CSMA/CD Bit Budget
Stephen Haddock
Extreme Networks
9/9/96
This is an ASCII version of the presentation given at the 802.3z Interim
in September 1996. Several of the diagrams do not translate to ASCII
well. The original presentation is included in the reprint book.
Example Calculation of the Worst Case Collision Event:
This assumes an 8 bit data path and 125 MHz state machines.
Byte Times Device Signal propagation
1 DTE-1 MAC Tx to TXEN
5 DTE-1 TXEN to MDI
62.5 Link Worst Case link delay
5 Repeater MDI to CRS assert
6 Repeater CRS to TXEN (Tx)
5 Repeater TXEN to MDI
62.5 Link Worst Case link delay
5 DTE-2 MDI to CRS assert
4 DTE-2 CRS to TXEN (Worst Case Non-defer)
5 DTE-2 TXEN to MDI
62.5 Link Worst Case link delay
5 Repeater MDI to CRS assert
4 Repeater CRS to TXEN (Jam)
5 Repeater TXEN to MDI
62.5 Link Worst Case link delay
5 DTE-1 MDI to COL assert
2 DTE-1 COL assert to MAC detect
-------
307 Worst Case Collision Detection
The following tables show the components of the 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
and Repeater with GMII connection to an 8 bit PCS/PMA.
3) 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.
Signal Propagation 8 bit 16 bit 16 bit
data path (Ext EnDec) (Int EnDec)
------------------- --------- ----------- -----------
MAC:
Tx to TXEN 1 2 2
CRS,COL to MAC detect 1-2 2-4 2-4
Worst case non-defer 3-4 6-8 6-8
PHY:
TXEN to MDI 4-5 4 7
MDI to CRS,COL assert 5 5 8
MDI to CRS,COL deassert 7-8 7-8 10-11
------------------- --------- ----------- -----------
MAC + PHY:
Tx to MDI 5-6 6 9
MDI to MDI non-defer 12-14 14-16 20-22
MDI to COL detect 6-7 7-9 10-12
Total (2 DTES):
Tx to COL detect 27 31 43
------------------- --------- ----------- -----------
Repeater:
CRS to TXEN 5-6 8-10 8-10
COL to JAM 3-4 6-8 6-8
Repeater w/PHY:
SOP (MDI to MDI) 14-16 17-19 23-25
SOJ (MDI to MDI) 12-14 15-17 21-23
Total:
SOP + SOJ 30 36 48
------------------- --------- ----------- -----------
Link:
2 x 100 meter fiber 250 250 250
(10 ns/m round trip)
Total (fiber):
Worst Case Collision 307 317 341
------------------- --------- ----------- -----------
Wild Guess at UTP budget:
2 DTEs 67 71
Repeater (SOP + SOJ) 70 76
2 x 100 meter UTP 285 285
(11.4 ns/m round trip)
Total (UTP):
Worst Case Collision 422 432
------------------- --------- ----------- -----------
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.