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  • 1.  Slow speed of Aspera Connect and Aspera for Desktop

    Posted Sat April 04, 2026 08:34 AM
    Edited by feng chen Sat April 04, 2026 08:38 AM

    Hi Community,

    With the help of some wonderful folks here on the forum, I recently managed to get both Aspera Connect and Aspera for Desktop up and running for downloading data from EBI. However, I've now run into a frustrating bottleneck - download speeds.

    When transferring files in the range of tens of gigabytes, I'm only seeing speeds of around 20–40 Mb/s, which honestly feels underwhelming for a tool that's specifically designed for high-speed transfers. For reference, here's the ascp command I'm currently using with Aspera Connect:

    

    

    




    

    ascp -QT -l 500m -P 33001 -i aspera_bypass_rsa.pem era-fasp@fasp.sra.ebi.ac.uk:/vol1/fastq/SRR131/080/SRR13199080/ .

    I'm a bit puzzled - is 20–40 Mb/s really the expected performance here, or am I leaving something on the table? Are there additional parameters or configurations I might be missing? Could this be a network routing issue on my end, or perhaps a server-side limitation?

    Any insights or suggestions would be greatly appreciated. Thanks in advance!

    image

    image


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    feng chen
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  • 2.  RE: Slow speed of Aspera Connect and Aspera for Desktop

    Posted Wed April 08, 2026 05:56 AM
    Edited by feng chen Sat April 11, 2026 11:11 AM

    You can have a look at ascp's log files to figure out what's happening.

    on Windows, logs are generated in a file named: aspera-scp-transfer.log

    see: https://www.ibm.com/support/pages/where-are-my-ibm-aspera-log-files-located

    You can activate logs in terminal by adding this option to ascp: -DDL- 

    The interesting line contains: Receiver bl

    Especially the part after rate

    Example:

    Mar 22 04:30:29 eudemo ascp[1330092]: LOG Receiver bl t/o/r/d/ts/c/h=1371/1371/0/0/1371/1372/1372 rex_rtt l/h/s/o=0/0/28/36 ooo_rtt l/h/s/o=0/0/28/36 rate_rtt b/l/h/s/r/f=28/0/0/55/5/1 ctl bm/bs=0/0 rex n/s/q/v/a/r=0/0/0/0/0/0 bl l/d/o/r/a/x/dl/df/dm/ds=0/0/0/0/0/0/0/0/0/0 disk l/h/b=0/0/0 vlink lq/lo/rq/ro=0/0/0/0 rate t/m/c/n/vl/vr/r=500000000/0/89347688/89347688/500000000/500000000/500000000 prog t/f/e=1928753/1928753/160310005645 rcvD=2

    so take this: rate t/m/c/n/vl/vr/r=500000000/0/89347688/89347688/500000000/500000000/500000000

    t = target (limit: option -l)

    m = minimum (option -m: do not use, stay at zero)

    c = computed : this is the actual sending side speed

    n = network

    vl = vlink local

    vr = vlink remote

    r = receiver disk

    Basically : c = max( m, min(t, n, vl, vr, r))
    But since m=0, we usually have: c = min(t, n, vl, vr, r)

    And since, usually Vlinks are not set, then we have: c = min(t, n, r)

    i.e. the minimum value of: target rate,  network bandwidth and receiver storage speed will be used as transmission speed.

    The "network speed" is computed by aspera's rate controller, based on its patented algorithm which detects congestion based on evalution of latency, and not affected by absolute value of latency or packet loss.

    So, you should first confirm your actual bandwidth by using some bandwidth evaluation tools (e.g https://www.speedtest.net)

    If you use Wifi, it's not very good for high bandwidth.

    And check the logs to see the values above.

    By the way:

    option -T deactivates encryption and integrity check, I recommend not using it. Impact on performance is only seen with speeds above multi Gbps.

    option -Q is deprecated. don't use (ignored)



    ------------------------------
    Laurent Martin
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  • 3.  RE: Slow speed of Aspera Connect and Aspera for Desktop

    Posted Fri April 10, 2026 10:09 PM
    I checked the logs file (see below) and ran a bandwidth test on my end (see attached image).
    Does this mean I need to upgrade my internet connection to get faster download speeds?
    "2026-04-11 09:53:46.753 [35d4-6808] LOG Receiver bl t/o/r/d/ts/c/h=47369/25633/21736/0/89580/46101/46420 rex_rtt l/h/s/o=245/648/258/262 ooo_rtt l/h/s/o=245/648/264/275 rate_rtt b/l/h/s/r/f=244/244/648/267/0/1 ctl bm/bs=0/0 rex n/s/q/v/a/r=17/43511/847/2623/0/967 bl l/d/o/r/a/x/dl/df/dm/ds=23409/967/2623/0/0/0/0/0/0/0 disk l/h/b=0/1/0 vlink lq/lo/rq/ro=0/0/0/0 rate t/m/c/n/vl/vr/r=300000000/0/35638076/35638076/300000000/300000000/300000000 prog t/f/e=65334016/65334016/16189330 rcvD=0"
    image
    image


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    feng chen
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    Original Message


  • 4.  RE: Slow speed of Aspera Connect and Aspera for Desktop
    Best Answer

    Posted Sat April 11, 2026 05:13 AM

    Right,

    Nothing can transfer bytes faster than IP-level speed.

    In your case, in case of download, it's 85Mbps.

    If you had a local internet connection like the following (ethernet and optical fibre), you would surely achieve better results.

    You can give that log line to some AI, to get some insight:

    https://claude.ai/share/0fff6da5-b9ee-4c77-ac38-03eb960811fa

    I checked, the analysis is not too bad.

    I see in the log is that you set a "limit" (i.e. target rate) at 300Mbps, but you achieve only 35Mbps.

    Note that better results are achieve if the target rate is not too high as compared to the actual network rate.

    Here , almost 10x, it's a bit high.

    Good practice is to not exceed twice the actual maximum bandwidth available, else higher packet loss decrease "goodput".

    So, yes, check why you have so small bandwidth, 35Mbps seem like Wifi and ADSL... not ethernet+fibre

    Lower target rate to about 100Mbps in your case, unless you manage to increase your raw bandwidth (speedtest).



    ------------------------------
    Laurent Martin
    ------------------------------

    Original Message


  • 5.  RE: Slow speed of Aspera Connect and Aspera for Desktop

    Posted Sat April 11, 2026 11:11 AM 
    Thank you for the suggestion! Setting the parameter to 100M did significantly boost the download speed, as you predicted.

I confirmed that I was indeed testing over WiFi, as you suspected. I'll switch to Ethernet and fiber for the next round of testing.

Thanks again for your help!


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    feng chen
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    Original Message


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