Layer Breakdown
Layer 1 - Physical
- Noise
- The PSTN can transfer both voice and data at the same time. The basics of this are that there are voice frequencies and a data frequencies. The router reserves the audible-frequencies for voice and then basically allocates whatever is left (and viable) for data. ADSL lives in the data frequencies. "Noise" is basically a measure of how much interference the data frequencies are getting. This can be from devices sharing the PSTN (i.e. fax/phone) or from poor ADSL filters (replace them) or from poor patching/cabling in premises or off-premises. Cisco routers track and show this value on "connected "ADSL services.
- Attenuation
- Is a measure of how much a signal degrades over distance. You may have heard that ISPs sometimes say "you're too far from an exchange". This value measures that distance. A signal enters a wire and is "loud". As the signals travels over the wire the signal loses strength. Attenuation is a measure of this loss over distance. When there is too much attenuation, the signal is not strong enough when it is reached by the other end. Cisco routers can track and show this value on connected ADSL connections.
- Filtering
- If there are other devices sharing the phone line (i.e fax or phone) and there is no filtering in place. The non-filtered non-ADSL device will potentially inject signals into the voice frequencies. The ADSL router will typically respond to this with varying noise readings/ADSL that drops while faxes are received. Filtering at all points where the phone is shared will prevent this. Filtering works by isolating non-ADSL devices to "voice-only" frequencies (i.e. audible frequencies). The ADSL plugs on filters do the inverse (i.e. prevent ADSL from using "voice-only" frequencies). From a router, the only way to spot this is varying noise margins (i.e. it is good, good, good, bad, good, good/etc).
- Sync/Exchange Configuration
- This is a little harder to explain, you get a PSTN from a provider. The provider then enables specific codes on the DSLAM/Exchange to enable ADSL codes for that PSTN (sometimes ISPs say the DSLAM isn't ADSL ready, which basically means the equipment at the local DSLAM is really old). Routers, before doing any authentication need to build "sync" with the DSLAM (basically a circuit). If the sync light on your router is flashing it usually means the router isn't able to "build" that circuit to the DSLAM over the PSTN. On Cisco routers this is the CD (Carrier Detect). Ring your ISP and/or make sure you have the right phone line.
Layer 2 - Link-layerCisco Commands
- ATM PVC
- There is a PVC ("circuit") between the router and the Exchange. On a Cisco router you can actually do some ATM pings to see if you can get to your local DSLAM ("segment") or to the exchange ("end").Layer 3 - Network
- PPP/Authentication -
The ADSL protocol in Australia is typically authenticated against a radius server using the PPP protocol. There are two varieties of this protocol PAP/CHAP. Telstra do both but some ISPs only do one or the other. Another thing to be aware of is there is PPPoA or PPPoE (these really just mean PPP over ATM or PPP over Ethernet). The configuration for each PPPoA or PPPoE is different on a router... basically the same thing. Slightly different packet structures/overheads on each. Settings on your router need to be correct though.
- IP Address -
Once the router authenticates successfully, the radius server should supply an IP address to the end-router (sometimes not depending on ISP). If no address is supplied this may mean that the ISP has not configured the radius server correctly. Are you getting a dynamic IP where you paid for a static? Ring the ISP.
- Routing
- If incorrect default route information is passed onto the router or no default route is configured on the router then the router will not be able to talk to the external world. No routes = no directions for where to go for internet/WAN.
Commands on Cisco Routers (837/877/887)
- "show user"
- show connected users VTY/dialer interfaces
- "show dsl int atm 0"
- shows ADSL information of connected interface ATM 0. Includes noise, attenuation, speed in both up/down varieties.
- "debug ppp authentication"
/ "debug ppp negotiation" - shows the ADSL authentication packets going back and forth between Telstra and the router. It is possible to see just authentication requests going out and no response from Telstra.
- "clear int ATM 0"
- Drops and brings back an ADSL connection. Good if you are remote and want to do this.
- "ping atm ..."
- If you have sync but nothing else, this allows you see whether you can reach the exchange or not
Hope it helps someone...
