1. Seamless Handoff Between
Heterogeneous Wireless
Networks
Haipeng Jin and Kameswari Chebrolu, UCSD
Aditya Pande and Xiaohua Chen, Stanford University
Javad Razavilar, Magis Networks Inc., San Diego
Barani Subbiah, 3Com Corp., Santa Clara

2. Introduction
 Future Wireless systems will be characterized by
their heterogeneity.
– Multiple access technologies provide access to internet
content.
 Indoor Environment – 802.11, Bluetooth,
HomeRF, IrDA etc.
 Outdoor Environment – GPRS, CDMA2000,
Satellite etc.
 Systems differ in terms of coverage, bandwidth,
delay, cost etc.

3. Universal Wireless
Communication System
 Anytime, Anywhere Coverage
 Three Layers of Coverage – coverage decreases
from top to bottom.
 Topmost Layer – Satellite Systems
– Global Coverage
 Middle Layer – Cellular Networks
– Metropolitan Areas
 Bottom Layer – Local Area Networks
– Hot Spot Coverage
 A common IP based network will connect them to
each other.

4. Requirements
 Multimode terminals
 Seamless inter-system (Vertical) handoff
– Service negotiation across different systems
 Seamless intra-system (Horizontal) handoff

5. Our Focus
 Vertical Handoff
– Identify issues related to handoff between
WLAN and WWAN
– Requirements for handoff: small delay, low
packet loss
– Implement a testbed
 Network Layer : Mobile IP with extensions
 Link Layer: Handoff Algorithm

6. Mobile IP
 IETF standard , handles mobility across IP subnets
at the network layer.
 Mobile Terminal (MT) communicates with other
nodes using its home address.
 MT acquires care-of address and registers with
Home Agent (HA).
 HA intercepts packets destined for the MT and
tunnels them.
 Route optimization - avoids triangle routing
– CH learns the MT’s current care-of address and tunnels
packets directly.

7. Mobile IP
CH
Packets destined for mobile Packets in reverse direction
HA FA
Encapsulated packets
MT

8. Architecture
 Seamless handoff between WLAN (IEEE 802.11)
and WWAN (CDMA PCS).
 Mobile IP based
 Extensions
– Before handoff, MT notifies old Foreign Agent (FA) to
stop transmitting and buffer packets.
– Connection established to new FA
– New FA contacts old FA which passes the buffered
packets.
– MT communicates with Correspondent Host (CH)
through the new FA.

9. Handoff Algorithm

10. Details
 Based on measurement of WLAN signal
strength, Signal-to-Noise Ratio (SNR)
 Connected to WLAN
– SNR above CThresh, MT stays connected to
WLAN
– SNR falls below CThresh, MT scans for
WWAN services and makes initial connections
– SNR falls below DThresh, MT will switch from
WLAN to WWAN

11. Cont…
 Connected to WWAN
– Searches for WLAN signal periodically
– SNR above CThresh, MT switches from WWAN to
WLAN
 Two thresholds defined to avoid “Ping-Pong”
effect
 An exponential moving average of the SNR is
used.
 Only SNR of the WLAN is monitored as MT
prefers WLAN because of the high data rate

12. Testbed

13. Testbed
 Testbed built at 3Com Corp
 A laptop equipped with 802.11 and CDMA PCS
network interfaces served as MT.
 Protocol – Mobile IP with extensions
 Home Agent was not implemented.
 Handoff related processes were implemented
using C under linux platform.
 A text based application was used in the
demonstration.

14. Results
 Handoff delay-time elapsed between the last
packet received on the old connection to the
first packet received on the new connection.
 Handoff delay was about 600ms when
switching from WWAN to WLAN
 Handoff delay was about 1040ms when
switching from WLAN to WWAN.

15. SNR measurement on WLAN

16. Switching from WLAN to
WWAN

17. Delay on WLAN

18. Delay while handoff

19. Conclusions
 A handoff algorithm to provide seamless
roaming between WLAN and WWAN was
proposed.
 Field test results show
– No unnecessary handoffs
– Small packet loss and delay