Larry Cook with a drawer of line cards on the new Nortel DMS-100 telephone switch; Each drawer in the switch holds 64 line cards, and each card provides service for two telephone lines (1FB or 1FR). Every telephone line that has dial tone has a line card. (As I understand things, the line cards themselves generate dial tone -- hence the relatively-short time it takes to get dial tone with the DMS -- and they are digital circuits, controlling PCM-encoded sound.)

They are the only ``single points of failure'' (SPOFs in High-Availability vernacular) in the entire system; that is, if one of them fails, then customers lose a phone line. Every other part of the switch is fully redundant with hot-failover design: should a memory unit fail, for example, another unit is put into use; in fact, it's already running, so only high-level control logic has to adjust for the failure.

(My question: since this is an SPOF, were 128 customers without phone service while we were looking at this drawer? I don't think it'd be like Mr. Cook to take customers down, though.)

Last update: Sun Jan 10 14:01:35 EST 1999, by mark@vielle