I found an interesting script in the Netgear DM200 firmware, /lib/cfgmgr/dsl.sh which appears to contain commands to configure the Lantiq DSL components appropriately for different countries and their ISPs and equipment.
It says what some of the commands are for, but it doesn't say what the UK VDSL2 ones do.
print_interop_bits_adjustment() # $1: country, $2: isp, $3: vdsl/adsl
{
#for NBN adjustment
echo -e "\tlfcs 0 1 1 1 1 -1"
echo -e "\tlfcs 1 1 1 1 1 -1"
if [ "$3" = "vdsl" ]; then
case "$1/$2" in
UK/*)
echo -e "\tdms 1562 0 1 8" # All UK ISPs need these commands
echo -e "\tdms 549 0 2 240B 0000"
echo -e "\tdmms 1C44 0 1 BFFF 4000"
;;
Sweden/Telia)
echo -e "\tdms 0x1562 0x0 0x1 0x6" # TELIASONERA Fixes
echo -e "\tdms 0xE843 0x2 0x1 0x1" # TELIASONERA Fixes
;;
esac
else
case "$1/$2" in
France/Orange)
echo -e "\tdmms 0x6743 0x1C 0x1 0x200 0x200" # Improve downstream performance & stability under REIN/Impulse noise
echo -e "\tdmms 0x6743 0x1C 0x1 0x100 0x0" # Improve downstream performance & stability for fixed RFI disturber
echo -e "\tdmms 0x6743 0x1 0x1 0x80 0x80" # FT bit
echo -e "\tdmms 0x6743 0x14 0x1 0x40 0x40" # Favour DS SNRM over DS INP
echo -e "\tdmms 0x6743 0x17 0x1 0x20 0x20" # Eanble the REIN adaptation in training phase.
echo -e "\tdmms 0x6743 0x1A 0x1 0x10 0x10" # DS performance at mid-long loops with BRCM DSLAM in ADSL2+ mode.
echo -e "\tdmms 0x549 0x0 0x1 0x49 0x49" # Reboot Criteria (1) for FT (LOS, LOM & LCD)
echo -e "\tdmms 0x549 0x1 0x1 0x3 0x3" # Reboot Criteria (2) for FT (SES30 & ES90)
echo -e "\tdmms 0x6743 0x1D 0x1 0x8000 0x8000" # Avoid declaring LOM from CPE side, in particular when no NearEnd CRCs are observed by removing the ceiling on the 15 bit loaded tones. This is required to avoid second retrain in FT evoultive test cases as well unnecessary LOM reboot
;;
Spain/Telefonica*)
echo -e "\tdmms 0x6743 0x17 0x1 0x8000 0x8000" # DS LATN reporting gap in Anx-A/M as compared to AR7
echo -e "\tdmms 0x6743 0x14 0x1 0x40 0x40" # Favour DS SNRM over DS INP
echo -e "\tdmms 0x6743 0x14 0x1 0x2000 0x2000" # Inproved US performance against GSPN DSLAMs
echo -e "\tdmms 0x6743 0x1A 0x1 0x20 0x20" # Telefonica bit 1
echo -e "\tdmms 0x6743 0x17 0x1 0x8000 0x8000" # Telefonica bit 3
echo -e "\tdmms 0x6743 0x1C 0x1 0x4000 0x4000" # Telefonica bit 4
echo -e "\tdmms 0x6743 0x1A 0x1 0x10 0x10" # DS performance at mid-long loops with BRCM DSLAM in ADSL2+ mode.
;;
Greece/OTE)
echo -e "\tdmms 0x6743 0x14 0x1 0x2000 0x2000" # Inproved US performance against GSPN DSLAMs
;;
Norway/Comlabs)
echo -e "\tdmms 0x6743 0x0 0x1 0x100 0x100" # Comlabs (Norway) bit
;;
UK/*)
echo -e "\tdms 549 0 2 204B 0000" # All UK ISPs need this command
;;
esac
fi
}
So the 3 commands for UK VDSL2 are:
dms 1562 0 1 8
dms 549 0 2 240B 0000
dmms 1C44 0 1 BFFF 4000
This one is listed for UK ADSL:
dms 549 0 2 204B 0000
The dms and dmms commands themselves don't really tell you anything, they stand for DeviceMessageSend and DeviceMessageModifySend.
But it may be possible to work out what the commands do from the Lantiq source code.
dms and dmms are similar commands that send low level messages to the device, but dmms adds a "mask" that "makes it possible to write single bits or bit groups instead of the complete data word".
The first clue is the comments for the France/Orange dmms 0x549 ... commands. It says they are "Reboot Criteria", which should be for adjusting the conditions under which the modem will re-sync. There are alternative rccg/rccs commands, RebootCriteriaConfigGet/RebootCriteriaConfigSet, which have the same settings for LOM, LOF, LOS, ES90, SES30 and others, and so appear to be for controlling the same thing, I'm not sure why they aren't used.
Searching the Lantiq drv_dsl_cpe_api for 549, I found
/** Message ID for CMD_ReInitNE_Configure */
#define CMD_REINITNE_CONFIGURE 0x0549
which the source code then says it "Configures re-initialization triggers for near-end failure conditions".
Searching the source code for 1562
/** Message ID for CMD_OperatorSelect */
#define CMD_OPERATORSELECT 0x1562
/**
The message selects a DSL operator. The information is used to configure
operator specific settings inside the DSL firmware.
*/
So presumably the dms 1562 0 1 8 command activates BT specific behaviour contained within the dsl firmware .bin file.
1C44 then is for:
/** Message ID for CMD_TestOptionsSet */
#define CMD_TESTOPTIONSSET 0x1C44
/**
The messages configures settings for test modes.
*/
Not so sure what this is for, but perhaps something to do with "fine gain control", whatever that is.
Summary: There are a lot of low-level settings to configure the device optimally or properly for specific countries and their ISPs.