TDMA synchronization
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From: ashu on 13 Jul 2010 09:35 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
From: Tim Wescott on 13 Jul 2010 11:30 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
From: Mark Borgerson on 19 Jul 2010 00:39
In article <9e430326-460f-480a-8c82-3c7ce870b5b6 @d37g2000yqm.googlegroups.com>, ashutosh.ghildiyal(a)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 |