Changes between Version 6 and Version 7 of 802.11/wlan_exp/app_notes/tutorial_hop_mac/fast_hopping

Timestamp:

Aug 3, 2015, 11:31:07 AM (9 years ago)

Author:

chunter

Comment:

--

802.11/wlan_exp/app_notes/tutorial_hop_mac/fast_hopping

@@ -6 +6 @@
 
 Rather than have an AP decree a hopping event by including the Channel Switch Announcement, instead we can let each node on the network have an ''a priori'' known frequency hopping schedule. This will not only dramatically reduce overhead, but will also increase reliability since client stations will not have to decode an explicit announcement prior to hopping to a new channel. If we can synchronize our nodes such that don't drift apart from one another, this approach can let us considerably speed up the hopping rate as compared to the prior approach (milliseconds of dwell time vs 10 seconds of dwell time).
+
+The 802.11 standard specifies the Timing Synchronization Function (TSF) for maintaining a common timebase among nodes in a network. The TSF requires every node implement a 64-bit counter which increments every microsecond. Each node maintains its own TSF timer and derives the microsecond clock from its own local oscillator.
+
+In an infrastructure network, the TSF synchronization scheme designates the microsecond timer at the AP as the authoritative time for the network. As part of its normal operation the AP transmits beacon frames at some pre-defined interval (typically 102400 µsec). Every beacon frame contains the 64-bit value of the AP's TSF timer at the instant the transmission occurs. Critically, the beacon's timestamp field must be set at the time of transmission, ''not'' the time of packet creation or enqueuing. This guarantees the timestamp value accurately reflects any transmission delays due to medium activity. Every STA in the network which receives a beacon from the AP must update the its local microsecond timer with the value of the timestamp contained in the beacon payload. With high probability this scheme provides a common timebase across the network, with inter-node time offsets bounded by the maximum oscillator drift in one beacon interval. In other words, with the TSF synchronization already in place as part of the 802.11 Reference Design, we do not need to worry about any kind of explicit synchronization in our fast hopping approach. We can just base out frequency selection on the TSF timer and trust that the value in that timer is synchronized with the other nodes in the network.
 
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:fast_hop_overview.png, width=800)]]  ||
@@ -103 +107 @@
 }}}
 
-Finally, just before the primary {{{while(1)}}} loop, add the following code snippet:
+This will also prevent the AP from including any Channel Switch Announcement tags in outgoing beacons. Finally, just before the primary {{{while(1)}}} loop, add the following code snippet:
 
 {{{
@@ -118 +122 @@
 
 
-
-
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:scope_plots.png, width=800)]]  ||
-||  ''Oscilloscope Plots'''  ||
+||  '''Oscilloscope Plots'''  ||
 
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:xput.png, width=800)]]  ||
-||  ''Fast Hopping 802.11 Throughput vs. Time'''  ||
+||  '''Fast Hopping 802.11 Throughput vs. Time'''  ||
 
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:standard_80211.jpg, width=800)]]  ||
-||  ''Standard 802.11'''  ||
+||  '''Standard 802.11'''  ||
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:slow_hop_80211.jpg, width=800)]]  ||
-||  ''Slow Frequency Hopping 802.11'''  ||
+||  '''Slow Frequency Hopping 802.11'''  ||
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:fast_hop_80211.jpg, width=800)]]  ||
-||  ''Fast Frequency Hopping 802.11'''  ||
+||  '''Fast Frequency Hopping 802.11'''  ||