Semester 1 v3'1'1: Networking Basics

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Semester 1 v3.1.1Networking Basics

• MODULE 6
• Ethernet Fundamentals

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Ethernet. Since 1973.

• Created at Xerox in 1973, released as an open
  standard in the early 80s.
• Later modified to comply with the OSI model,
  ratified as IEEE 802.3 in 1985.
• Ethernet has since evolved significantly
• Proved flexible as a technology, able to upgrade
  to new media and faster data transmission speeds.
• 10Gig Ethernet recently ratified as IEEE 802.3ae.
• Optical fibre has joined UTP copper as media of
  choice for the IEEE 802.3 family.
• Flexibility came through the simplicity of
  Ethernets structure.
• Ease of installation and maintenance of Ethernet
  networks guaranteed it as the network of choice.
• All varieties of IEEE 802.3 family are fully
  interoperable.

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The 802.3 Family

• Each member of Ethernet family has two naming
  conventions
• IEEE ratification supplement to 802.3 standard,
  eg. 802.3u, 802.3ae
• Abbreviated description transmission speed,
  signalling type, medium used.
• 100 BASE -TX

Maximum transmission speed Mbps
Type of signalling used Baseband
Medium used UTP copper (full duplex mode)
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IEEE 802.3 meets OSI Model

• Ethernet defines operations that take place in
  OSI Layers 1 and 2.
• Layer 1 issues addressed by Ethernet include
• How signals are transmitted down media
• What type of devices should be used to transmit
  data
• Topologies suitable for effective transmission
• Media Access Control (MAC) lower sub-layer of
  Layer 2 interacts with physical access
  protocols.
• Interfaces with the devices and media located at
  Layer 1.
• Logical Link Control (LLC) upper sub-layer of
  Layer 2 participates in encapsulation process,
  interfacing with Layer 3.

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Return of the MAC

• MAC Address space is a flat scheme.
• Unique Ethernet addresses for local delivery.
• 48 bits in length more often expressed as 12
  hexadecimal digits.
• First 24 bits is Organizational Unique
  Identifier, allocated by IEEE to vendors.
• Last 24 bits is Interface Serial Number
• MAC address is burned on to networking device by
  vendor.

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LLC MAC OSI Layer 2 PDU

• LLC takes data from upper OSI layers,
  encapsulates it into frames.
• Frame is the Protocol Data Unit (PDU) of choice
  for Layer 2.
• Add MAC address for local communication.
• Fields for start/end of transmission.
• Error detection with Frame Check Sequence
• Nodes on same segment can talk using MAC address
  alone.
• Ethernet is a broadcast technology, so all nodes
  on same segment see all frames.
• Nodes check to see if destination MAC matches its
  own.
• If not intended recipient, discard frames.
• If intended recipient, process frames.

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MAC Whos on First?

• Deterministic Media Access Control
• Each host on the network waits its turn to
  transmit data.
• Much like IEEE 802.5 Token Ring, and IEEE 802.8
  FDDI.
• Non-deterministic Media Access Control
• First come, first served basis.
• Wait for silence on the medium, then try to
  transmit.
• Carrier Sense Multiple Access/Collision Detect
  method utilised by IEEE 802.3 Ethernet.

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CSMA/CD Ethernet Rules

• CSMA/CD utilises a listen-before-transmit method.
• Node checks medium for traffic.
• If busy, retry randomly.
• Node enters transmit/listen mode, returns to
  listen mode after transmission.
• If a collision occurs, all nodes in a collision
  domain (one segment) must be aware of it before a
  back-off algorithm is invoked and all
  transmission halted.

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Need some time and space?

• Half-duplex mode uses a single circuit which can
  carry data in either direction but not both
  directions at once.
• Nodes must complete transmission of frames inside
  a slot time.
• If slot time exceeded, then late collisions may
  go unnoticed.
• Sending node must learn about any collisions
  before it finishes sending the smallest legal
  frame size.
• Full-duplex mode requires two pairs of wires,
  carrying data in both directions at once.
• CSMA/CD rules largely ignored and slot times not
  required.
• Full-duplex mode requires that each end of the
  link only connects to a single device.
• For all transmissions, a minimum inter-frame
  space of 96 bit-times is left between the sending
  of two frames
• Allows for receiving node to process incoming
  frames.

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Handling Ethernet Collisions

• A collision occurs when nodes in transmit/listen
  mode detect abnormally high signal amplitude.
• Manifested as collision fragments returning to
  sender.
• Any node transmitting halts immediately, then
  transmits a 32-bit jam signal.
• This extra transmission allows all hosts in the
  collision domain to see jam signal at least once.
• Colliding jam signals dont create more jam
  signals.
• Types of collisions local, remote, late.
• Only local and remote collisions are detected by
  sender, result in resending collided frame.

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Ethernet Errors

• Late collisions
• Sender doesnt detect collision after first 64
  octets of data transmitted.
• Frame not automatically retransmitted.
• Jabber and long frames
• Frames greater than maximum of 1518 octets.
• Short frames and runts
• Frames smaller than minimum of 64 octets.
• Usually fragments of collisions.
• Checksum/CRC error
• Checksum calculated by receiver doesnt match
  checksum entered by sender.
• Usually caused by faulty NIC/drivers, bad cable.

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Negotiation, Ethernet-style

• IEEE 802.3 over UTP supports a wide variety of
  link speeds, as well as half- and full-duplex.
• Upon initialisation, NICs have to decide what
  speed to operate at and what variety of IEEE
  802.3 to conform to.
• Link partners over UTP negotiate best speed to
  operate at using Normal Link Pulses (NLPs).
• Several NLPs in a short burst are termed as a
  Fast Link Pulse (FLP).
• If NLPs fail, then try re-negotiating at a slower
  link speed, or half-duplex mode instead of
  full-duplex.

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Welcome to my world )

• Daniel Comarmond
• CCNP, CCDP, CCSP, CCAI
• Cisco Networking Academy Instructor
• Systems Engineer Cisco Systems
• E-Mail dcom_at_it.uts.edu.au
• MSN dcom82_at_dcom82.com
• Phone [phone]
• Website http//www-staff.it.uts.edu.au/dcom
• Take care, and SMILE!!! )