Telstra to offer rural Australia free e-mail
Fri, 16 Oct 1998 17:44:44 +1000
George Michaelson wrote:
> Using modem layered over the ADPCM (or GSM or whatever) But isn't it
> possible> in CDMA and in fact in any of the standards to expose the digital
> carrier and achieve either slightly, or significantly better throughput by an
> all- digital encoding?
GSM uses voice coding called RPE-LPC which runs at 13kb/s, but this is
actually a primary 8kb/s coding, plus FEC. Then another 3kb/s is added for
in-band signaling, monitoring and control (a total of 16 kb/s). That is all
the slot will carry.
To shunt data over the link, it needs to bypass the voice coder at each end,
and it needs FEC even more. So the actual throughput rate is in the order of
8kb/s. They hope to be able to aggregate channels by allocating more than one
time slot, but no one is doing this yet to my knowledge.
CDMA cellular (in the 800MHz band - using 1.25MHz channels) also uses a
similar voice coder, but it gets a true throughput of about 9.6kb/s. However
the PCS version running at 1.8 or 1.9GHz uses a wideband vocoder for better
voice quality, and is currently able to shunt data at 13kb/s.
B-CDMA (Broadband) systems now appearing (IS-95b standard), can aggregate a
number of channels and get data rates (still using the standard cellular
channels) in the order of 56 and 64kb/s. They will be available later this
year. Next year the rate will jump to 128kb/s, and then go up to 215kb/s in a
few years. To get the extra channels to aggregate they only need to allocate
new codes, but the terminal equipment must be capable of handling the multiple
W-CDMA (wideband) uses wider channels (up to 5 and 10GHz), so it needs to be a
completely new system. This is the standard most likely for IMT-2000.
Stewart Fist - writer and columnist
http://www.abc.net.au/http/sfist/ (some archives)
http://www.electric-words.com (main archives)
70 Middle Harbour Road, Lindfield, 2070, N.S.W, Australia
Phone +61 2 9416 7458 Fax +61 2 9416 4582