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In contrast to Nortel's DMS-100 which uses individual line cards with a codec, most lines are on two-stage analog space-division concentrators or Line Units, which connect as many as 512 lines, as needed, to the 8 Channel cards that each contain 8 codecs, and to high-level service circuits for ringing and testing.

Both stages of concentration are included on the same GDX ( Gated Diode Access ) board.

Each GDX board serves 32 lines, 16 A links and 32 B links.

Limited availability saves money with incompletely filled matrixes.

The Line Unit can have up to 16 GDX boards connecting to the channel boards by shared B links, but in offices with heavier traffic for lines a lesser number of GDX boards are equipped.