The GDK range of controllers include:
| Model number | Lines & Extensions | Comments |
|---|---|---|
| GDK 16 | 16 | Flexibly used lines and extensions |
| GDK 34i | 34 | The I version indicates ISDN use |
| GDK186 | 186 | Maximum capability for this range |
Notice that this is the first range of controllers which can cope with ISDN.
Making the upgrades is via a series of plug-in cards. These include:
- SLIB cards: add up to 4 analogue phone lines into the controller with this Single Line Interface Board.
- STIB cards: add up to 2 ISDN 2 lines per card
- Along with LCOB cards and DTIB Cards
