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Connect Speed vs Carrier Speed 

Added: 04/12/2001, Hits: 3,310, Rating: 0, Comments: 0, Votes: 0
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By Brian Talbert

Question: I was just wondering, can anybody tell me the difference between modem connection speed and carrier speed? When I connect I see:

Carrier 31200

Answer: Your "connect speed" is the DTE to DCE speed, the speed at which your computer is communicating with the modem. For instance, with an external modem:


The Data Terminating Equipment (computer) is connected via an RS-232 port to the Data Communications (or circuit-terminating) Equipment (modem). The speed of this serial port has been configured via the control panel as 115,200 bps. This will be the maximum transmission possible, irregardless of the circuit speed.

The "carrier speed" is the DCE to DCE speed, the speed of the transmission on the circuit itself, between two modems. Again, with an external modem:


The 31kbps is the "carrier speed" here ... it reflects the mutual capabilities of the DCEs (modems) as well as line conditions.

Both settings are important because both can impact THROUGHPUT ... or the actual perceived rate of communication. Take the following as an example:


Now consider that the DCE's are each your typical 28.8kbps modem. These modems are capable of achieving 4:1 compression if conditions are good and the data being transmitted is highly compressible ... as with TEXT data. So, a large text file is sent from DTE-A to DTE-B. The data stream flows out the RS-232 port of DTE-A at 115kbps where it reaches DCE-A. DCE-A compresses the file at 4:1 and sends it to DCE-B. Because the file is getting compressed the effective throughput up to DCE-B is 115kbps! However, DCE-B now has all of this data and can't send it quickly enough to DTE-B because the connect speed is only 28.8kbps. Thus, even though data is flowing into DCE-B at a rate of 115kbps the OVERALL effective throughput is only 28.8kbps.

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