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

Vladimir www2008_2 at mail.ru
Fri Jul 24 12:29:27 EDT 2015


 Ian,

Thank you for the explanation! And what about Rx - is it also buffered similarly? We are mainly concerned about Rx buffering, since it probably can compensate some hitches in ethernet/pcie interface. The question is: does X310 have more memory that we could use to extend Rx buffer? How much is this difference, comparing to X300? We have to select one of them and need to estimate, how big is the difference from this point.

Vladimir


>Пятница, 24 июля 2015, 8:54 -07:00 от Ian Buckley <ianb at ionconcepts.com>:
>
>Vladimir, 
>both X300 and X310 using the factory FPGA images offer approximately 520K Bytes of SRAM FIFO that buffers Tx data before transmission. This buffer decouples the host and transport with it's inherent scheduling latency and jitter from the hard real-time nature of the DAC's consuming data. That amount is empirically enough to run practical applications at the highest sampling rates possible on the X300, clearly at much lower sampling rates it's overkill. This is actually an easily altered parameter during the build process, but typical users would not alter it, there is very little motivation. Very few applications can make practical use of over large FIFO's for Tx because typical radio's do not know in advance when and what they will be transmitting.
>We have a build option for the FPGA that uses DRAM instead of SRAM to build the large buffers but it's not currently able to reliably sustain throughput in the corner cases that can occur at the highest MIMO sample rates and is thus still the subject of development effort. The DRAM is far larger than any practical Tx FIFO's would need to be and it is included for the flexibility of FPGA developers who may use it for all sorts of things. Expect to see some RFNoC blocks using it for example.
>
>-Ian
>
>On Jul 24, 2015, at 7:05 AM, Vladimir via USRP-users < usrp-users at lists.ettus.com > wrote:
>>Marcus,
>>
>>To clarify the purpose of this question: if we get X310, will we have ability (maybe by modifying firmware) to have larger transfer buffer, and how much can be increase in size comparing to X300?
>>
>>I didn't realize that X3X0 has 1GB DRAM (Shown on the diagram in http://www.ettus.com/content/files/X300_X310_Spec_Sheet.pdf ). Is this the DRAM that you mentioned? Can you please summarize all the memory in both units?
>>
>>E.g., is this correct: both X300 & X310 have 1 GB DRAM, and SRAM size differs according the FPGA model (16,020 Kb and 28,620 Kb as specified here: http://www.ettus.com/product/details/X310-KIT )?
>>
>>Theoretically, if we alter FPGA firmware, can we increase xfer buffers, or their sizes are hardware defined? Didn't find the info about those two firmware versions you spoke about - where to read on this, or what is the difference btw them?
>>
>>Thanks!
>>Vladimir
>>
>>>Пятница, 24 июля 2015, 13:37 +02:00 от Marcus Müller via USRP-users < usrp-users at lists.ettus.com >:
>>>
>>>Hi Vladimir,
>>>
>>>no, from the computer's point of view, these devices are really
    identical.
>>>
>>>There is some buffering on the X310 and X300, and as far as I know
    they are absolutely identical (I'd have to read the FPGA source code
    [1]), yes, but it's not large enough to buffer seconds of sample
    data -- that would inherently lead to a lot of latency, and in
    almost all applications, that would be unwanted. We actually do have
    two different stock images for the FPGA, one using only SRAM (HGS),
    and one using DRAM (XGS); the first is the default, and most
    customers use it.
>>>
>>>Regarding accurate clocking: Both PCIe and 10GE are
    packet/burst-based, so there's some kind of inevitable buffering;
    however, the USRP keeps it's own reference oscillator-disciplined
    time, and gives you timestamps with the RX samples, as well as
    allowing you to specify the exact time when a sample should be sent,
    or a command (such as tuning, setting gain) should be exectuted.
    That's why in many cases, the interface latency doesn't matter at
    all -- you just make sure that the first bunch of samples of your
    transmission burst is sent "early enough" to the USRP, and add a
    timespec telling the USRP to hold these samples.
>>>
>>>Setting the device time can be done e.g. by a GPSDO, or via the
    interface on the edge of a PPS (pulse-per-second) signal (doesn't
    really need to be a pulse per second if you really just want to set
    the device time once -- a pull-up switch does work ;) ), or without
    external time reference by just instructing the X3x0 to set a new
    time in its timing registers.
>>>
>>>All this is done via standard UHD API calls.
>>>
>>>Best regards,
>>>Marcus
>>>
>>>[1]  https://github.com/EttusResearch/fpga/tree/master/usrp3/top/x300
>>>On 24.07.2015 13:19, Vladimir via
      USRP-users wrote:
>>>>Marcus,
>>>>
>>>>Yes, I understand this, but does it affect the connectivity with
      host, or its used only for signal processing algorithms? May be
      this is another question, but does X300 do any IO buffering at
      all, to provide accurate clocking of the samples when interfacing
      e.g. with ethernet? Is there some buffer there and how big is it?
>>>>
>>>>Thanks,
>>>>Vladimir
>>>>
>>>>>Пятница, 24 июля 2015, 13:09 +02:00 от
        Marcus Müller via USRP-users <usrp-users at lists.ettus.com> :
>>>>>
>>>>>The difference
                is that the X310 has the bigger FPGA, which only makes a
                difference if you want to do custom processing on it --
                for example, building a largish RF NoC application.
                Otherwise, X300 and X310 are absolutely identical.
>>>>>
>>>>>Best regards,
>>>>>Marcus
>>>>>
>>>>>On 24.07.2015 13:05, Vladimir via USRP-users wrote:
>>>>>>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/
>>>>>>>_______________________________________________
>>>>>>>USRP-users mailing list
>>>>>>>USRP-users at lists.ettus.com
>>>>>>>http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>_______________________________________________
USRP-users mailing listUSRP-users at lists.ettus.com
>>>>>>http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>>>>
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>>>>
>>>>
>>>>
>>>>
>>>>
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>>>
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>>
>>
>>
>>
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>>
>>
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