Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors

GSM logical vs physical channels…

Viewing 11 posts - 1 through 11 (of 11 total)
  • Author
  • #53532

    Can someone explain how all the GSM channels work?

    I understand the different types of channels (common channels vs dedicated (traffic & signalling)) but what I haven’t been able to figure out is how they are mapped to actual radio frequencies.

    Which ones actually share the same carrier and which ones have their own carrier (if any).

    Also how are they assigned to TRXs?
    My understanding is that each TRX can handle 8 TCH (since there is 8 timeslots in the TDMA frame) so does this mean there’s only one frequency per TRX? (well a pair for up & down links).



    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.


    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?



    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 :

    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 :



    Nice. Thanks Pix for the link.


    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.


    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.



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


    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


    nice one Pix


    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).

Viewing 11 posts - 1 through 11 (of 11 total)
  • The forum ‘Telecom Design’ is closed to new topics and replies.