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GDC NMS 520 Series
Premier Network-Managed
Universal Rate Data Service Units

Call us for Pre-sales and discount pricing

  301-924-7400 Support

GDC NMS 520 Series Premier Network-Managed
Universal Rate Data Service Units


•  Auto adaption to line rate, interface and service type

•  Full array of line measurements

•  Migration to SNMP using SpectraComm 5000 system

•  Up to 6 to 1 compression option saves on line costs

•  Optional compatibility with IBM LPDA-2 DSUs

•  Integral dial restoral via the PSTN, Switched 56, or ISDN services


The NMS 520 Series is General DataComm's premier family of network managed data service units (DSUs). It delivers the capabilities today's private line, digital networks need to remain agile and robust, from universal rate access and totally non-intrusive network management to three integral dial restoral choices— analog switched network, Switched 56 or ISDN. Distinguished by extensive line measurement functions, which allow precise assessment of circuit quality, the NMS 520 Series also provides a selection of powerful interface options. IBM networks, for example, will benefit from an LPDA-2 feature, while a synchronous/asynchronous data compression option can boost throughput by up to 6 to 1.

Models equipped with the Intelligent Front Panel put a simple and cost-effective management alternative at operators' fingertips. The NMS 520 Series is also ready for migration to SNMP via introduction of the SpectraComm 5000 System for T1 integrated access. Standalone and rackmount packaging in the modular DataComm format.

Legacy and LAN Applications

The NMS 520 Series can be used alone with the SpectraComm 5000 System, in pairs, or on secondary channel circuits with GDC's NMS 510 Series universal rate DSUs. It can operate not only in traditional point-to-point or multipoint applications with multiplexers and FEPs, but also in LAN environments with routers and servers. Equipped with the data compression option, the NMS 520 Series can be used to compress up to 384 Kbps of traffic for cost-effective transmission over 56 and 64 Kbps circuits. It also supports Link Problem Determination Aid (LPDA-2) diagnostics, providing compatibility with IBM 5822 series DSUs. LPDA-2-equipped NMS 520s can not only be mixed with 5822s on the same circuit, but also managed from IBM's NetView.

Interface Flexibility

The NMS 520 Series' interface flexibility is a valuable asset in controlling line and equipment upgrade costs and making installation easier. The NMS 520 Series supports synchronous data rates from 2.4 to 64 Kbps and asynchronous rates from 600 bps to 19.2 Kbps. It can operate over standard digital, 64K clear channel, and secondary channel circuits. In fact, it can detect and automatically adapt to service type (standard or secondary channel), interface (EIA/TIA-232-E,V.35, or EIA-530) and line data rates (up to 56 Kbps).

Non-intrusive Control

A key feature of the NMS 520 Series is its ability to provide totally non-intrusive management on multipoint digital circuits. On digital circuits with secondary channel, diagnostic and control data is transported on the channel provided by the carrier for auxiliary purposes. On conventional digital circuits, an on-demand, in-band derived secondary channel enables remote site conditions to be reported without "bit stealing" or otherwise impacting bandwidth—even with the data compression option.

IFP or SNMP Management

When remote NMS 520s are combined with a central-site SpectraComm 5000 System, SNMP management under HP OpenView is achieved via the LAN-based SNMP Manager. As an alternative, limited management can be accomplished via an optional Intelligent Front Panel (IFP), a simple-to-use yet powerful tool for basic configuration, testing, optioning, and installation.

Integral Restoral

The NMS 520 Series supports highly flexible, integral dial restoral options including the DBU-89 for point-to-point dial backup via the PSTN, and the DBU-56FW for point-to-point/multipoint restoral via Switched 56 services. Point-to-point restoral via ISDN services is also possible using the NMS 520 IFP/ISDN and its Intelligent Front Panel. Restoral can be manual, using the DSM, SSC, or IFP, or automatic, upon loss of Data Carrier Detect (DCD) for a designated time. In automatic restoral, after a given number of calls to the first of up to 10 stored numbers, the next stored number is automatically dialed. After the primary circuit's DCD has been active for a specified time, the call is terminated, and data is automatically transferred back to the private line. The DCD-loss and DCD-active time periods are user programmable. In units with the DBU-56 FW or DBU-ISDN option, the maximum time in restoral mode can also be set. Security features when entering dial backup mode include password protection, dialer identification and a call-back feature.


Mixed LAN and Legacy Access With SNMP Management







Conventional DDS and GDS, secondary channel services; 64K clear channel, wireline; PSTN, Switched 56 K, or ISDN in dial restoral mode



Synchronous:  2.4, 4.8, 9.6, 19.2, 56 and 64 kbps

Asynchronous:  600, 1.2, 2.4, 4.8, 9.6, 19.2 kbps



Synchronous:  Binary, serial

Asynchronous:  Binary, serial; 8, 9, 10 and 11-bit characters   (V.14 compliant)


Full duplex; point-to-point and multipoint; point-to-point only at 64 Kbps




Bipolar, return to zero




4-wire, non-loaded metallic circuits




135 ohms (nominal)



V.35/EIA/TIA-232-E (optional EIA-530 interface)

Internal, external or network timing; optional + 8-bit transmit buffer




8, 30, 60 or 90 ms at 2.4 Kbps

4 ms at 4.8 Kbps

2 ms at 9.6 Kbps

1 ms at 19.2 Kbps

0.4 ms at 56 Kbps



Signal Level, Bipolar Violations, Error Probability, Jitter, Receive Level, Transmit Level, Sealing Current, Signal to Noise Ratio, Round Trip Delay

SSC Controller; Intelligent Front Panel (Standalone only) or SNMP management via the SC 5000 System




DBU-89:  Point-to-point , single-call analog dial backup via the PSTN

DBU-56FW:  Point-to-point and multipoint 4-wire restoral via Switched   56K services

DBU-ISDN:  Point-to-point restoral via ISDN services, compliant with the   North American National-1 ISDN Network interface




Compression ratio of up to 6 to 1 (typically 4 to 1 for X.25); maximum rate of 384 Kbps synchronous, 230.4 Kbps, asynchronous

Accepts most standard protocols, with automatic recognition up to 5,000 bytes/block, including HDLC, SDLC, APPN, TCP/IP, ASYNC, X.25, DEC LAT (Async)




100-240 VAC, 50/60 Hz, 7.5 W nominal




Up to 95% without condensation



    Standalone Rackmount 
Height: 99 mm (3.9 in.) 267 mm (10.5 in.)
Width: 277 mm (10.9 in.) 483 mm (19.0 in.)
Depth: 18 mm (12.5 in.) 343 mm (13.5 in.)
Weight: 3.2 kg (7.1 lb) 18 kg (40.2 lb)
Shipping Weight: 3.6 kg (8.1 lb) 19 kg (42 lb)
Operating Temperature: 0 to 45 C (32 to 113 F) 0 to 45 C (32 to 113 F)
Storage Temperature: -40 to 70 C (-40 to 158 F) -40 to 85 C (-40 to 185 F)
  (full shelf with 16 standard front panel units)

UL recognized and CSA approved




Verified to comply with FCC Part 15, Class A requirements and FCC Part 68

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Please contact us 301-924-7400



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