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Ethernet Fundamentals
Introduction to Ethernet
The success of Ethernet is due to
the following factors:
•Simplicity and ease of
•Ability to incorporate new
•Low cost of installation and
•Bandwidth can be increased
without changing underlying
Essentially, Ethernet and IEEE 802.3 are the
same standards.
The original idea for Ethernet grew out of the problem of allowing
two or more hosts to use the same medium and prevent the
signals from interfering with each other.
This problem of multiple user access to a shared medium was
studied in the early 1970s at the University of Hawaii
A drawing of the first Ethernet system by Bob Metcalfe of Xerox
IEEE Ethernet naming rules
In BASE band signaling, the data signal is transmitted directly over
the transmission medium.
In BROADband signaling, not used by Ethernet, a carrier signal is
modulated by the data signal and the modulated carrier signal is
Layer 1 vs. Layer 2
Layer 2 deals with the limitations of Layer 1
• Layer 1
– Cannot communicate with
the upper-layer protocols
– Cannot name or identify
– Can describe only streams of
– Cannot decide which
computer will transmit data
from a group in which all
are trying to transmit at the
same time
• Layer 2
– Communicates with
upper-layer protocols
using logical link control
– Provides an addressing (or
naming) process
– Uses framing to organize
or group the bits
– Uses a system called
Media Access Control
(MAC) to control
IEEE Standard
• Divided OSI Layer 2 into two sublayers
– Media Access Control (MAC) – traditional L2 features
• Transitions down to media
– Logical link control (LLC) – new L2 features
• Transitions up to the network layer
Comparing LAN Standards
OSI Layer 1 and 2 Together Are the
Access Protocols
• These are the delivery
system protocols
• Independent of:
– Network OS
– Upper-level protocols
• Sometimes called:
Ethernet, Fast Ethernet, Gigabit
Ethernet, Token Ring, FDDI, Frame
Relay, ATM, PPP, and so on
Access methods
Access protocols
Access technologies
Media access
LAN protocols
WAN protocols
Logical Link Control (LLC)
• LLC allows part of the data link layer to
function independent of LAN access
technologies (protocols / methods)
– Provides services to network layer protocols, while
communicating with access technologies below it.
• LAN access technologies:
– Ethernet
– Token Ring
Logical Link Control (LLC)
• Participates in the data encapsulation process.
– LLC PDU between Layer 3 and the MAC sublayer
– Adds control information to the network layer data
to help deliver the packet. It adds two fields:
• Destination Service Access Point (DSAP)
• Source Service Access Point (SSAP)
• Supports both connectionless and connectionoriented upper-layer protocols
• Allows multiple higher layer protocols to share
a single physical data link
Media Access Control (MAC)
• Provides MAC Addressing (naming)
• Depending on access Technology
(Ethernet, Token Ring, FDDI), Provides:
Data transmission control
Collision resolution (retransmission)
Layer 2 frame preparation (data framing)
Frame Check Sequence (FCS – frame error detection)
Media Access Control (MAC)
• Ethernet (IEEE 802.3)
– Logical bus topology
– Physical star or extended star
– Nondeterministic
• First-come, first-served
• Token Ring (IEEE 802.5)
– Logical ring
– Physical star topology
– Deterministic
• Token controls traffic
– Older declining technology
• FDDI (IEEE 802.5)
– Logical ring topology
– Physical dual-ring topology
– Deterministic
• Token controls traffic
Ethernet and the OSI model:
The Media Access Control (MAC) sublayer is
concerned with the physical components that
will be used to communicate the information.
The Logical Link Control (LLC) sublayer
remains relatively independent of the
physical equipment that will be used for the
communication process to upper layers.
48 bits or 12 Hex
•The NIC uses the MAC address to assess whether the message should
be passed onto the upper layers of the OSI model.
•The NIC makes this assessment without using CPU processing time.
•The MAC address is burned into ROM on the NIC card
How the NIC Uses MAC
Legacy (Broadcast) Ethernet:
– Source host builds a Layer 2 data frame.
• Its own MAC is the source address.
• The MAC of the target is the destination address.
– All devices on the segment see the frame.
– Only the target’s NIC recognizes its MAC address
in the Destination Address field.
– Target host copies and processes the frame.
– Non-target hosts dispose of or ignore of the frame.
Layer 2 Address Encapsulation and
Source and
Destination MAC
address fields are part
of the frame header.
The following slides address the following topics:
• Why framing is necessary
• Frame format diagram
• Generic frame format
Why Framing Is Necessary
• Binary data is a stream of 1s and 0s.
• Framing breaks the stream into decipherable groupings:
Start and stop indicator fields
Naming or addressing fields
Data fields
Quality-control fields (FCS or CRC)
• Framing is the Layer 2 encapsulation process.
• A frame is the Layer 2 protocol data unit (PDU).
Generic Frame Format
Start Frame Field (multiple bytes in size – preamble + SFD)
Address Fields (Source & Destination MAC addresses)
Type / Length Field (depending on Ethernet standard)
Data Field (46-1500 bytes)
FCS (Frame Check Sequence) Field
Frame Stop Field (unique value)
Media Access Control (MAC):
(1st come 1st served)
Collisions occur and
slow transmission
(taking turns)
No Collisions
MAC rules and collision detection/backoff
Networking devices detect
a collision has occurred
when the amplitude of the
signal on the networking
media increases.
The devices that were
involved in the collision do
not have priority to transmit
(JAM) When a collision
occurs, each node that is
transmitting will continue
to transmit for a short
time to ensure that all
devices see the collision.
(transmit 32-bit jam signal)
MAC rules and collision detection/backoff:
Host wants to transmit
Is carrier sensed?
Assemble Frame
Start Transmitting
Is a Collision detected?
Keep Transmitting
Is the transmission done?
Transmission complete
Broadcast jam signal
10. Attempts = Attempts + 1
11. Attempts > Too many?
12. Too many collisions; abort
13. Algorithm calculates backoff
14. Wait for t microseconds
Chapter #6 Test Next Week
• This Class:
– Chapter 5 Labs due!
– Finish Labs:
• Chapter 5  no Labs for Chapter 6!
– Wall Outlet – Layout & cut
– Cable, Panel installation next week

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