Ethernet

1
Ethernet

• In 1976 a 2.94 Mbps a system named Ethernet using
  CSMA/CD was implemented at Xerox. It had over 100
  personal workstations on a 1-km long cable

2
IEEE802.3 Medium Access Control

• Random Access
• Stations access medium randomly
• Contention
• Stations content for time on medium
• It all began with ALOHA

3
ALOHA

• Packet Radio - The first Multiple Access network
  was the ALOHA packet radio system developed in
  University of Hawaii, back in 1971
• ALHOA true free for all - Transmit whenever you
  like without regard to any one else. When
  station has frame, it sends
• Station listens (for max round trip time) plus
  small increment
• If ACK, fine. If not, retransmit

4
ALOHA (continued)

• If no ACK after repeated transmissions, give up
• Frame check sequence (as in HDLC)
• If frame OK and address matches receiver, send
  ACK
• Frame may be damaged by noise or by another
  station transmitting at the same time (collision)
• Any overlap of frames causes collision
• Max utilization 18

5
Slotted ALOHA

• Time in uniform slots equal to frame transmission
  time
• Need central clock (or other sync mechanism)
• Stations can only start transmitting at the
  beginning of a time slot. Not at any other time
• Transmission begins at slot boundary
• Frames either miss or overlap totally
• Slotted ALOHA - Discrete Time ALOHA
• Less opportunity to collide
• Max utilization 37

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Aloha vs. Slotted Aloha

• Throughput efficiency increases dramatically for
  Slotted Aloha.

8
CSMA

• Propagation time is much less than transmission
  time
• All stations know that a transmission has started
  almost immediately
• First listen for clear medium (carrier sense)
• If medium idle, transmit
• If two stations start at the same instant,
  collision
• Wait reasonable time (round trip plus ACK
  contention)
• No ACK then retransmit
• Max utilization depends on propagation time
  (medium length) and frame length
• Longer frame and shorter propagation gives better
  utilization

9
Carrier Sensed Multiple Access (CSMA)

• CSMA/CD is an acronym for Carrier Sensed Multiple
  Access with Collision Detection.
• This is a way for many devices to access the same
  network (Multiple Access).
• The way it works is that every device needing to
  send data to the network first "listens" to the
  wire to determine if anyone else is sending
  anything (Carrier Sense).
• If there is any activity at the time, then the
  device needing to transmit will wait until the
  transmission ends, otherwise it sends its data.

10
CSMA Protocol

• Listen before you speak
• If someone else is speaking then wait and let
  them finish - backoff
• Persistency
• More efficient than Slotted ALOHA

11
If Busy?

• If medium is idle, transmit
• If busy, listen for idle then transmit
  immediately
• If two stations are waiting, collision

12
CSMA/CD improves CSMA

• With CSMA, collision occupies medium for duration
  of transmission
• Stations listen whilst transmitting
• If medium idle, transmit
• If busy, listen for idle, then transmit
• If collision detected, jam then cease
  transmission
• After jam, wait random time then start again
• Binary exponential back off

13
Collision Detection

• On baseband bus, collision produces much higher
  signal voltage than signal
• Collision detected if cable signal greater than
  single station signal
• For twisted pair (star-topology) activity on more
  than one port is collision
• Special collision presence signal

14
Collision Detection (CD)

• Once a device determines that the network is
  clear, it begins sending data.
• While sending, it monitors what data is actually
  on the wire, and compares it to what is being
  sent. It is possible for two devices to see that
  the network is open at the same time and for both
  to start sending. When this happens, the data on
  the wire will be garbled and neither device will
  see that what is actually on the wire matches
  what it sent. This is called a collision.
• The devices detect this collision, and both stop
  their current transmission, wait a random amount
  of time, and try again.

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TEBB

• A and B after the first collision
• both select random number from 0 and 1.
• if both select 0 and 0 or 1 and 1, there will be
  another collision.
• Thus collision probability is 1/2.

16
TEBB

• A and B after the second collision
• both select random number from 0 to 3.
• if both select the same number, there will be
  another collision.
• thus collision probability is 1/4.

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TEBB (cont.)

• A and B after the 10to 16th collision
• both select random number from 0 to 1023.
• if both select the same number, there will be
  another collision.
• thus collision probability is 1/1024.
• After 16th collision, transmission is aborted.

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Truncated Exponential Backoff Algorithm

• Slot time 512bits.
• r slots delay before nth retransmission.
• r is a uniformly distributed random integer.
• Kmin(n,10).
• 0lt r lt2K

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CSMA/CDOperation
20
CSMA/CD Flow Chart
21
Collision Detection Can Take As Long as 2?
22
The Collision Domain

• Collision detection can take as long as 2t, worst
  case.
• This round-trip delay defines the max Ethernet
  network diameter, or collision domain.
• Round-trip delay 512 bit times for all
  ethernets.

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Ethernet MAC (802.3)

• 802.3 MAC does the following
• Minimize access delay for low traffic
• Minimize collision for low traffic
• Maximize bandwidth utilization when few large
  data chunks are transmitted.
• Ensure fair access and bandwidth allocation by
  random access and minimizing the maximum packet
  size.
• Does not give a bound on the access delay.
• Efficiency degrades under heavy loads.

24
Ethernet

• Developed at Xerox PARC - Robert Metcalfe
• In 1976 a 2.94 Mbps a system named Ethernet using
  CSMA/CD was implemented at Xerox. It had over 100
  personal workstations on a 1-km long cable
• Was meant to be a part of the office of the future

25
DIX Ethernet

• DIX - Digital, Intel, Xerox
• First commercial Ethernet system
• Original Ethernet system was so successful that
  DEC and Intel collaborated with Xerox to build a
  10 Mbps Ethernet.
• This project was the basis for IEEE standard
  802.3, which includes specifications for 1 to 10
  Mbps networks using different physical media and
  topologies

26
IEEE 802 Series of Standards

• Why LAN Standards?
• 802.1 - Overview
• 802.2 - Logical Link Control
• 802.3 - IEEE Ethernet

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IEEE 802 Reference Model
28
LAN/MAN Standards
29
IEEE 802.3 at 10Mb/s
30
IEEE 802.3

• Uses Carrier sense multiple access with collision
  detection
• Four step procedure
• If medium is idle, transmit
• If medium is busy, listen until idle and then
  transmit
• If collision is detected, cease transmitting
• After a collision, wait a random amount of time
  before retransmitting

31
802.3 Medium Notation

• Notation formatltdata rate in Mbpsgtltsignaling
  methodgtltmaximum segment length in hundreds of
  metersgt
• e.g 10Base5 provides 10Mbps baseband, up to 500
  meters
• T and F are used in place of segment length for
  twisted pair and fiber

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IEEE Ethernet Nomenclature

• 10Base5 - Original Ethernet - Thicknet
• 10Base2 - Cheapernet - Thinnet
• 10BaseT - Twisted Pair, Star Topology Ethernet
• 10BROAD36
• 10BaseFL - Optical Fiber Ethernet

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Ethernet Topology

• Originally BUS
• Star and Tree

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Ethernet Media Types

• Coaxial Cable - Thick and Thin
• Unshielded Twisted Pair
• Optical Fiber

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Different Media and Wiring For Ethernet Types
36
Ethernet Signaling

• Differential Manchester
• -0.85 V and 0.85 V

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Ethernet Frame Format

• Preamble - 7 bytes of 10101010
• Start of Frame Delimiter - SOFD - 101010111
• Destination Address - 2 or 6 bytes - MAC
  Addresses, Physical Addresses - Broadcast Address
• Source Address - 2 or 6 bytes
• Length of Data Field - 2 bytes, valid frames must
  be at least 64 bytes long, 64-1500 bytes. Why?
• Data - 0 to 1500 bytes
• Padding - 0 to 46 bytes
• Checksum - 4 bytes or CRC-32 EDC

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Ethernet Frame - IEEE 802.3 Frame Format
39
10Base5

• Thick Ethernet, officially known as 10Base5
• Is the oldest form of Ethernet. It was originally
  developed in the late 1970's by Digital Equipment
  Corporation, Intel and Xerox
• Became an international standard in 1983

40
10BASE5 (Thick Ethernet)

• Original 802.3 medium specification
• 50-O coax and Manchester signaling
• Segment length can be extended past 500m with
  repeaters
• transparent at the MAC level
• maximum of 4 allowed
• No looping allowed--one path between any two
  stations

41
Topology Cabling

• 10 Base 5 is laid out in a bus topology, with a
  single coaxial cable connecting all nodes
  together
• At each end of the coaxial cable is a terminator
• Each node on the network physically connects to
  the coaxial cable through a device called a
  transceiver and an AUI cable is connected between
  the node and the transceiver

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Thick Ethernet Physical Topology
43
10Base5 PROS and CONS

• PROS
• Long Distances Possible
• Noise Immunity
• Conceptually Simple
• CONS
• Inflexible
• Fault Intolerant
• Susceptible To Ground Loops
• Very Difficult Troubleshooting

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10Base5 as a Backbone
45
10BASE2 (Thin Ethernet)

• Intended to provide lower-cost system for PC LANs
• Uses thinner cable and supports fewer taps than
  10BASE5
• Can combine 10BASE2 and 10BASE5 segments in the
  same network (but backbone must then be 10BASE5)

46
10Base2 Basics

• Thin Ethernet, officially called 10Base2, is a
  less expensive version of 10Base5 (Thick
  Ethernet) technology
• It uses a lighter and thinner coaxial cable and
  dispenses with the external transceivers used
  with 10Base5

47
10Base2 Topology Distance Limits

• 10 Base-2 uses an RG-58A/U coaxial cable and is
  wired in a bus topology. Each device on the
  network is connected to the bus through a BNC "T"
  adapter, and each end of the bus must have a 50
  Ohm terminator attached. Each node on the bus
  must be a minimum of 0.5 meters (1.5 feet) apart,
  and the overall length of the bus must be less
  than 185 meters (606 feet). Figure One
  graphically illustrates a sample 10 Base-2
  network.
• NOTE It is not possible to locate the node away
  from the BNC "T" adapter by connecting a coaxial
  cable between the "T" and the node's BNC
  connector. Doing so will detune the network, and
  will almost certainly result in major problems.

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10Base2 Network
49
10BASE-T

• Uses UTP, often prewired in buildings
• Star-shaped topology is well-suited to existing
  wires terminating in a closet
• Stations attach to central multi-port repeater
  (hub)
• Hubs can be cascaded
• Physical star, but logical bus (all transmissions
  are repeated)

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10BaseT

• Ethernet was originally designed to operate over
  a heavy coaxial cable, and was later updated to
  also support a thinner, lighter, coaxial cable
  type. Both systems provided a network with
  excellent performance, but they utilised a bus
  topology which made changing a network a
  difficult proposition, and also left much to be
  desired in regard to reliability.
• Also, many buildings were already wired with
  twisted-pair wire which could support high speed
  networks. Installing a coaxial-based Ethernet
  into these buildings would mean they would have
  to be rewired. Therefore, a new network type
  known as 10 Base-T was introduced to increase
  reliability and allow the use of existing
  twisted-pair cable.

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10BaseT Topology Cabling

• 10 Base-T utilizes Category 3 (or higher)
  Unshielded Twisted Pair (UTP) cable in a star
  topology. Each node on the network has its own
  cable run back to a common hub, and each of these
  cable runs may be up to 100 meters (330 feet) in
  length. Figure One shows a simple 10 Base-T
  network.

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10 Base-T Network
53
10BaseT Tree Topology

• 10 Base-T can also be wired in a tree topology,
  where one "main" hub is connected to other hubs,
  which are in turn connected to workstations.
  Please note that the depth of a 10 Base-T tree
  network is limited to one layer below the main
  hub. It is also possible to combine 10 Base-T
  with any combination of the other 10 Mbps
  Ethernet technologies in an infinite number of
  ways to meet nearly any requirement. Figure Two
  shows a combination of 10 Base-T and 10 Base-2.

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10 Base-T and Thin Ethernet Combination
55
Advantages Disadvantages

• 10 Base-T has various advantages and
  disadvantages which make it suitable for some
  applications and less suitable for others. Some
  of them are listed below
• Fault Tolerant
• Uses UTP Cable
• Easy Moves Changes
• Easy Troubleshooting

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Advantages

• Fault Tolerant
• Since each node on a 10 Base-T network has its
  own cable connecting it to a central hub, it is
  far less likely that any node can cause the
  entire network to fail. The hub also has a
  "partitioning" function built into it which
  allows it to detect a problem on any of its
  ports. If a problem is found, the node is
  disconnected from the rest of the network. This
  isolates the problem until the node can be
  troubleshot and repaired.
• Easy Troubleshooting
• Because of the partitioning function built in to
  the hubs and the star-wired topology, it is
  generally easy to troubleshoot a 10 Base-T
  network. In a worst-case scenario, one can be
  troubleshot by simply disconnecting nodes from
  the hub one at a time until the network recovers.
  Usually, the hub will give an indication as to
  which node is causing a problem, allowing the
  technician to troubleshoot that node as opposed
  to spending many hours finding where the problem
  is.
• Easy Moves Changes
• Disconnecting a node from the network has no
  effect whatsoever on the rest of the network.
  Therefore, moving an attached device is simply a
  matter of unplugging it from the hub and
  reconnecting it somewhere else.
• Uses UTP Cable
• Many buildings are already wired with UTP cable
  which can support a 10 Base-T network. Even in
  the event a building is not wired with UTP
  already, it is still preferable to install UTP
  than any other type of cable, as UTP will support
  other applications later, whereas other cable
  types will generally be specific to one network
  type. This allows leveraging the UTP cable
  investment for other applications many years
  later.

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Disadvantages

• Distance - 10 Base-T only allows distances from
  the hub to the node of 100 meters (330 feet). In
  some installations, this can be a major problem
  if nodes need to be located farther away.
• Sensitive To Noise - The nature of UTP cable
  makes it considerably more sensitive to
  electrical noise than coaxial cable. Generally,
  this rules 10 Base-T out as an option for
  installations on factory floor environments or
  other locations with a high ambient noise level

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10BaseT vs. 10Base2/5
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10BaseT Cabling Considerations

• 10 Base-T uses two pairs of wires one pair for
  transmission and the second pair for receive.
• The physical connector used is an 8 position
  modular plug, commonly referred to as an RJ-45.
• All cables must be rated at a minimum of Category
  3, and must be wired such that pins 1 2 are on
  one twisted pair and pins 3 6 are on a second
  pair. Common wiring standards which meet this
  requirement are EIA/TIA T568A and T568B.

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10BaseT Cabling Considerations - II

• There are two pinouts used MDI for DTE devices
  (such as computers, printers, etc.) and MDI-X
  (hubs). Connecting an MDI port to an MDI-X port
  requires a straight through cable, and connecting
  either MDI to MDI or MDI-X to MDI-X requires a
  crossover cable. Pinouts of the MDI and MDI-X
  interfaces are shown in Table One.

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Crossover Cable Applications

• There are several applications for crossover
  cables in 10 Base-T networks. The most common
  reason is to cascade hubs together in a tree
  topology. If both hubs have only MDI-X ports then
  a crossover cable is needed. Another application
  for a crossover cable is to connect two DTE
  devices together without a hub.

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Crossover Cable Pinout

• A standard 10 Base-T crossover cable wiring
  diagram is shown in Table

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10BaseT Hub Evolution

• Hub
• Active Hub
• Manageable Hub
• Switched Hub
• Switch

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10BROAD36

• Only 802.3 broadband spec
• Uses 75 -O CATV coax
• Maximum length of individual segment is 1800m
• Broadband is by nature analog, so analog encoding
  must be used (DPSK)

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10BASE-F

• Standard includes 3 specifications
• 10-BASE-FP Passive star topology, up to 1km per
  segment
• 10-BASE-FL Point-to-point link connecting
  stations or repeaters up to 2km
• 10-BASE-FB Point-to-point backbone link
  connecting repeaters at up to 2km
• All specs use two fibers, one for transmission in
  each direction
• Manchester encoding, converted to optical signal
  elements

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10BaseFL

• 10 Base-FL is basically a version of Ethernet
  which runs over fiber optic cable. In physical
  topology, it is very similar to 10 Base-T.
• 10 Base-FL runs over 62.5/125 micron multimode
  fibre optic cable. It supports distances of 2000
  meters (6600 feet).
• 10 Base-FL is wired in a star topology with all
  of the fibre optic runs originating from a
  central hub. It is also acceptable to connect a
  pair of 10 Base-FL devices directly together with
  a point to point link.
• One point which must be made is that although 10
  Base-FL links can be run up to 2000 meters, an
  Ethernet network has an overall maximum network
  diameter (distance between the two most widely
  separated nodes) of 2500 meters. Therefore, it is
  highly recommended that anyone planning to
  install a long 10 Base-FL cable run checks the
  overall network to ensure that adding the new run
  does not exceed the overall network size. If the
  overall size will exceed 2500 meters, then a
  local Ethernet bridge or a switch will need to be
  installed to split the network into multiple
  subnetworks, each of which can be built to the
  full 2500 meter limit.

67
10BaseFL Applications

• Since 10 Base-FL uses fibre optic cable, it is
  completely immune to any type of external
  electrical interference. This makes it a very
  attractive technology for use in areas such as
  factory floors where electromagnetic interference
  (EMI) levels are very high.
• Another property of fibre optics is that fibre is
  very difficult to tap into, thus providing a high
  level of security.
• Finally, 10BaseFL is very useful for use in
  interconnecting buildings in a campus environment
  where distances could be very long.

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Comparison
69
Problems with Ethernet

• CSMA/CD Protocol - Contention, Collisions
• Distance Limits
• Capacity Limits

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Pros and Cons of CSMA/CD

• Every Ethernet network, regardless of type,
  behaves in this manner. Naturally, this access
  method is most efficient under periods of light
  load, as there is a greater chance the network
  will be available when the device wants to
  transmit, and there is also less chance another
  device will try to transmit at the same time. As
  the load on the network increases, collisions
  increase with it. Eventually, it is possible to
  get to the point where there are too many
  collisions and devices spend more time
  retransmitting and waiting than they do actually
  sending data.
• The CSMA/CD protocol is usually robust enough to
  keep the network running very well, however there
  are situations which can cause it to fail. Proper
  design of the network is important to prevent a
  breakdown of the basic CSMA/CD protocol. Design
  mistakes which cause problems are most often
  either exceeding the maximum repeater count or
  exceeding the distance limit of a network
  segment.
• One thing which should be kept in mind is that
  CSMA/CD happens automatically in each machine's
  network interface card, and there is nothing the
  user needs to program or set up to implement it,
  nor is the user usually notified of collisions.