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Viewing 15 posts - 1 through 15 (of 17 total)
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  • #50142

    I have one doubt regarding SDCCH.
    1. Why the name SDCCH/8. It is due to the reason that 8 BLOCKS are there in one 51-MULTIFRAME OR anything like one SDCCH can control 8 TCH OR some other reason.
    2. Why four TS consecutively needed
    in case of SDCCH(not sure this is the case). Is it like 4 x .577 msec will do in all cases(viz LA,CALL SETUP ETC). I think this can’t be the case as how this will make the hopping possible on SDCCH.
    3 Is there any relation between no of SDCCH required vs no of TCH in a cell.


    Any how if SDCCH/8 is needs to be added in a cell. Then which TRX(and TS) will be a better choice and why.
    In case of failure of BCCH TRX we know that BCCH will go to next/other TRX but does this will be case with SDCCH also? or at least with the SDCCH/8 running on BCCH TRX?


    1. Yes, 8 blocks in one 51 mframe. As a consequence : 8 different users can be served at the same time on the same timeslot.

    2. One block equals to 4 radio bursts. It’s just how many bursts need to be used in order to transmit data contained in one radio block. It’s the same with TCH : one speech frame is sent over 4 radio timeslots (in average… not taking into account interleaving ๐Ÿ˜‰ )
    Same also in GPRS : 4 radio timeslots for 1 radio block…

    3. Yes : plan 1 SDCCH/8 for 2 TRX in standard network.
    For instance, for 4 TRX, you need 2 SDCCH/8.
    More might be needed in case of high usage of SMS.

    4. In case of failure of the physical unit that carries the BCCH, the whole logical TRX is transferred onto another unit. This is the reason why we must put one SDCCH/8 along with the BCCH. Other SDCCH can be put in different TRXs though.



    Pix, thanks for reply.but still iam not able to are saying that ” 8 different users can be served at the same time on the same time slot”.
    Let us assume i need a channel(signaling) for some xyz purpose from network. I request for it and he(network) assigns am told a frequency+TDMA frame+Time slot.After that i will use this TS for completing my need(it may be LA or SMS or getting a TCH).Now forgetting about the TCH part let us concentrate on the SDCCH part.Back to
    SDCCH…i am allocated a TS…i will send my signaling on this TS …but i have got only .577ms to do this….after that i have to weight for 4.615ms….again i will
    send my signaling for .577ms…an d so on….in the end i will release it ie TS.either my work is completed or an TCH may have been allocated…..This is my picture of SDCCH.

    But iam not able to understand how
    can it be at the same time.8 MS cannot used a SDCCH at the same time.


    GSM …is not a good technology..very complicated to understand…complicated to dimension….complicated to manage…a standard which was meant only for Europe…but the rest of world adopted it blindly without considering the fact that countries like India and China where population density is very high..this technology
    will fail one day .That is why finally they are moving towards WCDMA.



    I understand your question now ๐Ÿ™‚
    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 ๐Ÿ™‚ )

    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…)



    Thanks for reply.

    Please let me know:
    1. How may SDCCH BLOCKS are needed to
    send a 160 character SMS. OR if the characters in a SMS are less then 160
    , does that mean some reduction in the number of SDCCH BLOCKS?

    2. A requirement of user, say needs 5 SDCCH BLOCKS to be transmitted. Then does this means a MS has to weight for 51-4=47 frames? or 47×4.615 msec to send the second SDCCH BLOCK and so on.

    I think 47x 4.615 is a very long time for MS to remain idle. or even if he is collecting network information during this period to send them on SACCH on this 51 MF or NEXT 51 MF.


    1. I don’t know, one needs to check the useful data rate of SDCCH channel per block. One burst is about 110 bits, so 1 block is about 440 bits. 1 character is 8 bits… so you’ll need about 3 SDCCH blocks to send 160 characters.

    2. 47×4.615 msec to send the second SDCCH BLOCK and so on == correct

    it is a “long” time for a processor, but that’s the way it works. For a human user, this extremely short time is not perceived…

    Ain’t we human after all ? ๐Ÿ™‚



    i m using this forum first time.
    and i find here the answer of my query.

    can anyone help me on mapping of logical channel on physical channel.
    i m very confused on this.

    plz help me out


    Hi, Taqi!
    I have clear and really useful book about GSM. I can send it on your mail (pdf file ~ 6 Mbytes).


    Dear Pix,
    I’m from Ukraine. I work in UMC “Ukrainian Mobile Communication”. Alcatel BSS is used only in 3 region, but it’s ~ 14000 of cells. I’m an engineer in group of Network and service quality.


    hi Alexx!
    Could you inform me on more about logical and physical channels?


    Thank u very much.yes u can send ths on my e.mail id.i really need ths.


    – Physical channels are all the available TSs of a BTS, whereas every TS corresponds to a physical channel. Two types of channels need to be distinguished, the halfrate channel and the fullrate channel. For example,a BTS with 6 carriers, has 48 (8 times 6)
    physical channels (in fullrate configuration).
    – Logical channels are piggybacked on the physical channels. Logical
    channels are, so to speak, laid over the grid of physical channels. Each
    logical channel performs a specific task.
    Another aspect is important for the understanding of logical channels: during a call, the MS sends its signal periodically, always in a TDMA frame at the same
    burst position and on the same TS to the BTS (e.g., always in TS number 3.
    The same applies for the BTS in the reverse direction.


    Hello Taqi!
    What is your e-mail?
    My one is

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