Mobile Data Networks: Circuit Switched & Packet Switched
There are two main types of data connection available over the cellular mobile data networks;
Circuit switched gives you a radio timeslot to yourself, with the full data rate of that timeslot (9600 baud) available exclusively to you. The downside to this is that you are then paying for your connection per second. If you are doing an activity such as web browsing, your link may be idle for a significant period whilst you read the contents of a page - time that you are paying for at mobile rates.
HSCSD takes the circuit switched paradigm one step further. It modifies the error coding of a radio timeslot to give 14kbs, four per timeslot, but then allows you to use more than one timeslot. If you use two timeslots, you can get up to 28kbs. HSCSD can be symmetrical or asymmetrical - you can use two downlink and one uplink timeslot to give 28kbs downloads but 14kbs uploads. There are three reasons why you might want to do this - handset limitations, billing purposes, or the conservation of battery power. Every transmit timeslot takes a significant amount of power, so if you are more interested in downloading data than uploading, it can be worthwhile only using one uplink timeslot.
HSCSD handsets typically are able to use a certain number of timeslots - for example, four on the Nokia 6210 and Card Phone II. These slots can be split as needed between up and download timeslots, allowing you to use two up, two down (28kbs four in both directions) or three down and one up (43kbs two down 14kbs four up). The actual speeds and timeslots available will depend upon your network.
From a billing point of view, the more timeslots you use, the more your network may charge you. Some networks simply make a flat rate per-month charge then charge multi-slot connections the same as single slot. Other networks may charge you twice as much (or even more!) for using multiple timeslots. In the UK only Orange supports HSCSD.
Image source: http://www.glasbergen.com
Packet switched networks (such as GPRS) use a different data connection method. Your network will allocate a certain number of timeslots per base station for GPRS data. Rather than having exclusive use of that timeslot, you share it with all other users in your area. You send and receive data in short packets instead of maintaining a permanent link.
The advantage is that you can have a permanent connection, and are only billed for the amount of data transferred, not your connection time. If there is no-one else using GPRS on your cell, you will get very fast data rates. The disadvantage comes if there are many other users on your cell - you have no guaranteed minimum data rate, and may well find that you get data time-outs and extremely low download speeds.
Download and uploads speeds
Where you see items such as 9600 Baud, 14.4kbs, 28kbs: this refers to the speed of the data connection, in exactly the same manner as if you were describing the speed of your internet connection, e.g. 56k, 128k, 256k, etc.
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