Effect of Holdoff Time Base Value

As introduced earlier, the IEEE 802.16 mesh CDS mode regulates that every node should set the holdoff time base value to 4. With this regulation, radios compliant to the 802.16 mesh CDS mode co-cooperate using this fixed holdoff time base value. Our proposed dynamic holdoff time scheme, however, may require changing the holdoff time base value to operate. For this reason, a mechanism is required to notify network nodes of changes to the holdoff time base value. In this section, we explain the problems that may occur if this system parameter is dynamically changed. In Section 3.1.4, we will describe several mechanisms that can be used to change this parameter without causing problems.

The holdoff time base value contributes a constant time amount (2^base) to the holdoff time. Setting the holdoff time base value to 4 means that each node should suspend its contention for at least 2⁴ consecutive TxOpps after it has won one. This lower bound limits the smallest holdoff time value that can be assigned to nodes in the proposed dynamic holdoff time scheme. The proposed dynamic holdoff time scheme, therefore, cannot achieve its optimal performances, if the holdoff time base value is not reduced to 0. For this reason, in our proposed dynamic holdoff time scheme the holdoff time base value of the network is set to zero to provided the largest flexibility of control message

scheduling.

Instead of using a lengthy field, the standard uses two shorter fixed-length fields, exp and Mx, to represent a TxOpp number. The relationship between these two fields and a TxOpp number has been given as follows:

2^exp ∗ Mx < next TxOpp <= 2^exp ∗ (Mx + 1), (3.3) where next TxOpp denotes a node’s next TxOpp number.

On receiving a control message (such as an MSH-NCFG message or an MSH-DSCH message), a node should use the received exp and Mx fields to derive the next transmission interval of the transmitting node using Eq. (3.3). Fig. 3.4 shows the relationship between the holdoff time and the transmission interval (Tx Interval). The holdoff time comprises the Tx interval (denoted as α in the figure) and the ineligible interval (denoted as β in the figure). The Tx interval represents the duration in which a node may contend for one TxOpp. On the other hand, the ineligible interval is the duration in which the node is not allowed to contend for any TxOpp.

Based on Eq. (3.3), the length of the Tx interval is fixed to 2^exp because a node’s next TxOpp number is within a fixed-length interval ranging from (2^exp∗ Mx + 1) to 2^exp∗ (Mx + 1). Consequently, the length of the ineligible interval is (2^exp+base - 2^exp). If the base value is set to 0, the holdoff time and the Tx interval of each node will exactly overlap, causing the length of the ineligible interval to be zero. This means that, after winning a TxOpp, a node will contend for another TxOpp immediately. (This also means that a node will contend for TxOpps all the time.) Thus, a node should consider that all nodes in its two-hop neighborhood will contend for each TxOpp with itself. On the other hand, if the holdoff time base value is larger than 0, a node’s holdoff time will be larger than its Tx interval. In this condition, the contention time experienced by a node can be reduced due to a decreased number of contending nodes.

The choice of the holdoff time base value depends on the needs of a holdoff time scheme. For a static holdoff time scheme, using a positive holdoff time base value can reduce the contention time of each node. For a dynamic holdoff time scheme, however, the holdoff time base value must be zero for two reasons: First, if a positive holdoff time base value is used, the lower bound of the holdoff time value that can be assigned to nodes will be limited. Thus, to give the dynamic holdoff time scheme the largest freedom to set nodes’ holdoff times, the holdoff time base value should preferably be set to zero at all

exp+base

exp

exp exp

Figure 3.4: The relationship between the holdoff time and the Tx interval

time. Second, if the holdoff time base value is allowed to change during the operation of a network, after a node’s holdoff time has just been changed (due to the change of the holdoff time base value), MSH-DSCH control messages may collide. The reason for this phenomenon is explained below.

Fig. 3.5 and Fig. 3.6 show two cases after a node’s holdoff time value has just been changed. The former shows an example that changing the holdoff time value results in no message collisions while the latter shows an opposite example. Suppose that node A has changed its holdoff time value and broadcast the new holdoff time exponent value.

In Fig. 3.5, node B is the next one to transmit an MSH-DSCH message. In this case, node C will be notified of this change by node B’s MSH-DSCH message in time. Thus, node C will not schedule its MSH-DSCH message transmission to collide with node A’s MSH-DSCH message transmission.

In contrast, in Fig. 3.6 node C has scheduled an MSH-DSCH message transmission before node B can notify it of node A’s new holdoff time value. In this condition, node C’s MSH-DSCH message transmission may collide with node A’s MSH-DSCH message transmission because node A’s ineligible interval viewed by node C now becomes out of date. Figures 3.7, 3.8, and 3.9 illustrate three cases that can cause this problem.

In these figures, HTa denotes the holdoff time of node A viewed by node C (may be out of date) and HTa^′ denotes the holdoff time of node A viewed by node A itself (always up to date). The symbol γ denotes the vulnerable interval that results from node A’s changing its holdoff time and during which the MSH-DSCH messages of nodes A and C may collide. Suppose that node C’s transmission was scheduled within node A’s original ineligible interval β. Fig. 3.7 depicts a case that node A has just decreased its holdoff time base value and therefore its ineligible interval is just shortened from β to β^′. This operation generates the γ vulnerable interval because node C does not know that node A

A B C

1 2

Figure 3.5: An example showing that control messages will not collide after node A changes its holdoff time value

A B C

1 2

Figure 3.6: An example showing that control messages will collide after node A changes its holdoff time value

Node C

Figure 3.7: A case that node A has just decreased its holdoff time base value

Node C

Figure 3.8: A case that node A has just increased its holdoff time exponent value now will contend for TxOpps in the γ interval.

Fig. 3.8 depicts a case that node A has just increased its holdoff time exponent value and therefore both its Tx Interval and ineligible interval are lengthened. In this condition, node A’s ineligible interval will shift on the time axis, generating the vulnerable interval shown in Fig. 3.8. In contrast, Fig. 3.9 depicts a case that node A has just decreased its holdoff time exponent value and therefore its Tx Interval and ineligible interval are shortened, resulting in the shift of node A’s ineligible interval on the time axis. This shift generates the vulnerable interval shown in Fig. 3.9.

To prevent the collision problem from occurring, an additional mechanism to advertise nodes’ changes to their holdoff time base and exponent values in time is needed. For instance, in the case given in Fig. 3.6, after changing the holdoff time value, node A should defer its contention for TxOpps until its original ineligible interval has elapsed.

Such a mechanism, however, may increase the implementation complexity of a proposed

Node C

Figure 3.9: A case that node A has just decreased its holdoff time exponent value

dynamic holdoff time scheme and decrease its scheduling performances. To totally avoid the collision problem without wasting much network bandwidth, our proposed dynamic holdoff time scheme uses 0 as the holdoff time base value for all network nodes at all time.

Fixing the holdoff time base value to zero effectively eliminates every node’s ineligible interval (i.e., the length of each node’s ineligible interval now becomes zero.), resulting in each network node considering that it should always contend for TxOpps with all other nodes in its two-hop neighborhood. (These two-hop neighborhood nodes are considered to be always eligible to contend for TxOpps.) In this condition, if a node intends to win a TxOpp, it should win over all of its two-hop neighborhood nodes. This means that, for each node, the node list used as the input of MEA will always comprise its two-hop neighborhood nodes, despite the dynamic changes of the holdoff time exponent values of its neighboring nodes. Thus, packet collisions due to dynamic changes of the holdoff time exponent values can be avoided under the zero holdoff time base value condition.