Digital vs. Analogue Transmission
Digital transmission system transmits information by means of a series of ‘on’ and ‘off’, or more practically ‘less voltage’ for ‘off’ and ‘high voltage’ for ‘on’. Minor variations in voltage will not mean anything as only two states of voltage are considered. This means that the transmission is less errorsome; i.e. in technical terms, it contains less noise
In Analogue transmission system, signal is transmitted by varying voltages, current or phase. Here even a minor variation is meaningful and as a result analogue transmission has more noise.
How ADSL Works.
Public telephone System uses only Analogue transmission system. This was because the concept of digital data communications as we know it today did not exist earlier. The other reason is that voice is an analogue signal.
The normal telephone lines carry voice data using frequency less than 4 KHz (commercial bandwidth), however these lines have the capability to carry frequencies well beyond 4KHz.
ADSL utilizes this unused bandwidth for data transmission, simultaneously with the normal voice. This means that we can use telephone along with data transmission.
ADSL uses a modulation scheme DMT (Discrete Multitone Modulation) also called Orthogonal frequency-division multiplexing (OFDM) to modulate the data to about 26-138khz(upstream) and 138-1104khz (downstream) and transmits them. It is to be noted that what ADSL modems do is digital modulation. That’s what the digital in ADSL is all about. [Note:ISDN uses pure digital transmission and there is no modulation/demodulation]
Since the voice and ADSL signal frequencies are widely separated, the possibility of interference between them are very minimum.
Once the signal reaches the telephone company’s local office, the signal is filtered and split and the ADSL signal is stripped off and immediately routed onto a conventional internet network, while any voice-frequency signal is switched into the conventional phone network.
This filtering and splitting is also done on our side so that only voice is fed to phone.
Authentication scheme can also be provided in ADSL. Newer implementations of ADSL often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details.
Draw backs of ADSL
The ADSL signal however is distance dependant. That is, the speed of the service decreases with increase in distance from the telephone exchange. And the maximum possible distance is only about 5 KMs
The ADSL is asymmetric this means that the download and upload speeds are not same. Usually it can achieve a download speed of 1-2 mbps but the upload speeds are in the order of 256kbps. [Speed available in India]
ADSL modem
ADSL modem is a device that does the above specified things to enable a computer to use an ADSL service.
“Modem” is indeed the correct terminology since there is MOdulation and DEModulation of the signal, even though it doesn’t resemble a usual 56K modem [which uses analogue modulation].
For using an ADSL service with authentication [ie PPPoE], the user connects the ADSL modem to his computer usually via Ethernet/USB line and uses PPPoE dialer software to dial the connection. Once the connection is established the computer is assigned a public IP provided usually via DHCP. [Static IP can also be provided]
The ‘ipconfig’ command gives the following output:
PPP adapter <connection-name>:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 59.93.39.101
Subnet Mask . . . . . . . . . . : 255.255.255.25
Default Gateway . . . . . . . : 59.93.39.101
This shows that the computer is assigned a valid public IP.
ADSL Router
The ADSL connection will usually be used to share the internet to several computers. So most of the ADSL modems available now contain also a router and Hub/Switch. This type of modem is more appropriately known as an ADSL Modem+Router or simply ADSL Router.
In this setup the username and password are stored inside the router and the router automatically dials the connection when it’s switched on. Here it’s the router that gets the public IP and not the computer as in the previous case .
The advantage of this setup is that no computer is directly accessible from outside, so more security. This also eliminates the need to use the dialer software.
Note that to run a web server etc in one of the computers we will need to enable port forwarding in the ADSL Router.
We assign the internal IP of the ADSL Router [eg: 192.168.1.1] as the gateway IP of all internal computers to give them access to internet.
Bridged Mode of ADSL Router
There are times when we have a better router, and we would like to use it than the router in the ADSL Router. This means that we don’t want the ‘router functions’ of our ADSL router and just only the ‘modem functions’
To cater such cases, the ADSL Router has an option called bridged mode commonly called modem mode. Enabling this mode will make it a plain standard ADSL Modem.
