Module 6: Ethernet Fundamentals

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Module 6Ethernet Fundamentals

• James Chen
• ydjames_at_ydu.edu.tw

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Outline

• Ethernet Fundamentals
• Introduction to Ethernet
• IEEE Ethernet naming rules
• Ethernet and the OSI model
• Naming
• Layer 2 framing, Ethernet frame fields
• Ethernet frame structure
• Ethernet Operation
• Media Access Control (MAC)
• MAC rules and collision detection/backoff
• Ethernet timing
• Interframe spacing and backoff
• Error handling
• Types of collisions
• Ethernet errors
• FCS and beyond
• Ethernet auto-negotiation
• Link establishment and full and half duplex

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• 6.1 Ethernet Fundamentals

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Introduction to Ethernet

• The success of Ethernet is due to the following
  factors
• Simplicity and ease of maintenance
• Ability to incorporate (??)new technologies
• Reliability
• Low cost of installation and upgrade
• The original idea for Ethernet grew out of the
  problem of allowing two or more hosts to use the
  same medium and prevent the signals from
  interfering with each other.
• This problem of multiple user access to a shared
  medium was studied in the early 1970s at the
  University of Hawaii.
• A system called Alohanet was developed to allow
  various stations on the Hawaiian Islands
  structured access to the shared radio frequency
  band in the atmosphere.
• This work later formed the basis for the Ethernet
  access method known as CSMA/CD.
• The first Ethernet standard was published in 1980
  by a consortium of Digital Equipment Company,
  Intel, and Xerox (DIX).
• In 1995, IEEE announced a standard for a 100-Mbps
  Ethernet.
• In 1999, 1Gbps Ethernet.
• Now, 10Gbps Ethernet

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IEEE Ethernet naming rules

• When Ethernet needs to be expanded to add a new
  medium or capability, the IEEE issues a new
  supplement to the 802.3 standard.
• Naming Rule
• Speed 10, 100, 1G, 10G
• Signal method BASE, BROAD
• Medium 2, 5, T, TX, FX, CX, SX, LX, ZX

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IEEE Ethernet naming rules (cont.)
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Ethernet and the OSI model

• Ethernet operates in two areas of the OSI model
• the lower half of the data link layer (MAC
  sublayer )
• physical layer.

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Ethernet and the OSI model (cont.)

• Repeater
• A repeater is responsible for forwarding all
  traffic to all other ports.
• Traffic received by a repeater is never sent out
  the originating port.
• If the signal is degraded through attenuation or
  noise, the repeater will attempt to reconstruct
  and regenerate the signal.
• A collision domain is then a shared resource.
• Problems originating in one part of the collision
  domain will usually impact the entire collision
  domain.
• Stations separated by repeaters are within the
  same collision domain.
• Stations separated by bridges or routers are in
  different collision domains.

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Ethernet and the OSI model (cont.)

• Ethernet Layer 1 involves interfacing with media,
  signals, bit streams that travel on the media,
  components that put signals on media, and various
  topologies.
• Ethernet Layer 1 performs a key role in the
  communication that takes place between devices,
  but each of its functions has limitations.
• Layer 2 addresses these limitations.

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Ethernet and the OSI model (cont.)

• Data link sublayers contribute significantly to
  technology compatibility and computer
  communication.
• The MAC sublayer is concerned with the physical
  components that will be used to communicate the
  information.
• The Logical Link Control (LLC) sublayer remains
  relatively independent of the physical equipment
  that will be used for the communication process.

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Naming

• To allow for local delivery of frames on the
  Ethernet, there must be an addressing system, a
  way of uniquely identifying computers and
  interfaces.
• Ethernet uses MAC addresses that are 48 bits in
  length and expressed as twelve hexadecimal digits
  (6 Bytes).
• MAC addresses are sometimes referred to as
  burned-in addresses (BIA) because they are burned
  into read-only memory (ROM) and are copied into
  random-access memory (RAM) when the NIC
  initializes.

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Naming (cont.)

• At the data link layer MAC headers and trailers
  are added to upper layer data.
• The header and trailer contain control
  information intended (???)for the data link layer
  in the destination system.
• The NIC uses the MAC address to assess(??)whether
  the message should be passed onto the upper
  layers of the OSI model.
• The NIC makes this assessment without using CPU
  processing time, enabling better communication
  times on an Ethernet network.
• On an Ethernet network, when one device sends
  data it can open a communication pathway to the
  other device by using the destination MAC
  address.
• All devices that are connected to the Ethernet
  LAN have MAC addressed interfaces including
  workstations, printers, routers, and switches.

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Layer 2 framing

• Framing is the Layer 2 encapsulation process.
• Voltage vs. time graph (large and confusing)
• Frame format diagram (fields groupings of bits)
• Preamble field
• The pattern of ones and zeroes used for timing
  synchronization in 10 Mbps Ethernet
  (Asynchronous).
• The timing information is redundant but retained
  for compatibility in faster versions of Ethernet
  (Synchronous).
• Start Frame Delimiter field
• It consists of a one-octet field that marks the
  end of the timing information, and contains the
  bit sequence 10101011.
• Destination field
• Destination node MAC address (unicast, multicast
  (group), broadcast (all nodes))
• Source field
• Source node MAC address (unicast address, virtual
  entity)
• Length field / type field
• length of a frame in bytes (it implies the end)
• some frames have a type field, which specifies
  the Layer 3 protocol making the sending request.

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Layer 2 framing (cont.)

• Data field
• upper layer data
• user application data
• LLC bytes are also included with the data field
  in the IEEE standard frames.
• It adds control information to help deliver that
  IP packet to the destination node.
• Layer 2 communicates with the upper-level layers
  through LLC.
• padding bytes (minimum length for timing
  purposes)
• FCS field (Frame Check Sequence number)
• End of the frame.
• It contains a number that is calculated by the
  source node based on the data in the frame.
• When the destination node receives the frame the
  FCS number is recalculated and compared with the
  FCS number included in the frame.
• If the two numbers are different, an error is
  assumed, the frame is discarded, and the source
  is asked to retransmit.
• The ways to calculate the FCS number
• Cyclic Redundancy Check (CRC) performs
  calculations on the data.
• Two-dimensional parity adds an 8th bit that
  makes an 8 bit sequence have an odd or even
  number of binary 1s.
• Internet checksum adds the values of all of the
  data bits to arrive at a sum.

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Layer 2 framing (cont.)
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Ethernet frame structure
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Ethernet frame structure (cont.)
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Ethernet frame structure (cont.)

• The Ethernet II Type field is incorporated into
  the current 802.3 frame definition.
• If the value of Length/Type field is equal to or
  greater than 1536 or 0x600 (hexadecimal), then
  the frame is interpreted according to the
  Ethernet II type code indicated.
• The maximum transmission unit (MTU) for Ethernet
  is 1500 octets
• minimum-sized frame 64B (preamble is not
  included)
• Ethernet requires each frame to be between 64 and
  1518 octets

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• Ethernet Operation

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Media Access Control (MAC)

• Categories of Media Access Control
• Deterministic(????)(taking turns)
• In a Token Ring network, individual hosts are
  arranged in a ring and a special data token
  travels around the ring to each host in sequence.
  When a host wants to transmit, it seizes the
  token, transmits the data for a limited time, and
  then forwards the token to the next host in the
  ring. Token Ring is a collisionless environment
  as only one host is able to transmit at any given
  time.
• Non-deterministic (first come, first served)
• In a CSMA/CD system, the NIC listens for an
  absence of a signal on the media and starts
  transmitting. If two nodes transmit at the same
  time a collision occurs and none of the nodes are
  able to transmit.
• Ethernet logical bus topology, physical star or
  extended star, wired as a star
• Token Ring logical ring topology, physical star
  topology, wired as a star
• FDDI logical ring topology, physical dual-ring
  topology, wired as a dual-ring

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MAC rules and collision detection/backoff

• In the CSMA/CD access method, networking devices
  with data to transmit work in a
  listen-before-transmit mode. This means when a
  node wants to send data, it must first check to
  see whether the networking media is busy. If the
  node determines the network is busy, the node
  will wait a random amount of time before
  retrying. If the node determines the networking
  media is not busy, the node will begin
  transmitting and listening. The node listens to
  ensure no other stations are transmitting at the
  same time. After completing data transmission the
  device will return to listening mode.
• Networking devices detect a collision has
  occurred when the amplitude of the signal on the
  networking media increases. When a collision
  occurs, each node that is transmitting will
  continue to transmit for a short time to ensure
  that all devices see the collision. Once all the
  devices have detected the collision a backoff
  algorithm is invoked and transmission is stopped.
  The nodes stop transmitting for a random period
  of time, which is different for each device. When
  the delay period expires, all devices on the
  network can attempt to gain access to the
  networking media. When data transmission resumes
  (??)on the network, the devices that were
  involved in the collision do not have priority to
  transmit data.

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MAC rules and collision detection/backoff (cont.)
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MAC rules and collision detection/backoff (cont.)
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Ethernet timing

• In half duplex, assuming that a collision does
  not occur, the sending station will transmit 64
  bits of timing synchronization information that
  is known as the preamble. The sending station
  will then transmit the following information
• Destination and source MAC addressing information
  
• Certain other header information (Length / Type)
• The actual data payload (Data)
• Checksum (FCS)
• 10 Mbps and slower versions of Ethernet are
  asynchronous. Asynchronous means that each
  receiving station will use the eight octets of
  timing information to synchronize the receive
  circuit to the incoming data, and then discard
  it.
• If the attached station is operating in full
  duplex then the station may send and receive
  simultaneously and collisions should not occur.
• Full-duplex operation also changes the timing
  considerations and eliminates the concept of slot
  time.
• Full-duplex operation allows for larger network
  architecture designs since the timing restriction
  for collision detection is removed.
• 100 Mbps and higher speed implementations of
  Ethernet are synchronous. Synchronous means the
  timing information is not required, however for
  compatibility reasons the Preamble and SFD are
  present.

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Ethernet timing (cont.)

• For all speeds of Ethernet transmission at or
  below 1000 Mbps, the standard describes how a
  transmission may be no smaller than the slot
  time.
• Slot time for 10 and 100-Mbps Ethernet is 512
  bit-times, or 64 octets.
• Slot time for 1000-Mbps Ethernet is 4096
  bit-times, or 512 octets.
• Slot time is calculated assuming maximum cable
  lengths on the largest legal network
  architecture. All hardware propagation delay
  times are at the legal maximum and the 32-bit jam
  signal is used when collisions are detected.
• The actual calculated slot time is just longer
  than the theoretical amount of time required to
  travel between the furthest points of the
  collision domain, collide with another
  transmission at the last possible instant, and
  then have the collision fragments return to the
  sending station and be detected. For the system
  to work the first station must learn about the
  collision before it finishes sending the smallest
  legal frame size.
• To allow 1000-Mbps Ethernet to operate in half
  duplex the extension field was added when sending
  small frames purely to keep the transmitter busy
  long enough for a collision fragment to return.
  This field is present only on 1000-Mbps,
  half-duplex links and allows minimum-sized frames
  to be long enough to meet slot time requirements.
  Extension bits are discarded by the receiving
  station.
• On 10-Mbps Ethernet one bit at the MAC layer
  requires 100 ns to transmit. At 100 Mbps that
  same bit requires 10 ns to transmit and at 1000
  Mbps only takes 1 ns.

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Ethernet timing (cont.)

• As a rough estimate, 20.3 cm (8 inch) per
  nanosecond is often used for calculating
  propagation delay down a UTP cable. For 100
  meters of UTP, this means that it takes just
  under 5 bit-times for a 10BASE-T signal to travel
  the length the cable.
• 100 m / 20.3 cm per ns 492.6 ns
• 492.6 ns / 100 ns per bit 5 bits
• For CSMA/CD Ethernet to operate, the sending
  station must become aware of a collision before
  it has completed transmission of a minimum-sized
  frame. At 100 Mbps the system timing is barely
  (??)able to accommodate 100 meter cables. At 1000
  Mbps special adjustments are required as nearly
  an entire minimum-sized frame would be
  transmitted before the first bit reached the end
  of the first 100 meters of UTP cable. For this
  reason half duplex is not permitted in 10-Gigabit
  Ethernet.

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Interframe spacing and backoff

• The minimum spacing between two non-colliding
  frames is also called the interframe spacing.
• This is measured from the last bit of the FCS
  field of the first frame to the first bit of the
  preamble of the second frame.
• Ethernet devices must allow a minimum idle period
  between transmission of frames known as the
  spacing gap. It provides a brief recovery time
  between frames to allow devices to prepare for
  reception of the next frame.
• The minimum spacing gap is 96 bit times, which is
  9.6 microseconds for 10 Mb/s Ethernet, 960
  nanoseconds for 100 Mb/s Ethernet, and 96
  nanoseconds for 1 Gb/s Ethernet.
• http//www.usyd.edu.au/is/comms/networkcourse/USyd
  Net_mod3_ethernet.html

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Interframe spacing and backoff (cont.)

• After a collision occurs and all stations allow
  the cable to become idle (each waits the full
  interframe spacing)
• The stations that collided must wait an
  additional longer period of time before
  attempting to retransmit the collided frame.
• The waiting period is intentionally designed to
  be random so that two stations do not delay for
  the same amount of time before retransmitting,
  which would result in more collisions.
• If the MAC layer is unable to send the frame
  after sixteen attempts, it gives up and generates
  an error to the network layer. Such an occurrence
  is fairly rare and would happen only under
  extremely heavy network loads, or when a physical
  problem exists on the network.
• The slot time is a key parameter for half-duplex
  Ethernet network operation. It is defined as 512
  bit times for Ethernet networks operating at 10
  and 100 Mb/s, and 4096 bit times for gigabit
  Ethernet.
• Backoff is the process by which a transmitting
  interface determines how long to wait following a
  collision before attempting to retransmit the
  frame.

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Error handling

• The most common error condition on an Ethernet is
  the collision.
• Collisions are the mechanism for resolving
  contention for network access.
• Collisions result in network bandwidth loss that
  is equal to the initial transmission and the
  collision jam signal (32 bits), all stations have
  a chance to detect the collision.
• Jam signal is simply a repeating one, zero, one,
  zero pattern, the same as Preamble. When viewed
  by a protocol analyzer this pattern appears as
  either a repeating hexadecimal 5 or A sequence.

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Error handling (cont.)
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Types of collisions

• To create a local collision on coax cable
  (10BASE2 and 10BASE5), the signal travels down
  the cable until it encounters (??)a signal from
  the other station.
• The waveforms then overlap, canceling some parts
  of the signal out and reinforcing or doubling
  other parts.
• The doubling of the signal pushes the voltage
  level of the signal beyond the allowed maximum.
  This over-voltage condition is then sensed by all
  of the stations on the local cable segment as a
  collision.
• In the beginning the waveform in Figure
  represents normal Manchester encoded data.
• A few cycles into the sample the amplitude of the
  wave doubles. That is the beginning of the
  collision, where the two waveforms are
  overlapping. Just prior to the end of the sample
  the amplitude returns to normal.
• This happens when the first station to detect the
  collision quits transmitting, and the jam signal
  from the second colliding station is still
  observed.

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Types of collisions (cont.)

• Collisions are only recognized on UTP when the
  station is operating in half duplex.
• If the station is not engaged in transmitting it
  cannot detect a local collision. Conversely(???),
  a cable fault such as excessive crosstalk can
  cause a station to perceive(??)its own
  transmission as a local collision.
• A remote collision usually results from
  collisions occurring on the far side of a
  repeated connection.
• A repeater will not forward an over-voltage
  state, and cannot cause a station to have both
  the TX and RX pairs active at the same time.
• 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.
• As far as the NIC is concerned everything went
  out fine, and the upper layers of the protocol
  stack must determine that the frame was lost.
  Other than retransmission, a station detecting a
  late collision handles it in exactly the same way
  as a normal collision.

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Ethernet errors

• The following are the sources of Ethernet error
• Collision or runt Simultaneous transmission
  occurring before slot time has elapsed (??)
• Late collision Simultaneous transmission
  occurring after slot time has elapsed
• Jabber(????), long frame and range errors
  Excessively or illegally long transmission 
• Short frame, collision fragment or runt
  Illegally short transmission
• FCS error Corrupted (??)transmission
• Alignment error Insufficient or excessive
  number of bits transmitted
• Range error Actual and reported number of
  octets in frame do not match
• Ghost or jabber Unusually long Preamble or Jam
  event
• Jabber and Long Frames
• Jabber at least 20,000 to 50,000 bit times in
  duration
• Long Frames excess of the maximum frame size
  (1518 octets)
• both significantly larger
• whether or not the frame was tagged
• whether or not the frame had a valid FCS checksum

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Ethernet errors (cont.)

• Short frames
• less than the minimum frame size (64 octets)
• with valid(???)FCS checksums
• Some protocol analyzers and network monitors call
  these frames runts".
• The term runt is generally an imprecise
  slang(??)term that means something less than a
  legal frame size. It may refer to short frames
  with a valid FCS checksum although it usually
  refers (???)to collision fragments.

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FCS and beyond

• A received frame that has a bad FCS. The frame is
  then discarded.
• High numbers of FCS errors from a single station
• a faulty NIC
• a faulty software drivers
• a bad cable connecting
• FCS errors are associated with many stations
• a faulty software drivers
• bad cabling
• a faulty hub
• noise in the cable system

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FCS and beyond (cont.)

• A message that does not end on an octet boundary
  is known as an alignment error.
• bad software drivers
• Collision
• A frame with a valid value in the Length field
  but did not match the actual number of octets
  counted in the data field of the received frame
  is known as a range error.
• The length field value is less than the minimum
  legal unpadded size of the data field.
• Out of Range, is reported when the value in the
  Length field indicates a data size that is too
  large to be legal.
• Fluke Networks has coined the term ghost to mean
  energy (noise) detected on the cable that appears
  to be a frame, but is lacking a valid SFD.
• The frame must be at least 72 octets long,
  including the preamble.
• Ground loops and other wiring problems are
  usually the cause of ghosting.
• Most network monitoring tools do not recognize
  the existence of ghosts.
• The tools rely entirely on what the chipset tells
  them.

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Ethernet auto-negotiation

• Auto-Negotiation - This process defines how two
  link partners may automatically negotiate a
  configuration offering the best common
  performance level.
• 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.
• Example
• 10BASE-T required each station to transmit a link
  pulse about every 16 milliseconds, whenever the
  station was not engaged in transmitting a
  message.
• Auto-Negotiation adopted this signal and renamed
  it a Normal Link Pulse (NLP).
• When a series of NLPs are sent in a group for the
  purpose of Auto-Negotiation, the group is called
  a Fast Link Pulse (FLP) burst.

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Link establishment and full and half duplex

• Duplex mismatch
• one end is forced to full duplex
• the other is forced to half duplex
• result in collisions and errors
• 10-Gigabit Ethernet does not support half duplex
• The list is priority ranked, with the most
  desirable link configuration at the top.
• Fiber-optic Ethernet implementations are not
  included in this priority resolution list because
  the interface configuration is fixed.

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• END