ADSL and ISDN
Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires that gives good quality and high speeds. ISDN is a fixed-wire end-to-end digital telecommunications network.
Top notch and technologically advanced telephony capabilities, Automatic Call Back, Caller ID, Rapid establishing of calls, video conferencing, economical communication with scalable bandwidth that is robust. These features are a must have nowadays for ISDN users, and they cannot live without them. Some vendors are introducing a version of ADSL where the upstream and downstream frequency bands have been place above 0 to 80 kHz ANSI T1.601 basic rate ISDN transmission band. This reduces the ADSL bit rates but does permit simultaneous ISDN and ADSL service on one loop. ISDN in conjunction with ADSL are two techniques complement each other. This combination is used mainly in Germany and France where ISDN service is particularly widespread.Users today require simultaneous access to Internet, Telephony and fax. The best solution for this market has to provide the capabilities offered by ISDN in the LAN, and also offer ADSL connectivity for the LAN to the internet. This requires an integrated communication service that can carry all these types of communication over a single physical line. The following table compares some areas of application of ISDN and ADSL:
| ISDN | ADSL | |
| Telephony |
+ |
- |
| Fax |
+ |
- |
| Remote access |
+ |
+ via VPN |
| LAN-to-LAN |
+ |
+ via VPN |
| LAN-to-Internet |
+ |
+ |
| Video on Demand |
- |
+ |
In most countries ISDN uses a frequency range of 80 kHz. This follows from the 160 kbps (full duplex) bit rate. ADSL and ISDN transmit a bit stream over a twisted copper line. ADSL uses a frequency range of about 1.1 MHz for DMT The ADSL-ISDN combination is unlikely to provide the full 5.5 km reach normally provided by ISDN.
To prevent interference and to facilitate the mechanics of installation, ISDN and ADSL are split apart both at the subscriber premises and in the central office. This is done by frequency separators called “splitters”.
ISDN defines the protocols to be used up to the network layer. ISDN communication contains data type and switching information, which allows us to make automatic switching of voice calls and data channels.
ADSL was designed from the ground up to transfer mostly IP based communication. Therefore, it has not implemented universal methods for switching, and PC to PC communication. It cannot replace these features of ISDN, it can only add high speed networking capability over the switching capabilities of ISDN. They could be provided based on IP networks, but current protocols do not allow for sufficient QoS control, so old fashioned switching networks are preferred. Therefore, it is best to combine both ISDN and ADSL service.
