In all IP-wireless communication networks it is crucial to provide handoff triggers that initiate IP handoffs that are seamless in the sense of minimizing both latency and data loss. One of the major benefits in IP-based mobility protocols is their applicability over a wide variety of wireless/wire-line access technologies at open systems interconnection (OSI) Layers 1 and 2. Many research projects that relate to IP handoffs have focused on supporting fast movement across access routers/mobility agents of the same access technology.
Considering no single wireless access technology can simultaneously provide low latency, high bandwidth, and global coverage, supporting seamless heterogeneous movement across access routers/mobility agents providing different access technology of so-called “multi-access” mobile nodes is a very interesting research issue. Multi-access mobile nodes are generally considered to have at least two or more network interface hardware. Unlike in homogeneous movement, layer 2 of multi-access mobile nodes cannot be provisioned or controlled within a monolithic framework. As a pragmatic illustration, typical handoff measurement by comparing pilot strengths from different base transceiver stations is meaningless if different technology pilots are considered.
Low latency IP handoff requires timely arrangement of sequencing handoff processes. It is known that layer 3 handoff processes should be initiated in advance upon the special information collected and driven from the underlying access technology, layer 2 trigger, to minimize latency, data loss, and other disruptions at the IP layer. In other words, layer 2 triggers (or any other forms) play a key role in achieving seamless IP mobility by governing handoff timing and delivering identities of handoff candidates.
Heterogeneous handoff triggering is really a problem of how coherency between entities of different access technologies and networks can be ensured. As an example, we consider homogeneous mobility in a cellular radio access network (RAN). Inter-cell movement within cellular RAN subsystems is completely provisioned and governed through distributed (or central) control entities such as a radio network controller (RNC). In such cases, network controllers have absolute knowledge about the mobile node's movements. This is possible since base stations and mobile nodes are required to report handoff measurements and other information to network controllers. By combining multiple reports from base stations and mobile nodes, the network controller allocates radio resources for mobile nodes and maintains the coherency among access network entities. Thus, the layer 3 can only follow the layer 2 mobility. In this case, layer 2 triggers that come from the access network can solve the issue of seamless IP handoff.
On the other hand, generating handoff triggers within required timing is trickier in heterogeneous cases. Although the current work in layer 2 triggers specifies types and formats of layer 2 triggers required by various handoff protocols, significantly missing pieces in the layer 2 trigger puzzle are how to generate these triggers and how to resolve the trigger timing issues that are crucial for seamless IP mobility.
Heterogeneous access network handoff triggering problems include the problem of generating layer 2 triggers. Layer 2 triggers exhibiting dependencies on access network support cannot be expected if inter-technology movement is considered. Examples of such are source and target triggers. Secondly, there is a problem with trigger timing in heterogeneous access network handoff triggering. Difficulties are observed in comparative handoff measurement and how to deduce trigger timing from it. Simply, handoff parameters such as beacon pilot strengths of different access technologies cannot be compared. Handoff decision is trickier since the motivation for a heterogeneous handoff can be due to either the fading/loss of the current connectivity or a connection improvement possibility via discovery of a new access network. These issues converge to decision factors of when to trigger a heterogeneous handoff being sufficient and necessary.