|
Introduction
A network that utilizes a shared medium shall provide an efficient sharing mechanism. The PMP and mesh topology wireless networks are examples for sharing wireless media. The medium is radio waves in the space.
In the PMP mode of operation, the downlink, from the base station (BS) to subscriber stations (SSs), operates on a PMP basis. Within a given frequency channel and coverage of the BS sector, all SSs receive the same transmission or parts of it. The BS is the only transmitter operating in this direction. So it transmits without having to coordinate with other stations. The down- link is used for broadcasting the information. In cases where the message down link map (DL-MAP) does not explicitly indicate that a portion of the downlink subframe is for a specific SS, all SSs are able to listen to that por- tion. The SSs check the connection identifiers (CIDs) in the received protocol data units (PDUs) and retain only those PDUs addressed to them. SSs share the uplink to the BS on a demand basis. Depending on the class of service at the SSs, the SSs may be issued continuing rights to transmit or the transmis- sion rights granted by the BS after receipt of requests from SSs. In addition to individually addressed messages, messages may also be sent by multicast to a group of selected SSs and broadcast to all SSs. In each sector, SSs are controlled by the transmission protocol at the MAC layer. And they are enabled to receive services to be tailored to the delay and bandwidth requirements of each appli- cation. It is accomplished by four types of uplink sharing schemes, which are unsolicited bandwidth grants, polling, and bandwidth requests contention.
The transmission scheme at the MAC layer is connection-oriented. All data
communications are defined in the context of a connection. Service flows can be provisioned at an SS and connections are associated with these service flows, each of which is to provide transmission service at the requested bandwidth to a connection. The service flow defines the QoS parameters for the PDUs that are exchanged on the connection. The concept of a service flow on a connection is a key issue to the operation of the MAC protocol. Service flows provide a mechanism for uplink and downlink QoS management as bandwidth allocation processes. An SS requests uplink bandwidth on a per connection basis. Bandwidth is granted by the BS to an SS as an aggregate of grants in response to per connection requests from the SS. Connections may require active maintenance. And three connection management functions are supported by using static configuration and dynamic addition, modification, and deletion of connections. The termination of a connection is stimulated by the BS or SS.
Different from the PMP topology, in the operation of the mesh topology,
traffic can occur directly between SSs and be routed through other SSs. The
transmission can be managed by distributed scheduling, centralized schedul- ing, or a combination of both. Within a mesh network, a station that has a direct connection to backhaul services outside the mesh network is named a mesh BS. All the other stations of a mesh network are termed mesh SSs. Within mesh context, uplink and downlink are defined as traffic in the direction of the mesh BS and traffic away from the mesh BS, respectively. In a mesh network, there are neighbor, neighborhood, and extended neighborhood. The stations with direct links to a node are called neighbors of the node and neighbors of a node form a neighborhood. A node’s neighbors are only one hop away from the node. An extended neighborhood contains all the neighbors of the neighborhood.
In a mesh system, every node including the mesh BS cannot transmit without having to coordinate with other nodes. By distributed schedul- ing, all the nodes shall coordinate their transmissions in their two-hop neighborhood and shall broadcast their schedules to all their neighbors. Optionally, the schedule may also be established by directed uncoordinated requests and grants between two nodes. Nodes shall ensure that the result- ing transmissions do not cause collisions with the data and control traffic scheduled by any other node in the two-hop neighborhood. There is no difference in the mechanism for determining the schedule for downlink and uplink. By centralized scheduling, resources are granted in a more centralized manner. The mesh BS shall gather resource requests from all the mesh SSs within a certain hops range. It shall determine the amount of granted resources for each link in the network, both in downlink and uplink, and communicates these grants to all the mesh SSs within the hops range. Grant messages will not make any schedule, which should be determined by each node using a predetermined algorithm with given parameters.
All the communications are in the context of a link, which is established
between two nodes. One link is used for all the data transmissions between the two nodes. QoS is provisioned over links on a message basis. No service or QoS parameters are associated with a link, but each unicast message has service parameters in the header. Traffic classification and flow regulation are performed at the ingress node by upper-layer classification/regulation protocol.
Do'stlaringiz bilan baham: |
