Ethernet

1
Ethernet
Diagram drawn by Dr. Robert M. Metcalfe in 1976
to present Ethernet to the National Computer
Conference in June of that year.
2
Learning objectives

• After this lecture you should be able to
• Know what Ethernet is
• Understand why Ethernet is so popular
• Know how an Ethernet network operates

3
Ethernet

• Ethernet is a popular, vendor-neutral network
  technology
• There are several LAN technologies in use today,
  but Ethernet is by far the most popular.
• From the time of the first Ethernet standard, the
  specifications and the rights to build Ethernet
  technology have been made easily available to
  anyone.
• This openness,combined with the ease of use and
  robustness of the Ethernet system, resulted in a
  large Ethernet market and is another reason
  Ethernet is so widely implemented in the computer
  industry.

4
History

• Ethernet was developed at the Xerox Palo Alto
  Research Center in the 1970s by Dr. Robert M.
  Metcalfe, after earlier work on allocating the
  use of a shared channel at the University of
  Hawaii
• Called Ethernet after the luminiferous ether
  through which electromagnetic radiation was once
  thought to propagate.
• Formal specifications for Ethernet were published
  in 1980 by a multi-vendor consortium that created
  the DEC-Intel-Xerox (DIX) standard.
• The Institute of Electrical and Electronics
  Engineers (IEEE) standard IEEE 802.3 was first
  published in 1985
• Then adopted by the International Organization
  for Standardization (ISO), to make it a worldwide
  networking standard.
• IEEE 802.3 updated periodically to take account
  of changes in technology.

5
Elements in an Ethernet System

• The Ethernet system consists of three basic
  elements
• the physical medium used to carry Ethernet
  signals between computers,
• a set of medium access control rules embedded in
  each Ethernet interface that allow multiple
  computers to fairly arbitrate access to the
  shared Ethernet channel, and
• an Ethernet frame that consists of a standardized
  set of bits used to carry data over the system.

6
Physical Media

• Ethernet comes in different speeds
• 10 Mbps
• Fast Ethernet 100Mbps
• Gigabit Ethernet 1,000Mbs
• Four baseband media segments defined in the
  10-Mbps Ethernet standard (in order of
  definition)
• 10Base5
• 10Base2
• 10Base-T
• 10Base-F

7
10Base-T

• The most popular form of Ethernet
• The 10 refers to the speed in Megabits (millions
  of bits) that can be transmitted per second
• BASE stands for "baseband," meaning that Ethernet
  signals are the only signals carried over the
  media system.
• The T provides a rough indication of segment type
  or length
• T for Twisted Pair

8
Physical Media (cotd)

• 10Base5
• 5 because the maximum segment length for this
  cable is 500 metres
• Uses thick Coaxial cable
• BNC connection
• 10Base2
• 2 because the maximum segment length for this
  cable is about 200 metres
• Uses thin Coaxial cable
• 10Base-T
• Uses twisted pair copper cable
• RJ-45 Telephone type socket
• 10Base-F
• Uses fibre-optic strands

9
Wireless Ethernet (WiFi)

• 802.11 Standard
• Uses radio waves so that the signal can pass
  through walls etc.
• Original 802.11 operated at the frequency of
  2.4Ghz and had transfer speed of 2 Mbits per
  second
• 802.11b
• Increased speed to 11 Mbits per second
• Range of about 120 metres outdoors
• Ratified in 1999
• 802.11a
• Operates at 5Ghz
• Max speed of 54Mbits per second
• Shorter range of 30m outdoors
• 802.11g
• Operates at 2.4Ghz and 54Mbits per second, 50m
  range
• Backwards compatible with 802.11a

10
WiFi Equipment

• Need
• wireless network adaptors
• 2 adaptors can communicate with each other
• wireless access points
• Wireless hub connected to Ethernet network
• To share a broadband connection use a router
  connected to ADSL modem and a Wireless Access
  Point, or router with an built in modem. Some
  units come with a 4 port switch as well so you
  dont need an additional hub.
• The router means one computer doesnt need to be
  left on

11
Medium Access Control Rules

• Carrier Sense, Multiple Access with Collision
  Detection (CSMA/CD)
• Like polite conversation
• Wait for silence then try and send data
• If another node tries to send data at the same
  time there is a collision
• Retry sending after a random amount of time

12
The Ethernet Frame

• Used to deliver data between computers.
• The frame consists of a set of bits organized
  into several fields
• Address fields
• 2 48bit address fields
• The first is the destination address
• The second field is the source address
• The address of each Ethernet interface is
  commonly pre-assigned when it is manufactured
• a variable size data field
• carries from 46 to 1,500 bytes of data,
• error checking field
• checks the integrity of the bits in the frame to
  make sure that the frame has arrived intact.

13
Signal Topology

• The signal topology of the Ethernet is also known
  as the logical topology, to distinguish it from
  the actual physical layout of the media cables.
• The logical topology of an Ethernet provides a
  single channel (or bus) that carries Ethernet
  signals to all stations.
• Multiple Ethernet segments can be linked together
  to form a larger Ethernet LAN using a signal
  amplifying and retiming device called a repeater
• This may give the physical appearance of a star
  network topology but all the nodes will be
  connected to one bus receiving the same signal

14
Extending Ethernet Networks

• Hubs
• Devices with multiple Ethernet ports
• Repeater hub
• links individual Ethernet media segments together
  to create a larger network that operates as a
  single Ethernet LAN
• Switching hub
• provides packet switching
• each port of a packet switching hub provides a
  connection to an Ethernet media system that
  operates as a separate Ethernet LAN
• makes it possible to divide a set of Ethernet
  media systems into multiple LANs
• Greatly increases the potential number of nodes
  on the network