Advanced ADSL Troubleshooting
This document helps to explain a little bit about the Telstra Wholesale DSL network,
and how the service is provided to another ISP, such as Pacific Internet, IPrimus,
Netspace, Internode & IInet. It also aims to help users quickly identify faults on the DSL network, and how the network
integrates into their own. To start with, I have divided this document up into the 4 ways that
your phone line can be provisioned for DSL by Telstra.
A Type A Connection is a conventional PPP-based connection over Ethernet or ATM
About Telstra
The Telstra-based PPP over Ethernet/ATM looks like this.
A similar config is used for Pacific Internet / Netspace / IPrimus DSL connections that go
through the Telstra Wholesale Network. The most
common point of failure within this setup is the link between the IPSN and the CMUX. The CMUX
is located at your local exchange, whilst the IPSN is also reffered to as an "Access Concentrator".
There are currently 6 Access Concentrators use by Pacific Internet on the Telstra network
VET1-EXHIBITION Melbourne
VWT2-WINDSOR Melbourne
NKT1-KENT Sydney
NCT2-CHATSWOOD Sydney
QWT1-WOOLLOONGABBA Brisbane
QCT2-CHARLOTTE Brisbane
Each server has an internal IP address, starting with 172.31, that linux users will occasionally see, although this
means nothing in the real world as far as gateways and routing goes.
The authentication process relies on many points functioning within the Telstra Network. This is where the highest
point of failure is. When you authenticate using PPPoE/A, these are the devices that 'talk' to each other.
Your PC/Network
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Modem/Router
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Phone Line
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Exchange/DSLAM/CMUX
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Access Concentrator/IPSN/Shasta
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Telstra Radius Proxy
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ISP Radius Server
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Rest of the Internet
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Telstra have provided a definition of some of the terms used to describe their network.
Telstra authenticates users using a device called a "Shasta". This is made by a company called Nortel, who have a
website about their product The
current software being used is
V3199:T4:L35:Shasta 5000: iSOS (tm), 2.1.19.3\000
Some people make the mistake of thinking that just because an ISP resells the Telstra DSL service, they are also
using Telstra's overseas and domestic links. This not the case. In Pacific Internets situation, all data flows from the
telstra network in each state through to a Pacific Internet router. For example:
1 ADSL.mel.pacific.net.au (210.23.139.201) 32.845 ms 37.389 ms 45.695 ms
2 a6-0-1.mel001.pacific.net.au (210.23.140.85) 27.158 ms 26.011 ms 27.609 ms
3 FastEthernet0-0.mel003.pacific.net.au (210.23.140.162) 32.145 ms 26.874 ms 32.456 ms
4 comindico.gw.pacific.net.au (210.23.140.165) 32.596 ms 27.187 ms 32.416 ms
5 pos2-1.cor01-kent-syd.comindico.com.au (203.194.56.217) 40.774 ms 36.718 ms 39.497 ms
6 ge2-0.cw01-syd.comindico.com.au (203.194.29.249) 38.375 ms 35.295 ms 39.097 ms
7 ip-6.26.194.203.comindico.com.au (203.194.26.6) 41.149 ms 38.711 ms 37.347 ms
8 karl.planetmirror.com (203.16.234.20) 42.275 ms 39.275 ms 38.410 ms
This means that if you are having problems browsing to certain sites, or sending or recieveing email, the
problem is likley to be with the ISP itself, and have nothing to do with the DSL service Telstra provides. If
you are having a problem logging on the the network at all, then this is when Telstra may be the cause of
the problem.
You can find out more about the Telstra Network from this
Interoperabilty Testing Document.
(mirror). This is 46 pages long, and some fairly heavy reading. There is
also a similar 'DSL Service Interface Specification
Document' (mirror). The documents are designed for CPE suplliers,
but give a good idea of how the network works. There is also an unnoficial document written by a Telstra
tech which explains some of their own troubleshooting methods
(mirror)
A few words about L2TP
As of December 2001, Telstra began to allow ISP's to connect to its wholesale network using L2TP
(Layer 2 Tunnelling Protocol). This change has no visible effect on the way the
end user can connect or authenticate using PPPoE/A, It only effects the way the ISP connects with telstra on a
layer 2 level.
Some ISP's
have claimed that this makes the service more reliable. I cannot see any reason why this is the case. Data
still passes through the telstra network (on a Telstra phone line, through to an Alcatel CMUX, on to a Nortel Shasta)
where 99% of the current DSL problems lie. The ISP is the only real party to benefit from these changes. It allows the ISP
to offer "true" dynamic IP's, "boot off" PPP users, and bill timed usage accurately etc, thus it gives the ISP more control,
it does not give the end user any more reliablity.
An excellent summary of the current situation can be found here
I have gathered some relevant infomation relating to how LT2P works.
Known L2TP users are Ozemail and Internode. There are no known conflicts or issues for end-users who use an ISP
that connects using L2TP.
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About PPPoE
The majority of DSL connections are PPPoE. If you have an Alcatel ST Home modem, or a DLink DSL300, you
need some PPPoE software, which can be found here:
When you initiate a PPPoE connection, the session goes like this.
1 - [PADI] - PPPoE Active Discovery Initiation (client looking for Access Concentraitors)
2 - [PADO] - PPPoE Active Discovery Offer (the nearest AC responds with a service offer)
3 - [PADR] - PPPoE Active Discovery Request (the client decides on a PADO to respond to, and does so)
4 - [PADS] - PPPoE Active Discovery Session-confirmation (gives the ok session initiation)
5 - Session is Started by client (User & Pass)
This means that you should never have to try and configure or change any
settings on the Alcatel Home, or DLink 200/300 yourself. The entire setup
is done on the client software located on your server or PC.
Thanks to the Stallion EPipe debugger, we can see the PPPoE authentication
process in action. This log refers to a failed
authentication attmpts, but it shows us enough to see how PPPoE works.
Also of intrest here is the CHAP reference to "ppp@shastanets.com" which
is the default name/setting for ppp terminations at the Nortel Shasta
A correct PPPoE implementation can be seen in RFC2516
About PPPoA
PPPoA stands for PPP over ATM. I'm not going to start to get into ATM or various other protocols, as
there is no visible difference to the end user. Most small basic routers (such as the Alcatel ST Pro,
or the DLink 500) implement PPPoA.
An internal (ISA or PCI) or external (USB) adapter provides the user (transparently) with an ATM interface (as
opposed to an ethernet one for NIC/PPPoE). Due to the low cost of these devices, they are distributed
or sold with many of the cheaper residential DSL packages (
Pacific Internet or Internode). Thus, many end-users will end
up using PPPoA for the DSL connection.
Modems that need to use PPPoA:
In all instances, a PPP connection is made in the normal method of the operating system such as Windows'
Dial Up Networking (DUN). No configuration should be done on the device itself. All that is required is to install
the specific software drivers on the operation system of the PC. This sort of connection is not recommended
for low-spec machines, due to the fact that they will draw power and resources from an already-drained
host machine, and possibly interfere with the DSL connection.
Hardware Issues with D-Link 200 USB modems.
The D-Link 200 has some unconfirmed power management issues when used with some VIA motherboards.
(inparticular, variants of the VIA VT8366/VT8233 chipset). The modem intermittently 'drops dead' with no lights,
and no activity, a reinstall fails to find the modem, or the modem won't install, or be detected at all, or will be
detected incorrectly). We believe the problem is some of the latest VIA USB controllers, they don't seem to be able
to manage the resources required for a D-Link 200 USB modem. Its well known that the USB modems can use up
to 80% of the bandwidth of a USB controller, but no other motherboard seems to have the problem.
Workarounds for this problem are
ADSLGuide, UK - Finding USB Bus Bandwidth
D-Link 200 FAQ. - Why DSL-200 stops responding after leaving it on for a while?
Microsoft KB - Windows XP Does Not Detect Your New USB Device
And some other issues are mentioned in this rough document (Thanks Polly M)
PCI-DSL Modem cards
Generic PCI DSL modem-cards are made by d-link and alcatel, and are the cheapest DSL modem overall. The fact
that you have to open up your PC to install them seems to have limited their popularity a little. The only people I know
that use PCI modems use them on Debian/GNU Linux boxes. The modem comes
with a generic driver that provides a ethernet-style bridge (referenced as BRI1), and PPPoE is then used to terminate
the connection.
Router information:
The following devices are known to use PPPoA to terminate a DSL connection.
Some problems have been noted with Alcatels PPPoA implentation that appears to prohibit running a VPN through the
PPPoA interface. I believe its something to do with the way the fort fowarding is set up. Despite many attempts, I have
never seen this actually work, and would love somone to prove me wrong. There is also a Bigpond Direct document
which has some interesting sample configs for various routers that may be of interest
(mirror)
PPPoA is referenced in RFC2364. More PPPoA information is at
Everything2.net (used as a reference).
Troubleshooting : No Linesync
On an Alcatel ST Home modem, there is a light called the "linesync", which is located second from
the right-hand side. If this light is flashing, it means that the modem cannot locate the CMUX at the
Telstra End of your DSL Line. A linesync issue is universal problem, regardless of how the DSL is configured on
your phone line
Possible Issues
In Short : A linesync issue is almost definitely a Telstra Issue, as your ISP usually does not have access to this
part of the network. It is also something that can effect any DSL line, regardless of if it is provisioned as a type
A, B, C or D
Troubleshooting : No Data on Authentication (no comms)
If your linesync is ok, but your PPPoE client reports a 'session-timout' or 'Time out while waiting for PADO'
whilst logging on. We are encountering a different type of fault. It usually means the CMUX is ok, but the
connection between the CMUX and the IPSN is not.
There are 3 types of faults here:
In Short: It would seem that this part of the Telstra network/infrastructure is inherantly faulty
at this level. There has been some some
interesting discussion and observations made already with regards to the Nortel Shasta
and its capabilties in handling a large DSL Network. Officially, Telstra have never even
acknowledged that this issue exists, and this would seem to be the first step involved in solving
the issue, to avoid a general migration away from their wholesale network
by their largest customers.
Troubleshooting : Other Issues
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A Type B Connection is a pure Bridged Ethernet connection between you and Telstra
This is the type of connection I enjoy using at home. In my opinion its the most simple, reliable and trouble-free
DSL connection available (Its also slghtly fast as there is no PPPoE involved). Pacific Internet do these type of
connections for some corporate customers. Most ISP's will not do this for a residential account because of the 2
IP address's that are wasted making the connection.
A typical TypeB connection looks like this:
The configuration for this is done on your NIC. The settings are:
W2K : Start => Control Panel => Network & DUconnections => rightclick = Properties => TCP
IP Address : One from your allocated network
Default Gateway : Telstra End GW
NetMask : Your allocated Netmask (eg 255.255.255.252 for 2 usable address's)
DNS : Your ISP's settings
These settings should be given to you by your ISP.
My own connection was done on a RedHat Linux box, and
I have documented the results. The good thing about these connections is
that they do not suffer from 'data lockups' like PPP connections do, as there is no authentication process.
I have experienced 99.99% uptime with this configuration.
Troubleshooting : No Linesync
Troubleshooting : No Data Transfer (no comms)
eth0 Link encap:Ethernet HWaddr 00:10:4B:79:C4:30
inet addr:210.23.131.202 Bcast:210.23.131.255 Mask:255.255.255.252
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:79966 errors:0 dropped:0 overruns:0 frame:0
TX packets:77873 errors:0 dropped:0 overruns:0 carrier:0
collisions:1 txqueuelen:100
RX bytes:45481255 (43.3 Mb) TX bytes:13608460 (12.9 Mb)
Interrupt:10 Base address:0xde00
And my route table looks like this. (ADSL.mel is the Telstra gateway, default is the same as 0.0.0.0)
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
210.23.131.200 * 255.255.255.252 U 0 0 0 eth0
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default ADSL.mel.pacifi 0.0.0.0 UG 0 0 0 eth0
In Short : we need to ensure that
a) You PC can identify your network interface. (hardware detection)
b) Your network interface can idenitify itself, and the network that it is apart of. (interface config)
c) You network needs to have a place where it can send data to other networks. (routing table)
This type of conection forms the base of a Type D configuration. A Type D will attempt to
build on your current setup, using your current bridged ethernet to route another network down the DSL line.
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A Type C Connection is a way of routing networks down a DSL line using a router device
These connections must be made using a DSL modem/router. The have been tried and tested on an Alcatel
SpeedTouch Pro only, although they should work on most Cisco routers and the D-Link DSL 500.
A routed connection looks like this.
On your router, you need to:
(a) Configure your WAN interface with your User-End IP address, and set a gateway as your Telstra-GW. Set an
appropriate subnet mask (Pacific Internet allocate a /30 for these connections, so a subnet mask would be
255.255.255.252). From this point you should be able to ping the Telstra Gateway without any hassle
(b) Configure the route for your network, as your network will travel "through" the frame we have just created.
(c) Configure the router with another IP address, taken from your network. (Most people choose the lowest ip)
(d) Allocate another IP address to a client machine on your network, and set the gateway as the network IP you
have just given your router.
(e) Make sure the router can see both subnets. From your router you will be able to your Telstra GW, and your
client machines should be able to see the rest of the internet
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This style of configuration differs between routers, as do the troubleshooting methods. At the moment 3 modem/router
devices are supported. Aside from what I have mentioned here, I have been told by third partied that the Netopia r1600
will work with this configuration, as will the iprimus-supplied nokia's.
Troubleshooting : General
The only other real point of failure is the router itself. I have seen that the SpeedTouch Pro needs to
be rebooted on occasion, for no apparent reason. The first thing to check in any instance is the route table -
make sure its the same as when you left it.
The reason that there is not much documentation for this type of connection is because the user has
full control over what is done with his or her network. All telstra do is provide a gateway for
you to send data to, the rest of the configuration is done on your router. For further troubleshooting with the ST
Pro, I suggest you read my config document
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A Type D Connection uses bridged ethernet to route additional networks on a DSL line
The first thing to understand is that a Type D configuration is an extention of a Type B. With a Type B, a fairly
straightfoward Bridged Ethernet connection is established. A Type D uses this connection to route more networks
down the DSL line, in a similar fasion ot a Type C.
You can utilise this connection with any PC that has more than 1 network interface. It looks like this.
You should have been allocated your End-User address and gateway address by your ISP. You will need to
configure the network card appropratley, and then ping the GW end. Assuming that you get a response from the
gateway, you can proceed to configuring the second network card with an IP address from your allocated
range.
A customer set this up on his debian box, and emailed me to tell me how he did it
Troubleshooting : Networks
Destination Gateway Genmask Flags Metric Ref Use Iface
210.23.131.200 * 255.255.255.252 U 0 0 0 eth0
203.10.20.0 * 255.255.255.0 U 0 0 0 eth1
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
0.0.0.0 210.23.131.201 0.0.0.0 UG 0 0 0 eth0
Interface eth0 should have the IP address of 210.23.131.202 (network card connected to DSL modem)
Interface eth1 should have the IP address of 203.10.20.1 (network card connected to your LAN)
The routing table on a client machine is a little more simple. It also assumes that you choosen given the lowest IP
on your network to be second NIC on your server, and this is the gateway for the client machines.
Destination Gateway Genmask Flags Metric Ref Use Iface
203.10.20.0 * 255.255.255.0 U 0 0 0 eth0
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
0.0.0.0 203.10.20.1 0.0.0.0 UG 0 0 0 eth0
As with a Type B, we need to ensure that
a) You PC can identify your network interfaces. (hardware detection)
b) Your network interface(s) can idenitify themseves, and the network that they are apart of. (interface config)
c) You network needs to have a place where it can send data to other networks. (routing table config)
Once these connections are set up correctly, I have never seen them change dramatically, to the point where
they need serious troubleshooting. This is simple ment to give you an idea of how the setup should work.
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Other Links and resources of use.
Definition of Terms & End Notes.
I gathered some of these terms from this
document. A complete definition of terms can be found here
ADSL = Asymmetric Digital Subscriber Line
ATM = Asynchronous Transfer Mode
CMUX = Customer Multiplexer
CPE = Customer Premesis Equipment
DAC = Digital to Analog Converter
DSLAM = DSL Access Multiplexer
IPSN = Internet Protocol Services Node (also known as TAS / Access Concentrator)
LT2P = Layer 2 Tunneling Protocol
MTU = Maximum Transmission Unit
NIC = Network Interface Card
PPPoA = Point-to-Point Protocol over ATM
PPPoE = Point-to-Point Protocol over Ethernet
PSTN = Public Switched Telephone Service
PVC = Permenent Virtual Circuit
Disclaimer
This Document is to help you troubleshoot your own DSL connection, but it does not come with any warranty or liablity.
It is yours to play with. Any opinions conveyed in this document are mine, and not that of any other persons, including
my employer. This document may be freely distibuted so long as it is not used for profit, or modified in any way.
Credits
This document was written by myself, using the knowedge gained from working at Pacific Internet (Australia). This
document has ended up being more detailed and complex than what I first set out, so most lilkey I will be writing
an abridged, less-technical version at some stage. This document would not be here without the help of people
like Byron Brink and Fenn Bailey, and the technical staff at Pacific Internet. I am also greatfull for the help and
information contributed by some unnamed techincal staff at Internode, Telstra, and iPrimus.
Revision 1.3 12/06/02 | Changelog | Access Logs
Please send errors / questions to james [at] pacific.net.au
(c) James Mollison 2001-2002
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