating all received signals out all other ports
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Ethernet Operation
CSMA/CD and Collisions on 10BASE-T LANs with Hubs
10BASE-T NICs use a loopback circuit on the NIC to be able to recognize collisions on a hub. When the two signals combine, it is recognized as a collision.
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Ethernet Operation
Full Duplex, Half Duplex, and Collision Domains• CSMA/CD causes throughput over a LAN to degrade as
the LAN gets busier• Collisions cause devices to wait to retransmit, further
slowing the network• Originally, Ethernet LAN performance was restricted
because Ethernet NICs used half duplex logic – they could send or transmit (duplex), but not do both simultaneously
• To improve LAN performance, standards evolved where collisions cannot occur: full duplex (receive and transmit simultaneously)
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Ethernet Operation
Preventing Collisions with Switch Buffering• Switches prevent collisions by buffering frames• If several PCs send frames to the same address
at the same time, the switch holds the frames in memory - a process called buffering
• The switch then forwards the frames one at a time
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Ethernet Operation
Switch Buffering Example
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Ethernet Operation
Collision Domains with a Switch
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Ethernet Operation
Collision Domains and Full Duplex• Each port of a switch creates its own collision domain• A frame in one collision domain does not cause collisions
in another collision domain• The switch’s buffering process prevents the collisions• When full duplex is enabled:
– NICs/Interfaces can send and receive at same time– CSMA/CD is no longer needed as collisions can’t occur– Because CSMA/CD is disabled, the NIC disables its loopback
logic– Full duplex cannot be used if a hub is plugged into the port
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Ethernet Operation
Switches and Hubs: Case That Does Not Allow Full Duplex
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Ethernet Operation
Autonegotiation of Duplex and Speed• Ethernet supports a wide variety of speeds and two
duplex options• An IEEE standard sets the process by which Ethernet
NICs and switch ports can automatically negotiate the speed and duplex setting – called “autonegotiation”
• NICs and switch ports exchange information about their capabilities, using Fast Link Pulse (FLP) bursts [a series of Network Link Pulses (NLPs) that allow the devices to send each other a set of bits]
• These bits do not look like normal frames, so they are not processed like normal frames
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Ethernet Operation
Autonegotiation of Duplex and Speed• The preferred choices of autonegotiation
speed from fastest to slowest– 1000 Mbps, full duplex– 1000 Mbps, half duplex– 100 Mbps, full duplex– 100 Mbps, half duplex– 10 Mbps, full duplex– 10 Mbps, half duplex
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Ethernet Operation
Autonegotiation of Duplex and Speed• Speed and duplex may be manually configured or
automatically negotiated• If one end disables autonegotiation and the other end
uses a different speed the link will not work• If one end disables autonegotiation and the other end
uses a different duplex, the connection may work, but with excessive collisions
• Autonegotiation works well on copper cabling, but is not supported on fiber
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Ethernet Operation
Deterministic and Nondeterministic Media Access• Ethernet is a nondeterministic process – no guidelines in
place to determine the number of times a device is allowed to use the LAN cannot be determined ahead of time
• An Ethernet device might go several seconds before it can transmit, due to collisions
• Deterministic media access means that the amount of bandwidth available to devices can be accurately predicted – two formerly popular token-passing protocols are Token Ring and Fiber Distributed Data Interchange (FDDI)
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Ethernet Operation
Token Passing in Token Ring
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Summary
• The Ethernet family includes 10 Mbps, Fast Ethernet and Gigabit Ethernet
• Any new upgrades to Ethernet supplement the IEEE 802.3 standard
• Ethernet uses baseband signaling• IEEE 802.3 operates at the bottom two layers of
the OSI model– Lower half of the data link layer: the MAC sublayer– The physical layer
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Summary
• Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components and various physical topologies
• All types of IEEE-standard Ethernet use the same frame structure– Preamble– Start Frame Delimiter (SFD)– Destination Address– Source Address– Length or Type– Data and Pad– Frame Check Sequence
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Summary
• Address fields of the Ethernet frame contain Layer 2, or MAC addresses, for source and destination
• All frames are susceptible to errors – the FCS field contains a number that is calculated by the source – the destination performs the same calculation to determine if the frame is good
• Ethernet uses the CSMA/CD algorithm to manage access to shared media – it is a nondeterministic protocol
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Summary
• Other MAC algorithms, such as token-passing methods used by Token Ring and FDDI, are deterministic
• With CSMA/CD, a NIC waits for the absence of a signal on the media and the begins transmitting
• If two or nodes transmit at the same time, a collision occurs
• If nodes detect a collision, they wait a random time and then retransmit
• Autonegotiation detects speed and duplex mode (half or full duplex) of the device on the other end of the wire and adjusts to match the settings
