Cisco CCNA module 6

1© 2004, Cisco Systems, Inc. All rights reserved.
Module 6
Ethernet Fundamentals

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Objectives

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Ethernet
• Ethernet is a bus network in which
multiple computers share a single
transmission medium. While one
computer transmits a frame to another. All
other computers must wait.
• Ethernet is the dominant LAN technology
in the world.
• Ethernet is not one technology but a
family of LAN technologies.( E- FE-GE)

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Introduction to Ethernet
• Most of the traffic on the Internet originates and ends with Ethernet
connections.
• Competing and fighting to access the media -------Original idea for
Ethernet was developed in early 1970s at the University of Hawaii .
• 1970’s Alohanet was the basis Ethernet access method known as
CSMA/CD.
• DIX-1980 Robert Metcalfe+ The first LAN in the world. That is the
original version of Ethernet.
• 1985—IEEE---The Institute of Electrical and Electronics Engineers
published standards for LANs. These standards start with the number
802. The standard for Ethernet is 802.3. (IEEE –must be compatible to
OSI model). The standard for Ethernet is 802.3.
• Essentially, Ethernet and IEEE 802.3 are the same standards ( some
small modification )
• In 1995, IEEE announced a standard for a 100-Mbps Ethernet (called
Fast Ethernet).
• In 1998 and 1999 standards for gigabit per second (Gbps, 1 billion bits
per second)
• The bandwidth of the network could be increased many times without
changing the underlying Ethernet technology

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The Success of Ethernet
The success of Ethernet is due to the following factors:
• Simplicity and ease of maintenance
• Ability to incorporate new technologies
• Reliability
• Low cost of installation and upgrade
Together, Ethernet and IEEE 802.3 currently maintain the
greatest market share of any LAN protocol

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Inventor of Ethernet – Out of the Ether

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Introduction to Ethernet
Ethernet was originally developed to operate over radio in the
Hawaiian islands

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Defining Standard
• "The nice thing about standards is
that there's so many to choose
from."

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OSI Model

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The Physical layer
Layer 1 involves interfacing with:
• media, signals, bit streams that travel on the
media
• components that put signals on media, and
various topologies.

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Data Link Layer
• Data link layer provides reliable transit of data
across a physical link by using the Media
Access Control (MAC) addresses.
• Data link layer is concerned with physical (as
opposed to network, or logical) addressing.

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Data link layer– Two Parts (sub-layers)
Logical link Control (LLC) : (IEEE 802.2)
• The Logical Link Control (LLC) sublayer remains relatively
independent of the physical equipment
• Provides an interface between the MAC layer and the Network
layer which is independent of the hardware
Media Access Control (MAC): (part of IEEE 802.3)
• The MAC sub-layer is concerned with the physical components
that will be used to communicate the information..
• WHO can access the network when multiple computers are
trying to access it simultaneously.
• physical addressing (MAC addresses) and access control
methods.

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Ethernet and the OSI Model
• Ethernet operates in two areas of the
OSI model, the lower half of the data
link layer, known as the MAC sub
layer and the physical layer
• Trick question

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IEEE Ethernet Standards
IEEE 802 Committee Standards:
• 802.1 - Standards introduction
• 802.2 - Logical Link Control (LLC)
to prevent senders from overwhelming receivers (ACK)
• 802.3 – Ethernet
CSMA/CD, multiple media access methods, and packet (frame) format.
• 802.4 - Token Bus - 75 ohm CATV coax or Fibre
• 802.5 - Token Ring - physical standard and media access sublayer
• 802.6 - MAN (Metropolitan Area Network) - similar to FDDI
• 802.7 - Broadband
• 802.8 - Fiber Optics
• 802.9 - Integrated Voice and Data
• 802.10 - LAN Security
• 802.11 - Wireless
• 802.12 -100 VG AnyLAN

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Naming – The MAC or Hardware Address
• A 48-bit address
burned onto the NIC.
• It is a unique way of
identifying each
computer on a
network.
• Flat Address
All devices that are
connected to the
Ethernet LAN have
MAC addressed
interfaces including
workstations,
printers, routers, and
switches.

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Naming – MAC Address Format
The MAC address consists of 12 hex digits (48 bits)
The first six digits (assigned by the IEEE) represent the Organizational
Unique Identifier (OUI) which identifies the manufacturer
The last six are assigned by the manufacturer and represent a unique
hardware ID number for the NIC

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Framing
• Framing is the Layer 2 encapsulation process.
• Bit streams (data) 010101 alone can not be sent between devices.
• We must divide the data up and add:
a bit pattern to flag the start of each frame
relevant MAC addresses
a block of data
some bits for error detection
a bit pattern to flag the end of the frame
• A frame is the Layer 2 protocol data unit (PDU).

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Layer 2 Framing

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Ethernet Frame Structures

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Ethernet Frame Structure

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IEEE 802.3 Frame Structure

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IEEE 802.3 Preamble (Start Frame Field)

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Ethernet - IEEE802.3 Frame Field Comparison

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Layer 2 Technologies
• Token Ring - logical ring topology
(in other words, information flow
is controlled in a ring) and a
physical star topology (in other
words, it is wired as a star)
• FDDI - logical ring topology
(information flow is controlled in a
ring) and physical dual ring
topology (wired as a dual ring)
• Ethernet - logical bus topology
(information flow is on a linear
bus) and physical star or
extended star (wired as a star)

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Media Access Control (MAC)
• MAC refers to Protocols that determine
which computer on a shared-medium
environment, or collision domain, is
allowed to transmit the data.
• MAC, with LLC, comprises the IEEE
version of the OSI Layer 2.
• There are two broad categories of Media
Access Control
1. Deterministic
taking turns
2. Non-deterministic
first come, first served).

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Ethernet
•Ethernet performs three functions:
• Transmitting and receiving data packets
• Decoding data packets and checking them for valid
MAC addresses before passing them to the upper
layers of the OSI model
• detecting errors within data packets or on the
network
In the CSMA/CD access method, networking devices with data
to transmit over the networking media work in a listen-before-
transmit mode.

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MAC Rules and Collision Detection/Backoff

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CSMA/CD
Non-Deterministic
1. Listen then transmit
1 2
3 4
2. Collision occurs
3. Broadcast jam signal
4. Devices back off
appropriate amount of
time, goes back to listen
then transmit and then
retransmit
Ethernet is a broadcast transmission medium. This means that all devices on a network can see all
data that passes along the networking media.
When a device has verified the destination MAC address carried by the data, it then checks the
data packet for transmission errors. If the device detects errors, the data packet is discarded.
The destination device will not notify the source device, regardless of whether the packet arrived
successfully.
Ethernet is a connectionless network architecture and is referred to as a best-effort delivery
system.

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Ethernet Timing

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Interframe Spacing and Backoff

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Ethernet Timing
• Slot time – the longest time taken to do a round
trip of the largest legal network
– 512 Bits = 64 Bytes
• Interframe spacing
– 96 bit times for 10, 100, 1000 Mbps networks

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Error Handling

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Types of Collisions
• What is a Runt frame ?

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Answer
• Any frame which is received and which is less
than 64 bytes is illegal, and is called a "runt".
(14 bytes + 46+ 4)
• A receiver must discard all runt frames

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Types of Collisions

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Ethernet Errors

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FCS Errors

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Transmission Errors
Collision or runt – Simultaneous transmission occurring
before slot time has elapsed
Late collision – Simultaneous transmission occurring after
slot time (512 bits = 64 Bytes)has elapsed
Jabber, long frame and range errors – Excessively or
illegally long transmission (jabber 20000-50000 octets)
Short frame, collision fragment or runt – Illegally short
transmission
FCS error – Corrupted transmission

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Ethernet Auto-Negotiation
• NLP – Network Link Pulses (10 Mbps)
Pulses sent by NIC every 16 ms
Convey information about the capabilities of the card or
device
• FLP (100Mbps-1000Mbps)
Mandatory
Made up of NLPs so 10 Mbps cards can exist with 100
and 1000 Mbps

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Transmission Priority Rank

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IEEE Ethernet Naming Rules

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Why was Ethernet Successful
• Ethernet has been the most successful
LAN technology largely because of its
simplicity of implementation compared to
other technologies.
• Ethernet has also been successful
because it has been a flexible technology
that has evolved to meet changing needs
and media capabilities

Cisco CCNA module 6

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