- This topic has 6 replies, 1 voice, and was last updated 14 years, 7 months ago by Frood.
28th November 2007 at 13:19 #49882FroodGuest
Bit stuffing is used in gsm to keep the reciever in sync with the transmitter. And since bit stuffing when implemented means that a ‘0’is inserted after a continuous 5 bit of 1s. If the number continuous 1 bits in data can vary would not that mean that a variable length of information is transmitted through the channel so
how does the reciever negotiate with variable length of input?28th November 2007 at 19:52 #49883PixGuest
I don’t understand the problem.. never heard of bit stuffing, maybe it is a “transparent” layer 1 feature ? I stopped frying my brains at layer 3, never went below this 🙂
as additional info : the sync is done thanks to a dedicated block (the SCH), which is transmitted on the BCCH and the TCH logical channels.7th December 2007 at 14:41 #49884FroodGuest
My question of FCS-Frequency Check Sum is related to the LAPDm part of GSM. The Abis interface to be more exact, and the string of ones is used for synchronization on layer 2(which you have correctly pointed out).
The SCH is on the Um interface between the BTS and MS, as I understand. And not related to my question specifically.Thanks for your input so far.7th December 2007 at 19:55 #49885pixGuest
Thanks for the details. Indeed, i thought your question was about the Um interface. I can’t help about the Abis, I’m sorry.
Pix8th December 2007 at 13:49 #49886PANGuest
Any layer2 protocols (and LAPD too) uses frames for information transfer. Each frame is separated from other frames by means of Flags (01111110). To avoid flag imitation in information bit flow (in user data between two consequent flags) following is provided:
Transmitter shall to insert “0” after each block of 5 continuous 1s in user data.
Reciever shall to delete “0” after each block of 5 continuous 1s. If Reciever has detected 6 continuous 1s in combination 01111110, it comes to a conclusion this combination is Flag (opening or closing).18th December 2007 at 07:43 #49887FroodGuest
That is exactly my question, if for example the transmitter has added the 0 bit five time or four times, that means variable length of information. How does the reciever handle this variable length at its input, how does it know that the 0 bit has been added four times or five times?19th December 2007 at 04:54 #49888FroodGuest
Now I get it. Thanks.