Attenuation and Signal To Noise Ratio and ADSL and PPPoE things

Attenuation

Attenuation is the signal loss made by the transmitting medium. In short, the longer your cable is, the bigger the reduction of your signal is. It is sometimes referred as "noise", as it makes the signal weaker.

Lower is better (as the signal is stronger).

Signal to Noise Ratio (SNR)

This measures the external noise's rate compared to the signal's strength. It is like normalizing the nose to 0 dB, and comparing the signal strength. The decibel as a unit is just invented for this.

Higher is better (as the signal is more stronger than the noise).

Rates for copper ADSL

SNR Levels

• Over 20dB, it is good
• 10dB - 20dB, it is satisfactory
• 7dB - 10dB, it is bad, you'll disconnect often
• Below 7dB, don't bother to try, the noise is so strong, that interferes with the signal.

7dB means the ratio between the two power values is around 2.

If you see somewhere dBm, then it means, that the dB value was calculated from milliwatts.

Attenuation Levels

• Below 50dB, it is good
• Over 50dB, it is poor
• Over 60dB, the signal is not strong enough to detect it.

Some Terms

• "ATU-R" (ADSL Terminal Unit - Remote) - This is your ADSL Modem
• "ATU-C" (ADSL Termination Unit - Central Office) - This is the other endpoint of your DSL-enabled phone line

Why do ISP-s use PPPoE over DSL?

The answer is easy: They used to provide PPP over POTS, and PPPoE authentication can be integrated into their old, PPP authentication directories. They won't spend money if it isn't really necessary... Also, PPPoE can be encrypted and compressed before putting the packet onto the wire.

This means, if someone cuts your wire, can't use your DSL subscription, as they don't know your user/pass.

What your SOHO router does to initiate a PPPoE connection

Discovery

1. Client initiates a search for servers
1. Sends a PADI packet (PPPoE Active Discovery Initiation), which is an Ethernet broadcast packet, looking for an Access Concentrator (AC, later we'll talk to this).
2. Here, you can specify a Service Name, if multiple ISP-s use the same network, or your ISP is a dick, and requires additional configuration.
2. Server responds
1. PADO Offer returned (PPPoE Active Discovery Offer), which states the MAC of the AC
3. Client requests a session
1. Sends a PADR packet ( PPPoE active discovery request)
2. This in fact acknowledges that the session should be created (like in the TCP handshake)
4. Server confirms
1. PADS confirmation packet returned (PPPoE Active Discovery Session-confirmation)
2. This sends back the Session Identifier to use later.

After the Discovery is done, and the session is created, the PPP layer (above PPPoE) initiates the authentication, just like in the old dial-up times.

Problems

• "Err, my router says she sends the PADI, but no response..."
• It wouldn't try to connect, if it would not see a live medium, so there is a problem somewhere else. I had overheating issues XD
• To be sure, check the cable, and the connector at the end of the cable. If you have a cheap RJ45 plug crimped, it might be undersized (trollolo)...