Ethernet

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Ethernet

• Radovanovic Igor

Thanks to B.A. Forouzan
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Reminder
Application
Transport
Network
Data link
Physical
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Reminder IEEE standard
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Ethernet protocol(LAN protocol)
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Reminder Data Link layer
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Short history

• Invented in the midst of 1970s by Bob Metcalfe
  David Bogs in Xerox Palo Alto Research Center
• Norman Abramson developed an ALOHA network on
  Hawaii islands
• Bob (a Harvard PhD student) worked on ARPAnet at
  MIT
• 100 computers connected on a 1 km cable using
  CSMA/CD
• Original Ethernet 256 hosts running at 2.94 Mbps
• Xerox, Digital Intel established 10Mbps
  Ethernet
• IEEE standardization (802.3)
• 1979 Bob Metcalfe 3Com (left company in 1990)
• Ethernet has been to LAN as Internet to global
  networking

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Why is Ethernet so popular?

• First widely developed high-speed LAN
• Network administrators reluctant to switch to
  other LAN techn.
• Other technologies (Token Ring, FDDI) more
  complex expensive
• Ethernet evolved in speed
• Ethernet hardware of low cost
• CSMA/CD decentralized simple design

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Ethernet building blocks

• Ethernet frame (packet)
• Media Access Control protocol
• Signaling components
• Ethernet interface card, transceivers, repeaters
• Physical medium

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Traditional Ethernet frame-common to all
Ethernet technologies-

• MAC frame
• Preamble
• 7 bytes of alternating 0s and 1s receiver sync
• Not necessary in high-speed Ethernet systems
  (FastEth, GigE)
• Start of Frame Delimiter
• 10101011 not unique sequence -gt last chance to
  synchronize
• Source Address, Destination Address
• 48 bit unique address
• Length/Type
• Value up to 1518 length value larger tan 1536
  type of PDU encaps
• Data
• Frame Check Sequence (CRC) preamble and SFD
  excluded in calculat.
• Note no ACK mechanism provided

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MAC to PHY mapping

• Unaltered transmission in all but preamble field
• Start of Stream Delimiter and End of Stream
  Delimiter added

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Min Max Ethernet frame length

• Minimum length - used for CSMA/CD
• Every end station senses the frame within the
  correct time limits
• Historical requirement derived for bus topology
  with a coax cable
• Maximum length to assure fair access
• A station should not occupy the medium too long

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Collision domain

• a single CSMA/CD network in which there will be
  a collision if two computers attached to the
  system both transmit at the same time
• defined as all the Ethernet segments between a
  pair of bridges or other layer 2 devices

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Determining minimum frame length
10Mbps
Llt1500 m

• In practice, minimum packet size 512 bits
• allows for extra time to detect collisions
• allows for repeaters that can boost signal

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Three generation of Ethernet
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Physical layer implementation
sTYPE-t(l) s speed in Mbps TYPE broadband or
baseband signaling l cable distance in
multiple of 100 m t media type used
10Base-5 Original Ethernet large thick coaxial
cable 10Base-2 Thin coaxial cable
version 10Base-T Voice-grade unshielded
twisted-pair Category-3 telephone
cable 10Base-F Two optical fibers in a single
cable
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10BASE-T

• uses a physical star topology (logical bus)
• end stations connected to a hub using external or
  internal transceiver
• maximum distance 100 m using UTP cable
• maximum number of hubs is 4 (total span 500 m)
• Q How to increase the span?

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Bridged Ethernet

• Bridges increase the bandwidth and separate
  collision domains
• Attach more users and increase the distance
  between any 2 nodes

Q How to increase bandwidth further?
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Switched Ethernet

• Used to add bandwidth without replacing NICs
• Each end station uses a separate path to the port
  in the switch (no sharing medium- no need for
  CSMA/CD)

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Ethernet- Physical layer

• Medium Attachment Unit
• Medium dependant
• MDI
• External tap or a tee connector
• Internal jack

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Three generation of Ethernet
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Ethernet NIC with MII connector
Network Interface Card with the MII connector
Physical Layer Device attached to the NIC with
the MII connector
Optical MII transceiver - Physical Layer Device -
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Fast Ethernet

• Keep MAC layer untouched
• Increase the speed 10 times while keeping the
  compatibility with the Ethernet
• Quick deployment
• Autonegotiation
• negotiate the mode (full- or half-duplex) or data
  rate of operation (10 Mbps, 100 Mbps)
• Different encoding technique to transfer the high
  data rate signal
• Q What is the data rate of the transmitted
  signal if the Manchester encoding is used?

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Fast Ethernet implementations
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100BASE-TX

• uses two pairs of twisted-pair cable (Cat5 UTP or
  STP) in the physical star topology
• Maximum number of repeaters is 2 (Class II
  repeaters).
• Maximum cable length to a repeater is 100m.
• Maximum distance between repeaters is 5 m.

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MLT-3

• Uses 3 levels of signals (1,0,-1)
• Transition at the beginning of bit 1
• Used to decrease transmission frequency
• Q Why?

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100BASE-FX

• Two pairs of fiber-optic cables in a physical
  star topology
• Can support larger distances (unrepeated)
• up to 2 km full-duplex 412 m half-duplex
• single-mode fiber
• Long-wavelength lasers in NICs
• Problem not compatible with previous fiber
  standards

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100BASE-FX - encoding
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100BASE-SX

• Not an IEEE standard
• Only Physical layer changed
• Based on short-wavelength lasers (850 nm)
• Q Why is this important?
• A
• backwards compatibility
• lower cost of the NIC?
• Shortcoming
• shorter fiber span (300 m)

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Comparing different standards
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Three generation of Ethernet
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Gigabit Ethernet

• Once more, 10 times faster Ethernet
• what was the motivation for developing this
  standard?
• wanted to keep MAC layer unaltered did this
  happen?
• 802.3z (z indicates the end of the road)
• Design goals
• backwards compatibility
• offer unacknowledged datagram services
• same 48 bit address scheme
• All configurations are point-to-point
• Defined for both half- full-duplex
• Half-duplex supports CSMA/CD
• All implementations today are full-duplex

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Gigabit Ethernet (cntd)

• Requirement To implement CSMA/CD
• signal propagation time ( to Slot Time) must
  stay the same
• keeping the Slot Time the same requires
  decreasing the span (10 times)
• this would limit the span to 25 m only!
• possible remedy
• increase the minimum size packet
• Q What was the minimum size of the packet in
  Ethernet?
• by doing this we loose backwards compatibility
• introduce Carrier extension

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Gigabit Ethernet (cntd)

• Carrier extension is very inefficient for the
  short packets
• for example for 64 byte packet we have 448 bytes
  of padding
• low throughput
• Solution introduce Frame bursting
• Q What is the maximum burst size?

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GigEth implementations
Cat5e UTP 100 m
short wavelengths 260 m -gt MM fiber 550 m -gt SM
fiber
long wavelengths 550 m -gt MM fiber 3 km
-gt SM fiber
25 m
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10G Ethernet (IEEE 803.2ae)

• Only full-duplex (no CSMA/CD)
• Based on optical fibers
• Standard specified for both LANs and WANs

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Ethernet Retrospective

• Been around for 20 years
• Few technologies made it that long (OS,
  architectures etc)
• Pros
• simple, popular, flexible
• Cons
• non-deterministic, unreliable, no priorities, min
  frame size
• Q Why are these important?