Achieving All the Time, Everywhere Access in Next-Generation Mobile Networks

by Marcello Cinque, Domenico Cotroneo and Stefano Russo

Presented by Ashok Sahu

Goal

To leverage the availability of wireless connection thus providing the “All the time, Everywhere access” view of pervasive computing.

How?

Leveraging communication availability - by providing general and extensible vertical handoff schemes.

Providing applications with a connection awareness support.

Paper’s Contribution

A Novel Mobility Management architecture for NGMN (Next Generation Mobile Networks) that consists of

1. Last Second Soft Handoff (LSSH)2. Nomadic Computing SOCKetS

(NCSOCKS)- Connection aware transport layer API

What is a Handoff?

The process of transferring an ongoing call or data session from one channel connected to the core network to another.

Types of handoffs

Reactive and Proactive handoff schemes.Hard handoff and soft handoff.Vertical and horizontal handoff.

Author’s approach :- Proactive handoff based on Receiver Signal Strength Indicator (RSSI) , a combination of Hard and soft handoff Vertical handoff

Proposed handoff scheme

It consists of three phasesInitiation: the network status is monitored

to decide when to start a migrationDecision: Once the need to handoff is

triggered a new AP has to be selected.Execution: the connection to the selected

AP is established.

Primary focus

Primary focus is on minimizing unavailability periods assuming that mobile device is in a zone covered by Access points (APs).

Unavailability can be caused by two kinds of events i) handoff occurrence ii) cell overload during handoff

Event i) does not occur if a soft handoff scheme is adopted. For this reason, soft handoff scheme has been chosen.

Formula The define availability as

where Pr(H) is the probability that a handoff occurs and Pr(O) is the probability that the AP is overloaded.

Pr(O) has been farther defined as where Nap is the number of Access points, C is the maximum average number of connections that can be handled by APs. N is the total number of admitted connections.

N can be further defined as where Nc is the number of simultaneous connections required and U is the number of Mobile devices MDs

Initiation Phase

They noted that its crucial to discriminate between transient signal degradations from permanent ones.

Due to this they do not follow an initiation based on fixed RSSI value. They argue that initiation based on fixed RSSI value leads to a poor availability due to transient RSSI degradations. They experimentally confirm this.

The probability of handoff occurrence in case of simple fixed threshold mechanism is Pr(H) = Pr(RSSI< Srssi).

They follow an α-count mechanism instead.

α-count Mechanism for handoff initiation

α-count function is a count and threshold mechanism. It takes the L-th measured RSSI as the input than it is incremented by 1 if current RSSI is less then threshold. It is decremented by dec if current RSSI is greater that threshold.

Handoff is triggered if the degradation becomes permanent i.e. α-count reaches a threshold α-T

Experimental proof for α-count behaviour

Decision phase

The decision algorithm considers only neighboring APs.

To APs are neighbors if their distance d is less than a certain value dmax

The decision if taken based on score criteria.

Applications can easily specify their requirements via NCSOCKS API.

AP topology is provided by a specialized component, the Map Server.

Proposed Architecture

The major components are

Connection and Location Manager (CLM)

Nomadic Computing Sockets

Map Server The first two components

run on the device-side platform whereas the third is deployed on the core network.

Connection and Location Manager (CLM)

CLM is in charge of handling connections with the APs. It handles vertical and horizontal handoffs using

proposed LSSH scheme α-count parameters and score function weights are set

by applications via NCSOCKS API. CLM also manages information about the current

location of the mobile device. It is designed according to interface based approach, so

as to handle all the wireless channels through the same interface, which provides several common operations as searching APs, creating/destroying connections, building an IP interface upon the wireless media and getting current RSSI, delay, bandwidth and cost values.

Map Server Responsibilities

Responsible for accepting map requests from MD’s CLM.

Map server has to recognize topology changes and provide last updated information to MD’s

To avoid bottle neck APs have been grouped and there is a Map server for each cluster.

Clusters should be set up taking into account the physical topology.

Achieving connection awareness: the NCSOCKS NCSOCKS use both IP communication facilities

and information gathered from data link layer, through the CLM.

Information flows from/to the application to/from the CLM, via the NCSOCKS interface.

Applications can set there QOS requirements and can request current connection status.

The current symbolic location, the wireless technology and the cost are provided by the Map Server, knowing which AP is currently being used.

CLM and NCSOCKS class diagram

Testbed description

Pr(H) with different schemes

Pr(O) estimation with different handoff schemes

Conclusion

Experimental results have demonstrated that the proposed handoff scheme reduces unavailability periods, due to transient signal degradations and AP overloads, significantly.

I think it’s a useful contribution but there might be better ways.

Might suffer from Map server update problem. Session layer approach might be better.