Communication Bus Statistics

The following table contains some statistics about digital communication busses that I have recently seen in designs, notably the bus length to bit period statistic that I have talked about in the past (the higher the bus length to bit period ratio, the more complex the design of the bus is). 

Communication Standard	Signaling Type	Communication Architecture	Bus Delay (ns)	Signaling Rate (Mbps)	Data Rate (Mbps)	Bit Error Rate	Ratio of Bus Length to Bit Period	Ratio of Rise Time to Signailng Bit Period	Design Difficulty	Comment
HyperTransport	Differential LDT	Source Synchronous	1.81	800	800	NA	1.45	0.240	5
DDR SDRAM	SSTL2	Hybrid Souce Synchronous	0.773	400	400	NA	0.31	0.200	4
PCI-X	PCI-X	Synchronous	0.703	133	133	NA	0.09	0.085	3
GMII	3.3V CMOS	Source Synchronous	1.69	125	125	NA	0.21	0.069	3
SAS	Differential CML	Serial	49.05	3000	2400	1.00E-12	117.72	0.411	3
SATA	Differential CML	Serial	9.81	1500	1200	1.00E-12	11.77	0.410	3
Gigabit Ethernet	Differential PAM5 (1000BASE-T)	Serial	570	125	2000	NA	1140.00	NA	NA	Gig-E uses full duplex on same wires to achieve 2 Gbps
Gigabit Ethernet	Optical (1000BASE-SX)	Serial	1830	1250	1000	1.00E-12	1830.00	NA	NA
VCAP video interface	LVDS and LVTTL	Source Synchronous	16.71	148	148	NA	2.47	0.021	5	Timing margin was negative in this design (rise time specified for LVDS portion)
Transport Stream video interface	LVDS and LVTTL	Source Synchronous	19.98	90	90	NA	1.80	0.013	4	rise time specified for LVDS portion
I2S digital audio interface	LVCMOS	Source Synchronous	23.36	3.684	3.684	NA	0.09	0.001	5	This was a strange case where a long ribbon cable (6 ft.) with a large impedance discontinuity was used in the transmission channel.  This caused a lot of SI problems because of the fast edge rate of the CMOS driver.  No termination solution worked well.

The design difficulty ratings are described as follows:

1 - very easy (no routing rules needed, just hook it up)
2 - easy (limited routing rules based on rules of thumb and app notes)
3 - average (routing rules needed based on rules of thumb, app notes, or SI simulation)
4 - difficult (routing rules developed with extensive SI simulation and modeling)
5 - very difficult (routing rules developed with extensive SI simulation and modeling, plus a very narrow solution space or a lot of difficulty getting it to work)