[USRP-users] Hardware selection for X310-based system

Vladimir www2008_2 at mail.ru
Fri Jul 24 07:05:31 EDT 2015


 Hi guys,

I wish to thank all of you for valuable information, it's very helpful for us! Looking at all this, seems that we could start from 10 GBE link and then see if we need more latency or whatever else to try PCIe.

I forgot one question yesterday. From the point that has been discussed, how big is the difference btw X300 and X310? One of the diffs is that X310 has more memory - ~3.5 MB if I don't mistake, comparing to 2 MB in X300. Does it use this memory to do some sort of buffering when transferring data to/from host? Will it help to withstand any short hitches/delays that happen during streaming? For now, it's the only reason I could imagine to prefer X310 from this point of view.

Again, thank you very much for the info!
Vladimir

>Пятница, 24 июля 2015, 1:22 +02:00 от Marcus Müller via USRP-users <usrp-users at lists.ettus.com>:
>
>Hi Vladimir,
>
>although he claimed not to answer your question, I think Rob did an
    excellent job of providing the info you've asked for; thank you!
>
>Hence, I'll keep this as short as possible
>On 23.07.2015 22:31, Vladimir via USRP-users wrote:
>>1. We already have an Intel X520-DA2 board which Ettus recommends for 10GBE input. Would it suffice to order just 783343-01 (10 Gb SFP+ ETHERNET CABLE, 1M) to X310 kit to get complete setup to be able to stream data into/from PC? That, or any direct attach SFP+ 10GE cable, or a pair of
    transceivers for whatever electrical or optical medium you plan to
    use.
>>2. From what I read earlier here and saw at Ettus website, I understand that PCI-Express Connectivity Kit (PCIe – Desktop) is NI PCIe-8371 board with the cord. NI states its speed to be 798 MB/s. But to stream 100 MS/s we need 800 MB/s. Is their number - 798 - approximate (which would look kind of strange for me given its precision :)) and this won't in practice lead to speed problems, or do they actually mean 798 MiB/s, or some other explanation? Could anyone shed some light on this? I must admit that this is news to me. But yes, PCIe is not the bus of choice for maximum throughput. In fact, most PCIe
    10Gigabit network adapters, and especially the Intel devices, have
    drivers that are very good of distributing work across multiple CPU
    cores and reducing interrupt pressure -- it's in fact progressively
    hard for some customers to get high rates (>ca. 150MS/s total)
    with the PCIe interface, mainly because the NI driver for
    compatibility reasons is single threaded, and a single core can only
    spin so fast.
>>3. Would it make sense to look at the NI-recommended alternative - 8381 board? Given that the price difference on NI site for 8371 and 8381 is just $50, is it a good idea to go with 8381 to be sure it definitely won't be a bottleneck? Or it was not tested with X310 and we might encounter some compatibility issues, or any other reasons against it? If it was tested, can we order it from Ettus instead of 8371 at comparable price? I must admit I'm really not overly familiar with the NI side of PCIe
    / PXI backend equipment. Generally, you only need to use either 10GE  or PCIe. Use PCIe if you want to combine things into
    large LabVIEW-commanded clusters of USRPs  [1], or if you know that
    you need the possibility to reduce USRP-host latency of the expense
    of more CPU consumption. For the general user, I recommend 10GE.
>>4. Do I get it right that if we encounter problems on 100 MS/s, the next sampling rate we can use would be 50 MS/s, as we must use integer decimation values from the max one? And in this case we'll get 100 MHz bandwidth and limit X310 potential of 120 MHz. Am I correct here? You're quite right. So the point is that you can only use integer
    factors of the master clock rate as sampling rates. For the default
    Master Clock Rate of 200MHz, these are 200MS/s, 100MS/s,
    (66.666..MS/s), 50MS/s, ..., but there is also the 120MS/s master
    clock rate, which is predominantly used by LabView, and can hence be
    used to get sampling rates of 120, 60, (40), 30 MS/s. Also, we have
    an cellular-friendly 184.32 MHz master clock rate. Notice that I put
    some potential sampling rates into parentheses; these are odd
    decimations of the master clock rate, which hence don't exhibit the
    same quality of flatness/anti-aliasing as the even decimations.
>>5. Am I coerrect that the frequency accuracy of stock X310 oscillator is much better than that of previous models, so we don't need external clock if we don't plan to sync several devices, work at large distance, etc? Uh, to be honest, I'm not quite sure about the "much better" part.
    "much better" depends very much on your needs; if you couldn't use
    e.g. the N210 for its frequency offset or drift being much too
    large, then you'll want to use an external reference here, too.
    However, most customers are quite happy with the oscillators we have
    on board -- simply because frequency offset is a given fact in
    digital communications, and typically, the oscillator outperforms
    things like mobile terminals very significantly. This doesn't
    necessary apply to things like high-performance interferometry, or
    very accurate signal detection.
>>It won't be used for any 'real-world' cases like network deployment etc, - still, speaking about GSM/UMTS lab experiments (like connecting one-two MS in the room), may we need additional clock source? No. Base Stations/eNodeBs define the clock of the cells they supply,
    so you don't have to worry. The oscillators really aren't that bad
    -- they just aren't perfect.
>>Am I missing any critical parts to build up the working SDR set? Well, you mentioned using UBX-160, but you can use both
    daughterboard slots with one daughterboard each, allowing you to do
    2x2 MIMO.
>A few typical frequently answered questions' answers:
>* all RF interfaces are 50 Ohm matched
>* the output of the UBX daughterboard can go up to roughly 20dBm; for details, see [2]; safe input range is -infty to -15dBm.
>* UHD itself doesn't need any special privileges and runs completely
    in userland. Only the network card or PCIe bridge card have kernel
    drivers.
>* Ettus support will be on your side.
>
>>I would also like to hear from people who tried to get about 100-120 MHz RX-TX bandwidth from X3X0, e.g. just simply streaming at 100 MS/s to/from disk and doing processing offline later  - which is the most stable and reliable way to do it - 10GBE or PCIe? In my experience: 10GE. The problem here definitely is, as Rob
    mentioned, that it's in no way "simple" to save 2x 100MS/s to disk.
    There's been (and will always be, and that's a good and just thing)
    long discussions on the needs, feasibility and example
    implementation support of sustaining such hard drive write rates.
>A quick calculation shows that, for 16bit short integer fixed point
    complex recording:
>datarate(100MS/s) = 16bit/number * 2numbers/complex *1complex/S *
    100MS/s =3.2Gb/s for a single channel!
>Now, you need to guarantee these rates, meaning, that unless
    you can come up with ridiculous amount of write buffering, the hard
    drive minimum write rate must be above this value, and that doesn't
    even account for the fact that data is typically written to file
    systems that add even further overhead and latency. Now, I just came
    up with this less than reliable source[2]; bad news is that the
    fastest hdd has an average write rate of 193MB/s (ca
    8*200Mb/s = 1.6Gb/s) according to their benchmark -- meaning that
    the minimum rate will be significantly lower. So even a raid of 2 or
    3 hard drives will not be sufficient, and of four only if there
    weren't such things as seek latency. In short: Writing that bulk of
    data to disk in realtime isn't easy anymore; but RAM disks work
    fine, so be sure to have enough RAM.
>
>Best regards,
>Marcus Müller
>
>[1] http://www.ni.com/white-paper/52382/en/
>[2] http://files.ettus.com/performance_data/ubx/UBX-without-UHD-corrections.pdf
>[3]  http://hdd.userbenchmark.com/
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