[hpsdr] A note about consumer-grade Ethernet switch/router throughput

[Tom McDermott]

One item to consider in home-network performance issues is the throughput of some consumer-grade ethernet switches and routers.
 
The HPSDR protocol puts 126 IQ samples per 1032-byte Ethernet frame (one Rx), and the same for transmit.
 
This makes the data rate for one 192k receiver equal to 12.58 megabits/sec, and the data rate for the 48k transmitter equal to 3.15 megabits/sec. Thus one T+R is 15.73 megabits/sec (ignoring some minor ethernet overhead)
 
Some common terminology: an ethernet switch in consumer products usually is just a dumb switch with only LAN ports. The router usually refers to a device that has at least one WAN port intended to connect to the outside world (i.e. your DSL modem, cable modem, whatever), plus usually some number of LAN ports.
 
My ethernet router is about 8 years old. It has 8 x 100M LAN ports, and one 100M WAN port. Back when it was built, the manufacturer (a very large well known supplier) used a low-power embedded microprocessor and firmware to do switch and route each individual frame. The switching effort is minor, but packets destined to / from the WAN port undergo more processing. Usually there's a NAT, a firewall, TCP port checking, etc. Since my Windows computer already has a firewall, and the DSL IP provider modem also has a firewall, I am blessed with triple redundancy in the firewall department.
 
However because of the lack of horsepower, my ethernet switch does not come anywhere close to actually being able to forward at 100M/s.  In fact the WAN throughput on my device maxes out at 5.6 megabits/sec (reported on the internet for my device, and I've confirmed it). Note that this is much slower than the HSPDR frame rates. Makes you wonder why the manufacturer even bothered to put a 100M port on the WAN jack.
 
The LAN throughput is higher.  I've been able to almost run 15.73 megabit/sec from LAN port to LAN port through my router. While I cannot measure LAN-LAN throughput, I suspect that 16 megabits/sec is right on the hairy edge of what my switch can do. Turning on 2 HPSDR receivers, or having any other local network traffic would drop a lot of packets through my home Ethernet switch.
 
More modern ethernet switches and routers (particularly gigabit switches) have migrated to using an actual VLSI switch chip for the fast-path ethernet frames. The specification on these devices lists throughput equal to the actual line rate (i.e. 1 gb/s per port). I don't know if that's accurate, but it's probably fast enough not to be a concern for HPSDR.
 
-- Tom, N5EG
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