Modem unit NAG : custom development

NAG is a unit in RBC(Radio Block Central). RBC is used to control movements of trains in ETCS (European Train Control System) and was custom developed for AŽD Praha s.r.o.. NAG modem serves as protocol stack (CFM - Communication Functional Module) of EuroRadio. Protocols: ITU-T V.110, HDLC, T.70 and X.244. This protocol stack is set of protocols specially modified for communication between trains and RBC. NAG modem allows communication with up to 30 trains at the same time.

The modem unit NAG is 6 layers PCB in size 220x100mm. It is designed as a plug-in card with front panel compatible with 19'' case which is 3U high.
There are following system interfaces towards to Backplane :

IFIX E1 interface defined as ISDN PRI with 2,048 Mbit/s speed.
IRBC Ethernet interface 10Base-T with 10MBit/s speed and 100Base-TX with 2,4/4,8/9,6 kbit/s speed.
Sbus I2C halfduplex interface with 400kbit/s speed for configuration memory in data bus acces.
ID 1-Wira interface according the Maxim Integrateg Products standard for board identification.
Pcap USB interface for configuration, supervision and debug is on the front panel.


24VDC unit's power supply, 2,5W usage

Basic functional blocks: processor with ARM AT91SAM9260B architecture including SDRAM 64MB memory, ethernet interface AMD7001, ISDN PRI interface with 2,048 Mbit/s PEF2256-HV2.2 and circuit FPGA Spartan 3A XC3S400A-4FTG256. This architecture and modification of operational system is also used in our well-established Combi module.

In FPGA circuit are lower layers of CFM protocol stack. Specifically V.110 protocol and part of HDLC protocol. These protocols are difficult to process and their realization in CPU wouldn't be efficient. Higher level protocols HDCL - LAPB, T.70 and X.224 are realized by programs. The operational system is Linux. The application is written in C/C++ and contains following processes. Processes communicate with each other through shared memory.

 DSS1  Realizes communication with PBX (Private Branch Exchange) by DSS1 protocol
 Realizes link layer protocol
 T.70 a X.224  Realizes net protocols (T.70) and transport layers (X.224)
 APL  Realizes net interface to RBC core