Ethernet

1
Ethernet

• Context
• History
• Bus-based (coax)
• Star-coupled (10BaseT)
• Switched

2
Ethernet context

• Layer 2 of protocol hierarchy
• Arbitration to determine sharing of physical
  medium
• Addressing within local area (not at network
  layer)
• Error detection (but not correction what do you
  think happens to any message received with an
  error?
• Identification of type of data being carried at
  network layer

3
Ethernet history
Invented 1973-76 at Xerox PARC - Robert Metcalfe
credited with the invention. Metcalfes task
was to figure out how to share a high-speed
printer among a number of microcomputer
workstations. Metcalfe and Boggs Ethernet
Distributed Packet-Switching For Local Computer
Networks. 1976 Metcalfe persuaded Xerox, Digital
and Intel to promote Ethernet as a standard.
Original Ethernet slightly different from current
IEEE 802.3 standard. One of the most successful
technologies in world of telecom.
4
Ethernet history
Bus-based, with terminators at each end to absorb
an electronic signal Each station puts its
message onto the bus (the ether) Message
propagates in both directions Eventually reaches
the station for which it is intended.
T
T
T
NIC
NIC
NIC
5
Bus-based Ethernets
6
Ethernets
Every NIC has a unique-in-the-Universe 48-bit MAC
(Media Access Control) address.
IEEE 802-controlled MAC addresses
24 bits specify vendor (OUI)
24 bits allocated by vendor
OUI Organizationally Unique Identifier
7
Ethernets
Every NIC has a unique-in-the-Universe 48-bit MAC
(Media Access Control) address.
IEEE 802-controlled MAC addresses
24 bits specify vendor (OUI)
24 bits allocated by vendor
OUI Organizationally Unique Identifier http//www
.iana.org/assignments/ethernet-numbers
8
Ethernets
Ethernet Frame
http//www.wildpackets.com/compendium/EN/EN-FrFSn.
html
See Figure 3-12 for comparison SAP Service
Access Point SNAP Subnetwork Access Protocol
9
Ethernets
Internet Assigned Numbers Authority (IANA)
http//www.wildpackets.com/compendium/EN/EN-FrFSn.
html
See Figure 3-12 for comparison SAP Service
Access Point SNAP Subnetwork Access Protocol (3
bytes of OUI 2 bytes of EtherType) http//www.net
worksorcery.com/enp/protocol/802/ethertypes.htm
10
Bus-based Ethernets
The cable propagates the signal in both
directions, so that the signal (eventually)
reaches all four of the computers. Termination
resistors at the ends of the cable absorb the
frame energy, preventing reflection of the signal
back along the cable.
http//www.erg.abdn.ac.uk/users/gorry/course/lan-p
ages/csma-cd.html
11
Bus-based Ethernets
All NICs receive the frame. Only the red NIC
recognizes the destination address and forwards
the contents of the frame to the computer's
network layer. The other computers ignore the
frame.
http//www.erg.abdn.ac.uk/users/gorry/course/lan-p
ages/csma-cd.html
What's an obvious problem?
12
Bus-based Ethernets
Stallings Data and Computer Communication 6th ed.
13
Bus-based Ethernets
Works extremely well as long as there is minimal
contention. Degrades with increased
contention Of the 10 Mbps, get about 4 Mbps
throughput maximum! Extremely difficult to
troubleshoot (why?)
14
Bus-based Ethernets
www.delmar.edu/Courses/ ITNW2313/throuput.jpg
15
Hub-based Ethernet 10BaseT (10Mbps, Baseband,
Twisted pair)
Think of the bus as shrinking into a piece of
hardware and the T connectors becoming RJ45
jacks. Now think of the connections to each
computer getting fairly long, and being Cat 5
twisted pair.
16
10BaseT
Do the NICs know the difference??
No, they are still running CSMA/CD
What is the role of the hub?
Are there still collisions?
17
10BaseT
Send on one pair of the twisted pair receive on
another.
10BaseT
HUB
Notice the Star topology - "Star-coupled" Ethernet
Notice the limit on stations.
Station
Station
Station
Station
Station
http//www.ertyu.org/steven_nikkel/ethernetcables
.html
18
10BaseT
Fa means "a frame from station A
Note the mistake in the label on the arrow from
HHUB to the left IHUB - it should be Fa
19
10BaseT
CP means a "Collision Presence" signal. Notice
they are issued by the hubs! But they get to
every station.
20
10BaseT
A, B and C all began to send when there was
silence on the medium. Now they are all aware of
the collision, and will back off a random number
of ms before trying again.
21
10BaseT
A, B and C all began to send when there was
silence on the medium. Now they are all aware of
the collision, and will back off a random number
of ms before trying again.
How many collision domains??
22
Switched Ethernet
Collision domains?
Collision domains?
23
Switched Ethernet
Transparent to stations/NICs Scales
easily Eliminates collisions - can reach 10Mbps
http//www.networking.ibm.com/mse/mse0c01.htmlSwi
tched Ethernet LANs
24
Switched Ethernet
25
Switched Ethernet
Once there are no collisions, a station can
simultaneously send and receive -gt Full Duplex
(FDX)
http//www.networking.ibm.com/mse/mse0c01.htmlSwi
tched Ethernet LANs
26
Switched Ethernet