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Convergence Sublayer
Convergence sublayer (MESA, 2005) is designed to map services to and from
802.16 MAC. 802.16 has two services—the ATM convergence sublayer and packet convergence sublayer. Packet convergence sublayer provides support for IPv4, IPv6, Ethernet, and VLAN. The main task of the convergence sub- layer is to map higher protocol data units into proper service data units. Additionally, it is responsible for bandwidth allocation and QoS, as well as header suppression and reconstruction to enhance air-link efficiency.
Common Part Sublayer
802.16 is designed to support PMP network architecture. However, mesh operation, also known as point-to-point architecture, is left as an optional feature.
2.4.2.2.1 802.16 PMP
IEEE 802.16 MAC protocol was designed as PMP connection from the base station (BS) with sectorized antenna to multiple subscriber stations (SSs). TDD multiplexing is used to divide transmission time into up- and down- link periods. On downlink, data to SSs are multiplexed in TDM fashion and generally broadcasted to all SSs capable to listen to the downlink frame. Each SS checks the connection ID in the protocol data units (PDUs) and retains the PDUs addressed to it. The uplink is shared between SSs implementing TDMA on demand bases.
802.16 MAC is connection oriented even for connectionless transmissions such as IP. Connectionless transmission is mapped into a connection, which is used as a pointer to destination and context information. In the 802.16 standard, SSs are identified by a 48-bit universal MAC address. This address is unique and is normally used for authentication and during ranging process to establish connections. Connections are referenced with 16-bit connection identifiers (CID). Upon joining the network, three management connections and at least one transport connection are established between the BS and SS in the up- and downlink direction. The third management connection is optional. The three management connections reflect different levels of QoS as follows:
Basic connection: short-time urgent MAC management messages
Primary management connection: longer, more delay-tolerant messages
Secondary management connection: standard-based delay toler- ant management messages such as Dynamic Host Configuration Protocol (DHCP), Trivial File Transfer Protocol (TFTP)- and Simple Network Management Protocol (SNMP)
Transport connections are used to facilitate different QoS communication levels for the up- and downlink. The contracted level services are unidirec- tional, thus, the QoS level may differ between the up- and downlink. In addition to the aforementioned connections, there are three additional spe- cial purpose connections. One is reserved for connection-based initial access while the other two are broadcast and multicast connection based polling.
802.16 defines the concept of service flow. Once an SS joins a network and connection is established, the connection is mapped into the service flows; each connection is mapped to one service flow. Service flows provide a mechanism for up- and downlink QoS management, mainly the bandwidth allocation process. Bandwidth is allocated to an SS by a BS as a response to a per connection request from the SS. Bandwidth allocation may be constant depending on the type of service, for example, T1 unchannelized services or it may be adaptive such as that granted for the IP bursty services. As with connection establishment, connections may undergo maintenance or termination.
2.4.2.2.2 802.16 Mesh
The key difference between the PMP and mesh topology is that in the PMP mode communication is based on a direct connection between the BS and SSs, while in the mesh mode multihop communication is allowed, where traffic can be routed through other SSs and can occur directly between SSs. Hence, an SS may operate as a router to relay traffic between SSs until it arrives to a BS, called mesh BS. Mesh BS has a direct connection to backhaul services outside the mesh network. All the other systems of a mesh network are termed mesh SS nodes. In mesh node, the term up- and downlink has a different meaning. Uplink is defined as traffic in the direction of the mesh BS while downlink is defined as traffic away from the mesh BS.
In mesh mode the up- and downlink transmission is still based on TDMA. However, mesh mode defines another type of scheduling mechanism in addi- tion to the centralized scheduling, distributed scheduling and a combination of both distributed and centralized scheduling. In centralized scheduling, a mesh BS gathers resource requests from all the mesh SSs within a certain hop range. It determines the amount of granted resources for each link in the network, both in down- and uplink, and communicates these grants to all the mesh SSs within one hop range (LAN/MAN committee, 2004). In distributed scheduling, all nodes including the mesh BS coordinate their transmissions in their two-hop neighborhood and broadcast their available resources, requests, and grants to all their neighbors. 802.16 defines a node neighbor to be nodes one hop away (forming nodal neighborhood). Addi- tionally, distributed scheduling can be established by directed uncoordinated requests and grants between two nodes. Hence, communicating nodes are required to ensure collision-free transmission within two hops proximity.
We remark that the 802.16 mesh operates in the licensed and unlicensed
2–11 GHz NLOS communication spectrum. We also remark that QoS over the mesh is link based; there is no end-to-end QoS guarantees. QoS is provisioned over links on a message-by-message basis, where each message has service parameters in its header.
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