Chapter 6 Ethernet Fundamentals

1
Chapter 6Ethernet Fundamentals
By Steven P. Luse
2
6.1.1 Introduction to Ethernet

• From its beginning in the 1970s, Ethernet has
  evolved to meet the increasing demand for high
  speed LANs. the same protocol that transported
  data at 3 Mbps in 1973 is carrying data at 10
  Gbps.
• 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

3
6.1.1 Introduction to Ethernet

• 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.

4
6.1.1 Introduction to Ethernet

• In 1985, the Institute of Electrical and
  Electronics Engineers (IEEE) standards committee
  for LANs published standards. They started with
  the number 802. Called Ethernet 802.3. This had
  to be compatible with the ISO/OSI model. To do
  this, the IEEE 802.3 standard had to address the
  needs of Layer 1 and the lower portion of Layer 2
  of the OSI model. As a result, some small
  modifications to the original Ethernet standard
  were made in 802.3.

• The differences between the two standards were so
  minor that any Ethernet network interface card
  (NIC) can transmit and receive both Ethernet and
  802.3 frames. Essentially, Ethernet and IEEE
  802.3 are the same standards.

5
6.1.2 IEEE Ethernet naming rules

• Ethernet is not one networking technology, but a
  family of networking technologies that includes
  Legacy, Fast Ethernet, and Gigabit Ethernet.
  Ethernet speeds can be 10, 100, 1000, or 10,000
  Mbps. The basic frame format and the IEEE
  sublayers of OSI Layers 1 and 2 remain consistent
  across all forms of Ethernet.

• The abbreviated description consists of
• A number indicating the number of Mbps
  transmitted.
• The word base, indicating that baseband signaling
  is used.
• One or more letters of the alphabet indicating
  the type of medium used (F fiber optical cable,
  T copper unshielded twisted pair).

6
6.1.3 Ethernet and the OSI model

• Ethernet operates in two areas of the OSI model,
  the lower half of the data link layer, known as
  the MAC sublayer and the physical layer.

7
6.1.3 Ethernet and the OSI model

• A collision domain is then a shared resource.
  Problems originating in one part of the collision
  domain will usually impact the entire collision
  domain.

8
6.1.3 Ethernet and the OSI model

• maps a variety of Ethernet technologies to the
  lower half of OSI Layer 2 and all of Layer 1.
  Ethernet at 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.

9
6.1.3 Ethernet and the OSI model

• Layer 1 involves media, signals, bit streams that
  travel on media, components that put signals on
  media, and various topologies. Each of its
  functions has its limitations. Layer 2 addresses
  these limitations.
• For each limitation in Layer 1, Layer 2 has a
  solution.
• Layer 1 cannot communicate with the upper-level
  layers Layer 2 does that with logical link
  control (LLC).
• Layer 1 cannot name or identify computers Layer
  2 uses an addressing (or naming) process.
• Layer 1 can only describe streams of bits Layer
  2 uses framing to organize or group the bits.
• Layer 1 cannot choose which computer will
  transmit binary data, from a group in which all
  computers are trying to transmit at the same
  time Layer 2 accomplishes this by using a system
  called Media Access Control (MAC).

10
6.1.3 Ethernet and the OSI model

• 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.

11
6.1.4 Naming

• Ethernet uses MAC addresses that are 48 bits in
  length and expressed as twelve hexadecimal
  digits. The first six hexadecimal digits, which
  are administered by the IEEE, identify the
  manufacturer or vendor. This portion of the MAC
  address is known as the Organizational Unique
  Identifier (OUI). The remaining 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) because they are burned into read-only
  memory (ROM) and are copied into random-access
  memory (RAM) when the NIC initializes.

12
6.1.5 Layer 2 framing

• Framing helps obtain essential information that
  could not, otherwise, be obtained with coded bit
  streams alone.
• Which computers are communicating with one
  another
• When communication between individual computers
  begins and when it terminates
• Provides a method for detection of errors that
  occurred during the communication
• Whose turn it is to "talk" in a computer
  "conversation"

13
6.1.5 Layer 2 framing

• The frame format diagram shows different
  groupings of bits (fields) that perform other
  functions.
• The names of the fields are as follows
• Start frame field
• Address field
• Length / type field
• Data field
• Frame check sequence field 

14
6.1.5 Layer 2 framing

• All frames contain naming information, such as
  the name of the source node (MAC address) and the
  name of the destination node (MAC address).
• In some technologies, a length field specifies
  the exact length of a frame in bytes. Some frames
  have a type field, which specifies the Layer 3
  protocol making the sending request.
• Data
• The Frame Check Sequence (FCS) field contains a
  number that is calculated by the source node
  based on the data in the frame. This FCS is then
  added to the end of the frame that is being sent.
  
• There are three primary ways to calculate the
  Frame Check Sequence 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.

15
6.1.6 Ethernet frame structure
16
6.1.7 Ethernet frame fields

• fields permitted or required in an 802.3 Ethernet
  Frame are
• Preamble - is an alternating pattern of ones and
  zeroes used for timing synchronization in the
  asynchronous 10 Mbps and slower implementations
  of Ethernet.
• Start Frame Delimiter - one-octet field that
  marks the end of the timing information, and
  contains the bit sequence 10101011.
• Destination Address
• Source Address
• Length/Type
• Data and Pad
• 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.
• Extension

17
6.2.1 Media Access Control (MAC)

• There are 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 MAC protocols use a first-come,
  first-served approach. CSMA/CD is a simple
  system. The NIC listens for an absence of a
  signal on the media and starts transmitting.

18
6.2.1 Media Access Control (MAC)

• The specific technologies for each are as
  follows
• Ethernet logical bus topology (information flow
  is on a linear bus) and physical star or extended
  star (wired as a star)
• Token Ring logical ring topology (in other
  words, information flow is controlled in a ring)
  and a physical star topology (in other words, it
  is wired as a star)
• FDDI logical ring topology (information flow is
  controlled in a ring) and physical dual-ring
  topology (wired as a dual-ring)

19
6.2.2 MAC rules and collision detection/backoff

• The access method CSMA/CD used in Ethernet
  performs three functions
• Transmitting and receiving data packets
• Decoding data packets and checking them for valid
  addresses before passing them to the upper layers
  of the OSI model
• Detecting errors within data packets or on the
  network

20
6.2.3 Ethernet timing

• The electrical signal takes time to travel down
  the cable (delay), and each subsequent repeater
  introduces a small amount of latency in
  forwarding the frame from one port to the next.
  Because of the delay and latency, it is possible
  for more than one station to begin transmitting
  at or near the same time. This results in a
  collision.
• 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.
• 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
• The actual data payload
• Checksum (FCS) used to ensure that the message
  was not corrupted along the way
• Stations receiving the frame recalculate the FCS
  to determine if the incoming message is valid and
  then pass valid messages to the next higher layer
  in the protocol stack.

21
6.2.4 Interframe spacing and backoff

• The minimum spacing between two non-colliding
  frames is also called the interframe spacing.

• The minimum spacing between two non-colliding
  frames is also called the interframe spacing.

22
6.2.5 Error handling
23
6.2.6 Types of collisions

• Collisions typically take place when two or more
  Ethernet stations transmit simultaneously within
  a collision domain. A single collision is a
  collision that was detected while trying to
  transmit a frame, but on the next attempt the
  frame was transmitted successfully.
• Three types of collisions are
• Local - collision on coax cable (10BASE2 and
  10BASE5), the signal travels down the cable until
  it encounters a signal from the other station.
• Remote - 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. This
  sort of collision usually results from collisions
  occurring on the far side of a repeated
  connection.
• Late - Collisions occurring after the first 64
  octets. 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.

24
6.2.7 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

25
6.2.8 FCS and beyond

• A received frame that has a bad Frame Check
  Sequence, also referred to as a checksum or CRC
  error, differs from the original transmission by
  at least one bit. In an FCS error frame the
  header information is probably correct, but the
  checksum calculated by the receiving station does
  not match the checksum appended to the end of the
  frame by the sending station. The frame is then
  discarded.

26
6.2.9 Ethernet auto-negotiation

• 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. Each FLP burst is
  sent at the same timing interval as an NLP, and
  is intended to allow older 10BASE-T devices to
  operate normally in the event they should receive
  an FLP burst.
• 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.

27
6.2.10 Link establishment and full and half duplex

• Link partners are allowed to skip offering
  configurations of which they are capable. This
  allows the network administrator to force ports
  to a selected speed and duplex setting, without
  disabling Auto-Negotiation. 

28
The End

• Good Luck on the Chapter Test