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Data Connections: Circuit Switched vs. Packet Switched

David Carlson

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 Circuit Switched > CSD > Modem-to-Modem line connection.  This is the type of connection your old 28.8 kilo-baud modem used with dial-up internet accounts.  The modem calls the 10-digit phone number of the Internet Service Provider (ISP).  This is essentially an analog connection; the data is sent as if it were a voice signal, in real time.  CSD requires an entire voice comm. line connection for the duration of the call.

Packet Switched > PSD: GPRS (GSM) and 1XRTT (CDMA) > This is a digital-type of communication used by cellular networks.  Here is an important point: for PSD connections, the cellular network is the ISP - they provide the onramp to the internet.  Each file or data stream that is sent is broken into small packets that are numbered and reassembled at the other end.  Several PSD connections can be made simultaneously on a single comm. line.  This type of connection is initiated by using a character string instead of a phone number:

  • CDMA (Verizon, Telus, Alltel*) - #777
  • GSM (AT&T, Rogers, et all) -*99***1#

 In recent years, the availability of cellular CSD connections has been in decline in favor of PSD connections.  This is because the PSD connections can send data faster, more reliably, and at a lower cost to the service provider. The decline in CSD connections can be seen most in Europe and North America, where AMPS cellular networks have been decommissioned.

For iPacks, it is better to use PSD connections. This is true for a few reasons:

  1. Voice calls are not required, so a data only cellular plan can be purchased (usually less expensive)
  2. Since the cellular provider is the ISP, internet connect time is reduced
  3. Data transfer time is reduced

Generally, with a daily call schedule, the iPack will transfer less than 5Mbytes of data per month.
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