Dual-Linked BS Model | NETWORK ARCHITECTURES

WiMAX and 3G-LTE systems consider flat network architectures but mobility is still implemented in a hierarchical manner, as shown in Figure 1. An MS traveling within the subnet while changing BSs performs L2 handoff only without changing the MIP attachment. When it moves into another AR area, it triggers L3 handoff. On the other hand, the pure all-IP network suffers from a long handoff latency and high signaling overhead since it incurs L3 protocol at each handoff.

The subnet-based network reduces the frequency of L3 handoff that is accompanied by relatively long latency. Nevertheless, an MS still experiences a long latency when it performs L3 handoff. For seamless L3 handoff, we develop a dual-linked BS model where some BSs are connected to two neighboring ARs at the same time as shown in Figure 2. Obviously, this approach can be extended to support the case where a BS is linked with more than two ARs by adding more links as many as neighboring ARs to that BS. Here, we will use the terminology of "dual" as the general implementation term.

 

Figure 1: The subnet-based access network that has a dual-linked BS

In the conventional subnet-based model, an MS performs L2 and L3 handoffs at the same time when it crosses the boundary of a subnet. This may cause a serious problem of communication blackout because L2 handoff typically exploits a conservative method in preventing the ping-pong effect. It happens like this. An MS starts an L2 handoff when the signal power of the corresponding BS is weak. As an L3 handoff is accompanied by a long latency, the signal may turn too weak during the L3 handoff, resulting in a blackout.

In contrast, the presented network model with some dual-linked BSs decouples L2 and L3 handoffs, thereby providing a flexible handoff mechanism. Since each dual-linked BS can access both ARs of new and old, it helps L3 handoff to be performed independently of L2 handoff when an MS stays in its coverage. An MS entering the area of the dual-linked BS will prepare the L3 handoff. More explanations are given next.

Future Movement Prediction

Generally an MS is able to sense the presence of neighboring BSs since each BS broadcasts its pilot signal. When the MS enters the service area of a dual-linked BS, it triggers L2 handoff. Completing the handoff, it predicts the movement by detecting the pilot strengths of neighboring BSs. When it is likely to move into some other subnet, it prepares L3 handoff. The handoff can be initiated by either the MS or the BS. If L3 handoff is triggered too early, there exists a possibility of too many L3 handoffs, resulting in the pingponging effect. On the other hand, if too late, L2 and L3 handoffs are incurred at the same time. In this case, the handoff delay may not be reduced, because L3 handoff dominates the overall delay.

This motivates to design an algorithm that initiates early L3 handoff following the concept of the existing L2 handoff algorithm. The graph in Figure 2 shows an example of L2 and L3 handoff triggers. In this scenario, if the measured pilot signal strength at the MS from a new BS (BS3) exceeds that of the old BS (BS2) by Th₁ for the time interval I₁ the L2 handoff towards the new BS is triggered (Holma & Toskala, 2000). If the new BS belongs to a different subnet, the L3 hand off is initiated according to the thresholds Th₂ and I₂. In this case, the L3 handoff must start before the next L2 handoff. Deciding the threshold values is an implementation issue.

 

Figure 2: An example of handoff in the dual-linked BS model

Mobile IP Handoff

Following a proper movement detection, an MS performs MIP handoff. The MS begins MIP handoff by sending a request message. After the MS obtains a CoA (care-of-address) for the new subnet, the AR forwards the request message to the MS's Home Agent (HA) to update the MS's location information. During this process, packets arriving at the BS via the old AR will be transferred to the MS by using the new CoA. This is possible because the BS can access the two ARs, which is a unique feature in this scheme. In contrast, in a conventional network, some packets arriving at the old BS or AR will be dropped or forwarded via old and new ARs, so the forwarded packets will experience some latency.

In the dual-linked BS model, each dual-linked BS maintains a table for mapping between old and new CoA during the handoff procedure. Indeed, the handoff latency in MIP is mainly incurred by HA registration. In the new architecture, however, there exists only a little handoff latency since every packet arrives at the same BS via either old or new AR during the handoff. Figure 2 shows an example of the handoff scheme. An MS and its corresponding BS perform L2 handoff whenever it crosses a cell boundary, while performing L3 handoff separately from L2 handoff.

The advantage of the dual-linked BS model is exhibited in Figure 3. In the conventional model, the new BS or AR may receive packets for an MS to which it does not have a direct connection while the MS is performing the handoff between subnets. In this case, the new BS or AR may drop or buffer packets destined for the MS. This case also occurs when the whole handoff process has not completed yet even if the MS has established a new connection already. Therefore, as shown in Figure 3, some packets should be forwarded to the corresponding location during the handoff process. Unless the system supports packet forwarding, packets will be dropped. In contrast, the dual-linked BS model has almost no packet loss during the handoff, because the BS has a connection to each of old and new ARs.

 

Figure 3: Packet forwarding comparison between the conventional and new models