and data storage functions on magnetic tape and hard disk. The Message Controller provides communications links between the DMS Core and the DMS Bus. DMS Bus is used to interconnect the DMS Core, the switching network and the Input/Output controller (IOC) and manage message flows between these units and consists of redundant Message Switches. The Message Switches of the DMS Bus operate in a load-sharing mode and one of them provides the main clock source for the DMS-100 system while the others are synchronized to it. Messages between all SuperNode units are carried by optical DS512 links. The operating system used by both generations of the DMS-100 switch was called Support Operating System (SOS) and was written in a high level language called PROTEL which stood for PRocedure Oriented Type Enforcing Language developed at Bell Northern Research (BNR). Hardware and maintenance are administered locally through cathode-ray terminals, through a multilevel menu system called MAPCI. There are various methods used to access the DMS remotely as well, including modem and telnet. Backups and other hard drive work are administered through a DISKUT command line program. Line Module Analog lines are terminated on individual line cards, each with its own codec, in pull-out Line Drawers in frames. The original design of such frames was called the Line Module (LM) with 32 lines per drawer. LMs were not able to send caller-id information (CLASS services) and became rare late in the century, having been supplemented or replaced by the newer Line Concentrating Module (LCM). Duplicated ringing generators serve each LM or pair of LCM. For DC testing, each line card has a relay to connect it to a test bus. Line Concentrating Module LCM have smaller line cards, serving 64 lines per physical drawer (2 logical drawers) of the same size as in the LM. An LCM require only half the space as an LM for the same number of lines. The majority of line cards are NT6X17, with three relays for loop start lines. Others include NT6X18 which have four or more relays and a slide switch for ground start lines (a newer version of the NT6X18 has eliminated the ground switch). The NT6X18 also has the ability to provide current reversal required for some coin phones and business systems. The NT6X17B and the NT6X18B is known as the world line card and is software configurable to more than 15 million functional