glownt
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Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Wed Mar 14, 2018 6:39 pm

Hi,

First of all, congrats for the new arrival to the Raspberry Pi family.

I was very interested in this 3B+ model when I first saw it this morning because it's the first enhancement in the Eth speed since the first versions of the Pi, and I awaited for long that inprovement.

Sadly enough, I also discovered today (altough I suspected it) that the hardware is completely closed in the case of the Pi. That prevents me from finding out if the only USB2.0 is shared between the USB hub and the ethernet. I can safely assume that it is shared due to the use of the LAN7515 from the infamous Microchip... Anyway, and here comes the real question, what about the wifi module, Does it use the same USB 2.0 or a different bus? Can I achieve reasonably good throughput for a NAS system using the WIFI instead of the Eth? (I know that the Pi is not the best hardware out there for a NAS, but call me nostalgic, I like it anyway!)

PD: I am not asking for the full schematic, but a brief architecture of the hardware...

jamesh
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Wed Mar 14, 2018 7:55 pm

Architecture is the same at the Pi3 (and all previous Pi models with ethernet)

There is one USB2 port on the SoC which handles the USB ports and the ethernet.

The wireless chip is connected via SDIO, the Bluetooth part is connected to a UART.


In our tests, the move to gig ethernet gives a threefold improvement in bandwidth in uncontended conditions, although clearly not to the entire capabilities of the ethernet. This will drop if using the USB at the same time.


Note that this information is freely available - you don't need the schematics or open HW to find it out - Google would have worked.
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glownt
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Joined: Wed Mar 14, 2018 6:28 pm

Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Wed Mar 14, 2018 8:25 pm

Thanks for the quick reply jamesh, but you didn't clarify if it would be better to use wifi instead of Ethernet to improve the throughput for a NAS system that uses USB hdd.
About the closeness that you point out below:
jamesh wrote:
Wed Mar 14, 2018 7:55 pm
Note that this information is freely available - you don't need the schematics or open HW to find it out - Google would have worked.
I prefer to go to the sources instead of Google gossip, when the sources are available of course...
Cheers,
A.

jamesh
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Wed Mar 14, 2018 8:40 pm

glownt wrote:
Wed Mar 14, 2018 8:25 pm
Thanks for the quick reply jamesh, but you didn't clarify if it would be better to use wifi instead of Ethernet to improve the throughput for a NAS system that uses USB hdd.
I don't know. I'm sure benchmarks will appear at some point.
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thagrol
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Wed Mar 14, 2018 11:36 pm

Don't forget that every piece of your network infrastructure has to support gigabit or you likely won't see any improvement.

That means cables, switch, client machine NICs etc.
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tkaiser
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Thu Mar 15, 2018 12:56 pm

glownt wrote:
Wed Mar 14, 2018 8:25 pm
you didn't clarify if it would be better to use wifi instead of Ethernet to improve the throughput for a NAS system that uses USB hdd.
Here you find numbers for both: viewtopic.php?f=63&t=207897#p1285923

Please keep in mind that Wi-Fi is always 'shared medium' so if you live in a crowded area wireless performance might drop a lot. With GbE it's at least predictable.

tkaiser
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Thu Mar 15, 2018 1:09 pm

thagrol wrote:
Wed Mar 14, 2018 11:36 pm
Don't forget that every piece of your network infrastructure has to support gigabit or you likely won't see any improvement.
Better replace 'likely' with 'definitely' ;)

If there's good cause for your fear (RPi users not getting that every piece of a network connection must be GbE capable to get faster networking) then at least I fear soon RPi users will struggle with strange GbE problems caused by bad cables.

Due to the nature of Gigabit Ethernet utilizing all 4 cable pairs instead of two as with Fast Ethernet users might run into troubles now when switching to GbE with bad cables (that work just fine with older RPi). In such a situation it might help to 'downgrade' from Gigabit to Fast Ethernet:

Code: Select all

sudo ethtool -s eth0 speed 100 duplex full autoneg on
This is meant as a temporary workaround or as a diagnostic measure since the only solution once the cable is identified as the culprit is to replace the cable with a new one (Cat5E or better)

hippy
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Thu Mar 15, 2018 1:21 pm

glownt wrote:
Wed Mar 14, 2018 8:25 pm
Thanks for the quick reply jamesh, but you didn't clarify if it would be better to use wifi instead of Ethernet to improve the throughput for a NAS system that uses USB hdd.
With WiFi via SDIO and ethernet via USB there will be less USB contention using WiFi but there are so many factors which could affect things only someone benchmarking would probably be able to say whether there was any actual advantage to using WiFi.

Best bet is perhaps to buy a Pi 3B+ and see which works best for you.

tkaiser
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Re: Raspberry Pi 3B+ USB, WiFi, Eth shared/common bus

Thu Mar 15, 2018 1:58 pm

hippy wrote:
Thu Mar 15, 2018 1:21 pm
With WiFi via SDIO and ethernet via USB there will be less USB contention using WiFi but there are so many factors which could affect things only someone benchmarking would probably be able to say whether there was any actual advantage to using WiFi.
How should this be possible? There's just one antenna --> we're able to use just one spatial stream --> the number is 433Mbps (keep in mind that these numbers used with wireless technologies mean upload + download at the same time unlike with wired numbers like 100Mbps or 1000Mbps).

If we divide by 2 we get the maximum theoretical bandwidth and if we then remove all the overhead and look at what we get at the TCP/IP layer then I would assume we're talking about slightly more than 100Mbits/sec with Wi-Fi on the new board. If distance to AP increases, a few walls are in between and if 802.11n and not 802.11ac is used (only the latter allows for beamforming, then the AP can locate the client and provide better performance) then those numbers will decrease even more.

The new dual-band Wi-Fi is a great improvement for sure but since there's no MIMO (multiple antennas and spatial streams) the Wi-Fi is already the bottleneck regardless of host side bus limitations (SDIO or USB).

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