Novatel vs Sierra CDPD Modems UPDATE
Nov 08 '00 (Updated Aug 30 '02)
The Bottom Line CDPD works well in many areas, but is slow, so if you need more speed, try the new 2.5G 60Kbs services from Sprint, or find a nearby Wi-Fi "hotspot".
First a little background info (if you're already familiar with CDPD you can skip this part):
CDPD, or Cellular Digital Packet Data, is currently the most popular way to get wide-area wireless Internet connectivity into your laptop or PDA.
While quite slow (theoretical maximum data rate is 19.2Kbits/sec, reality far slower, see below), it piggybacks on the existing 800 Mhz analog cellular telephone infrastructure, and so it does have excellent coverage-between the two major carriers, AT+T and Verizon, you can have usable roaming service in 98% of US population centers. Note, however, that CDPD requires the carrier to install one or more X.25 packet radio access points in your local analog cell tower, so not every place that has analog cellular service necessarily offers CDPD.
Since CDPD is a TCP/IP packet protocol, it does have many of the advantages of traditional hardwired wide-area networks, including connectionless data transfer and, in theory, "guaranteed" data delivery. The most notable performance differences you'll see are in turn-around (ping) times, which are typically around 400 to 500 Msecs (compared with 10 to 80 Msecs in hardwired wide-area networks),transfer rates, and, with portable use, intermittent availability as you move in and out of areas with adequate RF signal level. You also get a static IP address (at least for now) which has some performance, footprint, and reliability advantages.
The most common way of obtaining the CDPD service is to buy a PCMCIA card from one of several suppliers (Sierra and Novatel being among the most popular), install the card, register with either AT+T or Verizon (avoid GoAmerica), and you're on-the-air with nationwide roaming using either your laptop or PDA. Both carriers offer reasonably attractive all-you-can-eat plans.
Comparing the two modems:
Recently, I needed to compare the data-rate performance of the Novatel Merlin CDPD wireless PCMCIA modems with the equivalent offering from Sierra Wireless, the Aircard 300. Here's what I found, using AT&T's CDPD service and running WS_FTP to transfer several test files from our FTP server to the IBM Thinkpad on which I installed first one card, and then the other.
With conditions as nearly identical as possible, I transferred both 200K and 2Meg test files in both directions (typically downloads would be expected to be faster than uploads, due to RF signal strength issues). I used "uncompressable" binary files to avoid artificially jacking-up the true data rates.
The results were that The Sierra Aircard 300 was over twice as fast as the Novatel Merlin (but it's important to remember that the Sierra, at around $500, is also twice as expensive as the Novatel at around $250):
Novatel average sustained transfer rate 2.4K (upload) to 3.4 Kbits/sec (download)
Sierra average sustained transfer rate 5.5K (upload) to 8.0 Kbits/sec (download)
However, neither of them was anything like as fast as the theoretical 19.2Kbits/sec, despite fully adequate RF signal strength levels of around -75 dbi.
From all of this, I suppose you can conclude (a) you get what you pay for, and (b) if you need real speed forget CDPD- instead look to the new 2.5G services from Sprint, probably coming to a city nearby shortly, or my all-time favorite, soon to be rising from the ashes of Metricom, the Ricochet network.
UPDATE: Novatel sent us a firmware patch to address a major cause of their 50% transfer-rate performance disadvantage- unnecessary de and re-registrations that were invoked whenever the RSSI (received signal-strength indication) changed by more than around 2 db (such fluctuations happen all the time). I've just tried the new firmware, and it does indeed dramatically improve the performance, to the point where, on preliminary tests, it now performs every bit as well as the Sierra. Hopefully they will post this new patch on their website. They should-Sierra is making a really big deal of their data-rate advantage at every wireless trade show I visit.
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sharples
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Member: Tom Sharples
Location: Vancouver, WA 98664
Reviews written: 4
Trusted by: 0 members
About Me:
President of Qorvus Systems, Inc., wireless consultants and interactive product development.
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