Ethernet
Network Fundamentals – Chapter 9
Sandra Coleman, CCNA, CCAI
Version 4.0
Objectives
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Identify the basic characteristics of network media used in Ethernet.
Describe the physical and data link features of Ethernet.
Describe the function and characteristics of the media access
control method used by Ethernet protocol.
Explain the importance of Layer 2 addressing used for data
transmission and determine how the different types of addressing
impacts network operation and performance.
Compare and contrast the application and benefits of using
Ethernet switches in a LAN as opposed to using hubs.
Explain the ARP process.
Characteristics of Network Media used in Ethernet
 Alohanet – 1970 – digital radio network to connect Hawaiin
islands.
 1st LAN – Robert Metcalf – Xerox – 30 yrs. Ago
DIX (Digital Equipment, Intel,
Xerox) – released stds. in 1980
IEEE 802.x – 1985
Ethernet operates at Layers 1
& 2 of OSI Model (Physical &
Data Link)
Ethernet operates at Network
Access layer of TCP/IP model
Physical and Data Link Features of Ethernet
 Layer 1 – involves signals, bit streams that travel on the
media, various topologies
 Layer 2 – MAC sublayer – concerned w/physical
components (802.3)
LLC sublayer – (802.2)
independent of physical
equip. Concerned with
upper layer transitions
Physical and Data Link Features of Ethernet
 Know the IEEE numbers for Layer 1 & 2 protocols
Physical and Data Link Features of Ethernet
 Logic Link Control – Connecting the Upper Layers
 IEEE 802.2 standard is represented here
Physical and Data Link Features of Ethernet
 Media Access Control (MAC)
Characteristics of Network Media used in Ethernet
 Success of Ethernet is because of: simplicity/ease of
maintenance, ability to adapt new technologies,
reliability, and lower cost to install and upgrade
Physical and Data Link Features of Ethernet
 2 most
common types
of media are
copper UTP
and optical
fiber.
Characteristics of Network Media used in Ethernet
 Migration from hubs to
switches to increase
throughput while
minimizing collisions
 Each port on a switch is a
collision domain
Layer 2 Addressing and Its Impact on Network Operation and
Performance
 DATA field contains the layer 3 (network layer) packet
Layer 2 Addressing (MAC)
 The Ethernet MAC Address
MAC Address specifications
Copied to RAM from ROM during POST (startup)
6 bytes (12 hex digits) long (3 bytes OUI, 3 bytes vendor
assigned)
Hexadecimal Numbering system
 Base 16 – 0-9, A-F..
Another layer of addressing
 MAC – used locally in the frame, layer 2 address
 IP – used to cross a WAN in a packet, layer 3 address
Addressing
 IP addresses NEVER change in a packet (layer 3)
 MAC addresses DO change within a frame (layer 2)
 If a device doesn’t know the MAC address for an IP
address, it will broadcast an ARP request for this
information.
Unicast – delivers a packet to ONE single destination host
Broadcast – delivers a packet to all hosts on a single broadcast
domain
Multicast – delivers a packet to a group of hosts
MAC in Ethernet
 All devices have guaranteed access to the medium, but no
prioritized claim on it. Explain CSMA/CD.
Carrier Sense Multiple Access w/Collision Detection
 Random delay (backoff algorithm) is used to try to help prevent
another collision. First expired timer gets to transmit data first.
 Once a collision has occurred, devices return to listen before
transmitting mode – THIS DECREASES network performance!
 Devices DO NOT interrupt others that are in the process of sending
data!
Ethernet Timing
 Every device that data travels (propagates) through
adds delay (latency) because of the error checking,
decision making involved.
 This delay will actually cause some devices to transmit
thinking there is NO signal on the wire, hence;
collisions!
Interframe spacing
Gives the media time to stabilize between frames. Allows
slow devices time to process a frame and prepare for the
next frame.
All devices are required to wait 96 microseconds before
transmitting again.
Overview of physical layer
Legacy Ethernet – Using Hubs
 Created LOTS of collisions
which will decrease
network performance
 Not scalable – just results
in larger collision
domains!
 Increased latency
Ethernet – Using Switches
 Each port is its on
collision domain;
full bandwidth
available to that
port – full
duplexing
capabilities –
virtually collision
FREE!
 Switch will isolate
segments and
limit collisions!
Address Resolution Protocol (ARP) Process
 Purpose: resolve IPv4 addresses to MAC addresses
 Maintained dynamically by monitoring traffic that occurs
on that segment of the network or by broadcasting an
ARP request packets with the desired IP address, looking
for the MAC address.
Receiving device
responds by
sending an ARP
reply back as a
unicast frame with
its MAC address.
It can then be
added to the ARP
table.
YEAH… Only 2 more chapters!!!
-Study Guide – Do this NOW and show me...
- Pg. 232-233 Matching
- Pg. 234-235 – Multiple Choice
- Pg. 238 – Matching
- Pg. 239-240 – Multiple Choice
-Discuss online Test – take it before next class, open
book. Will count as a test grade.
-Labs…None