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TUCoPS :: Phreaking General Information :: dms.htm

Digital Multiplexing System (DMS)

Digital Multiplexing System                                                   Digital Multiplexing System

--This file will attempt to explain the DMS (Digital Multiplexing System). Think of this file as more of a compilation of the material I have read, rather than something I authored completely from scratch. Special thanks to Control-C for most of the information found here.


DMS was/is made by Northern Telecom. It was first introduced in 1979. To date, DMS has been able to interface with such switches as ESS #1-4, Xbar, TSPS, and EAX. The DMS switch itself is physically smaller than a Xbar switch, and usually smaller than most AXE switches. This is because the DMS switch is more spread out, as opposed to other types of switches which are all located in one switch house. The use of remote modules give the CO more space to install a Line Concentrating Module (LCM) or Main Distribution Frame (MDF). Many versions of DMS exist. DMS versions and systems are as follows:
 1) DMS-10 - a C5 switch which can be used with up to 10,800 lines. Designed for rural areas and large businesses. Almost always connected with a larger DMS-100 or -100/200 switch.

 2) DMS-100 - a C5 local office able to be used with 1,000 to 100,000  lines. Very widely used today to handle residential areas' phone lines.  A DMS-100 local office can also be adapted to Equal Access End Office (EAEO)

3) DMS-200 - can be used with up to 60,000 trunks. Can also serve a AT (Access Tandem) function. The Auxiliary Operator Services System (AOSS)     is a part of DMS-200 that controls Operater-assisted calls, such as Directory Assistance. AOSS is made possible by Traffic Operator Position System (TOPS) and Operator Centralization (OC). These 2 functions allow transfer operator services from other DMS-200 toll centers.

4) DMS 100/200 - Uses functions such as the toll and local systems mentioned above, but also includes the EAEO/AT combination. Can handle either 100,000 lines or 60,000 trunks. Used instead of  using -100 and -200 seperately.

5) DMS-250 - Not very widely used. Used in association with specialized common carriers that need tandem switching.

6) DMS-300 - Designed for international use. The number of DMS-300 switches that are used is in the single digits.

7) Remote Switching Center (RSC) - Used instead of DMS-100, it has the ability to switch up to 5,760 lines.

8) Remote Line Concentrating Module (RLCM) - Able to switch up to 640 lines. Can be used with RSC or DMS-100 with assistance from the Line  Concentrator Module (LCM).

9) Outside Plant Module (OPM) - Able to switch up to 640 lines. Can also be used in association with RSC or DMS-100.

10) Subscriber Carrier Module (SCM or SCM-100) -
-a) Subscriber Carrier Module (Rural (SCM-100R)) -   Eliminates the CO Central Control Terminal (CCT) by being integrated with a DMS-100 switch.
-b) Subscriber Carrier Module SLC-96 (SCM-100S) - gives a direct link between DMS-100 and SLC-96 loop carriers.

-c) Subscriber Carrier Module Urban (SCM-100U) - Used to interact  with DMS-1 Urban (DMS version specialized for use in urban areas.)

11) DMS-Mobile Telephone Exhange (DMS-MTX) - A special type of DMS-100 that is used with Cellular switching. It can serve up to 50,000 people in up to 50 cells.

12) Supernode
-a) DMS-Supernode - Revision of the DMS-100 that supports faster processing.

-b) DMS-Supernode SE - same as above, except in a reduced physical size, and uses the Link Peripheral Processor  (LPP).

Important Features of DMS-100:
1) Automatic Route Selection - automatically detects the best trunk for routing toll and LD calls.
2) Station Message Detail Recording - an enhanced call logging system,keeps track of times, dates, duration, etc.
3) Direct Inward System Access (DISA) - allows maintenance and administration from remote terminals.
     Operator Features included with DMS-200 and -100/200:

1) Traffic Operator Position System (TOPS) - gives certain functions to handle incoming and outgoing calls.
2) Operator Centralization (OC) - Lets an operator interface with the switch equipment itself. Allows calls to be routed from a remote DMS switch to a host.

     DMS is divided into 4 areas that each handle special operations:

1) Central Control Complex (CCC) - Controls the functions that are used   in the other 3 areas. The CCC contains 4 units:
-a) Central Processing Unit: Each DMS switch contains 2 CPUs. The CPUs have access to memory banks where stored programs and network data are located. Consider the CPUs the "engines" of the switch. They process all incoming data from outside lines.

-b) Program Store Memory Module: Associated with one CPU to contain the program instructions needed to run programs on the switch. The second PS contains duplicate instructions.
-c) Data Store Memory Module: Contains information such as customer information and office data. The second DS is a duplicate that is used with the second CPU.
-d) Central Message Controller: Controls the messages between the other areas of the CCC and the Network Message Controller (NMC) in the various Network Modules or the I/O controller. Both CPUs have access to the CMC.

2) Network (NET) - Network Modules handle the vocal aspect between the Peripheral Modules and the Central Control Complex (CCC).

3) Peripheral Modules (PM) - Interface between analog trunks, subscriber lines, and digital carrier spans (DS-1). Responsible for creating dialtones, sending/receiving signalling, and checking the network.
Before 1984, the following types of PMs existed:
-a) Trunk Module - Changes speech into digital format to be sent through the line. The TM also handles MF tones, test circuit announcement trunks, etc.

-b) Digital Carrier Module - gives a digital interface between the DMS switch and the DS-1 digital carrier.  The DS-1 signal consists of 24  voice channels.
-c) Line Module - gives an interface for a maximum of 640 analog lines and condenses the voice and signaling into two, three, or  four DS-30, 32-channel speech links.
-d) Remote Line Module - same as above, except it controls the DMS switch remotely. Can be used up to 150 miles away.

Since 1984, 10 more types were added:

-a) Digital Trunk Controller - Interfaces up to 20 DS-1 lines, then sends the DS-1 lines to the network.
-b) Line Group Controller - Can interface up to 20 DS-30 lines, and can serve RSCs, RLCMs, or OPMs.
-c) Line Trunk Controller - has the ability to give interfaces to a maximum of 20 outside ports from DS-30A speech links or DS-1 links to 16 network side DS-30 speech links.
-d) Line Concentrating Module - An expanded version of the LTC, it can serve up to 640 subscriber lines interfaced with 2-6 DS-30 speech links.

-e) Remote Switching Center - interfaces subscriber lines at a remote location to a DMS-100 host. The RSC consists of the Line Concentrator Module, Remote Cluster Controller,  Remote Trunking, Remote-off-Remote, and Emergency Stand-alone.
-f) Remote Line Concentrating Module - an LCM used from a remote location from the DMS-100 host. Can handle up to 640 lines, sometimes used as replacement for PBXs.

-g) Outside Plant Module - Outside plant remote unit. Handles 640 lines over 6 DS-1     Links.
-h) Subscriber Carrier Module - Remote interface for remote concentrators.
-i) SCM-100R - Can interface up to five DMS-1R Terminals. Each terminal can handle up to 256 lines.

-j) SCM-100U - Can interface up to three DMS-1 Urban RTs.  Each RT can interface up to 576 POTS or special service lines.

4) Maintenance and Adminstration - DMS provides different ways to maintain and administrate the network. M&A is divided into 4 major groups:

-a) Administrative: Provides for the interrogation, collection and modification of data.
-b) Internal Maintenance: Includes all DMS hardware (to the MDF) and software.
-c) External Maintenance: Includes circuits on the transmission facility.
-d) Reporting: Include I/O  facilities and the alarm system.

    Common Channel Interoffice Signalling (CCIS) uses a dedicated line to transmit data between offices, trunks, or trunk groups. CCIS-6 uses the International Consultative Committee on Telephone and Telegraph (CCITT) No. 6 international standard. CCIS-7 added the ability to use CCIS with almost all common DMS versions such as DMS-100, -200, -100/200, and -100/200 with TOPS. CCIS-6 uses 2 types of Serving Offices (SO):
1) CCIS-BS: used for trunk signalling between COs. Transmits data such
           as numbers dialed, number dialed from, and other routing information.
           CCIS-BS put an end to Blue Boxing.
2) CCIS-DS: enables the use of touch-tone menu administration, such as
           voice mail, calling card input, and so forth.

     Access Tandems:
1) Equal Access (EA) gives a connection between Local Access and
           Transport Areas (LATA). It provides such services as ANI, Automatic
           Message Accounting (AMA) for both originating and terminating calls,
           and operator service signaling.
2) Equal Office End Office (EAEO) gives a connection between interLATA
           carriers and international carriers' POP.
3) Access Tandem with Equal Access End Office gives a connection from a
           trunk tandem to ICs/INCs POP inside a LATA. It uses a two-stage
           "overlap output pulsing" method which makes dialing quicker and
           easier. The first stage identifies the INC dialed and picks a
           reliable outgoing trunk. A connection is established from the INC to
           the EAEO through the access tandem. The second stage processes ANI
           and makes a connection to the called number through your specific DMS
           switch type.
4) Access Tandem with a Non-Equal End Office uses Feature Group A, B, or
           C to connect to an IC/INC. It uses standard Central Automatic Message
           Accounting (CAMA) to place a call through an AT.

     Other services provided with DMS switches used in urban areas:
1) Auxiliary Operator Services System (AOSS) - used primarily for
           directory assistance, and the intercept needs not included with TOPS.
2) Integrated Business Network (IBN) - commercial concept designed for
           business to have a small, private PBX. IBN can be installed into a
           business to a Centrex Control Office or a Centrex Costumer Unit with
           minor hardware adjustments. Features of IBN include the ability to
           handle 30,000 lines, customer call records, centralized attendant
           maintenance, administration functions, and direct inward dialing.
3) Electronic Switched Network (ESN) - designed to meet needs of multi-
           location complexes. Used with SL-1 or -100 Digital Business
           Communications Systems with networking features or a DMS-100 IBN
4) Specialized Common Carrier Service (SCCS) - provides conversion of
           analog and digital signals. Must be used with older analog lines,
           sometimes also used with newer digital lines.

     DMS-MTX is a DMS switch used for switching radio and cellular signals. DMS switches provide 3 basic types of cell switching:
1) Stand-alone switching is used by a MTX which is interfaced with one
           or more C5 EOs with DID trunks. MTX is used with urban areas, MTXC
           for suburban areas, and MTXM for rural areas.
2) Combined switching is the most cost-effective type of MTX and is easy
           to install. It can be incorporated into a DMS-100 switch and used
           with cellular software.
3) Remote switching is accomplished by the Remote Switching Center (RSC)
           alongside a Cell Site Controller (CSC). A Remote or Stand-alone
           switch hosts the remote switch. Remote switching is not used in urban

Suggested Reading:
   Understanding DMS; Control-C; 1987 (Most of my information came from here!)
   DMS Family of Digital Switching Systems; Erudite; ????
   DMS-100; Jester Sluggo; ????
   DMS-100 Family System; Northern Telecom; 1978


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