TDMA synchronization

Hi All,
I had a small question on synchronization in TDMA protocols.

Lets assume the case where we have a master and some nodes and each of
the nodes have their own pre-assigned time slot. Supposing each TDMA
node has a crystal-clock which has some deviation(lets say few tens of
ppms), how do we manage to wake the nodes up correctly at their pre-
assigned time slot ? One way is to somehow extract master's clock and
then synchronize all the slave clocks with master. But supposing clock
extraction is too costly and cumbersome ...is there any other way ?
How is this managed in 802.15.4 protocols ?

regards
ashutosh

On 07/13/2010 06:35 AM, ashu wrote:
> Hi All,
> I had a small question on synchronization in TDMA protocols.
>
> Lets assume the case where we have a master and some nodes and each of
> the nodes have their own pre-assigned time slot. Supposing each TDMA
> node has a crystal-clock which has some deviation(lets say few tens of
> ppms), how do we manage to wake the nodes up correctly at their pre-
> assigned time slot ? One way is to somehow extract master's clock and
> then synchronize all the slave clocks with master. But supposing clock
> extraction is too costly and cumbersome ...is there any other way ?
> How is this managed in 802.15.4 protocols ?

Part of the reception & demodulation task requires synchronizing to the 
master far more tightly than is required for the protocol, so that 
synchronization comes mostly for free.  If the travel time from slave to 
master is a significant proportion of a packet time then the slave needs 
to be controlled for delay, so that it's response arrives at the 
transmitter in the correct timeslot, but that's not what you're asking 
about.

I have no clue how it's managed in 802.15.4 -- I'm just familiar with 
GSM (cell phone) usage.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

In article <9e430326-460f-480a-8c82-3c7ce870b5b6
@d37g2000yqm.googlegroups.com>, ashutosh.ghildiyal@gmail.com says...
> Hi All,
> I had a small question on synchronization in TDMA protocols.
> 
> Lets assume the case where we have a master and some nodes and each of
> the nodes have their own pre-assigned time slot. Supposing each TDMA
> node has a crystal-clock which has some deviation(lets say few tens of
> ppms), how do we manage to wake the nodes up correctly at their pre-
> assigned time slot ? One way is to somehow extract master's clock and
> then synchronize all the slave clocks with master. But supposing clock
> extraction is too costly and cumbersome ...is there any other way ?
> How is this managed in 802.15.4 protocols ?
> 
> regards
> ashutosh
> 
In one UAV application, we synchronized the TDMA slots to the 
1Hz pulse from the GPS system.  Of course, that relies on
a GPS system---which can be gotten for under $40 in small quantities,
and a clear view of the sky.  We  needed about 50 slots covered
at a 4Hz rate, so our requirements weren't too stringent as the
actual transmissions only occupied about 3mS of the 5mS slot.


Mark Borgerson