This configures which telco central-office ISDN switch protocol variant, AT&T 4ESS, AT&T 5ESS, or Nortel DMS-100, this card's ISDN signaling should be tuned to match. Because ISDN signaling details differ subtly between these CO switch types, a mismatch between this setting and the carrier's actual CO switch causes call setup or signaling failures on ISDN PRI or BRI circuits that look like a line problem but are actually a protocol-variant mismatch. For example, ISDN calls failing to set up on a newly turned-up PRI get resolved once the admin discovers cardISDNswtype was left at att-4ess(1) when the carrier's actual CO switch is a dms100(3).