**3. IEEE 802.16 and IEEE 802.11e: An overview**

### **3.1. Overview of IEEE 802.11e**

The multiple access mechanism in 802.11e is arisen in super-frames which start with beacon frames having the same duration as beacon intervals. The super-frame comprises an optional CFP (Contention Free Period) followed by a CP (Contention Period) divided into equal duration SIs as shown in Figure 2. At each SI (Service Interval), each QSTA (QoS Station) should transmit its own traffic streams with respect to its QoS constraints. This mechanism is called HCCA function which defines a centrally-controlled polling-based medium access scheme for IEEE 802.11e WLANs. Each SI is divided into a CAP (Controlled Access Phase) period and an optional EDCA period in which the traffic streams having less stringent QoS constraints contend for access to the medium. Usually best effort traffic streams such as HTTP use this period which offers no QoS guarantee. The CAP period is further divided into a number of TXOPs (Transmission Opportunity). Each TXOP is granted by QAP (QoS Access Point) to each QSTA and each QSTA is responsible for sharing this period among its traffic streams.

**Figure 2.** HCF super-frame structure

#### **3.2. Overview of IEEE 802.16 mesh mode**

IEEE 802.16 MAC PDUs (Protocol Data Unit) (Figure 3) begin with a fixed-length generic MAC header (6 bytes). The MAC header field contains a 2 bytes CID (Connection Identifier) field which carries 8 bits *Link ID* used for addressing nodes in the local neighborhood. The header is followed by the Mesh sub-header (2 bytes) which includes *Xmt Node Id*. Mesh BS grants *Node Id*s to candidate nodes when authorized to the network. After the variable length payload there exists a 4 bytes CRC. The medium in IEEE 802.16 mesh mode is divided into equal duration frames (Figure 4), consisting of two sub-frames:


The control sub-frame is divided into MSH-CTRL-LEN transmission opportunities indicated in the ND (Network Descriptor). Each transmission opportunity comprises 7 OFDM symbols, so the length of the control sub-frame is fixed and equal to 7×MSH-CTRL-LEN OFDM symbols.


**Figure 3.** MAC PDU format

6 Wireless Mesh Networks – Efficient Link Scheduling, Channel Assignment and Network Planning Strategies

**3. IEEE 802.16 and IEEE 802.11e: An overview** 

**3.1. Overview of IEEE 802.11e** 

period among its traffic streams.

**Figure 2.** HCF super-frame structure

 Data sub-frame, Control sub-frame.

**3.2. Overview of IEEE 802.16 mesh mode** 

previous works in two aspects. First, we take into account the underlying network traffic related to each MSS. Second, the algorithm proposed in this chapter is such that shrinking the link duration doesn't affect the minimum QoS requirements of real-time traffic types.

The multiple access mechanism in 802.11e is arisen in super-frames which start with beacon frames having the same duration as beacon intervals. The super-frame comprises an optional CFP (Contention Free Period) followed by a CP (Contention Period) divided into equal duration SIs as shown in Figure 2. At each SI (Service Interval), each QSTA (QoS Station) should transmit its own traffic streams with respect to its QoS constraints. This mechanism is called HCCA function which defines a centrally-controlled polling-based medium access scheme for IEEE 802.11e WLANs. Each SI is divided into a CAP (Controlled Access Phase) period and an optional EDCA period in which the traffic streams having less stringent QoS constraints contend for access to the medium. Usually best effort traffic streams such as HTTP use this period which offers no QoS guarantee. The CAP period is further divided into a number of TXOPs (Transmission Opportunity). Each TXOP is granted by QAP (QoS Access Point) to each QSTA and each QSTA is responsible for sharing this

IEEE 802.16 MAC PDUs (Protocol Data Unit) (Figure 3) begin with a fixed-length generic MAC header (6 bytes). The MAC header field contains a 2 bytes CID (Connection Identifier) field which carries 8 bits *Link ID* used for addressing nodes in the local neighborhood. The header is followed by the Mesh sub-header (2 bytes) which includes *Xmt Node Id*. Mesh BS grants *Node Id*s to candidate nodes when authorized to the network. After the variable length payload there exists a 4 bytes CRC. The medium in IEEE 802.16 mesh mode is

divided into equal duration frames (Figure 4), consisting of two sub-frames:


**Figure 4.** Frame structure for the mesh mode

Nodes can transmit based on the granted bandwidth and a transmission schedule which is worked out using a common distributed algorithm. The data sub-frame is used for this purpose which is divided into transmission opportunities comprising 256 mini-slots based on the standard. However, there may be fewer than 256 mini-slots depending on the frame size and the size of the control sub-frame. Frame duration which is indicated in ND is determined by MBS to avoid losing synchronization with the connecting nodes. MSH-CSCH-DATA-FRACTION indicated in ND specifies the fraction of data sub-frame which can be used for centralized scheduling. The remaining part of the data sub-frame is used for decentralized scheduling.

## **3.3. QoS Comparison between WiFi and WiMAX mesh mode**

Providing QoS in IEEE 802.11e comes with a new coordination function called HCF. The HCF controlled channel access is for the parameterized QoS, which provides the QoS based on the contract between the AP and the corresponding QSTA(s). First, a traffic stream is established between the AP and an QSTA. A set of traffic characteristics and QoS requirement parameters are negotiated between the AP and QSTA and the traffic stream should be admitted by the AP. The QoS control field in the MAC frame format is a 16 bits field which facilitates the description of QoS requirements of application flows. Its TID (4 bits) identifies the TC (0-7) or the TS (8-15) to which the corresponding MSDU in the FB field belongs. The last eight bits are used usually by QAP to receive the queue size of QSTAs. After admission, the AP specifies the TXOP duration for the QSTA based on the traffic characteristics. So, the QoS is provided based on connections established between AP and QSTA(s).

Unlike WiFi, the QoS in the mesh mode of IEEE 802.16 is provided in a packet by packet basis. Each transmitted packet contains the mesh CID. Figure 5 shows the structure of mesh CID used in unicast messages. In order to enable differentiated handling of packets, the queuing and forwarding mechanisms deployed at individual nodes may make use of the values for the *Type, Reliability, Priority/Class*, and *Drop Precedence* fields. The *Type* field is used to distinguish between different categories of messages. This field may be used to

prioritize the transmission of management messages transmitted in the data sub-frame (e.g., messages for uncoordinated distributed scheduling). The *Reliability* field is employed to specify unacknowledged transmitted packets (when ARQ is enabled). This allows the packet to be retransmitted for up to four times. The *Priority/Class* field allows the classification of the messages into eight priority classes. This can be used by the queuing and forwarding mechanisms at each node to differentiate the packet treatment for different classes. The *Drop Precedence* field indicates the likelihood of a packet being dropped during congestion.

Application of Genetic Algorithms in Scheduling of TDMA-WMNs 9

MSSs or the MBS. The MBS, MSSs, and MTs share the same frequency band. The routing tree that is made by the MBS is a binary tree [14], and we assume that it's known in

For better support of QoS, we consider MBS and MSSs use TDMA-based scheduling in their MAC layer; however, because of IEEE 802.11 deployment in most mobile devices (laptops and cell phones), MTs use contention-based medium access method. In a given TDMA frame (of the length for example 20ms), some MSSs are sending frames upward or downward the network, while the others are collecting (distributing) frames from (to) MTs. Since the tree rooted at the MBS is a binary one, each MSS has maximum of six logical links to its neighbor MSSs; three for sending and another three for receiving packets (Figure 1). As wireless transceivers are usually half duplex [8], they can't be used for reception and transmission at the same time. So there are six queues in each MSS, three of these are to store the outbound packets and three others are for inbound packets to queue for reception. However, in our model receiving queues are ignored as they are considered in sending nodes; hence at most three queues are considered for scheduling. It's worthy to note that we schedule only one queue at each leaf node and two queues at the MBS. Moreover we schedule only the links that have non-empty queues. Each queue is filled by MTs (shown in Figure 6) or the receiving links at that node. For example in Figure 1, the queue of *e2* can be

Let, *M* be the set of all the stations (including MSSs and the MBS) in the system, indexed by *m*=1,2,…,*M*. We consider *M*>1; i.e., there is at least one MSS. In most of the mesh networks, the frame length is fixed and may not be changed; otherwise the whole system should be restarted [8]; hence the frame length is fixed at *L* milliseconds. Each transmission in the frame is along with some overheads, so the number of transmissions for each link should be

 be the set of all the links in the system. We take a subset *I* of � (*I*��) , in which the links have non empty queues. Each *i*∈*I* has one queue per traffic type which are assumed to

Each queue has some restricted QoS traffic specifications; this means that each queue should be scheduled appropriately and get emptied in a desired time. Since the frame length is fixed and all of the links in the system should be scheduled at each frame (because of their restricted QoS requirements), there is a limited interval for each queue to get scheduled. Nevertheless, some of the nodes may not find enough transmission opportunity to evacuate all of their queues, causing the system not to be able to fulfill QoS constraints of delay sensitive traffic types. So, a scheduling method is strongly necessary to satisfy QoS requirements of voice and video traffic. On the other hand, bandwidth allocation to more stringent QoS traffic types may cause starvation for elastic traffic. As such, we define a threshold (*k*), to

Let *k*m*,i,j* be the length of the *j*th queue (filled by MTs or other MSSs), associated with *i*th outgoing link, related to *m*th MSS. So, [*k*m*,i,j*] is an *M* × *I* × *J* matrix. Each queue should be

limited to one per frame to minimize transmission overhead.

assure the elastic traffic types to be scheduled at each *k* frames.

have unlimited sizes for the sake of simplicity.

advanced.

filled by the queue of *e4*.

Let �


**Figure 5.** Mesh CID format
