Seamless Handoff Between Heterogeneous Wireless Networks

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.

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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

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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 MTs current care-of address and
  tunnels packets directly.

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

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Handoff Algorithm
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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

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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

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Testbed
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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.

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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.

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SNR measurement on WLAN
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Switching from WLAN to WWAN
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Delay on WLAN
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Delay while handoff
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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