Chapter 10Handoff in mobile &

wireless network

Introduction• When mobile user moves from one AP to another,

handoff is required.• To minimize interruption of cell transport, efficient

switching of active VC (Virtual Circuit) from old data path to the new data path is needed.

• The switching should be fast enough to make the new VCs available to mobile users.

• During handoff, old path is released then new path established.

Signaling handoff protocol in WATM network

• Signaling is a problem area in WATM.• Possible solution, integration of the required mobility

extension with the standard signaling protocols.• Protocol stack that includes mobility function for handoff :-

– MMC – Mobility Management and Control– RRM – Radio Resources Manager– SAAL – Signaling ATM adaptation layer– CCS – Call control and signaling– UNI – User network interface

Signaling handoff protocol in WATM network

– WMAC – Wireless medium access control– S-channels – Permanent Virtual Circuits (PVC)

intended for standard signal– M-channels – PVCs intended for mobility signaling– U-plane – User plane

Signaling handoff protocol in WATM network

Protocol stacks

Signaling handoff protocol in WATM network

• CS_MMC module is responsible for handling all mobility related procedures.

• CS_MMC task :-– Establishment of M-channels through which the

mobility-related messages are exchanged– Coordination of wireless and fixed resources, during

the execution of mobility and standard signaling procedures.

– Switching signaling and data connections whenever an MT crosses the boundaries of a cell

– The updating of the location of an MT in the CSU- hosted DB.

Signaling handoff protocol in WATM network

• Smoothness of handoff depends :-– The number of connections requiring handoff– Time between initiation of a handoff and loss of

contact with previous AP/BS.

Crossover switch discovery• Basic step to most handoff scheme.• COS is an intermediate switch along the current path of

a connection that has nonoverlapping path to both current or next AP/BS.

• Selecting COS for connection can be initiated at the previous or next AP/BS.

• COS selected, appropriate resources for the connection are procured along the new subpath (between COS and the new AP)

Crossover switch discovery• After the COS start forwarding packets onto the new

subpath, the existing subpath to the previous AP is torn down.

• Handoff process complete.• Several factors to select particular switch as COS :-

– Switch capability– Selection policy– New access point– Previous path.

Crossover switch discovery

Crossover switch-based rerouting during handoff

Rerouting methods• Important concerns in performing connection rerouting :-

– Limiting handoff latency– Maintaining an efficient route– Limiting disruption of continuous media traffic– Limiting network switch update rates due to rerouting.

• Handoff scheme in WATM :-– Switched Virtual Circuit (SVC)– Permanent Virtual Circuit (PVC)

Rerouting methods• General approaches toward connection rerouting :-

– Optimistic handoff approach– Ordered handoff approach– Predictive handoff approach– Chaining handoff approach

• Goal of Optimistic handoff scheme – To perform simple and fast handoffs with the optimistic view that disruption to user traffic will be minimal.

• COS simply reroutes data traffic trough a different path to the new BS with the connection from the source to the COS remaining unmodified.

Rerouting methods• Goal of Ordered handoff scheme – Provide ordered

lossless data delivery during handoffs.• Predictive approach – Predict next BS of the mobile

endpoint and perform advance multicasting of data to the BS.

Scheduled-Assisted handoffs• MMUP – Mobile Multi-User Platform.• Schedule provides the MMUP with information about the

upcoming cell in advance of its intercell moves.– MMUP close enough to the next AP for the

mechanism to be triggered, it initiates a COS discovery for some or all existing connection (or group) through the current AP.

– Connections are maintained if the call proceeding sends signaling to the next AP until the MMUP establishes contact with it, upon which all pending connection request are forwarded onto the MMUP.

Scheduled-Assisted handoffs– After MMUP confirms successful reestablishment of

the connections, the COS begins switching data along the new path and initiates tear down of the old subpath.

• Using Footprint Handover Rerouting Protocol (FHRP).• Two phases :-

– Augmentation– Footprint rerouting (FR)

• Augmentation phase– Direct link from the new end satellite to the existing

connection routes is found– Route can be updated with minimum signaling delay

and at low signaling cost.

Handoff in Low Earth Orbit (LEO) Satellite networks

• FR phase– Connection route is migrated to a route that has the

same optimality feature with the original route– The goal of rerouting is to establish an optimum route

without applying the optimum routing algorithm after a number of handovers.

– The optimality of the original route is maintained after FR phase.

Handoff in Low Earth Orbit (LEO) Satellite networks

• PRP – Consist in dynamically reserving channels when number of communications in progress grows in a given cell.

• Reserve resources that will be freed when the flow is less important.

• Dynamically reserves radio resources according to the local topology.

Predictive Reservation Policy (PRP)

• The basic idea – to extend the connection from the old to the new BSs, somewhat like a chain.

• Two type :-– Hop-limited handoff scheme– Chaining followed by make-break

• Hop-limited handoff scheme – PVC-based scheme.• BS in each cell is connected to neighboring cells by a

number of PVCs.• The number of successive traffic path elongations by

PVCs connecting neighboring cells is restricted to less than predetermined numbers.

Chaining approaches

• If the number is reached, the traffic path is rerouted by the network to the PVC between the network and the new cell.

• Reduce the required number of PVCs between neighboring cell

• Increase traffic path efficiency.• Keeps the networking processing load light.

Chaining approaches

• Chaining followed by make-break1. The mobile host sends a handoff request message

to the new BS, identifying the old BS and its connection server

2. The new BS adds local translation table entries for its internal routing

3. The new BS asks the old BS to forward packets pertaining to the mobile host

4. The new BS sends back a handoff response message to mobile host, instructing the mobile host to transmit/receive through the new station

Chaining approaches

5. The new BS passes the updated route information to the connection server

6. The connection server performs necessary QoS computations on the new route.

7. The connection server identifies the best route to the COS, allocate resources along the new route, and sets up the new routing entry in COS. The switch now multicasts cells received from the source to both the BSs

Chaining approaches

8. The connection server informs the new BS of the completion of route change, which then starts using the new route

9. The connection server exchanges messages with ATM switch, tearing down the old routing entry. The connection server also requests the old and the new BSs and switches in the old route to deallocate the old resources.

Chaining approaches

Chaining approaches

Connection management architecture