In the 802.16 mesh CDS mode, each node’s control message transmission is sched-uled by the Mesh Election Algorithm (MEA) [1], which is hash-based algorithm with an exponential holdoff mechanism. In this mode, each node needs to maintain (i.e., learn) the information about its hop neighborhood (which is an input of MEA). The two-hop neighborhood of a node is defined as the set comprising the node itself, its one-two-hop neighboring nodes, and its two-hop neighboring nodes, i.e.,
nbr(i) = {i} ∪ nbr1(i) ∪ nbr2(i), (2.1) where nbr1(i) and nbr2(i) are defined as follows, respectively.
nbr1(i) = {k | node k ∈ node i’s one-hop neighboring nodes,
when omnidirectional antennas are used.} (2.2) nbr2(i) = {k | node k ∈ node i’s two-hop neighboring nodes,
when omni-directional antennas are used.} (2.3) The purpose of maintaining the two-hop neighborhood is to avoid the hidden terminal problem when transmitting control messages. By using the same MEA in every node, this algorithm guarantees that in the network only one node in any two-hop neighborhood will
win the access to a specific TxOpp. Thus, when nodes transmit their control messages, no message collisions will occur. Since the transmission of control messages are collision-free and these messages are used for scheduling a collision-collision-free minislot allocation, the transmission of data can also be guaranteed collision-free.
The scheduling process of the MSH-DSCH message in the 802.16 mesh CDS-mode network is explained here1. Each node should perform MEA to determine the TxOpp on which to broadcast its next MSH-DSCH message. MEA takes a given TxOpp number and an eligible node list (which is a list of nodes that are eligible to contend for the given TxOpp) as input. It then iteratively computes a hash value for each node in the given eligible node list on the give TxOpp number. The hash function is given in [1].
Finally, it outputs the ID of the winning node whose computed hash value i s the largest among all of the competing nodes. Because nodes within two hops use the same MEA and consistent eligible node lists, every node knows which node will win a given TxOpp within its two-hop neighborhood.
To achieve the consistency of neighboring nodes’ eligible node lists for each TxOpp, each node should periodically broadcast the next MSH-DSCH TxOpps used by itself and its one-hop neighboring nodes. In addition, for node i, if the information about the next MSH-DSCH message transmission of its neighboring node j is unknown, node i will conservatively consider that node j will contend for every TxOpp and put node j into the eligible node list for the following TxOpps until it receives the information about the next MSH-DSCH TxOpp used by node j. Using this design, no message collision will occur on any TxOpp. If a node cannot win a given TxOpp, it repeats the above process with the next TxOpp (i.e., the previous TxOpp number plus one) as input until it eventually wins one TxOpp.
The eligibility of a node for contending for a TxOpp is determined by the holdoff time mechanism [1]. The holdoff time mechanism first defines the control message transmission cycle of a node as the time interval between the node’s two consecutive control message transmissions. Recall that, as shown in Fig. 2.2, the transmission cycle of a node comprises 1) the holdoff time and 2) the contention time. The former is defined as the number of consecutive TxOpps during which a node must suspend its contention for TxOpps after
1The scheduling process of the MSH-NCFG message is the same as that of the MSH-DSCH message.
Thus, we only explain the latter in this chapter for brevity.
Figure 2.2: The transmission cycle of a node
wining a TxOpp, while the latter is defined as the number of consecutive TxOpps for which a node may contend to win a TxOpp. Thus, by obtaining the holdoff times of the nodes in its two-hop neighborhood, a node can know for which TxOpps these neighboring nodes will and will not contend. Based on such information, it can construct an eligible node list of its two-hop neighborhood for every TxOpp.
In the 802.16(d) mesh CDS mode, the holdoff time of each node is fixed and defined as follows:
HT(i) = 2exp+base, (2.4)
where HT(i) denotes the holdoff time of node i. The base value is fixed to 4 and the range of the exp value is between 0 and 7. In the 802.16 mesh mode standard, the exp value of each node is the same. Thus, the holdoff times of all nodes in the network are the same.
In contrast, the contention times of nodes may vary depending on which and how many nodes are eligible to contend for TxOpps.
Before broadcasting an MSH-DSCH message, a node first executes MEA to calculate (win) a TxOpp for its next MSH-DSCH message transmission. It then puts the following information into the MSH-DSCH message that is going to be sent out: 1) its calculated next MSH-DSCH TxOpp number; 2) its holdoff time; 3) the learned next MSH-DSCH TxOpp numbers of its hop neighboring nodes; and 4) the holdoff times of these one-hop neighboring nodes. After this, it broadcasts this MSH-DSCH message to all of its neighboring nodes.
By this design, a node can learn in advance the next MSH-DSCH TxOpp numbers and holdoff times of all nodes in its two-hop neighborhood. Because every node knows such information about every other node in its two-hop neighborhood, for any given TxOpp, the output of every node’s MEA in any two-hop neighborhood is consistent. The consistency of the MEA output means that: For nodes i and j, suppose that {a, b} = nbr(i) ∩ nbr(j),
Figure 2.3: The THP defined in the IEEE 802.16(d) mesh CDS mode
it is impossible that a and b simultaneously win the same TxOpp. That is, in a two-hop neighborhood, there is only one winning node for a given TxOpp and every node knows who wins it. As a result, when nodes transmit their MSH-DSCH messages, no collision will occur. It is this advance announcement design that enables MEA to generate collision-free control message scheduling.
Another advantage of the holdoff time design used in the 802.16 mesh CDS mode is that, using this design, a node must refrain from contending for TxOpps after winning a TxOpp (i.e., after transmitting its current MSH-DSCH message) until its holdoff time has elapsed. This ensures that nodes other than the winning node will have a chance to win subsequent TxOpps and all nodes can fairly share TxOpps in the long run.