Ethernet Fundamentals

1
Ethernet Fundamentals

• Module 6 Exam Review

2
6.1.1 Introduction to Ethernet

• The success of Ethernet is due to the following
  factors
• Simplicity and ease of maintenance
• Ability to incorporate new technologies
  scalability
• 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.
• 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 Local and Metropolitan Networks published
  standards for LANs. These standards start with
  the number 802. The standard for Ethernet is
  802.3.
• Essentially, Ethernet and IEEE 802.3 are the
  same standards.
• The bandwidth of the network could be increased
  many times without changing the underlying
  Ethernet technology. This makes Ethernet
  scalable

5
6.1.2 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.
• 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

• 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.

7
6.1.4 Naming

• Ethernet uses MAC addresses that are 48 bits in
  length and expressed as twelve hexadecimal
  digits.
• 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.

8
6.1.5 Layer 2 Framing

• A single generic frame has sections called
  fields, and each field is composed of bytes.
  The names of the fields are as follows
• Start frame field Beginning of frame
• Address field Includes source and destination
  MAC address.
• Length / type field
• Data field
• Frame check sequence field 

9
6.1.6 Ethernet Frame Structure

• At the data link layer the frame structure is
  nearly identical for all speeds of Ethernet from
  10 Mbps to 10,000 Mbps.
• In the version of Ethernet that was developed by
  DIX prior to the adoption of the IEEE 802.3
  version of Ethernet, the Preamble and Start Frame
  Delimiter (SFD) were combined into a single
  field, labeled Type.
• The field labeled Length/Type was only listed as
  Length in the early IEEE versions and only as
  Type in the DIX version.
• These two uses of the field were officially
  combined in a later IEEE version, as both uses of
  the field were common throughout industry

10
6.1.6 Ethernet Frame Structure

• If the two-octet value is equal to or greater
  than 0x600 (hexadecimal), then the frame is
  interpreted according to the Ethernet II type
  code indicated.

11
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).
• Examples of deterministic protocols include Token
  Ring and FDDI.
• Token Ring is a collisionless environment as only
  one host is able to transmit at any given time.

12
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

13
6.2.2 MAC Rules and Collision Detection/Backoff

• Networking devices detect a collision has
  occurred when the amplitude of the signal on the
  networking media increases.
• 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.

14
6.2.3 Ethernet Timing

• Slot Time
• 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.
• Undefined at 10 Gigabit Ethernet
• 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.

15
6.2.4 Interframe Spacing and bBackoff

• 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.

16
6.2.7 Ethernet Errors

• Collision or runt Simultaneous transmission
  occurring before slot time has elapsed. A large
  number of runts on a network indicates
• Significant number of collisions are occurring
• Throughput on the network is significantly
  reduced due to excessive collisions

17
6.2.7 Ethernet Errors

• Ethernet standard specifications limit network
  service interruption by setting the following
• Maximum segment length
• Maximum number of stations per segment
• Maximum number of repeaters between segments