Enhancing network access for highly mobile nodes

With the proliferation of mobile devices, the demand for mobile computing has

called for better mobile network access technology. In particular, network

access for highly mobile nodes such as vehicles is envisioned to have great

significance to future transportation systems and help to foster many

attractive applications from disseminating safety information among vehicles to

downloading of multimedia infotainment content from the fixed network

infrastructure. Mobile nodes can form Mobile Ad hoc Networks (MANETs) on demand

and communicate among themselves without infrastructure support. On the other

hand, they can also exchange information with the surrounding network

infrastructure while moving. However, the high node mobility causes frequent

topology changes among mobile nodes and brief contacts between mobile nodes and

roadside infrastructure. This poses great challenges to communication within

the MANET and the data exchange between mobile nodes and roadside

infrastructure in highly mobile networks such as vehicular networks.

The author first focuses on mitigating the impact of high node mobility on the

routing in highly dynamic mobile ad hoc networks. Existing literatures propose

to use mobility based clustering schemes to form stable communication structure

among mobile nodes by exploiting group mobility. To understand the degree of

group mobility in common mobility models, particularly Manhattan mobility model

which represents urban vehicle movements, the author first proposes a group

mobility metric based on the underlying network topology to measure the degree

of group mobility for various mobility models. The author observes that in some

mobility models, including Manhattan model, no significant group mobility is

demonstrated. This suggests mobility based clustering can be ineffective in

these models.

To find an alternative solution for scenarios in which significant group

mobility is absent, the author continues to investigate the effectiveness of

deploying mobile relays with stable uplinks. The simulation results show that

the mobile relays can significantly improve network connectivity. The author

compare the effectiveness of deploying mobile relays against adding more user

nodes. The simulation results from both Manhattan and Random Waypoint mobility

models suggest that mobile relays are much more effective than adding more user

nodes. It is also observed that the path duration is usually brief in highly

mobile networks even with mobile relays. This suggests that the clustering and

routing algorithms should incur minimum delay to maximally utilize the

established paths.

Based on the above observation, the author proposes a new cluster based routing

protocol FASTR which utilizes mobile relays as backbones to mitigate the impact

of node mobility for dense networks with high node mobility and low group

mobility. The proposed scheme eliminates the delay caused by cluster head

election and enables nodes to start communication immediately after joining a

cluster. Through simulation and analysis, the protocol is shown to possess good

scalability, incur lower control overhead and achieve higher packet delivery

ratio than existing routing protocol. The control overhead of FASTR is shown to

be independent of node mobility and consumes less network resources.

On the other hand, mobile Internet access through roadside Access Points (APs)

has emerged as an alternative to cellular networks due to its high

bandwidth and low cost. However, due to the limited range and sparse deployment

of roadside APs, several issues could arise when vehicles roam between APs.

Besides the brief contact between mobile nodes and roadside infrastructure

caused by high node mobility, other issues include intermittent network

connectivity and changes in IP address. To address these issues, the author

proposes a mobile network access protocol (mNAP) for highly mobile nodes. mNAP

introduces a Terminal Local Proxy (TLP) to shield application connections from

connectivity disruptions and change of IP address such that the application

connections can be preserved across network disruption when roaming between

different APs. By enabling cooperative relaying, mNAP exploits opportunistic

contacts for additional data transfer. Through simulation, mNAP is shown to be

able to exploit both direct and indirect contact opportunities and delivers

more than existing scheme.

In addition, when vehicles roam across several APs with overlapping coverage,

it is important to provide fast and transparent mobility management for the

vehicles. To this end, the author proposes a network based local mobility

management scheme COAP for 802.11 wireless mesh networks. COAP makes use of

802.11s to forward traffic to the correct location after mobile nodes have

roamed to a different AP. Furthermore, COAP proposes a cooperative DHCP

service to ensure mobile node always obtain the same IP address within the

mesh network. It has several advantages compared to existing schemes. COAP

does not require modification on the mobile node and the mobility management

is entirely handled by the network. In addition, unlike existing Proxy

Mobile IPv6 (PMIPv6) protocol, COAP allows network based mobility across

different operator domains. In summary, COAP facilitates easy deployment

and helps to enable transparent roaming between roadside APs for mobile

nodes.