# GSM

#### RLC

Joined Jul 20, 2011
13
In the traffic channels of GSM there are 8.25 bits reserved for guard PERIOD
can someone tell me, how bits is in point (0.25)??
i mean when we say a 2 bit no, there are 4 possibilities(i.e. 2^n) like 00,01,10,11.
But how will 8.25 have possibilities

#### Von

Joined Oct 29, 2008
65
The key to your question is located in the question...i.e. Guard PERIOD (or time).

GSM's Gaussian Minimum Shift Keying modulation has a data throughput of 270.833 kilobits/second (kb/s).

This works out to be 3.69 microsec./bit.

Thus 8.25 bits would take 30.44 microseconds.

This is the Guard PERIOD.

This period is referenced to bits when in the context of frames.

This guard time is needed due to the unknown distance of the MS from the BTS (propogation time/delay).

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#### kubeek

Joined Sep 20, 2005
5,736

Hint: try "gsm 8.25 bit"

#### RLC

Joined Jul 20, 2011
13
Everywhere it is written that 8.25 are GUARD bits

So how can BIT be rational,it has to be a natural NO like 1,2,3...etc

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#### RLC

Joined Jul 20, 2011
13

Hint: try "gsm 8.25 bit"
how can BIT be rational,it has to be a natural NO like 1,2,3...etc

#### RLC

Joined Jul 20, 2011
13
how can BIT be rational,it has to be a natural NO like 1,2,3...etc
The key to your question is located in the question...i.e. Guard PERIOD (or time).

GSM's Gaussian Minimum Shift Keying modulation has a data throughput of 270.833 kilobits/second (kb/s).

This works out to be 3.69 microsec./bit.

Thus 8.25 bits would take 30.44 microseconds.

This is the Guard PERIOD.

This period is referenced to bits when in the context of frames.

This guard time is needed due to the unknown distance of the MS from the BTS (propogation time/delay).
how can BIT be rational,it has to be a natural NO like 0,1,2,3...etc

#### AlexR

Joined Jan 16, 2008
732
how can BIT be rational,it has to be a natural NO like 0,1,2,3...etc
You are talking about the guard period i.e a period of time not a count of bits sent or received. The units of this time period happen to be bits which is defined as the time that it takes to transmit 1 bit and obviously a time period can be any length you like so it can be some fraction of a bit long.

RLC

#### RLC

Joined Jul 20, 2011
13
So it means that in GSM 148 bits of ONE burst are bits send or received
and rest 8.25bit is GUARD period NOT send or received bits
that makes it TIME SLOT=148(one burst)+8.25= 156.25