Ethernet

1. Ethernet
Network Fundamentals – Chapter 9
ITE I Chapter 6
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2. Objectives

Identify Ethernet

Identify the function and characteristics of the media
access control method used by Ethernet protocol.

Identify different types of layer 2 addressing and
impacts network operation and performance.

Identify the application and benefits of using Ethernet
switches in a LAN as opposed to using hubs.

Identify basic switch operations

Identify the ARP process.
ITE 1 Chapter 6
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3. What is Ethernet?
 A family of LAN products described by the IEEE standards
 Developed in the early 1980s and first published by the consortium
Digital equipment, Intel and Xerox (DIX) as an open standard
 To maintain compatibility with the ISO the IEEE publish the 802.3
standard in 1985
 Many technologies have challenged Ethernets dominance.
However Ethernet has continued to be the dominate LAN product
ITE 1 Chapter 6
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4. Physical and Data Link Features of Ethernet
 Standards and Implementation for Ethernet include
802.2 and 802.3 both defined by the IEEE
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5. Physical and Data Link Features of Ethernet
 Ethernet operates across two layers of the OSI model
ITE 1 Chapter 6
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6. Physical and Data Link Features of Ethernet
 Logic Link Control – Connecting the Upper Layers
ITE 1 Chapter 6
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7. Physical and Data Link Features of Ethernet
 Media Access Control (MAC)
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8. Ethernet – media changes
 Ethernet went from Bus type Coaxial based cable to
Star based UTP/STP cables
Coaxial
Thicknet, (10Base5)
Thinet(10Base2)
CSMA then CSMA/CD
Low Bandwidth
Difficult to scale
UTP (10Base-T)
UTP (100Base-TX)
Fiber(100Base –FX)
CSMA/CD
Higher Bandwidth
Scalable
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9. Ethernet Hubs versus Switches in a LAN.
 Legacy Ethernet – Using Hubs suffered multiple
collisions Hubs not intelligent devices, Bandwidth is
shared, not dedicated per host
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10. Ethernet – media changes
 The movement of Hub based LANs to switched based
Ethernet LANs reduced the size of collision domains
and improved network availability
Multi-port Bridge
Intelligent deviceforwards frames based
on MAC address
Reduce the size of
collision domains
Increased Bandwidth
Communication channel
inside switch is
dedicated
Can support Virtual
LANs
ITE 1 Chapter 6
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11. Ethernet Switches versus Hubs in a LAN.
 Ethernet – Using Switches
ITE 1 Chapter 6
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12. Ethernet Switches versus Hubs in a LAN.
 A switch can eliminate collisions, backoffs and retransmissions, the leading factors in reduced
throughput on a hub-based Ethernet network
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13. Switch operations
 Switches forward frames based on their destination MAC
addresses. To accomplish their purpose switches use:
 Learning (Populate MAC address table with MAC addresses of
connected hosts)
 Aging (Removes old MAC addresses from table, 5 mins)
 Flooding (If a destination entry is not found in the MAC address
table that matches the destination in the received frame, the switch
floods the frame out all its ports except the port on which the frame
entered the switch)
 Selective forwarding (Forwarding a frame based on its destination
MAC address)
 Filtering (Un-forwarded frames based on failed CRC and Port
security)
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14. Physical and Data Link Features of Ethernet
 Physical Implementations of the Ethernet
ITE 1 Chapter 6
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15. Ethernet – media changes
 High bandwidth Ethernet cabling is used to connect
MANS and WAN
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16. Ethernet Media Access Control Method
 CSMA/CD
ITE 1 Chapter 6
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17. Carrier Sense Multiple Access with Collision
Detection
 Listen before sending – if a device has data to send it
must listen for the carrier before sending, if the carrier
is detected it must wait a specified time before
transmitting
 Detecting a collision - If two devices or more fail to
detect the carrier they will transmit at the same time.
Their data will propagate across the media until a
collision occurs, destroying the data. Any device
detecting a collision will then transmit a Jam signal
 Jam and Random Backoff - Other devices detect the
Jam signal and invoke a back off signal for a random
amount of time allowing the original collision to subside
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18. Ethernet Delay (Latency, Bit time, Slot time)
 Ethernet data bits don’t move in zero time
 Data passing through devices experience delay, can result in
collision because the receiving device transmits because it doesn’t
see the frame
 Bit Time = time for 1 bit to be sensed and placed on the media
 Slot time determines Min size of Frame and Max media length
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19. Ethernet and IEEE frame fields
 The IEEE included the extra field (Start of Frame) to allow for extra
frame size to accommodate the frames used in switching
technology i.e. frames that carry a VLAN field
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20. Layer 2 addressing is used to identify hosts
 The Ethernet MAC Address
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21. Layer 2 addressing uses Hexadecimal
 MAC addresses use Hexadecimal Numbering
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22. Layer 2 MAC addressing
 MAC addressing is used in LANs
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23. MAC addresses 48 bits or 12 Hex digits
 Ethernet Unicast, Multicast and Broadcast
 First 6 hex digits OUI + 6 hex digits serial number
Organizational Unique
Identifier (OUI)
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24. MAC Address communication types
 Unicast – A unique frame sent form a single source to a
single destination
 Multicast – sent from a single host to a group (or set) of
host a LAN segment. Multicast MAC addresses begin
with 01-00-5E (Consumes Bandwidth)
 Broadcast – sent from a single host to all hosts on the
local segment or LAN on Ethernet networks the
broadcast address is FF-FF-FF-FF-FF-FF in
hexadecimal or 48 1’s in binary (Consumes more
Bandwidth)
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25. Address Resolution Protocol (ARP) process.
 Mapping IP to MAC Addresses required to build all
frames before frame is converted to bits and placed
onto media
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26. Address Resolution Protocol (ARP) process.
 ARP – Destinations Outside the Local Network
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27. Address Resolution Protocol (ARP) process.
 ARP – Removing Address Mappings
 When hosts are no longer available mappings have to
aged out
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28. Address Resolution Protocol (ARP) process.
 ARP Broadcasts are issued if mapping is not in table
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29. ARP - process
 ARP table maps Destination IP to MAC addresses
 Can be populated Dynamically by sensing devices on
network or via ARP request (Broadcasts)
 Can be populated Statically not recommended as
entries don’t age out
 To build a Frame the table is first consulted if an entry
is not found the source issues an ARP broadcast.
 If the destination IP is on a different network an ARP
request is sent to determine the MAC address of the
gateway
ITE 1 Chapter 6
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30. Labs
 9.8.1- Address Resolution Protocol- ARP (ARP
command using Wireshark to examine ARP
exchanges)
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