3 Context Description

High rate data originates at AMS on-board computer and will be transmitted by the NASA Ku-band signal processor subsystem (KUSP). The signal is made available at Customer Interface Panel (CIP) as a synchronous serial stream of either NRZ-L or NRZ-M encoded bits with RS422 voltage levels using a bit rate of 2Mbits/s. RS422 allows multiple receiving circuits on the same signal wire, and thus this presents a natural place for connecting multiple redundant systems to the same CIP signal connector.

The total amount of data that can be transferred using HRDL is limited to an average of 1Mbits/s, in cycles of about half an hour on, half an hour off.

1. STS mission of 100h of AMS active time.
2. 50% duty cycle means a total of 46GBytes of data during STS. (256kB/s * 50% * 100h * 3600 s/h = 46080 MB)

Data on the link will be a continuous synchronous serial stream of fixed-length frames. (It needs to be determined whether there will be dummy data or no data transitions during loss of signal, i.e.~during off-cycle.)

The start of every frame will be signified by a pre-agreed sync pattern. Use of CCSDS-suggested 32-bit pattern is recommended, but other sync words (up to 38 bits) can be accommodated with a firmware change. Frames will follow each other without any gaps, except the gaps created by LOS cycles.

All frames will be of equal, pre-determined fixed length. The optimal frame size will be determined later, but it will most probably lie within 255--4090 octets (bytes). At least the following affect the selection of frame size:

1. CCSDS telemetry downlink recommendations suggest 1275 octets (bytes) plus the sync word.
2. ISS draft document SSP50184 limits frame data size to 94--4090 octets, total packet size being 8 octets more.
3. Reed-Solomon encoding usually implies the frame data size to be a multiple of 255 octets.
4. Having an even number of octets may simplify the design of the interface card.
5. High-rate (2--8Mbits/s/track) magnetic tape recorders used in VLBI radio astronomy successfully use a frame length of 2500 octets.
6. Long frames will experience a bit error more likely and in case of a corrupt frame the amount of data lost will be larger.
7. Short frames introduce more frame overhead but bit errors cannot do as much damage as with long frames.

GSE on-line analysis computers would prefer to get access to captured data with a time delay less than one minute. The required throughput is essentially the same as on HRDL, 2Mbits/s, roughly 256kBytes/s peak rate; when averaged over all available time, 1Mbits/s, roughly 128kBytes/s.