Carrier Extension Issues
Stephen Haddock
Extreme Networks
5/21/96


Introduction:

Scaling 802.3 MAC to 1Gb/s without modifying the slot time
results in an extremely small network diameter.  The carrier
extension proposal allows a larger network diameter by
increasing the slot time from 64 to 512 byte intervals.
Increasing the slot time has undesirable side effects.
This presentation examines those side effects and proposes
some mechanisms to mitigate them.


Implications of 512 byte slot time:

-- Larger transmit and receive FIFOs.
-- End of packet may precede end of carrier.
       When is packet transmission successful?
-- Reduced network efficiency:
       When transmitting short packets and
       When collisions occur.
-- Amplification of capture effect


Larger Tx/Rx FIFOs:

-- Transmitter must be prepared to retransmit up to 512 bytes.
-- Receiver must be prepare to flush up to 512 bytes.


Whole Packets within Fragments:

Station A transmits short packet with carrier extension.
Station B (at maximum distance from station A) collides with A.
Fragment received by station C may contain the entire packet
   but must still be discarded to avoid duplication of data
   when A retransmits.


Network Efficiency:

Efficiency = Useful Data / (Useful Data + Overhead)
    For calculations below, "useful data" includes the 802.3
    data field, and "overhead" includes IPG, preamble, MAC
    header, and FCS.
Carrier Extension on small packets increases overhead.
Collisions reduce efficiency due to:
    Time spent in collision itself.
    Idle time when all stations in backoff.


Network Efficiency:  Small Packets

    Packet distribution            Efficiency
                                64 byte     512 byte
                               slot time    slot time
  ----------------------------------------------------

    All minimum size packets      55 %          9 %
    50% min and 50% max           95           75
    Workgroup Avg distriburion    94           72


Network Efficiency:  Collisions

    Collision frequency            Efficiency
     (Workgroup Avg packet      64 byte     512 byte
      size distribution)       slot time    slot time
  ----------------------------------------------------

    No collisions                 94 %         72 %
    Mohan's simulation of         86           56
      15 station, 200 m network



Enhanced Carrier Extension:

Increase minimum carrier event to 512 byte times.
Allow multiple packets within a carrier event.
    Minimal incremental burden on MAC Tx/Rx.
    Virtually eliminates efficiency loss on small packets.
No changes to MAC service interface.


Expected Benefits:

Expect the most benefit on networks with relatively 
    few station offering high loads.
       This is typical load distribution on segments
       providing switch-switch and switch-server
       connectivity -- primary application anticipated
       for Gigabit Ethernet.
Little benefit on networks with a large number of 
    stations each offering a light load.
       Typical load distribution fo segments providing
       desktop connectivity.



Packet Packing:  Transmission:

Tx Rule 1:  Transmitter may begin a new packet if current
   carrier duration is less than the slot time (512 bytes).
    -- Transmitter does not need to wait until it has 
        multiple packets to being transmission.
    -- Must keep at least min. IPG between packets.
    -- Results in up to 7 packets within carrier event.
    -- Max. carrier event approximately 2 KBytes (slot
        time plus max. packet)
    -- PCS encoding must allow a (properly aligned) S after
        R without intervening idle.


Packet Packing:  Collisions:

Tx Rule 2:  If a collision occurs, the transmitter must
   retransmit all packets included in the carrier event.
       Implies transmitter cannot report Tx status to MAC
       service interface until carrier active for a slot
       time.

Rx Rule:  Receiver must discard all packets received within
   a single carrier event if total carrier duration is less
   than the slot time.
       Implies receiver cannot report Rx frames to MAC
       service interface until carrier active for a slot
       time.


Packet Packing:  Errors:

Coding/CRC Errors:  Assuming carrier duration exceeded slot
   time, discard only frames containing errors.

Excessive Collisions:  Transmitting stations MAY retransmit
   all packets in carrier event.

Late Collisions:  Receiver may keep any complete packets.
   Transmitter may only retransmit the last packet.


Network Efficiency:  Packet Packing

    Collision frequency             Efficiency
     (Workgroup Avg packet  64 byte   512 byte   Packed
      size distribution)   slot time  slot time  Packets
  -------------------------------------------------------

    No collisions             94 %      72 %       94 %
    Mohan's simulation of     86        56         74
      15 station, 
      200 m network


Conclusions:

The Carrier Extension proposal enables maintaining a 
    200 meter network diameter for CSMA/CD operation,
    however it reduces network efficiency with small
    packets.
Allowing the transmission of multiple packets within
    a carrier event will recover most of the loss in 
    network effieciency.
     -- Without packet packing, Gigabit Ethernet
         throughput is 6.5 times Fast Ethernet through-
         put with the workgroup average packet size
         distribution.
     -- With packet packing, Gigabit Ethernet through-
         put is 8.6 times Fast Ethernet.
     -- Carrier Extension with Packet Packing has 32%
         higher throughput than without Packet Packing!