On Fri, 23 Mar 2007, Chris Houser wrote:
> On 3/23/07, Gerhard Fasol <fasol@eurotechnology.com> wrote:
>>
>> I guess that the packet size of 128byte comes comes from the
>> ATM switching technology used in the initial phase of
>> 3G here in Japan
>
> But didn't "packet" billing preceed 3G?
Read up on the history of ATM (Asynchronous Transfer Mode); it far
predates 3G, though it appears to me to be pretty much dead, nowadays.
(Telcom being telcom, of course, there's no doubt a whole lot of ATM
still hiding in the insides of various carriers.)
That said, Docomo's packet size is probably nothing to do with ATM, for
a couple of reasons:
1. ATM used 64-byte packets (datagrams, to be more precise) with a
56 byte payload.
2. ATM was designed around the idea of building a packet network
optimized for handling traffic that, at the time, was carried via
TDM (time-domain multiplexed) links such as DS1/DS3/etc., and would
let you converge your TDM and packet data networks. In the end, we
ended up using networks optimized for packet data and making the TDM
stuff work as best it could over them, which turns out to be plenty
good enough for most people, given the cost savings.
All that aside, 128 bytes is certainly not any kind of an odd number for
a packet network, especially one running at 9600 bps over shared media,
which is exactly what Docomo's PDC data network was. This network was
built long before i-Mode ("long before" being "Internet time" here, of
course), was what i-Mode happened to be built on top of, and was thus
probably quite a reasonable choice for the billing unit.
If you look around at protocols designed in the 70s, you'll probably
find a fair number that used 128 byte packets. The most famous would
probably be XModem, which was designed for use over 300 bps modem links.
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974
Mobile sites and software consulting: http://www.starling-software.com
Received on Mon Mar 26 08:17:58 2007