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ISDN | Tech Info Home Page |
Integrated Services Digital Network (ISDN) -
A standard for a technology that integrates data, voice and video into a digital signal which is transmitted over telephone lines.
The older narrow band ISDN has a maximum limit of 2Mbps. The newer broadband (b-ISDN) is much higher towards Gbps.
ISDN modems are configured to operate at 64Kbps to 128Kbps. One line only or both 64Kbps lines can be used either alone or in conjunction to achieve 128Kbps.
Since ISDN modems typically have 2 lines, and each line has a phone number, you could use one line to connect to the internet via ISDN and have a telephone connected to the other line and use it independently.
The ISDN modem connects to a local ISP by dialing in but unlike a modem, the ISDN is usually left connected for extended times (unless minute charges apply).
The downside of ISDN modems is that the phone company charges for each of the two lines as they were 2 phones, and the ISP also charges separately for having the ISDN account and possibly for excess bandwidth.
It is limited to 128Kbps but can achieve higher virtual speeds with compression. Typically this bandwidth can be sustained, unlike broadband cable which depends on the local area load conditions.
ISDN Terminal Adapters are more commonly known today as ISDN Modems.
The "ISDN Modems" page has many models available for purchase.
The Basic Rate Interface (BRI ) is formed with 2 64Kbps "B" bearer channels and one 16Kbps "D" channel which provides signaling for the "B" channels. ISDN can also be used for connections to X.25 Frame Relay packet switching networks
Primary Rate Interface (PRI) is also available at 1.544Mbps configured as 23 B channels and one D channel
Typically ISPs use Routers to combine two (64Kbps) B channels into a single (128Kbps) link using Multilink Point to Point protocol (MLPP ).
BACP (Bandwidth Allocation Control Protocol ) -
A protocol used by ISDN which allows a 2 channel ISDN circuit to use one channel to connect to the other host, and as traffic increases to activate the second channel as needed.
ISDN Modem Connections -
The ISDN modem has to be connected on one end to the ISDN wall plug and on the other end to the computer.
The ISDN connection
The back of the modem has a BRI connector which is used to connect the modem to the ISDN wall outlet. The connection can be of either "BRI U" or "BRI S/T" type. A pin-out can be found by using our FAQ search page to search for either "BRI U" or "BRI S/T".
The Computer connection
a) internal type
Typically an internal ISDN modem has a PCI type slot which directly connects it to the computer.
b) external type
1) USB connection - A cable connects from the external ISDN modem to the computer's USB port.
2) Serial Connection - A cable connects from the external ISDN modem to the computer's Serial port in most cases the DB-9 Male connector.
It is also possible that an auxiliary serial connector is provided for connecting to the modem to perform maintenance while the primary interface Data connection is either a USB or Network connection.3) Network Connection - Either a 10base-T or 10/100base-T connector is provided so that a cable can connect the ISDN modem to the computer.
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