Doubt in GSM Concepts

[karthik537]

If in a cell(area defined by a BTS), has 20 mobile stations (let us say). Those 20 mobile stations scanned and found the downlink frequency 935.2 MHz(lets say) which has good signal strength. In GSM, from the TDMA Scheme as we know that a frequency channel can support only 8 mobile stations, what abou the remaining 12 mobile stations as they are also locked the frequency 935.2.
Will they change to next best frequency? and if so how they know that the freq 935.2 MHz is already occupied by 8 users?

[s52d]

If in a cell(area defined by a BTS), has 20 mobile stations (let us say). Those 20 mobile stations scanned and found the downlink frequency 935.2 MHz(lets say) which has good signal strength. In GSM, from the TDMA Scheme as we know that a frequency channel can support only 8 mobile stations, what abou the remaining 12 mobile stations as they are also locked the frequency 935.2.
Will they change to next best frequency? and if so how they know that the freq 935.2 MHz is already occupied by 8 users?

Hi!
Terminals "use" timeslots only when they talk (dedicated mode).
Most of the time they just silently listen paging ch. for incoming calls.
So, yes, you can have 1000 terminals listening for incoming calls, while 7 are talking.

BTW, most cells have more TRXes (up to 18), so more terminals can talk.
For most of the networks, only 1 out of 50 terminals is talking, others just wait.

BR
s52d

p.s. Once, with some beer, we found several "proofs" that GSM can not work at all.
There are only 5*10^9 terminals who do not bielive us.

[choubeyb]

If in a cell(area defined by a BTS), has 20 mobile stations (let us say). Those 20 mobile stations scanned and found the downlink frequency 935.2 MHz(lets say) which has good signal strength. In GSM, from the TDMA Scheme as we know that a frequency channel can support only 8 mobile stations, what abou the remaining 12 mobile stations as they are also locked the frequency 935.2.
Will they change to next best frequency? and if so how they know that the freq 935.2 MHz is already occupied by 8 users?

Basically all mobiles will camp on the same cell. If the cell has 1 trx, then considering the 1st Timeslot for BCCH, 7 Mobiles can be on a call at a time ( Full Rate channels). If the 8th user wants to initiate a call, he won't be able to do so due to congestion

Normally cells have more than 1 trx. Also techinques such as Congestion relief is used to offload the cells.

Currently AMR and half rate, The capacity of the cell also can be increased