HWN* Routing Management

Efforts must be invested on HWN* to avoid congestion, compute the next route, discover medium resources and gather data. This section proposes a novel cascaded Adaptive Distributed Cross-layer Routing (ADCR) scheme for the HWN* framework, using a minimal number of hops and considers dynamic routing models aimed at reducing latency, preserving communications and delivering good overall and per node throughput. A cross-layer network design that seeks to enhance the system performance by jointly designing MAC and network layers has been adopted.

The cascaded ADCR includes three sub packet transmission modes labeled as One-Hop Ad-Hoc Transmission (OHAHT) for point-to-point direct communication, Multi-Hop Combined Transmission (MHCT) for radio resource relaying using fixed RNs or MTs, and Cellular Transmission (CT) for a traditional cellular service. The RNs of the core network compose a mesh structure with fixed routing tables using ad hoc frequencies, while BSs are connected to each other via wires and the link between the RN and BS are established using cellular channels and directional antennae. In areas without infrastructure support, two MTs may communicate directly, or through intermediate MTs. When a MT transmits packets to a BS through RNs, the RNs extend the signalling coverage of the BSON thus enhanced resource sharing performance is expected. The QoS flows can consume all the bandwidth on certain links, thus creating congestion for, or even starvation of best effort sessions. Statically partitioning the link resources can result in low network throughput if the traffic mix changes over time. Thus, a mechanism that dynamically distributes link resources across traffic classes based on the current load conditions in each traffic class is critical for performance. By proposing the cascaded ADCR for HWN*, the framework discourages applications from using any route that is heavily loaded with low priority traffic. Traditional routing strategies that use global state information are not considered. Problems associated with maintaining global state information and the staleness of such information is avoided by having individual MTs infer the network state based on route discovery statistics collected locally, and perform traffic routing using this localised view of the network QoS state. Each application, categorised by the service class with the choice of three possible transmission modes, maintains a set of candidate paths to each possible destination and route flows along these paths. The selection of the candidate paths is a key issue in localised routing and has a considerable impact on how the ADCR performs. The high priority traffic is given high priority in accessing comparatively expensive cellular resources, while low priority traffic tries to access lower cost ad hoc resources. Per MT bandwidth is used as the metric for local route statistics collection since it is one of the most important metrics in QoS routing. Furthermore, important metrics such as end-to-end delay can be improved by an increased bandwidth as long as the traffic load is not largely increased. As in QoS based investigation for inter system handover management. Traffic sessions are divided by HPUs, NPUs and LPUs. In case of network congestion, CT mode may temporarily become unavailable to NPUs when HPUs are not fully accommodated, while LPUs sessions may be only granted MHCT and OHAHT mode access to mitigate network congestion, reduce transmission delay and improve per MT throughput.

To avoid having higher traffic classes being influenced by lower traffic classes in terms of queueing delay, a waiting time limitation is placed on each traffic class using a forced starvation packet switch model. A traffic flow maintains two queues: a slot queue and a packet queue. The slot queue is decoupled for traffic class identification from the packet queue for transmission. The RN is specially designed so that it reserves QoS guaranteed free channels for signalling information exchange in the slot queue. Each MT and RN maintains a routing table. A RN's routing table contains the information of other fixed RNs thus the routing delay and multi-hop signalling overhead in MANET RN mode are largely reduced. On the other hand, a RN can also analyse the current system traffic load condition through feed-back from other fixed RNs. The purpose of bandwidth reservation is to let RNs that receive the relaying discovery command in the slot queue check if they can provide the bandwidth required for the connection. Start and finish tags are associated with slots but not with packets. When a packet arrives for a flow, it gets added to the packet queue, and a new slot is added to the slot queue. Corresponding start and finish tags are assigned to the new slot. The way to raise priority in slot queue is that the packets related to a high profile have shorter back-offtime to increase the probability of early medium access. As for the status table maintenance, information flooding is restricted to a limited scope. Once a positive acknowledgment message is confirmed by a requesting RN, the relay paths will not be changed unless resource contention happens. Given the fact that maintaining global RNs channel status in each RN slows down RN response time, each RN only updates neighbouring RNs' information, periodically.

In OHAHT, the requesting MT A first broadcasts SEARCH messages to every node in its transmission range including its associated RN and BS. If the destination MT B is within its transmission range and there is no ad hoc based media contention between MT A and MT B, MT B can respond to MT A with an ACK message. Once MT A confirms the acknowledgment, it starts a connection SETUP session immediately. The OHAHT transmission model can be extended to multi-hop ad hoc communication and it is only activated on demand.

The MHCT involves RNs acting as intermediate nodes for message relaying. BS provides assistance on cellular link establishment using cellular location registers. In the connection setup process for communication between MT A and MT B via the RN infrastructure, MT A first broadcasts SEARCH messages to every node to find MT B. After the SEARCH session, MT A may find that the cellular resources can be used through RNs by receiving three ACK messages from the serving BS of MT B, RNs and the MT B. It may also find that the ad hoc frequency based RN mesh is adequate for communications by receiving two ACK messages from the serving RN of MT B, and the MT B, respectively. These positive acknowledgments require MT B to send an ACK to its serving BS and serving RN, then the serving BS and the serving RN feedback the ACKs to MT A. Once the positive ACK is confirmed, MT A can either start a connection SETUP from MT A → RN, then RN → BS, and finally BS → MT B, or from MT A → RN then RN → M T B. The DATA transmission process follows the same packet delivery route, and further route discovery is prohibited to reduce the signalling overhead.

The label routing concept originated in Asynchronous Transfer Mode (ATM) network is introduced to MHCT mode since the position of RN is fixed and label based RN switching provides faster packet forwarding than routing because its operation is relatively simple.

The label is a fixed-length identifier. Multiple labels can identify a path or connection from source MT to destination MT. The structure of label has four parts. The first part CAST of the label is 3 bits and relates to cast options. Only uni-cast is considered in the current research. As practically the RN mesh can be very large, the second part of the Label is 20 bits which includes a node identifier and is unique in the network ensuring conflicts do not occur. The third part is 3 bits QoS, which means class of service. The last part of the label is TTL. All label information has a time-related restriction. After time out for a label, all corresponding entries will be deleted from the label routing table. Label routing uses a label to directly index into a connection table entry to determine the next hop, lending itself to a simpler lookup implementation than the complex IP routing and hop-by-hop IP address lookup. All intermediate nodes in the virtual connection can forward packets more efficiently. The path from a source MT to a destination MT is identified by multiple labels. The protocol distributes labels and set up new route after the path is computed by the routing protocol. The path finding process dynamically initialised by the label request packet carrying a unique label and flow information, where low path setup delay is guaranteed. The path between MT and destination MT is composed of multiple segments. The path is separated by segments and all data packets are relayed by these segments. Each segment is a real connection between two nodes and labeled by the sending-side node of label relay message in this segment. Figure 1 presents an example routing presentation and routing tables for three nodes MT A, RN B and MT C. The relay nodes only need to find the available entry indexed by a label in the packet, swap it with the respective Label out of this entry, and then send it out to the next relay node. Furthermore, the fixed RN placement reduces the frequency of label entry changes with reliable service.

 

Figure 1: Presentation of routing presentation and routing tables for three nodes MT A, RN B and MT C

In the CT mode, the source node MT A first broadcasts SEARCH messages to every node to find destination MT B. After the SEARCH session, the MT A finds that it is able to communicate with MT B directly via BSs, while the connection can be setup through a virtual wireless backbone. The positive acknowledgment of a connection requires MT B to send an ACK to its serving BS, then the serving BS informs the serving BS of MTA or the BS feedbacks the ACK to MT B when both MT A and MT B share the same serving BS. Once the positive ACK is confirmed, MT A starts connection SETUP from MT A → BS, then BS → BS, and finally BS → M T B. The DATA transmission process follows the same packet switched delivery route. The cellular transmission also includes cellular transmission with RN support. For cellular based RN connection, the packet tries to establish a link with a BS first then the traffic will be connected to BS RN mode. Dynamic channel allocation can be realised in a distributed manner given that the channel usage does not break the two channel interference constraint which are cosite constraint where there are minimum channel separations within a cell and non-cosite constraint where minimum channel separation between two adjacent BSs is kept.