*3.1.1. IEEE 802.16 mesh frame*

In the mesh mode, bidirectional links can be established between any of the WiMAX nodes, and the information is transmitted on a hop-by-hop basis. The system access follows a framebased approach where each channel is divided in time into a series of frames. The number of frames in a series is defined during process of creating the network..

A frame is divided into two subframes: a control subframe and data subframe (Figure 5). The control subframes are used for carrying the information necessary for access control systems, bandwidth allocations, connection establishment and connection maintenance. The data subframes are used for carrying the packets of upper layers. The control subframe is divided into a number of transmission opportunities. The data subframe is similarly divided into a number of minislots.

There are two types of control subframes depending on their function. The first type of control subframe is the scheduling subframe in which nodes transmit scheduling messages. The second is the network configuration subframe in which nodes broadcast network configura‐ tion packets containing topology information, network provisioning information, and network management messages.

**Figure 5.** Mesh frame structure

bandwidth requirements of the subscriber stations and the parameters for coding and

In a WiMAX mesh network, a "Mesh BS" (MBS – mesh base station) provides the external backhaul link. The backhaul links connect the WiMAX network to other communication networks. There may be multiple Mesh BSs in a network; other nodes are known as ''Mesh SSs" (MSS – mesh subscriber stations). In point-to-multipoint mode, the SSs are under the direct control of the BS. In Mesh mode, the uplink and downlink is not clearly separated and

In the mesh mode, bidirectional links can be established between any of the WiMAX nodes, and the information is transmitted on a hop-by-hop basis. The system access follows a framebased approach where each channel is divided in time into a series of frames. The number of

A frame is divided into two subframes: a control subframe and data subframe (Figure 5). The control subframes are used for carrying the information necessary for access control systems, bandwidth allocations, connection establishment and connection maintenance. The data subframes are used for carrying the packets of upper layers. The control subframe is divided

SSs can communicate with each other without communicating with the BS.

frames in a series is defined during process of creating the network..

modulation. Figure 4 illustrates the WiMAX QoS Architecture in PMP mode.

**Figure 4.** Architecture for IEEE 802.16 QoS

68 Selected Topics in WiMAX

*3.1.1. IEEE 802.16 mesh frame*

**3.1. QoS in WiMAX networks in mesh mode**

The IEEE 802.16 mesh standard uses a combination of a 16-bit mesh node identifier (node ID) and a 16-bit connection identifier (CID) to identify the source and destination of every transmission. The CID in mesh mode is a combination of an 8-bit link ID and an 8-bit QoS description for the connection. All the communications occur in the context of a link, which is established between two nodes. One link will be used for all the data transmissions between two nodes. QoS is provisioned over links on a message-by-message basis. No services or QoS parameter are associated with a link, but each unicast message has service parameters in the header. Figure 6 shows the Mesh connection identifier (CID) construction which contains these service parameter fields.

**Figure 6.** QoS bits in the mesh CID

The 8-bit QoS in the CID contains three definable fields: Reliability, Priority/Class, and Drop Precedence. Reliability refers to retransmit or not (0 indicates no retransmit while 1 indicates retransmit). Priority/Class refers to the priority of the packet. Drop Precedence refers to the probability of dropping the packet when congestion occurs [12] [13].

Distributed Scheduling (Mesh DS): The neighboring Mesh SS responds to a request with a corresponding grant for a link between two Mesh SSs. Mesh distributed scheduling messages

A Mobile WiMAX Mesh Network with Routing Techniques and Quality of Service Mechanisms

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The scheduling policy for accessing data slots in coordinated distributed fashion, is not specified in the IEEE 802.16 standard. The standard only defines the Mesh Distributed Scheduling Message (MSH-DSCH message), and specifies the scheduling to avoid collisions between messages of different nodes. The MSH-DSCH message contains the scheduling information organized in Information Elements (IE): Request IE, Availability IE, Grant IE and

The scheduling procedure follows a three-way handshake to reserve the minislots. First, a node sends an MSH-DSCH message to one of its 1-hop neighbors, requesting a set of data slots. In the message, the node also includes the set of data slots that it has available for reservation. The 1-hop neighbor grants the request by replying with another MSH-DSCH message that specifies a set of data slots that confirms the availability of data slots at both nodes. Finally, the first node confirms the reservation of this set of data slots by repeating the grant in another

In contrast with point-to-multipoint WiMAX networks, the standard does not define sched‐

Currently, one of the main areas of mesh networks that is being studied, is the routing protocol used to find the best path to the base stations (or access points). This allows customers who use this type of technology to take advantage of their services in a more effective way and with efficient communication, as well as transferring their data stream through the wireless communication environment [15]. Routing is a service in which the router evaluates the possible paths to transmit packets to their destination, and determines the best route this packet

The concept of network performance optimization is carried out through the construction of the routing tree selection which is characterized by the topological properties that are inde‐ pendent when the network is being formed. The construction of the tree and arrangement of the nodes allows a distribution of the nodes that leads to a better chance of routing and optimization. The correlations between the topological parameters of the tree and the efficiency of the network must be estimated, and those that show the strongest correlations should allow the creation of the best trees and thus provide some routing and topology optimization [17].

Currently there are a number of routing protocols with several differences and similarities between them, that show the particular advantages and disadvantages when applied to mesh networks. Among these various routing protocols, there is no exists single protocol that can be claimed as the best. The reason for this is that they have several peculiarities and there not

are exchanged to perform this operation.

uling services for Mesh WiMAX networks [13].

**4. Routing protocols in wireless mesh networks**

Scheduling IE.

MSH-DSCH message.

should follow [16].
