BT FTTC Sync - B.FOCUS and Plusnet Routers Low Sync Speed

I have PlusNet FTTC service. Using old HG612 (unlocked) I get 80Mbps sync with 101Mbps attainable listed.

However, using ECI B.Focus modem or Plusnet HH (both with OpenWRT) I get 66Mbps at ~3.5dB SNR and about the same rate attainable. Both these devices use xRX200 rev 1.2 modem.

Cabinet is listed as Broadcom 164.154.

Any ideas? Is my hardware missing support for some particular standard maybe?

Many thanks.

Do you have software offloading enabled?

Without it, you'll probably attain around 80-85 MBit/s, with software-flowoffloading you can get up to 115 MBit/s (if your line and contract allow it).

Thanks for the reply. Not enabled that yet. However the issue is with the DSL sync rather than measured throughput, at this point.

Mmmh, did you use the exact same firmware blob for the dsl-modem between the different xrx200 modems?

I'm using the default modem blobs provided with OpenWRT currently. Both modems with this chipset perform the same.

On one of the ECI modems I have I did run the firmware updater utilities to extract the updated firmware with vectoring - but that didn't seem to connect at all!

The HG612 has a different (broadcom) chipset I think.

Ah, okay. At least in Germany broadcom modems have/had a reputation to work better with the invariably broadcom linecards in the DSLAMs, at least for vectoring lines (this discussion only came up after the switch to vectoring and bi-directional G.INP/retransmissions). The issue seems to be different for profile 35b "super-vectoring" lines where currently lantiq/intel/maxlinear seem to have a slight edge.
So you might need to try different firmware blobs for the lantiq modems to find one that harmonizes sufficiently with your linecard....

@moeller0 thanks. The DSLAM PDF lists:

• 48 VDSL2 and POTS splitter ports with up to 17a profile
• System Level Vectoring with Auto-sense for legacy CPE
• Any two ports Bonding
• Dynamic Line Profile

I know that the OpenWRT supplied firmware does not support vectoring.

Made some progress here. The modem needed Annex.B vectoring firmware, which can be obtained from here and extracted using Freetz from here.

I used the latest release version

I saved the file 'vr9-b-dsl.bin' to /etc/config and set the firmware through the Web UI (Network/Interfaces/DSL/Firmware file "/etc/config/vr9-b-dsl.bin"), then power cycled the router.

Now the DSL status is reporting 'G.993.5 (VDSL2 with down- and upstream vectoring)' and the attainable rate has increased again to 101Mbps. Sync rate is currently 76Mbps at 7dB, which is close to the HG612.

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Interesting, I always thought, that in the UK vectoring was not used. The reason for the lack of a vectoring firmware blobbing default OpenWrt seems to be a simple licensing issue, for distribution of such firmware blobs....

Seems to be so, yes.

For completeness: I've now set 1500MTU across the PPPoE using some manual configuration from open bug report FS#2667 and enabled software flow offloading via the WebUI (Network/Firewall).

Overall result achieving about 70Mbps throughput on 76Mbps sync, which I think is the best I've ever managed on this line.

Considering the PlusNet routers are available brand new on eBay for £10 seems like quite a nifty little box of tricks :slight_smile:

Theoretically you could expect the following goodput/speedtest result, assuming a VLAN tag, PPPoE, baby-jumbo frames , IPV4, TCP and no options (also ignoring the HTTP-overhead):

76 * 64/65 * ((1508-8-20-20)/(1508+26)) = 71.22 Mbps
[gross-rate * PTM-encoding * ((MTU - PPPoE - IPv4 - TCP) / (MTU +( partial ethernet header + PTM-header)#))

#) PPPoE-VDSL2-PTM:  2 Byte PPP + 6 Byte PPPoE + 6 (dest MAC) + 6 (src MAC) + 2 (ethertype) + 4 Byte VLAN + 4 Byte Frame Check Sequence (FCS) + 1 Byte Start of Frame (S), 1 Byte End of Frame (Ck), 2 Byte TC-CRC (PTM-FCS): 2+6+6+6+2+4+4+1+1+2 = 34 Bytes

Any competent ISP will run a traffic shaper on its side of the actual VDSL-link, so the true upper limit should be <= the calculated theoretical threshold from above, so ~70 from around ~71 seems pretty decent.

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