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I understand your question now 🙂
I’ll come back to your remarks of GSM later.

The SDCCH is a 51 multiframe. 51 consecutive timeslots = 51*4.615 = duration of the whole frame.

when the BTS allocates a SDCCH to someone, whichever the purpose, actually it means that :
The reference of 1 SDCCH block (UL & DL) + the reference of 1 SACCH block is allocated to 1 user, for as long as he needs it. “Reference” means address : TRX (0…i) , TS (0…7) and sub-channel (0…7)

1 SDCCH block = 4 consecutive timeslots (or rather, 4 radio bursts) of 0.577ms. For this block, each burst is sent consecutively. It takes 4 TDMA frames to send the whole block.

The same for the SACCH (which carries the signalling, like power control, timing advance, measurement reports)

After this 4 TS, the MS has to wait for the next 51-multiframe in order to send the remaining data.

During the rest of the 51 multiframe, 7 users can send/receive their SDCCH & SACCH.
Altogether, 1 51-mframe contains 8 SDCCH blocks in UL & DL, and 4 SACCH blocks in UL & DL.
I hope I was clear..

So why only 4 SACCH ? because actually in odd 51mframe, the users 0,1,2,3 will use their SACCH, and in even 51mframe, the users 4,5,6,7 will use their SACCH.

Now, regarding GSM :
1. cheap
2. not so complicated (to install ? to set the radio parameters ? .. oh ok, frequency planning can be a bit tough sometimes)
3. provide enough throughput for most applications.
4. was available at the time the world asked for a wireless technology.

Of course, sailing boats are not the best way to cross the atlantic ocean, but Columbus didn’t have an Airbus A330 available…

The only limiting factor is the frequencies : this is the reason why the band 1800 is available, and it should be used widely in the dense urban areas of india, china, or europe.

There is no doubt the WCDMA is a very clean and smart solution, but it costed SO MUCH in terms of R&D… and in the end, does it bring much more than GSM ? It’s a little easier to plan, maybe provide a little more capacity… (yet, i want to see how it behave in a dense area, WITHOUT a GSM network next to it to carry most of the voice traffic 🙂 )

How is WCDMA under congestion ? Does it provide good capacity, good quality and good throughput ? (that really is an innocent question, I would really want to know that…)