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GSM logical vs physical channels...

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Deepak - 11th January 2011  (13:27 GMT)

Well Talking in a layman's lang Physical channel means Your Road on which u can carry ur vehicle for the Communication.Logical channel means the sign boards on that Road like take left take right,blind curve and distance etc.Technically Physical channel(Road) means the frequency for carrying Communication Logical Channel means(Sign Boards) the information encoded i.e the bits on that frequency to guide ur Communication (Vehicle).

anshu - 11th January 2011  (12:21 GMT)

nice one Pix

pix - 8th January 2011  (06:04 GMT)

physical channel = timeslot, ie. tch timeslot or sdcch timeslot or bcch timeslot or pdch timeslot
logical channel = what is actually being transmitted on a timeslot: tch/sacch or sdcch/sacch, or bcch/pch/agch/rach, or pdtch/pacch

Rucha - 8th January 2011  (04:02 GMT)

Can anyone will tell me the exact explanation of physical and logical channels in simple language

SHELDON - 27th May 2010  (20:05 GMT)

Hi Gopa,

FCCH, SCH, BCCH are always on timeslot 0 on the BCCH frequency. However, it's possible to configure some CCCHs on other timeslots on the BCCH frequency. I think the feature is called Extended BCCH. You can research more on it.

Regards,
SHELDON

Gopa - 27th May 2010  (03:37 GMT)

Can any one tell me , Does FCH+SCH+BCCH +CCCH are always in 0th timeslot of the beacon frequency , or is it possible to configure it in any other time slot of the beacon frequency.

Nagra - 31st August 2008  (10:16 GMT)

Nice. Thanks Pix for the link.

Pix - 31st August 2008  (05:22 GMT)

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.

Nagra - 31st August 2008  (00:02 GMT)

So for each cell, one TRX is assigned the BCCH? And this BCCH uses one timeslot out of the 8?
I also understand that another timeslot or two will be assigned to SDCCH. All other timeslots are for TCH?

Is this correct?

That's what I'm trying to figure out.

For instance, what carries the FCCH, SCH and all the CCCH (RACH, AGCH, NCH, PCH). Separate timeslots or are these channels combined somehow?

RH - 28th August 2008  (13:57 GMT)

It doesnt mean that each TRX is assigned only with 1 frequency.
Only BCCH TRX is always assigned with 1 frequency.
But TCH TRXs can use more then 1 frequency (if you use SFH).
See documents for logical channel mapping.....and also for hopping.

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