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octobre 14, 2005
ConnexLink 900 MHz link: Planning
I rediscovered the Aerocomm ConnexLink modems today, I had read about them in the past, but for some reason I didn't find them interesting. I certainly find them interesting today.
I would like to use them for a 2.9 Km link in a rural area. I did a path analysis of the link, I would have 30 dBi of fade margin without considering interference or obstruction (mainly by trees), this link definitely looks feasible.
The ConnexLink is a serial radio modem, to use it for Internet connectivity, I'd need a convertion from TCP/IP to serial -- I guess using PPP. I'm looking into setting a Linux router with real-time compression on both ends of such a link.
Throughput Estimate
The manufacturer gives an over-the-air throughput of 78 Kbps. I know that PPP does data compression automagically. I did a little experiment to get an idea of what the gzip compression can do. All my mailboxes are stocked in Maildir format, most of them containing plain text messages. The PC I'm doing this experiment on is an AMD Athlon XP2000+ (1.3GHz) with 512 MB RAM running Linux Debian testing.
gfk@ali:~$ tar -c Maildir/* > maildir.tar
gfk@ali:~$ ls -lh maildir.tar
-rw-r--r-- 1 gfk gfk 252M Oct 13 21:02 maildir.tar
gfk@ali:~$ time gzip --best maildir.tar
real 1m2.976s
user 0m49.160s
sys 0m2.660s
gfk@ali:~$ ls -lh maildir.tar.gz
-rw-r--r-- 1 gfk gfk 155M Oct 13 21:04 maildir-best.tar.gz
gfk@ali:~$ gunzip maildir.tar.gz
gfk@ali:~$ time gzip --fast maildir.tar
real 0m46.027s
user 0m32.470s
sys 0m3.080s
gfk@ali:~$ ls -lh maildir.tar.gz
-rw-r--r-- 1 gfk gfk 161M Oct 13 21:07 maildir.tar.gz
That gives between 36% and 38% compression. For a 78 Kbps link that would raise it to about 110 Kbps for plain text.
My mother is paying $25 per month for a 128 Kbps link to the Internet with videotron, if I'm able to set up a PtP link from my house to hers with this setup, it would pay for itself pretty fast.
Price list
To give me an idea of how much such an adventure would cost me, I made a preliminary part list.
Radios
Price: US$250 for both radios at Mouser
Serial-Ethernet bridge
Linksys NSLU2 Requires a 256 MB USB Memory Stick.
Price: CAN$115 for the NSLU2 at FutureShop CAN$50 for the memory sitck at WallMart.
Weatherproof enclosure
7"x6"x2" NEMA 6 (Submersible) box made by Pacific Wireless
Price: US$21.75 at FAB-Corp.
Antenna
Cushcraft PC9013N 13 elements 900 MHz yagi
Price: CAN$135 at Hutton.
POE Injector
IEEE 802.3af compliant POE injector/spliter.
Price: US$24.95 for the injector, US$24.95 for the splitter at Pacific Wireless
Connectors
Weatherproof ethernet cable passthrough Price: US$10 (10 pack)
Pigtail RPSMA Male-N Female Bulkhead pigtail. Price: CAN$30
Cables
15' LMR-400 with N-Male connectors on both ends. Price: CAN$60 (2x)
50' Weatherproof CAT5e cable. Price: ?
Posted by gfk at octobre 14, 2005 3:26 PM
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