Rerouting for Handoff in a Wireless ATM Network

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Rerouting for Handoff in a Wireless ATM Network

• Akyol and Cox
• IEEE Personal Communication Magazine
• Oct. 1996

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• New era in telecommunications
• Wireless communications ATM ? Wireless ATM
  (WATM) Portable ATM Terminal fixed ATM
  LAN/WAN
• WATM Needs
• Higher Bandwidth Mobility
• Handoff
• The procedure by which a users radio link is
  transferred from one radio port to another
  through the network without an interruption of
  user connection.
• In this article
• Summarize the handoff procedure in WATM
• Propose Nearest Common Node Rerouting (NCNR)
• Comparison of NCNR with 5 other rerouting
  algorithms

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Handoff Procedure

• WATM consists of
• radio ports (RP)
• user terminals (UT)
• network interface equipment (NIE)
• Zone
• A group of radio ports that is connected to the
  same WATM interface equipment
• Managed by the zone management process (zone
  manager)

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• Two levels in a handoff event
• Radio-level handoff
• actual transfer of the radio link between two
  radio ports.
• Network-level handoff
• supports the radio-level handoff between one or
  more network interface equipment by rerouting and
  buffering.
• Two kinds of handoffs
• Intrazone Handoff (not discussed in this paper)
• rerouting only at the same WATM NIE within the
  same zone
• Interzone Handoff (discussed in this paper)
• rerouting at one or more WATM NIEsacross
  different zones

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• Assume
• The zone manager
• maintains a local lookup table of the network
  addresses of the neighboring zones
• updates the table periodically by means of an
  update protocol between neighboring NIEs
• The handoff transaction (not discussed in this
  paper) may be implemented using
• either the current ATM signaling protocols (UNI
  3.1)
• or a migratory wireless ATM signaling protocol
• These zones are interconnected by WATM switches.

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Rerouting for Interzone Handoff

• NCNR(Nearest Common Node Rerouting)
• To perform the rerouting at the nearest ATM
  switching node that is common to both zones
  involved in the handoff transaction
• Goal
• By eliminating unnecessary connections
• Minimize the resources
• Conserve bandwidth

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• Services that the WATM users subscribe
• Time-sensitive traffic types
• e.g. audio, video.
• may not tolerate delay
• may tolerate cell loss
• Throughput-dependent traffic types
• e.g. data, file transfers, WWW access.
• may tolerate delay
• may not tolerate cell loss ?
• Two different NCNR strategies
• for Time-Sensitive Traffic (NCNR-TS)
• for Throughput-Dependent Traffic (NCNR-TD)

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• Assume
• The transmission delay and latency of the links
  from the NCN to the zones involved in the
  handoff are negligible compared to the the radio
  transmission medium.

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NCNR for Time-Sensitive Traffic

• 1. A handoff session between zones A and B. Zone
  A - the present zone Zone B - the candidate
  zone.
• 2. The zone manager of A checks to see if a
  direct physical link between A and B
  exists (Case 2A) If A is a parent of B... (Case
  2B) If B is a parent of A...
• if no direct physical link between A and B
  exists
• go to Step 3.

A
B
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• 2A. If A is a parent of B
• Note Parent
• - flat network end point of user connection as
  root
• - hierarchical network

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• 2B. If B is a parent of A
• Note Parent
• - flat network end point of user connection as
  root
• - hierarchical network

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• 2A. If A is a parent of B
• A notifies B
• Established a new connection
• A acts as an anchor
• Once the handoff is stable, the connection stays.

• anchor
• a network point that the user connection (end
  point) is forwarded through to candidate zone
  until the handoff is completed

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• 2B. If B is a parent of A
• A notifies B
• Established a new connection
• B acts as an anchor
• Once the handoff is stable, B deletes the
  connection

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• 3. If A and B are not directly connected
• A sends a handoff start message to the end
  point (containing the ATM addresses of A,
  B, and the endpoint)

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• 4.
• Upon receiving the handoff start message, the ATM
  switches along the path (from A to the end point)
  check to see whether all three ATM addresses are
  routed on different egress ports of the switch.
• When the node is found, it is designated as the
  NCN.

• The NCN sets the NCN bit in the handoff start
  message.
• The rest of the switches on the path do not
  perform the egress port test.

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• 5.
• The NCN forwards a reroute message to all of the
  switches located between B and itself.
• The switch first checks for resource
  availability
• if available, then setup the connectionsif
  not available, then handoff fails and the
  involved parties are notified.

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• 6.
• When the reroute message is received by B, B
  sends a reroute acknowledge message to A.
• The radio-level handoff is started.
• The NCR starts to forward the user information to
  both A and B in a point-to-multipoint manner
  until the radio-level handoff is stable.

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• 6 (cont.)
• In a fading environment,a small motion of the
  mobile terminal may cause the radio link to
  switch back and forth between the two radio
  ports hence, a point-to-multipoint link from NCN
  to both A and B ensures the timely delivery of
  time-sensitive information.
• In the uplink direction, the information may be
  transmitted through either zone and correctly
  routed to the endpoint by the NCN.
• The user information may be discarded at the zone
  which is not in contact with the user
  terminal.(if a zone has not received an uplink
  transmission from the portable in a given radio
  transmission frame.)
• Occasionally, the portable may receive duplicate
  information. The duplicate information may be
  determined by the time sequence information and
  discarded accordingly.

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• 7.
• If the radio-level handoff is successful
  (stable), the connection between A and the NCN is
  cleared by A by sending a clear connection
  message to the NCN.
• Any buffered time-sensitive data that has not
  expired will be transmitted to the current zone
• Expired data are discarded.

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NCNR for Throughput-Dependent Traffic

• Differs from the NCNR-TS
• 1. In the downlink direction- As the
  radio-level handoff (RLH) is started, user
  information is buffered at both A and B.
• - No downlink transmission until the RLH is
  completed.
• - Once the RLH is completed, the information is
  transmitted in a FIFO manner.

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NCNR for Throughput-Dependent Traffic

• 2. - If As buffer is non-empty before the RLH
  is started, As buffer is transmitted to the
  user terminal if possible otherwise, these
  data is transmitted to B and go in front of all
  other cells buffered in B. This preserves the
  cell sequence.
• 3. - In the uplink direction
• - The traffic is transmitted through A if
  possible otherwise, it is buffered at the
  terminal.
• - As the RLH is started, the user terminal
  starts to buffer the user information.
• - Once the RLH is completed, the buffered
  information is transmitted.

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NCNR for Throughput-Dependent Traffic

• Multiple connections may be routed using the
  virtual path connection.
• Assigning a virtual path identifier for
  connections between a user and multiple endpoints
• Performing the rerouting on a virtual path
  instead of on a virtual circuit basis.

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1. Yuan-Biswas Rerouting Scheme

• Rerouting of connections at designated Handoff
  switching equipment (HOS).
• It does not specify how the HOS is
  determined.Use the initial WATM switch as the
  HOS.
• Assume the base stations are interconnected by
  permanent virtual circuits.
• Handoff ports on the same WATM switch
• updating translation table in one switch
• Handoff ports on the different WATM switches
• new connection is established before handoff is
  completed
• forwarding cells to the users new WATM switch
• Cell sequence is preserved
• since the first switch acts as a handoff server
  (switch)

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2. BAHAMA Rerouting Scheme

• For wireless LAN
• For flat network
• The initial radio port acts as an anchor
• After the handoff is completed, the initial radio
  port migrates the user connection to an optimal
  route provided that the portable stays in the
  coverage area of the new radio port for an
  extended period of time.
• Cell sequence is preserved
• since the cells are always routed by the anchor
  during the handoff
• Use virtual path indicator in the ATM cell header
  for routing simple (only the VPI needs to be
  changed)

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Comparison of NCNR with Yuans and BAHAMA

• Sameness
• all use cell forwarding ? ease of cell sequencing
• all use the previous switch as an anchor ? cell
  sequencing
• all do buffering either at anchor switch or the
  new switch
• differences
• In NCNR, the buffering is only performed for
  throughput-dependent traffic and only when the
  radio-level handoff is being performed.
• Yuans and BAHAMA do not allow connection between
  two radio ports while the handoff stabilizes.
  This causes delay problem for time-sensitive
  traffic.

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Comparison of NCNR with Yuans and BAHAMA

• Differences (cont.)
• Yuans and BAHAMA fit in flat networks only.NCNR
  fits in either flat networks or hierarchical
  networks. ? NCN minimize the Bandwidth,
  buffering is performed at the edge of network
• For fast moving user in small coverage areas,
  rerouting usually occurs at the same NCN in
  NCNR? minimize the bandwidth minimize the
  number of rerouting
• BAHMA uses virtual path indicator. This is
  suitable for LAN but not scale well for WAN.

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3. VCT Virtual-Connection-Tree-Based Rerouting
Algorithm

• Virtual-Connection-Tree
• A root node is attached to the backbone ATM
  network.A number of radio ports (leaves) are
  connected to the root.
• When a mobile terminal established an WATM
  connection, a VCT is formed. The mobile utilizes
  only one of the leaf nodes at a time.
• When the mobile terminal moves within the tree, a
  new leaf node becomes active, using the
  pre-established connection.
• When the mobile terminal moves out of the
  coverage area of the VCT, a new VCT is
  established.

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Comparison of NCNR with VCT

• Sameness
• The zone in NCNR is similar to a VCT.
• differences
• In NCNR, only one virtual circuit is needed at
  any given time. In VCT, multiple virtual
  circuits are pre-established and reserved for a
  single connection.
• NCNR guarantees cell sequence preservation. VCT
  does not implement cell sequence
  preservation.The mobile is responsible for cell
  sequencing.
• NCNR accommodates time-sensitive and
  throughput-dependent traffics.VCT does not
  address this issue.

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4. SRMC Source Routing Mobile Circuit Rerouting

• An improvement of VCT Rerouting.
• Use a tethered point (TP) to server as the root
  in the connection tree.
• All potential network routes from TP to the
  leaves due to possible handoff attempts are
  pre-established.
• But, unlike VCT, no resources are reserved.
• Once the handoff is initiated, only the resources
  of the active handoff connection are reserved.

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Comparison of NCNR with SRMC

• differences
• NCNR is at worst comparable to, and for neighbors
  better than, SRMC.
• In SRMC TP may locate at the highest level upIn
  NCNR NCN may locate at one level up (handoff
  between neighbors) or at the highest level
  (handoff between farthest switches)
• The higher level the TP/NCN locates, the more
  bandwidth and delay are consumed.
• For every handoff, a pair of messages, for
  notification and for resource allocation, are
  sent between TP and the leaf node.
• SRMC centralized ? more overheadNCNR
  distributed ? less overhead
• SRMC does not address the constraints of TS and
  TD traffic.
• SRMC hierarchical networksNCNR flat or
  hierarchical networks

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5. Tohs Hybrid Handover Protocol

• A hybrid handover scheme
• For WLAN
• Implemented using Cambridge Fairisle ATM switches
  in a LAN environment
• Similar to NCNR. But differs in that
• NCNR is proposed for wireless WAN
• NCNR works with current ATM signaling
  specifications using the overlay signaling
  proposed in 9.
• NCNR does not require any buffering in the
  network switches. Buffering is performed only at
  the zones only for throughput-dependent traffic.
• NCNR provides support for TS and TD traffics and
  point-to-multipoint support for TS traffic.

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• Nc - Number of user connections established for
  rerouting
• B - user bandwidth allocated for handoff
• CF (cell forwarding), DY (dynamic ), T(tree)
• Robustness - 1 (worst) 5 (best)

• Nh - Number of signaling messages exchanged
  during handoff
• Nr - Number of signaling messages for rerouting
  exchanged during handoff
• Nn - Number of network nodes involved in the
  rerouting

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Conclusion

• Comparing NCNR with five other schemes,NCNR is
  seen as a promising scheme for rerouting a
  wireless ATM connection for handoff.