In this section, we present the proposed semi-soft scheme in IEEE 802.11 wireless LAN and our architecture in mobile node. For simplicity of
presentation, we assume that a mobile node is in the coverage of two APs during handoff.
Figure 6 illustrates the author’s proposed architecture. We add some functions in the wireless LAN card. The sequence of events is as follows:
1. DHCP server: When the mobile node asks the IP address, the DHCP server will assign an IP address for it. It should be always the same IP address.
2. WNAT(Wireless NAT): It will decide which link we want to transmit the data and it will change the IP address of the packets to IP address 1 or IP address 2 (based on the link). It always transfers to the IP address (the DHCP sever assigns) when it receive a packets from the wireless.
3. DHCP client: It will try to get the new IP address when the mobile node connects with the new AP. We combine the network layer and IP layer algorithm. In network layer, when it connects with the new AP, it will trigger the DHCP client to start the IP assignment procedure. DHCP client will get the IP address for each links.
APP1 APP2 APP3
Figure 6 System Architecture of mobile node
In reference to appendix 1, there are a lot of latencies occurred during
handoff. We set out to address layer 2 and layer 3 latencies, with the following objectives: First, reduce the discovery time. Second, recude the handoff time of layer 2 and layer 3.
1. Reducing Discovery time
In Figure 7, it shows a generic wireless environment, the mobile node (STA) connects to AP1 in channel 1. When it start the scanning procedure, it send a null frame with power management bit on to AP1, AP1 will change STA’s mode to doze and it will keep STA’s data in the buffer before STA change its status. STA can change its channel to scan the other channels.
After the scanning, STA changes back the current channel, and then it send a null frame with the power management bit is off to AP1. AP1 changes STA’s status to awake. AP1 sends the data to STA if it keeps any data in the buffer. Based on this procedure, the STA does not lost any data from AP1 and it will keep the newest wireless information in the meantime. (please refer to Figure 8.)
AP2 in channel 2
AP1 in channel 1 AP3 in Channel 3 STA
Figure 7 Generic wireless environment
STA
No data traffic in STA
AP1
Null data(1) Scan the prefer
Probe Request Get the information
AP3
Probe Response Active AP sends the
Null data(0)
AP1
Figure 8 mobile node scanning procedure
2. The handoff time of layer 2 and layer 3
When the mobile node detects the radio signal strength of the old
AP, and find it to be less than the threshold while the radio signal strength of the new AP is increasing, then it will start the handoff procedure by initiating the following sequence of events:
1. Send a null frame with power management bit is on to the old AP, the old AP will change STA’s status to doze and it will keep the STA’s data in the buffer.
2. STA change its channel to the new AP’s channel. It sends the authentication frame to the new AP. The new AP will reply its authentication frame.
3. STA sends the association frame to the new AP. The new AP will reply its association frame. The STA is connected with the new AP.
4. STA sends a null frame with the power management bit is on to the new AP. The new AP will keep STA’s data in its buffer.
5. STA changes its channel to the old AP’s channel then send a null frame with the power management bit is off to the old AP, the old AP will send the STA’s data which it keeps in its buffer.
6. After receiving all of the data from the old AP, the STA sends a null frame with power management bit on to the old AP again, and then it triggers the DHCP client to send the DHCP DISCOVER frame.
7. When the STA receive the DHCP DISCOVER frame from the host, it sends the packet to the new AP with power management bit off. Thereafter, it will receive the DHCP OFFER from the DHCP server.
8. The DHCP client then sends the DHCP REQUEST and STA will transmit it to the new AP.
9. The DHCP server replies with the DHCP ACK, and then the STA will get the new IP address. The STA will record the IP address with the BSSID of the AP.
10. The STA sends a null frame with the power management bit on to the new AP, then it changes its channel to the old one.
11. The STA sends a null frame with the power management bit is off to the old AP, then it receive the data from the old AP.
12. The STA sends a null frame with the power management bit off to the new AP. After which, it keeps the
connection with the new AP.
Please refer to Figure 9 for illustration of the series of events.
STA
Figure 9 Handoff procedure in layer 2 and layer 3
Data from old AP Old AP’s signal strength is less than
the threshold Null data(1) Old AP
Authentication
Authentication STA send management frames to
the new AP
New AP
Association
Association
Old AP
STA get the new IP address from the new AP
Null data(1) New AP keeps the data of the STA
STA receive the DHCP OFFER frame
Null data(0) Old AP sends the data to STA
DHCP client sends out the DHCP
DISCOVER frame Data from old AP
Null data(1) DHCP DISCOVER
DHCP OFFER New AP
DHCP REQUEST
DHCP ACK DHCP client sends out the DHCP
REQUEST frame
Null data(0)
Old AP
STA connect with the old AP to get the last packets
New AP
Null data(0) STA keeps the connection with the
New AP