XVI. DSL Diagnostics

Using BeThere's ADSL2+ at full speed with the Vigor2800

Products:
VigorBX 2000
Vigor IPPBX 2820
Keywords:
INP
Impulse Noise Protection
Vigor 2800

ADSL2+ (ITU-T G.992.5) provides DSL services of downstream speed up to 24Mb/s. With ADSL2+ in practice, speeds of around 15Mb/s are achievable (before overheads). With BeThere, a Vigor2800 may not achieve the maximum possible connection speed in its default configuration. BeThere use a different setup from all other ISPs. VPI/VCI are 0/101 (instead of 0/38) and they use MPoA instead of PPPoA. Ensure you set those correctly on your router. They also use an INP setting of '2' which is not currently supported by the Vigor 2800. (It is supported by the Vigor2700 and Vigor2820).

For speed, you can now skip forward to Solutions.

IMPORTANT NOTE: This information and document refers only to the Vigor 2800 ADSL router which was discontinued in 2008. The issue or problem described does not affect later models such as the Vigor 2710 and 2820 which support all settings of INP and allow full speed operation with BeThere/O2 without any changes needed by the ISP.

What is Impulse Noise Protection?

Impulse Noise is random intermittent interference on a line typically caused by mechanical effects, for example machinery, electrical storm activity etc. Inpulse Noise Protection (INP) is a system within G.992.5 which is intended to reduce the effects of Impulse Noise on data. It uses a Reed Solomon Coding method to apply Forward Error Correction (FEC). Put more simply, by adding a 'polynomial checksums' to the transmitted data, errors can be corrected automatically by the receiving device. This method of FEC is already widely used for CDs and DVDs as it can recover data which has been lost (for example a scratched CD).

Most ISPs in the UK use an INP setting of 0, 0.5 or 1 (FEC codewords per symbols). ITU-T standard G.992.5 (ADSL2+) limits the maximum achievable bitrate with INP=2 and maximum permitted delay of 16ms (latency) to 7.5 Mb/s. These values are all directly proportional. Newer DSLAMs use (for example) a Broadcom chipset which implements a newer optional (i.e. non-mandatory) extension to G.992.5 allowing the ISP to reduce the number of DMT symbols (mux frames) per RS FEC codeword (down as far as 1/16) and increase the interleaving (interleaving depth values can be set from 64-511).

When this extension is implemented, the INP value can be increased so the gross bitrate (measured before overhead) can be increased. In lay terms, this leads to an increased connection speed reported by your CPE (modem) using the higher INP setting.

In order to do this, both the ISP's DSLAM and the CPE need to support these optional extensions by including a suppporting Broadcom chipset, or equivalent (whereby the increased interleaving depth and decreased symbol rate are supported). The Vigor 2800 does not currently support this extension and therefore restricts bitrates above 7.5Mb/s with INP values of 2 or higher, as per the ADSL2+ specification (ITU-T G.992.5).

Another method used for Inpulse Noise resilience is 'Interleaving' which is enabled by some ISPs. Interleiving is fully supported by the Vigor (automatically). .

Performance Effects of using INP

There is a performance/resilence balance to be considered when using INP. i.e. a there exists a tradeoff between Impulse Noise Protection, coding gain and data latency, so INP, and particularly higher settings of INP should be used only where the benefits are considered greater than the effect on other areas of performance. On 'good' lines, or those which rarely suffer from Impulse Noise, the use of INP would have a nett detrimental eaffect on performance. i.e. Increasing the INP setting increases latency. The maximum bitrate of standard ADSL2+ is 24Mb/s, but that is only achieveable without INP. You cannot use INP without affecting performance, even if the Broadcom extensions are used and the higher you set your INP value, the greater the effect. Interleaving without INP can also be used to reduce the effects of impluse noise, but also reduces nett performance. Interleaving also adds some latency (estimated 20ms)

Solution

If you are using any ADSL2+ compliant modem or modem-router which does not support the Broadcom-type extension and hence does not report a connection speed above 7.5Mb/s with INP settings of 2 or higher, your ISP can change the INP settings to be lower than 2. You are unlikely to notice any difference in performance (although latency may reduce slightly, which is a good thing!). An INP setting of 1 should be adequate, but you can request a setting of 0.5 or 1 if you have a good line and want the lowest possible latency (transmission delays). Connection speed does also depend on your line quality, so in some scenarios, your limited line speed may be due to physical limitations.

For BeThere, you can request an INP change. They have provided the following instructions for how to request it from their customer services dept:

Information Correct as of 17th July 2007:

Call BeThere's customer service/technical department and request "Customer ISAM port be configured with the Service-Profile 20". This information has been providided by BeThere.


ADSL2+ WAN Disconnections

If you are noticing WAN disconnections on a Vigor2800 series router that is connected to a O2 / BeThere line then it could be worth changing a setting in the WAN detection mechanism. If the WAN detection mechanism does not receive replies to packets it sends as part of the detection mechanism (either ping or ARP - depending on the current setting) then it could trigger a disconnection. If this is a cause of a WAN disconnection then changing the setting could prevent disconnections; The setting can be changed via the telnet interface.

>wan detect ?
 wan detect <on/off/always_on> : enable/disable link detection
            <on>: ping detect(default) : 168.95.1.1, TTL=2
            <off>: arp detect
            <always_on>: disable link detect, always connected
 wan detect <status> : show the currest status
> wan detect always_on
>

For the Vigor2800 series, the recommended ADSL firmware for use on ADSL2+ lines is version E67.3.61 . The firmware is available for download in the downloads section.

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