User manual

WideFly Ltd. WF28 User Manual
http://www.widefly.com rev 1.1
P.20
Always-on Wi-Fi
Start > Settings > Control Panel > WLAN > Advance (Tab page)
Always-on Wi-Fi
WF28 is the first POS PDA features always-on Wi-Fi even PDA is in
sleep mode, i.e. screen & touch panel are off, CPU is in sleeping
status for maximum power saving while Wi-Fi connection is kept
connected.
To enable the feature, click “keep connected when OS sleeps”
Developer Note: Unicast & Broadcast
When PDA is in sleep mode, it could be woken up by incoming
network events so that application would be able to process the
network packets timely.
In most cases, unicast network packet, i.e. those packets with
designated device, is concerned by most POS application. This is
true as long as the application is a point-to-point network
application, e.g. client / server application. Therefore, “Woken by:
Unicast” setting is optimal.
If application requires boardcast type of network packet, it is
required to change to “Woken by: Unicast + Broadcast”. However,
it may defeat the purpose of power saving as single broadcast
message would wake up all PDAs in the whole subnet.
Developer Note: ARP & Cache Timeout
ARP is an IP address to MAC address resolution protocol using broadcast type of network packet. When
POS server sends TCP packets to a PDA, network subsystem would look up ARP cache table. If it finds
the cache entry for the PDA, it would forward the packets to the designated MAC address. Otherwise,
ARP would send out a broadcast message and waits for reply from the device, then build up the ARP cache
entry.
ARP cache entries will be expired. For Windows, default unused ARP entry would be expired in 2 minutes
and reused ARP entry would be kept for maximum 10 minutes. When the entry is expired, ARP broadcast
packets are required. Because of this behavior, if the PDA sleeps for more than 2 minutes and if it only
accepts unicast packet (i.e. ARP request would not be listened), server may not be able to send network
data to the PDA. This scenario would be fixed by increasing the ARP cache lifetime, e.g. few hours that
equals to a daily operation.
In Windows, simply change the registry below and restart the system to make it effective:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\ArpCacheLife
Default value is 120 (2 minutes). It may be defined as 72000 (20 hours).
For details, please refer to
http://www.microsoft.com/mspress/books/sampchap/2453.aspx