ITCC 1302 Module 6

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ITCC 1302 Module 6

• Ethernet Fundamentals

Always bear in mind that your own resolution to
succeed is more important than any other one
thing. - Abraham Lincoln
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Module 6 Objectives

• At the end of this module you will be able to
• Describe the basics of Ethernet technology
• Explain Ethernet naming rules
• Define how Ethernet and the OSI model interact
• Describe the Ethernet framing process and
  structure
• List Ethernet frame field names and purposes
• Identify the characteristics of CSMA/CD
• Describe the key aspects of Ethernet timing,
  interframe spacing and backoff time after a
  collision
• Define Ethernet errors and collisions
• Explain auto-negotiation in relation to speed and
  duplex

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Introduction to Ethernet

• Why Ethernet?
• Simplicity ease of maintenance
• Scalability
• Reliability
• Low cost of installation and upgrade
• Gigabit Ethernet is extending out to
  metropolitan-area network (MAN) and wide-area
  network (WAN) standards
• Ethernet conceived by Robert Metcalfe and
  coworkers at the DIX Palo Alto Research Center
• Institute of Electrical and Electronics Engineers
  (IEEE) adopted it as a standard for LANs (802.3)
• 10-Mbps Ethernet adequate in 1980s legacy now
• Faster PCs, file sizes types demand more
  bandwidth
• IEEE 100 Mbps Fast Ethernet standard in 1995
• IEEE Gigabit Ethernet standards in 1998 and 1999

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IEEE Ethernet Naming Rules

• Ethernet family includes Legacy, Fast Ethernet,
  and Gigabit
• Basic frame format and IEEE sublayers of OSI
  Layers 1 and 2 consistent across all forms of
  Ethernet
• AS Ethernet expands to add new mediums or
  capability, IEEE issues a new supplement to the
  802.3
• Abbreviated description (identifier) is assigned
  to the supplement
• This abbreviated description consists of
• A number indicating number of Mbps transmitted
• The word base, indicating baseband signaling
• Alpha character indicating type of medium (F
  fiber optical cable, T copper unshielded
  twisted pair)

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Naming

• Must be an unique addressing system for local
  frame delivery
• 48 bit MAC addresses, expressed as 12 hex digits
• First six hexadecimal digits identify the vendor,
  AKA Organizational Unique Identifier (OUI)
• Last six hexadecimal digits represent the
  interface serial number, or another value
  administered by the specific equipment
  manufacturer
• MAC addresses are sometimes referred to as
  burned-in addresses (BIA) and are copied into RAM
  when the NIC initializes
• Layer 2 MAC headers and trailers are added to
  upper layer data
• Upper layer data is encapsulated by layer 2
  header/trailer
• NIC uses the MAC address to assess whether the
  message should be passed onto the upper layers of
  the OSI model

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Layer 2 Framing

• Framing is Layer 2 encapsulation
• Frame is the Layer 2 protocol data unit (PDU)
• Single generic frame has sections called fields,
  with each field composed of octets
• Field Names
• Start frame
• Address
• Length / type
• Data
• Frame check sequence  
• Frames contain naming information, such as the
  name of the source node and destination node (MAC
  addresses)
• The reason for sending frames is to get upper
  layer data, ultimately the user application data,
  from the source to the destination

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Ethernet Frame Structure

• Data link layer frame structure is nearly
  identical for all speeds
• Physical layer Ethernet versions of are different
  with each speed having a distinct set of
  architecture design rules
• Original DIX Ethernet had the Preamble and Start
  Frame Delimiter (SFD) combined into a single
  field
• Length/Type field was only listed as Length in
  the early IEEE versions and only as Type in the
  DIX version
• Ethernet II Type field is incorporated into
  current 802.3 frame
• Receiving node determines which higher-layer
  protocol is present in an incoming frame by
  examining the Length/Type field
• If two-octet value is equal to or greater than
  0x600 the frame is interpreted according to the
  Ethernet II type code

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Ethernet Frame Fields

• 802.3 Ethernet Frame Fields are
• Preamble - alternating pattern of 1s and 0s used
  for timing synchronization in the asynchronous 10
  Mbps and slower implementations of Ethernet.
  Faster versions of Ethernet are synchronous,
  while redundant is retained for compatibility.
• Start Frame Delimiter - consists of a one-octet
  field, marks the end of timing information.
• Destination Address - MAC destination address,
  can be unicast, multicast (group), or broadcast
  (all nodes).
• Source Address - MAC source address, generally
  unicast address of transmitting node
• Length/Type - supports two different uses. If
  the value is less than 1536 decimal, 0x600
  (hexadecimal), then the value indicates length.
  If the value is equal to or greater than 1536
  decimal (0600 hexadecimal), the value indicates
  that the type and contents of the Data field are
  decoded per the protocol indicated
• Data and Pad - may be of any length that does not
  cause the frame to exceed the maximum frame size.
  The maximum transmission unit (MTU) for Ethernet
  is 1500 octets
• FCS - contains a four byte CRC value that is
  created by the sending device and is recalculated
  by the receiving device to check for damaged
  frames

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Ethernet Frame Fields
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Media Access Control

• MAC protocols determine which computer is allowed
  to transmit data
• Two broad categories of Media Access Control,
  deterministic (taking turns) and
  non-deterministic (first come, first served).
• Deterministic protocols include Token Ring and
  FDDI
• Non-deterministic protocols use a first-come,
  first-served approach such as Ethernet
• Three common Layer 2 technologies are Token Ring,
  FDDI, and Ethernet

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MAC Rules and Collision Detection / Backoff

• Ethernet is a shared-media broadcast technology
• Access method CSMA/CD uses performs three
  functions
• Transmitting/receiving data packets (from layer
  3)
• Decoding data frames checking them for valid
  addresses
• Detecting errors within data frames or on the
  network
• With CSMA/CD, devices with data to transmit must
  listen-before-transmit
• Networking devices detect a collision when the
  amplitude of the signal on the networking media
  increases
• When a collision occurs, each transmitting node
  will continue to transmit for 56 timing bits to
  ensure that all devices see the collision
• Once all devices have detected the collision a
  backoff algorithm is invoked and transmissions
  stop
• When the delay period expires, devices attempt
  access

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MAC Rules and Collision Detection / Backoff
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Interframe Spacing and Backoff

• Minimum spacing between two non-colliding frames
  is called the interframe spacing
• Measured from last bit of the FCS field of the
  first frame to the first bit of the preamble of
  the second frame
• After frame is sent, stations on a 10-Mbps
  Ethernet wait a minimum of 96 bit-times (9.6
  microseconds) before they may legally transmit
  the next frame
• Faster Ethernet spacing remains 96 bit-times,
  time interval is shorter
• After a collision, stations allow media to become
  idle (full interframe spacing), then stations
  that collided must wait an additional and
  potentially progressively longer period of time
  before attempting to retransmit the collided
  frame
• Waiting period designed to be random so two
  stations do not have same time out period
• If MAC layer is unable to send the frame after
  sixteen attempts, it gives up and generates an
  error to the network layer

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Types of Collisions

• Local - collision is detected on the local
  segment only when a station detects a signal on
  the RX pair at the same time it is sending on the
  TX pair
• Remote With a remote collision, a frame that is
  less than the minimum length, has an invalid FCS
  checksum, but does not exhibit the local
  collision symptom of over-voltage or simultaneous
  RX/TX activity
• Late - Collisions occurring after the first 64
  octets are called late collisions". The most
  significant difference between late collisions
  and collisions occurring before the first 64
  octets is that the Ethernet NIC will retransmit a
  normally collided frame automatically, but will
  not automatically retransmit a frame that was
  collided late

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Ethernet Errors

• Local and remote collisions are normal operation,
  late collisions are an error
• Jabber is defined in the 802.3 standard as a
  transmission of at least 20,000 to 50,000 bit
  times. Jabber is called a long frame
• A long frame is longer than the maximum legal
  size and takes into consideration whether or not
  the frame was tagged. This error usually means
  that jabber was detected
• A short frame is a frame smaller than the minimum
  legal size of 64 octets, with a good frame check
  sequence. Some protocol analyzers and network
  monitors call these frames runts".
• Runt may refer to short frames with a valid FCS
  checksum although it usually refers to collision
  fragments

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Ethernet Auto-Negotiation

• As Ethernet grew from 10 to 100 to 1000 Mbps, it
  had to be interoperable
• Auto-Negotiation of speeds at half or full duplex
  was developed
• Auto-Negotiation defines how two link partners
  may automatically negotiate a configuration
  offering the best common performance
• Auto-Negotiation is accomplished by transmitting
  a burst of 10BASE-T Link Pulses from each of the
  two link partners
• The burst communicates the capabilities of the
  transmitting station to its link partner
• After both stations have interpreted what the
  other partner is offering, both switch to the
  highest performance common configuration and
  establish a link at that speed
• If anything interrupts communications and the
  link is lost, the two link partners first attempt
  to link again at the last negotiated speed
• If that fails, or if it has been too long since
  the link was lost, the Auto-Negotiation process
  starts over

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Module 6 Summary

• You should now be able to
• Describe the basics of Ethernet technology
• Explain naming rules of Ethernet technology
• Define how Ethernet and the OSI model interact
• Describe the Ethernet framing process and frame
  structure
• List Ethernet frame field names and purposes.
• Identify the characteristics of CSMA/CD
• Describe the key aspects of Ethernet timing,
  interframe spacing and backoff time after a
  collision
• Define Ethernet errors and collisions
• Explain the concept of auto-negotiation in
  relation to speed and duplex

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Any questions???