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# Reply To: GSM logical vs physical channels…

#53535
Pix
Guest

nagra,

if you search on internet, you’ll be able to find drawings and graphical explanations that will certainly b easy to understand.

Let’s keep it simple and talk only about “speech” in GSM.

a GSM cell is defined by a certain amount of TRX’s that are “controlled” by only one BCCH. So there is one BCCH timeslot for a whole group of TRX’s.

what is the BCCH ? it contains broadcast information, synchro and frequency correction info and allows users to access the network. But befor all, it is a physical channel 🙂 In order to do so… This physical channel is consecutively sending different logical channels.

The physical BCCH will alternatively send FCCH, SCH, the CCCH’s and the … BCCH, but the logical one this time. Of course you’ll wonder what’s the point of this “bcch” ? But first, let’s see how all those logical channels can be send on only one physical channel.
A physical channel is one timesot. The BCCH, for example, is usually located on the first timeslot of the first trx of the cell.

So it’s going to be sent once every 8 TS. If you collect those BCCH bursts only, you’re going to build the BCCH-multiframe and there you’ll see the way the logical is mapped on the physical. And every 51 bursts, you’ll notice (if you’re really focused) that the pattern repeats itself ! That’s why it’s called 51-multiframe.

In the bcch 51 multiframe, you’ll count 51 bursts. Ahem. Wait, it gets more interesting. 10 “blocks” of 4 consecutive bursts, 10 “lone” bursts and 1 idle burst. 40 + 10 + 1 = 51. Ok, I guess it’s time to draw it :

gsmfordummies.com/tdma/logical.html#mapping

the lone timeslots are FCCH and SCH, while the blocks are used for bigger data. 1 block is used for the BCCH (which will alternatively send systen information msg), while the 9 others are used either by PCH or by AGCH. It depends on your radio parameter settings.

For richer information, now you know the basics, i recommend you go there :

gsmfordummies.com/tdma/tdma.html
gsmfordummies.com/tdma/logical.html

etc.