Some long-held ADSL beliefs seem incorrect!

[#1]

While setting up yesterday my spiffy new Asus DSL-AC86U modem/router (which I bought with a view to upgrading to fibre in future), I discovered that two of my long-held beliefs regarding ADSL seem to be wrong:
a) I always understood that BTOR's DSLAMs were Broadcom based, and it was hence recommended to have a modem which also uses a Broadcom chipset, as do both my old and new ones. However, my new modem/router reports the DSLAM on my exchange (St. Albans) to be Infineon/Lantiq; so things have changed it seems;
b) likewise, I have always gathered that the correct MTU value for PPPoA ADSL is 1500 (as against 1492 for VDSL), and have posted to that effect elsewhere in these Forums. I decided to test this using the PING command posted here http://community.plus.net/forum/index.php/topic,106592.0.html/ (and also here http://www.asus.com/uk/support/FAQ/1007914/). To my surprise, using a value of 1472 (= 1500 MTU) consistently showed 100% packet loss, whereas 1464 (= 1492 MTU) had 0% loss. Smaller values in the PING command also resulted in varying rates of loss. So it seems that an MTU of 1492 is best for ADSL as well as VDSL, at least for Plusnet at this exchange.

[#2]

my MTU is currently set at 1500 on my ADSL connection, and haven't noticed any issues.
but as you say, it may be down to individual cabinets/exchanges.
how can you tell what the DSLAMs are based on ?
i use a billion 8800nl with a broadcom chipset.

[#3]

My 20CN ADSL exchange is using a "TSTC:0x0510" Texas Instruments chipset.
The advantage of having a modem with a Broadcom chipset, is generally the Broadcom chips have good performance, but they also have better than most reporting of detailed connection stats (particularly via Telnet), and provide best compatibility with monitoring programs such as RouterStats.
I have found, over the years, by experimenting with about 20 modem/routers, that the optimum MTU is often down to the device you are using.
Some just don't work at 1500 as you have discovered, I've had some that won't work over 1470 !
On my connection the optimum MTU for fastest speed and minimum losses, is 1430.
Other MTUs that I've found work well are 1458 and 1478.
These days my MTU has become more complicated because I'm running a 6in4 IPv6 tunnel which carries more than 85% of my traffic.
I've chosen to leave the IPv4 MTU at 1492 so that I can have the IPv6 tunnel MTU optimised instead !
I'm currently waiting for the imminent release of the next firmware for my router, which will allow my router's WAN PPPoE connection to have an MTU of 1508, and therefore allow my modem to run at the full 1500 MTU over PPPoA, which in turn allows me another 8 on the IPv6 tunnel !

Quote from: chenks76

how can you tell what the DSLAMs are based on ?

Try to 'telnet' into your device, on my modem (with Broadcom BCM6358+BCM6301 chipset) the command is -

adsl info --vendor

[#4]

> adsl info --vendor
adsl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    8000
Last initialization procedure status:  0
Max:    Upstream rate = 1224 Kbps, Downstream rate = 16104 Kbps
Bearer: 0, Upstream rate = 1219 Kbps, Downstream rate = 16136 Kbps
ChipSet Vendor Id:      IFTN:0x71c8
ChipSet VersionNumber:  0x71c8
ChipSet SerialNumber:
>

[#5]

So you have IFTN = Infineon (now Lantiq)

[#6]

interesting.... is it possible it has changed?
i'm sure it was reporting as broadcom a while ago.
not that i've had any connection issues or pin issues though.

[#7]

For anyone else looking up their exchange chipset, there is a code translation list on Kitz forum - "Vendor Identification Codes"

[#8]

My 21CN ADSL2 is also TSTC 0510. I think most BTWholesale ADSL is TSTC or IFTN, although the very recently upgraded to ADSL2+ exchanges appear to have Broadcom equipment.
Also, is there an Asus DSL-AC86U, or was that supposed to be DSL-AC68U? I think most Asus DSL models, including the DSL-N66U and DSL-AC68U, use MediaTek (company previously called TrendChip) DSL chipsets, although I've not looked at the newer VDSL2 models, which there seem to be several of (DSL-AC56U, -AC52U, -N17U, -N16).

[#9]

Quote from: NorthPole

To my surprise, using a value of 1472 (= 1500 MTU) consistently showed 100% packet loss, whereas 1464 (= 1492 MTU) had 0% loss. Smaller values in the PING command also resulted in varying rates of loss. So it seems that an MTU of 1492 is best for ADSL as well as VDSL, at least for Plusnet at this exchange.

The loss with lower sizes is a bit worrying, but then it's going to depend on who you are pinging + maybe wireless loss.
I don't think you can conclude that 1492 is best for anyone else from this test. Results will depend on who you ping and the settings on your router plus what it does with them.
I assume this is with 1500 set on the router (plus saved OK and restarted to get a new connection) and also on the device doing the pinging?

[#10]

Quote from: ejs

Also, is there an Asus DSL-AC86U, or was that supposed to be DSL-AC68U? I think most Asus DSL models, including the DSL-N66U and DSL-AC68U, use MediaTek (company previously called TrendChip) DSL chipsets

Yes, sorry - I did mean AC68U. I'm sure it does have a Broadcom chipset, though - Asus' advertising made a feature of it, which was one reason I bought it.

[#11]

Quote from: legume

Quote from: NorthPole "at least for Plusnet at this exchange." I don't think you can conclude that 1492 is best for anyone else from this test. Results will depend on who you ping and the settings on your router plus what it does with them.

Not what I suggested - I repeat my qualifier.

[#12]

Quote from: NorthPole

Quote from: ejs Also, is there an Asus DSL-AC86U, or was that supposed to be DSL-AC68U? I think most Asus DSL models, including the DSL-N66U and DSL-AC68U, use MediaTek (company previously called TrendChip) DSL chipsets Yes, sorry - I did mean AC68U. I'm sure it does have a Broadcom chipset, though - Asus' advertising made a feature of it, which was one reason I bought it.

The Asus DSL-AC68U is one of these interesting boxes, which actually contains two separate systems, which must be networked together internally somehow. Examining the firmware reveals two filesystems and two Linux kernels, one of each for an ARM cpu, and the other for a MIPS cpu. The wireless is indeed Broadcom, running on the ARM cpu. But the DSL is done by the MIPS cpu, this part is the MediaTek (TrendChip) part.
The Asus page only mentions Broadcom regarding the wireless:

Quote

Broadcom® TurboQAM™ technology super-charges 2.4 GHz 802.11n performance from 450 Mbps to 600 Mbps with compatible devices.

[#13]

Quote from: NorthPole

Quote from: legume Quote from: NorthPole "at least for Plusnet at this exchange." I don't think you can conclude that 1492 is best for anyone else from this test. Results will depend on who you ping and the settings on your router plus what it does with them. Not what I suggested - I repeat my qualifier.

I don't  think you can conclude the "at least for Plusnet at this exchange" either.
I think it's your router causing these results.

[#14]

Quote from: ejs

The Asus DSL-AC68U is one of these interesting boxes, which actually contains two separate systems, which must be networked together internally somehow. Examining the firmware reveals two filesystems and two Linux kernels, one of each for an ARM cpu, and the other for a MIPS cpu. The wireless is indeed Broadcom, running on the ARM cpu. But the DSL is done by the MIPS cpu, this part is the MediaTek (TrendChip) part. The Asus page only mentions Broadcom regarding the wireless: Quote Broadcom® TurboQAM™ technology super-charges 2.4 GHz 802.11n performance from 450 Mbps to 600 Mbps with compatible devices.

ejs - I'll read the advertising more carefully in future!