The Effect of the Holdoff Time Value

We use the ten random connected topologies generated in Section 4.2.5 to compare the performances of all studied holdoff time schemes in general networks. The simulation setting used here is the same as that used in Section 4.2.5. In [22], we pointed out two reasons why the initialization of an IEEE 802.16 mesh network may fail. In this section, we applied the revised network initialization process proposed in [22] to all studied schemes, including the original fixed-value schemes. This revised process can significantly alleviate the MSH-NCFG message collision problem.

After this revised process is applied, message collisions now only result from excessive MSH-NCFG message transmissions by a new node’s neighboring nodes. A typical example is shown in Fig. 5.6. Suppose that node C is a new node trying to enter the network, and nodes A and B are its neighboring nodes that have attached themselves to the network.

The dotted circles represent the signal coverage of nodes A and B, respectively. Before node C attaches itself to this network, nodes A and B transmit their own MSH-NCFG messages without considering whether their MSH-NCFG messages can be successfully received at the location of node C. Consequently, many MSH-NCFG messages transmitted by nodes A and B may be collided at node C. However, since node C so far has not been a functional node in this network, such message collisions does not hinder node C’s normal operation at this moment.

However, in case nodes A and B transmit their own MSH-NCFG messages very fre-quently (for example, these two nodes use a very small holdoff time value to schedule their MSH-NCFG message transmissions), it is very likely that node C cannot successfully re-ceive any MSH-NCFG message transmitted by these two nodes. In this condition, node C cannot proceed its network initialization process because it cannot obtain necessary information required to start its network initialization process.

A C B

Figure 5.6: Example of MSH-NCFG message collisions

The other reason that causes the network initialization process to fail is the absence of routing paths from a new SS node to a BS node. For an SS node, the success of its network initialization process relies on the availability of a routing path from itself to the BS node.

On performing the registration procedure (one of the necessary procedures in a node’s network initialization process), a new node must have a routing path to communicate with a BS node. If no available routing path exists, the new node’s network initialization process will fail.

To eliminate the effect of the above routing problem from the performance results, we adopted a design to guarantee that 1) every new SS node has a sponsor node and 2) every new SS node has a routing path to the BS node. To provide such a guarantee, we first generated routing paths among all nodes using Dijkstra’s shortest path algorithm.

Then, we let an SS node choose the first-hop node on its routing path to the BS node as its sponsor node. Such a design guarantees that when an SS node is performing the registration procedure, at least one routing path exist for the SS node to communicate with the BS node. With this design, the problem that network initialization processes fail due to lack of routes to the BS node no longer exists. Thus, the simulation results reflect solely the effect of different holdoff time values rather than the mixed effects of different holdoff time values and the used routing protocol.

In the following, we compare four different fixed-value holdoff time setting schemes.

As mentioned previously, the standard regulates that the holdoff time base value be set to 4. Therefore, here we set the holdoff time base value used by all schemes to 4 while varying the holdoff time exponent value used by these schemes from 0 to 3. The resultant holdoff time values are thus 16, 32, 64, and 128, respectively.

Table 5.3 shows the performances of the four fixed-value holdoff time schemes. In

Table 5.3: The performances of the four fixed-value holdoff time setting schemes

SRNI ATOUN ATHPT (ms) NetUI

Avg. Avg. Std. dev. Avg. Std. dev. Avg. Std. dev.

Holdoff Time 16 93% 0.481 0.164 47.423 25.220 95.336 6.083 Holdoff Time 32 98% 0.363 0.165 66.453 15.706 150.262 8.252 Holdoff Time 64 100% 0.237 0.131 104.146 7.518 226.507 10.077 Holdoff Time 128 100% 0.132 0.077 192.719 4.903 318.771 10.629

total, fifty runs of simulations were conducted. The success rate of network initialization (SRNI) is defined in (5.1).

SRNI = NCsuccess

NCtotal

, (5.1)

where NCsuccessdenotes the number of cases in which the network succeeds in initialization and NCtotal denotes the number of total cases. The success of a network initialization is defined as follows. For a network case, if all of its nodes successfully initialize and attach themselves to the network, the initialization of this network case succeeds. In contrast, if any node fails to perform its initialization and attachment procedures, the initialization of this network case fails. The ATOUN, ATHPT, and NetUI metrics are defined in Section 4.2.1. Each value presented here is the average across the fifty simulation runs.

From Table 5.3, one sees that when the holdoff time value decreases, the ATOUN-Avg.

increases and both the ATHPT-Avg. and the NetUI-Avg. decrease. As discussed before, all of these trends are expected and reasonable. These trends show that using a smaller holdoff time value results in better performances when the network is un-congested.

However, the SRNI results reveal a serious problem when small holdoff time values are used. One sees that using large holdoff time values (e.g., 64 and 128) results in a 100%

success rate of network initialization. However, using small holdoff time values (e.g., 16 and 32) results in a success rate less than 100%, which means that some SS nodes cannot successfully initialize and attach themselves to the network.

These simulation results show that a fixed holdoff time value cannot provide good scheduling performances while guaranteeing the success of network initialization processes.

Based on this observation, we propose a new two-phase holdoff time scheme to achieve both of the above goals. In the following, we explain the proposed two-phase holdoff time scheme in detail.

Figure 5.7: The proposed two-phase holdoff time scheme