Copyright © 2011 Open Geospatial Consortium
110 Many projects are ongoing for the next 10 years, on both LEO Iridium Next and MEO
google project called O3B and should bring high speed internet access for mobiles using omnidirectionnal antennas so aviation should be a target.
8.1.1.4 Cost of airborne embeded software
As software assurance level SWAL asked for avionic software is higher onboard than for what you can design for ground IT service, the quality of the libraries components
brought onboard is predominant and have to comply to ED12B and DO-178C common to both EUROCAE and RTCA. So compression is mandatory but not at any cost,
because you need to prove the stability and reliability of your software.
8.1.1.5 Bandwith constraint
In the case of this study, uploading a D-NOTAM on an en-route “standard” plane asks for low bandwidth datalink and using the scarce remaining bandwidth let to AOC
communications by ATM communications
That’s the reason why compression is mandatory; the smaller will be the data to send the better. As most of datalink techno are using framespaquets, the goal would be to keep
data small enough to fit in one single framepaquettime slot.
POA: 2400 bps by frequency shared between all planes on a 100 miles radius AOA: 31kbps by frequency shared by all plane in a 100 miles radius and with ATN.
Inmarsat BGAN: between 200 and 432 kbps by channel up to 2 and by plane, depending on the antenna type active or not or up to four 64Kbps channel for swift 64.
Iridium: 2400 bps by plane
8.1.1.6 Message size for each datalink
As POA limits messages to a maximum of 16 blocs of 220 characters, it’s difficult to imagine sending a message bigger than 3KB. As VHF communication is noisy, the more
frames you send, the less probable it is to retrieve all pieces.
For ATN which relies on X.25 and connected communication with detection of lost messages, you can send bigger messages. As ATN relies on AVLC and on VDL2, burst
frames contains only 249 bytes, so the less burst you use, the better.
SatCom use slots, and as SatCom is mainly thought for phone communications, you find small slots.
Copyright © 2011 Open Geospatial Consortium
111 The SBB service of Inmarsat provide both IP and packet modes ISDN, but no data is
publicly available concerning a frame size. Iridium uses messages up to 1960 bytes SBD cf. [IRIDUM_SBD]
8.1.2 Best candidate
Regarding the message size data, and the result obtain for compression of D-NOTAM, we recommend using EXI with both schema and deflate post-compression.
Anyway, even if the compression provided by this candidate is the best one around reduction from 5321 bytes to 711 bytes, a D-NOTAM will still span on 3 VDL2 burst
frames, or 4 POA blocs. As the probability to have a corrupted message increases with the numbers of frames used, the goal is to fit into only one frame. D-NOTAM won’t fit in
less than 220 characters.
But this compression level is enough to send a full D-NOTAM through Iridium’s SBDS.
8.2 Best compaction for synchronization of databases across ground network
The recommendations given in this chapter uses intensively the measures of maximum throughput of the candidates given in 7.2.3.
8.2.1 Slow network links 1Mbitss
As such a low speed of 1Mbps like an anemic DSL line, you could use almost any algorithm of compression. If your server doesn’t serve more than 1 client at a time
average, you could go for EXI with both schema and deflate 0.15 MBps will just make enough to fulfill the bandwidth.
8.2.2 Fast network links 1Mbits
Between 1 and 10 Mbitss regular DSL line, or a rented EtherLink access with limited bandwidth, EXI with deflate is not more an option, excepted if you have between 5 and
10 clients connected simultaneously in average and a server with a least 8 cores HT included or more.
But in most cases, for regular server and a limited number of clients with no overlap of requests, we recommend to use Fast Info Set with deflate or simply deflate alone with
level between 5 and 9. As this latest choice is already implemented in HTTP servers, it will cost you nothing.
