802.3z CSMA/CD Repeater 

Stephen Haddock
Extreme Networks
11/11/96

This is an ASCII version of the presentation given at the 802.3z Working 
Group in November 1996.  Several of the diagrams do not translate to 
ASCII well.  

Comments received after the September bit budget presentation:
1) The budget should use the "worst case collision response" instead 
of "CRS to collision detect".
2) UTP cable delay should use Cat-5 (11.12 ns/m round trip) instead 
of Cat-3 (11.4 ns/m round trip).
3) 8 ns for CRS sync (in 8 bit wide MAC implementation) is marginal 
for metastable resolution.
4) Encoder needs more latency to detect End Delimiter (not for CRS as 
defined but perhaps for RX_DV, RX_ER).
5) Repeater needs more time.
6) Guesses for UTP latencies look achievable.


The following tables show the components of the updated bit budget base 
on three implementation assumptions:

1)  Assuming an 8 bit data path and 125 MHz state machines in all 
devices.
2)  Assuming a 16 bit data path and 62.5 MHz state machines in the 
MAC, Repeater, and all PCS blocks, with 10 bit fibre channel 
interface to 10 bit SerDes chips.  (This results in larger numbers 
than assuming a 16 bit MAC with a GMII interface to an external 
PCS/PMA implementation.)
3)  Proposed budget for 802.3z which was arrived at by generously 
distributing the margin achieved by the carrier extension.


Signal Propagation         8 bit        16 bit     Proposed
                         data path   (Int EnDec)    802.3z
-------------------      ---------   -----------  -----------
MAC: 
  Tx to TXEN                1            2            2
  CRS,COL to MAC detect    2-4          2-4           6
  Worst case non-defer      6            8           12
  Worst case collision      6            8           10


PHY: 
  TXEN to MDI              4-5           7           16
  MDI to CRS,COL assert     5            8           16
  MDI to CRS,COL deassert  7-8         10-11         19

-------------------      ---------   -----------  -----------

MAC + PHY:
  Tx to MDI                 6            9           18
  MDI to MDI non-defer     16           23           44
  MDI to COL detect        11           16           26

Total (2 DTES):
  Tx to COL detect         33           48           88

-------------------      ---------   -----------  -----------
Repeater:
  CRS to TXEN              6-8          8-10         20
  COL to JAM               4-6          6-8          20

Repeater w/PHY:
  SOP (MDI to MDI)         18           25           52
  SOJ (MDI to MDI)         16           23           52 

Total:
  SOP + SOJ                34           48          104

-------------------      ---------   -----------  -----------
Link:
  2 x 100 meter fiber     250          250          250
    (10 ns/m round trip)

Total (fiber):
  Worst Case Collision    317          346          442

-------------------      ---------   -----------  -----------
Wild Guess at UTP budget:
  2 DTEs                   73           88           88
  Repeater (SOP + SOJ)     74           88          104
  2 x 100 meter UTP       278          278          278
(11.12 ns/m round trip)

Total (UTP):
  Worst Case Collision    425          454          470

-------------------      ---------   -----------  -----------








OTHER CONSIDERATIONS:

IPG Shrinkage:
  Caused by CRS assertion and deassertion variability in a PHY:
    In this case the only component is the synchronization of CRS 
    to transmit clock.  Can result in removing one Idle symbol pair
    the IPG.

  Causes fairness problems if lose more than 4 bytes of IPG
    In single repeater topology can restrict to no more than one
    Idle symbol pair removed by the repeater, and one pair by the
    receiver.

Collision After Defer:
  Case to consider when one station transmits two packets back-to-back
    with minimum IPG.  A second station defers to the first packet and
    collides with the second.  If the IPG between the two transmissions
    gets shortened by the mechanism mentioned above, then the "worst 
    case non-defer" of the second station increases by two byte times.
  There is sufficient margin in the 512 byte carrier extension that
    this is not a problem.


CONCLUSIONS:
  A 512 byte carrier extension allows CSMA/CD operation on a 200 meter
    diameter network.  There is plenty of margin to allow a wide range 
    of implementations of MACs, PHYs, and Repeaters.  There is 
    insufficient margin to allow even a very small network without
    carrier extension.

  A single repeater topology is mandated by the bit budget and by 
    potential IPG shrinkage.