Friday, 2 September 2011

ADSL SNR vs Attenuation vs Distance

Below is a quick info sheet for ADSL when it comes to those Signal to Noise Ratio or SNR and Attenuation numbers usually found on ADSL routers:

Signal to Noise - The higher the better, this indicates the ratio of signal power to noise power, ie. the background noise should be as low possible and the signal power as close to desired as possible.

SNR can be influenced by various factors, these include weather conditions and sources of interference eg. electrical cables running over a phone line.

6dB - No sync or intermittent sync
7-10dB - Fair service level
11dB-20dB - Good service level
20dB-28dB+ - Excellent service level

Attenuation is the gradual loss of signal over a distance, this occurs due to the resistance of the copper wire being used and various other telecom goodies between you and the exchange. The lower the attenuation the better and the more likely you become to sync at at higher data rates.

The graphs below give a good indication of the influence of both distance and attenuation will have on the average ADSL, ADSL2, and ADSL2+ line:






Postfix Smarthost Configuration

If you require a postfix setup which delivers e-mail using another mail server and authenticating by using an e-mail account on the other server, follow these steps:

1. Create a password file in /etc/postfix eg. /etc/postfix/smarthost which contains
otherserver.example.com username:password

2. Create the postfix hash file from the password file (needs to be done with any changes to the password file) using the command:
postmap /etc/postfix/smarthost

3. Modify the /etc/postfix/main.cf file by adding/changing the lines below:   
relayhost = [otherserver.example.com] #Add : to indicated port, example :2525 to use port 2525
smtp_sasl_auth_enable = yes 
smtp_sasl_password_maps = hash:/etc/postfix/smarthost 
smtp_sasl_security_options =   

4. Restart your postfix: 

/etc/init.d/postfix restart

If all goes well you should see the messages in your queue (if any) become less and go out via the smarthost, the /var/log/mail.log file should also give you an indication if your configuration is working correctly...

Cisco ASA 5505 PPPoE Config File

vpdn group pppoegroup request dialout pppoe
vpdn group pppoegroup localname *myisp*
vpdn username *myisp* password *letmein*
vpdn group pppoegroup ppp authentication PAP
ip address outside pppoe setroute