Integrated Services Digital Network
Intergrated Services Digital Network (ISDN) as the name suggests, is a digital telephone service that supports simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are two types of ISDN channels that are used for communication, the 64 Kbps 'B' (Bearer) channel and 16 or 64 Kbps 'D' (Delta) Channel. The B-channel can be used for voice, video, data, or multimedia calls. B-channels can be joined together for higher bandwidth applications such as video conferencing. The D-channel can be used primarily for communications (or "signaling") between switching equipment in the ISDN network and the ISDN equipment at your site.
ISDN is available in 2 configurations, ISDN BRI (Basic Rate Interface), or ISDN PRI (Primary Rate Interface). An ISDN BRI connection supports two 64 kbps B-channels and one 16 kbps D-channel over a standard phone line. BRI is often called "2B+D" referring to its two B-channels and one D-channel. An ISDN PRI connection supports 30 64 kbps B-channels and one 64 kbps D-channel (or 30B+D) over a high speed E1 or T1 circuit. North America have a different configuration where PRI supports 23 B + D. 30B + D is the european standard.
ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s.
In the case of ISDN BRI, a pair of copper wires run between the switch (CO) and the Network Termination Device (NT1). Unlike PSTN, the ISDN circuit does not get its power from the phone company but instead from the NT1. Therefore the NT1 is an important piece of equipment that acts as a test point. The pair of cable from the switch and power are connected to the NT1, and a 4 wire connection is the output that gets connected to the ISDN device or PABX. This 4 wire connection is generally a patchlead. The 2 wire connection from the phone company to the NT1 is the U-Point interface, where as the 4 wire connection between the NT1 and the ISDN device/PABX is the S/T-Point interface.
Multiple Subscriber Numbers (MSNs) are additional numbers that can be associated with the main number of your ISDN circuit. They allow you to allocate different numbers to ISDN devices such as telephones, faxes and computers. You will need to program the equipment you connect to the ISDN line so that only the correct device responds to its allocated MSN. MSNs are available for ISDN lines operating in point-to-multi-point configuration. Point-to-multi-point working is normal for ISDN terminal adapters, PC cards and telephones. If you are using a PBX, you may need to use DDIs.
Direct Dial In (DDI) Numbers are additional numbers that can be associated with the main number of your ISDN circuit. They are normally used with a switch/PBX. DDIs can be allocated to extensions, providing separate numbers for individual staff. You will need a switch/PBX that can be programmed to route incoming calls using DDI numbers to the correct extensions. Although you can have many DDI numbers, they can only be used if there is a spare channel in the ISDN Line. for example if you have 2 x ISDN BRI connected to your PABX, this would mean you have 4 x B channels, hence a maximum of 4 DDI calls can be active and the fifth caller will get a busy tone as no spare channes are available. Outgoing calls would further reduce the number of available channels. DDIs are applied to ISDN lines operating in point-to-point configuration.
Primary rate connections are presented to the user on four wires - one pair for each direction. They are usually 120 ohm "balanced" twisted pairs using standard network cable, but older installations may use 75 ohm "unbalanced" coaxial cables. Devices called "baluns" can convert between the two types. The connection from the NTU to the network may use additional pairs or optic fibre, depending on the distance. Primary rate connections are always point-to-point.
ISDN is available in 2 configurations, ISDN BRI (Basic Rate Interface), or ISDN PRI (Primary Rate Interface). An ISDN BRI connection supports two 64 kbps B-channels and one 16 kbps D-channel over a standard phone line. BRI is often called "2B+D" referring to its two B-channels and one D-channel. An ISDN PRI connection supports 30 64 kbps B-channels and one 64 kbps D-channel (or 30B+D) over a high speed E1 or T1 circuit. North America have a different configuration where PRI supports 23 B + D. 30B + D is the european standard.
ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s.
In the case of ISDN BRI, a pair of copper wires run between the switch (CO) and the Network Termination Device (NT1). Unlike PSTN, the ISDN circuit does not get its power from the phone company but instead from the NT1. Therefore the NT1 is an important piece of equipment that acts as a test point. The pair of cable from the switch and power are connected to the NT1, and a 4 wire connection is the output that gets connected to the ISDN device or PABX. This 4 wire connection is generally a patchlead. The 2 wire connection from the phone company to the NT1 is the U-Point interface, where as the 4 wire connection between the NT1 and the ISDN device/PABX is the S/T-Point interface.
Multiple Subscriber Numbers (MSNs) are additional numbers that can be associated with the main number of your ISDN circuit. They allow you to allocate different numbers to ISDN devices such as telephones, faxes and computers. You will need to program the equipment you connect to the ISDN line so that only the correct device responds to its allocated MSN. MSNs are available for ISDN lines operating in point-to-multi-point configuration. Point-to-multi-point working is normal for ISDN terminal adapters, PC cards and telephones. If you are using a PBX, you may need to use DDIs.
Direct Dial In (DDI) Numbers are additional numbers that can be associated with the main number of your ISDN circuit. They are normally used with a switch/PBX. DDIs can be allocated to extensions, providing separate numbers for individual staff. You will need a switch/PBX that can be programmed to route incoming calls using DDI numbers to the correct extensions. Although you can have many DDI numbers, they can only be used if there is a spare channel in the ISDN Line. for example if you have 2 x ISDN BRI connected to your PABX, this would mean you have 4 x B channels, hence a maximum of 4 DDI calls can be active and the fifth caller will get a busy tone as no spare channes are available. Outgoing calls would further reduce the number of available channels. DDIs are applied to ISDN lines operating in point-to-point configuration.
Primary rate connections are presented to the user on four wires - one pair for each direction. They are usually 120 ohm "balanced" twisted pairs using standard network cable, but older installations may use 75 ohm "unbalanced" coaxial cables. Devices called "baluns" can convert between the two types. The connection from the NTU to the network may use additional pairs or optic fibre, depending on the distance. Primary rate connections are always point-to-point.