Section 6: Ethernet Fundamentals

• The only real disadvantage to the at the time new 10BASE-T network in the 1980s was it could only cover distance of up to a 100 meters

  before they needed to have their signal repeated

  by a switch or a hub.
• Now, the big question when it comes to network devices,

  especially when they start operating at high speeds, is,

  how are they going to access the network and communicate?

  And this is really one of the core questions

  that ethernet had to answer.

  Should the network be deterministic?

  Or should it be contention-based?
• Methods of accessing the network and communicating: Deterministic


  Very organized and orderly. Requires an electronic token to transmit

  
  

  Used by token ring networks

  
  

  If a device wants to transmit data they must wait for its turn to get the electronic token to get to it
• Deterministic example

  Classroom setting where students MUST raise their hand to speak

  The student (device) waits until the instructor gives them permission (handing them a token) then they may speak
• The great benefit of a deterministic network is that there will never be a collision because everyone waits their turn
• Methods of accessing the network and communicating: Contention Based


  Very chaotic and can transmit whenever possible
• Contention based example

  A night out at bar with friends, everyone talks and if

  you hear a gap in the conversation and you begin to speak.

  If somebody else is speaking, you listen

  until you find an appropriate time for you to transmit,

  or in this case say whatever it is you wanted

  to add to that conversation
• The problem with using

  a contention-based method

  is that you can have collisions
• Ethernet chose to work as a contention based network because they have lower overhead

  
  

  You do not have to pass around an electronic token allowing you to make full use of all the bandwidth in the network because anyone can talk at anytime
• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

  Prevents collisions by using carrier sensing to defer transmissions until no other stations are transmitting
• CSMA/CD is like being at the bar with freinds and when you both go to speak at the same time, you both stop, say

  "Oh, I'm sorry. We must have spoken over each other.

  Let's try again."

  And then you wait and we try again.
• Breaking down CSMA/CD: The CS stands for Carrier Sense


  Listen to the wire, verify it is not busy
• Breaking down CSMA/CD: The MA stands for Multiple Access


  All devices have access at any time

  ethernet is going to have lots of devices on the network,

  and they're all going to be able to listen before they speak
• Breaking down CSMA/CD: CD Stands for Collision Detection


  If two devices transmit at the same time, a collision occurs

  ● Back off, wait a random time, and try again
• Under CSMA / CD, if a collision is detected, both devices will stop transmitting and pick a random number and then count to that number

  The first device to get to that number will start to transmit again.

  This number is called a random back off timer
• Collision Domain

  Each area of the network that shares a single segment
• With ethernet, anytime you have devices on the same cable,

  or they're all connected to the same hub,

  you are sharing the same collision domain.

  Because of this,

  all the devices need to operate in half duplex mode

  because they have to listen and then talk.

  They can't do both at the same time.
• Ethernet Switch

  Increases scalability of a network by creating multiple collision domains
• Every single switch port is actually considered

  its own collision domain.
• If there are NO other devices sharing a switch port ( a collision domain ) then that port can now operate in full duplex mode

  
  

  The device does not have to bother listening before talking, it has a full time dedicated channel
• The only two exceptions to an ethernet cable going not 100 meters max are

  CAT 8 which only goes 30 meters,

  and CAT 6, which only goes 55 meters

  if you want to reach the full 10 gigabits per second of speed.
• Depending on the type of cable,

  this is going to determine the capacity

  for the bandwidth of your network.

  So, if you replace all your old CAT 3 cables

  and switches with newer CAT 7 ones,

  you're going to increase your bandwidth

  from 10 megabits per second to 10 gigabits per second,

  making it 1,000 times faster.
• Things to remember for ethernet / fiber standards

  1 - Copper cables have a max distance of 100 meters
• Things to remember for ethernet / fiber standards

  2 - Using CAT 6 at 100 meters will limit the speed from 10 Gbps to 1 GBps
• Things to remember for ethernet / fiber standards

  3 - Using CAT 6 at under 55 meters can reach 10 Gbps of speed
• Things to remember for ethernet / fiber standards

  4 - Multimode fiber (MMF) deal with shorter distances - something in the 200 to 500 meter range
• Things to remember for ethernet / fiber standards

  5 - Use single mode fiber (SMF) for long distances

  
  

  Distance in kilometers instead of meters
• For the exam know that copper is great for short distances

  Fiber if you need short distance you will use MMF

  Fiber if you need long distance you will use SMF
• Remembering which fiber standard is MMF or SMF


  S is not single

  So, if you see an S there in the fiber ethernet standard,

  like 100BASE-SX, 1000BASE-SX, 10GBASE-SR,

  you know it's using a multimode fiber

  An S in the name denotes it is MMF
• The only exception to the S is not single rule is

  1000BASE- LX

  Because this one can be either SMF or MMF
• This icon is for a router

  
• this icon is for a switch

  
• This icon is for a bridge

  
• this icon is for a hub

  
• Hub

  Also known as a multi port repeater, a layer 1 device that connects multiple network devices and workstations
• Types of Hubs: Passive

  Repeats signal with no amplification
• Types of Hubs: Active

  Repeats signals with amplification
• Amplification - boosting signal to overcome the 100 meter limitation that we have with twisted pair copper cabling
• Types of Hubs: Smart

  Active hub with enhanced features like SNMP

  With Simple Network Management Protocol it enables you to monitor its status from a distance
• In modern networks hubs are essentially completely replaced by switches