Ethernet, Fast Ethernet, and Gigabit Ethernet by ibt12826

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									  Ethernet, Fast Ethernet, and
       Gigabit Ethernet

In this presentation, we will take a closer look
at Ethernet, Fast Ethernet, and Gigabit Ethernets.
Ethernets
Collision Domains Should Be Small




The packets sent by the hosts on the same collision
domain may collide with each other.
   2 Pairs of Wires in a UTP Cable Are
               Always Used



                                                     2 pairs




No matter whether the device is a hub or a switch, 2 pairs of wires
in a UTP cable are always used to connect a host to the device.
 Ethernet
MAC flow
 in Half-
 Duplex
  Mode
  Switching Between Full and Half-
      Duplex Modes Is Simple




No matter whether full or half-duplex mode is used,
2 pairs of wires in a UTP cable are always used to
connect a host to a hub/switch port.
 Full-Duplex Ethernet Is No Longer
             Ethernet
• Ethernet is characterized by its CSMA/CD
  MAC protocol when operating in half-
  duplex mode.
• In full-duplex mode, because CSMA/CD is
  disabled and not used, actually full-duplex
  Ethernet is no longer Ethernet.
  – This is especially true for Gigabit Ethernet
    because Gigabit Ethernet mostly operate in full-
    duplex mode.
         Transmitted Bit Order




Little Endian: The least significant bit in a byte is
transmitted first.
          Transmitted Byte Order




The bytes of a multi-byte field are sent from first
byte to last byte, with each byte sent in Little Endian
bit order.
      Ethernet Address Format




Every vendor (e.g., 3COM) is assigned a vendor
block code. Therefore, every globally administered
address is globally unique.
IEEE 802.3 Ethernet Frame Format

                    If length is less
                    than or equal to
                    1500, it
                    represents the
                    length of data.
                    Otherwise, it
                    represents the
                    higher-level
                    protocol type.
Fast Ethernet
Encoding Method
     Flow Controls Needed When
     Ethernets Switches Are Used
• When a hub is used to connect all sending hosts
  (half-duplex mode), Ethernet‟s CSMA/CD MAC
  algorithm is an effective flow/congestion control
  (backoff) and retransmission (up to 16 times)
  method.
• However, when a switch is used and full-duplex
  mode is used, each host now has its own collision
  domain and CSMA/CD no longer works. Frames
  now may be lost due to buffer overflow inside the
  switch.
   – Hosts no longer know this type of “collision.”
     Congestion control cannot automatically be performed.
   – Also, retransmission cannot automatically be performed.
Backpressure Can Be Used In Half-
          Duplex Mode
• If a switch uses half-duplex mode to connect to
  hosts, although each host has its own collision
  domain, we can use some methods to ask sending
  hosts to slow down their sending rates:
   – Force collisions with incoming frames
   – Make it appear as if the channel is busy
• If a switch uses full-duplex mode, the above two
  methods won‟t work. We need an explicit flow
  control for switched full-duplex Ethernet switches.
       IEEE 802.3x Flow Control
• Introduced for Fast and Gigabit Ethernets.
• Introduce special MAC-layer control packets
  (PAUSE packets) to enable or disable frame
  transmission.
• Thus a more generic MAC framework is defined
  and an entity called (MAC Control) is introduced.
• MAC Control is responsible for generating,
  sending, receiving, and performing the PAUSE
  operation.
IEEE 802.3x Architecture
MAC Control Frame Format
             PAUSE Function
• Implement a simple “stop-start” flow control
  scheme.
• If a device wants to temporarily inhibit incoming
  frames, it sends a PAUSE frame to the full-duplex
  partner. This PAUSE frame contains a parameter
  indicating the length of time the partner should
  wait before sending more frames.
• If the device wants to cancel the timer at its
  partner which is set up by its previous PAUSE
  frame, the device can send another PAUSE frame
  that contains a parameter of zero time.
PAUSE Frame Implementation

                   PAUSE frames have
                   higher priority over
                   normal data frames.
Must Stop Transmission in 512
          Bit Time
                   • After receiving a
                     PAUSE frame, the
                     receiver has 512 bit
                     time to decode it.
                   • No more new
                     transmission is
                     allowed after this 512
                     bit time period.
Buffer Thresholds to Control When
     to Issue PAUSE Frames
                    • The buffer space
                      (above the high water
                      mark and below the
                      low water mark)
                      should be larger than
                      link RTT * link BW.
                    • Otherwise, frames
                      may be dropped or
                      link utilization may
                      not be 100%.
Input-Buffered Switches Assumed
        for IEEE 802.3x
              • IEEE 802.3x is a link-level
                flow control scheme.
              • It uses the buffer
                occupancy level of an
                input port to determine
                whether to inhibit frames
                from incoming.
              • Unfortunately, this scheme
                does not work for output-
                buffered switch. (Why?
                because there will be no
                queue in input ports.)
 IEEE 802.3x May Cause Low Link
            Utilization
• IEEE 802.3x is a link-level flow control applied to
  input-buffered switches.
• If the input-buffered switch has the HOL blocking
  problem, then using IEEE 802.3x will not further
  lower the link utilization. (It is already low .)
• However, if the input port uses virtual output
  queuing (one queue for each output port) to avoid
  the HOL blocking problem, using IEEE 802.3x
  may further lower the link utilization.
   Low Link Utilization Example

  Output port 1 is busy or blocked all the time.
          Switch B                                 Switch A

          P1, P1,                                   P1 P2
          P1, P1                                    P2
                                                       P2
                           PAUSE
  Output port 2 is idle.

After receiving the PAUSE frame, switch A cannot send any
more frame to switch B. However, sending P2 should be
allowed because P2 can be immediately forwarded when it
arrives at switch B.
Auto-Negotiation Eases Configuration
• Starting from Fast Ethernet
• For UTP cables
• Auto negotiation about duplex mode (half or full),
  maximum speed (10 or 100 Mbps), and flow
  control support.
• Avoid configuration headache.
   – The same RJ45 jack can support 10 or 100Mbps and
     half or full duplex mode, how do you know which
     setting you should use when you plug your RJ45 into a
     hub or a switch‟s port?
   – So, the best setting that you should choose is auto-
     negotiation.
Auto-Negotiation Message Format
Auto-Negotiation Message Transfer




Because link speed is one of the parameter that needs
to negotiate, the auto-negotiation message cannot be
transferred at a chosen speed (e.g., 10 or 100 Mbps).
Instead, it is transmitted at a predetermined clock
rate.
Gigabit Ethernet (IEEE 802.3z)
Gigabit Ethernet Architecture
Encoding Method
        1000BASE-X Physical Media
                      Fibers
Shortwave Laser




Longwave laser
Ethernet Technology Migration
Ethernet Technology Migration
    Minimum Ethernet Frame Length
         Needs to be Increased
• When link speed increases from 100 Mbps to
  1000 Mbps, for half-duplex CSMA/CD to work
  well (to be able to detect collision), we can:
  – Reduce the network size by a factor of 10 to only 20
    meter
     • Not acceptable
  – Or increase the minimum Ethernet frame length by
    a factor of 10
     • Network efficiency will decrease.
     • Higher layer protocols needs to be modified.
               Carrier Extension
• Gigabit Ethernet does not want to modify
  application programs.
   – We want application programs to still use 64 bytes as
     the minimum frame size. Ideally, they should not need
     to know whether they are using Ethernet, Fast Ethernet,
     or Gigabit Ethernet.
• However, to be able to detect collision, the
  minimum frame size must be enlarged from 64
  bytes to 512 bytes (4096 bits).
• Carrier extension is introduced to insert padding at
  the MAC layer so that every transmitted frame is
  at least 512 bytes.
Carrier Extension
              Frame Bursting
• Although carrier extension solves the
  collision detection problem, the link
  utilization can be very low.
  – If every transmitted frame is a minimum-sized
    frame, because in every transmitted 512 bytes,
    there are only 64 useful bytes. The effective
    link utilization is only 64/512 = 12.5%
• Frame bursting is introduced to improve
  link utilization.
                    Frame Bursting




• If the source host has many small frames to send, it can send
  them in a burst after a MAC arbitration.
• Except for the first frame, other frames sent in a burst need not
  be carrier-extended.
• If the first frame is larger than 512 bytes, it need not be carrier-
  extended.
• The burst can be as long as 8192 bytes to improve throughput.
            However, the livelock problem is not solved!
   Why Doing Carrier Extension for the
        First Frame in a Burst?
• First, if there is no carrier extension, when collision
  happens, the sending host may have sent out many small
  frames. If the sending host resends all frames that have
  been sent, two problems may occur:
   – They may already be removed from the buffer.
   – The receiver may receive duplicate frames.
• Second, if there is carrier extension and collision is
  detected in this period, the sending host knows that only
  the first frame should be retransmitted.
• Third, if there is carrier extension and there is no collision
  detected in this period, the sending host can be assured
  that no collision will happen in the future.
 Gigabit Ethernet Mostly Operates in
         Full-Duplex Mode
• Although half-duplex (hub) Gigabit Ethernet‟s
  MAC performance is academically interesting,
  commercially they are rarely used.
   – Under 1000 Mbps high speed, CSMA/CD‟s performance
     is bad when the number of participating hosts increases.
   – Why not using full-duplex mode (switch) to get much
     more bandwidth at slightly increased cost?
• Nowadays, almost all Gigabit products are switches.
   – Since CSMA/CD is totally disabled, Gigabit Ethernet
     actually no longer is Ethernet.
   – The sending host just pumps its frames into the cable as
     fast as it can.
Physical Layer Architecture
Various Line Coding Methods

                       Gigabit
                       Ethernet
                       uses NRZ.


                       Manchester
                       consumes
                       too much
                       Bandwidth.
                       Gigabit
                       Ethernet
                       cannot afford
                       to use it.
1000BASE-X Uses 8B/10B Block Code
 • Unlike Manchester encoding, whose baud rate is
   200% of the data rate, in 8B/10B, the baud rate is
   only 125% of the data rate.
    – Supporting 1000 Mbps is already difficult for some
      media such as UTP. Asking UTP to support 2000 Mbps
      baud rate is too much.
 • In 8B/10B, an 8-bit byte is encoded into a 10 bit
   code word.
    – Provide sufficient signal transitions for clock recovery
      at the receiver
    – Allow easily detecting single or multi-bit errors
    – The number of one and zero bits transmitted are almost
      the same (DC balance).
          8B/10B Block Coding




• No more 4 consecutive ones or zeros will be generated in a
  10-bit code.
• The imbalance between the number of ones and zeros in a
  10-bit code is at most 1.
1000BASE-X Physical Layer
          1000BASE-X Connectors




• One jack is for transmission. The other jack is for reception.
• To connect two hosts together, you need to „cross-over‟ the
  transmission and reception jacks by yourself.
            1000BASE-T over UTP
• Product is already available, but not too long ago.
  (introduced to Taiwan just in March, 2001)
• Use four (all) pairs of wires in an UTP cable to together
  support 1000 Mbps.
• The quality level of UTP cables can be CAT 5.
• The cost of a 1000BASE-T network adapter is much
  cheaper than that of a 1000BASE-X network adapter.
  (7,400 vs 25,000 NT dollars)
• We will purchase and test 1000BASE-T NIC soon (for
  the “Design and implementation of high-speed network
  systems” course.)

								
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