TD220X_manual_v8.doc
Page 5 of 48
1/27/2017
2 Introduction
The GE MDS TD220X is a 25-Watt 220 MHz GMSK data radio intended for bridging messages over
the air between locomotives and wayside devices. The data interface is Ethernet and uses the
UDP/IP-based Simple Timeslot/Frequency/Power Protocol (STFP), defined elsewhere. STFP is capable
of supporting various payload protocols.
A time division channel access method is used by the TD220X for wireless communication. Each
second is divided into 8 133-byte time slots. The radio further defines a multi-second epoch to allow
the effective number of time slots to be scaled according to system design. Radios must be
configured with the same epoch size.
Precise synchronization of timing amongst radios is necessary for operation. Each radio can be
configured to use one of three timing sources: GPS, Precision Time Protocol (PTP), and over-the-air
(OTA). While system design can be flexible, a base radio installation typically uses either GPS or PTP
timing and a mobile radio typically uses OTA to synchronize to the wireless transmissions of a base
radio. Radios configured for GPS timing must be connected to an external GPS unit to receive NMEA
sentence information and the PPS signal. Radios configured for PTP timing will interact with a PTP
Grandmaster Clock over the Ethernet port using IEEE 1588 (PTPv2). Radios configured for OTA timing
use messages received wirelessly in the first time slot of each second to maintain timing.
With its time slot definition and variable epoch size, the TD220X provides a generic TDMA
implementation that can be used by an external Communication Manager (CM) to support a variety
of TDMA schemes. A CM is responsible for making decisions regarding timeslot, frequency, power,
and payload organization. The TD220X is responsible for requesting data from a CM for upcoming
timeslots, forwarding messages received wirelessly to a CM, and wirelessly transmitting messages
received from a CM using the specified timeslot, frequency, and power. All communication between
the TD220X and a CM is done using STFP.
