A Cross-Layer Routing Design for Multi-Interface Wireless Mesh Networks
© T.-C. Tsai and S.-T. Tsai. 2009
Received: 15 April 2009
Accepted: 7 August 2009
Published: 29 September 2009
In recent years, Wireless Mesh Networks (WMNs) technologies have received significant attentions. WMNs not only accede to the advantages of ad hoc networks but also provide hierarchical multi-interface architecture. Transmission power control and routing path selections are critical issues in the past researches of multihop networks. Variable transmission power levels lead to different network connectivity and interference. Further, routing path selections among different radio interfaces will also produce different intra-/interflow interference. These features tightly affect the network performance. Most of the related works on the routing protocol design do not consider transmission power control and multi-interface environment simultaneously. In this paper, we proposed a cross-layer routing protocol called which coordinates transmission power control and intra-/interflow interference considerations as routing metrics. Each radio interface calculates the potential tolerable-added transmission interference in the physical layer. When the route discovery starts, the will adopt the appropriate power level to evaluate each interface quality along paths. The simulation results demonstrate that our design can enhance both network throughput and end-to-end delay.
WMNs are dynamically self-organized and self-configured, with the nodes in the network automatically establishing and maintaining mesh connectivity among themselves and compatible with conventional Wi-Fi clients. Multi-interface WMNs provide multiple radio interfaces of a node that can improve the throughput capacity . This feature enables nodes to transmit and receive simultaneously, hence nodes can use nonoverlapping channels to transmit and receive at the same time via different interfaces. WMNs technologies accede to the advantages of ad hoc networks. Traditional ad hoc network routing protocols may not be suitalbe for WMNs since they do not fully consider the features of WMNs such as multi-interface. In IEEE 802.11s, it presents the prototype of default path selection protocol-HWMP (Hybrid Wireless Mesh Protocol) and routing metric-airtime cost. The implementation details can be based on user demands. Several routing protocol designs for WMNs [4, 5] focus on single layer of network protocol stacks and do not consider coordinating with different protocol layers. Specifically, in the physical layer, the transmission power level decides the signal strength and determines the neighbor nodes which can hear the packet. This thus affects the network layer to select the forwarding nodes at the route discovery. The transmission power also causes the interference that affect the link quality among nodes. The appropriate transmission power level selection can improve network performance [6–8]. Traditional transmission power control problems in wireless ad hoc networks mainly focus on reducing energy consumption. Some researches address power selection problems but still use minimum hop-count as the routing metric. Power control indeed impacts multiple network protocol layers. Transmission power control tightly affects network performance . The theoretical studies  have demonstrated that transmission power control can improve wireless network capacity. The result in  presents that the need to design future protocols is based on variable-range power control, not on common-range transmission power control. The higher transmission power increases network connectivity and gives lower end-to-end delay in the low traffic load with slight interference. However, the higher transmission power will create high interference when concurrent transmissions in the vicinity are increased. This will decrease the spatial reusability. In this high loading case, using lower transmission power will result in lower interference, and thus increase the throughput. The motivation of our cross-layer routing protocol development is inspired from the above features we observed.
We previously proposed the MiRii (Multi-Interface Routing with Intra/Inter-flow Interference)  routing protocol that measures intra-/interflow interference and applies to routing path selection in the network layer. The multi-interface feature is utilized by considering the channel diversity of the routing path. By contrast with AODV , ETX , and WCETT  (will be introduced later in Section 2.2), the simulation results demonstrate that our MiRii routing protocol can improve packet delivery ratio and decrease end-to-end delay. To further improve the performance, the routing protocol has to work together with the physical layer. In this paper, we propose our cross-layer routing protocol, namely, Multi-power, Multi-interface Routing with Intra/Inter-flow Interference), that incorporates MiRii routing protocol with perflow transmission power control. routing protocol jointly coordinates the transmission power at each traffic flow and route selection among multi-interface nodes. The protocol interplays between the network and the physical layers. It aims to select appropriate transmission power to reduce the noise interference more efficiently when the traffic loading in the network is increased.
The rest of the paper is organized as follows. Section 2 reviews the past work related to link quality routing, load-balancing routing, and the transmission power control on the physical layer. Section 3 describes our cross-layer routing protocol in detail. Section 4 presents the simulation result and analysis. Finally, Section 5 concludes the paper with a summary and proposes the future work.
2. Related Work
2.1. Transmission Power Control
Previous transmission power control schemes for ad hoc networks have focused on throughput improvement or power consumption. We do not consider the power consumption in our work becasue WMN backbone does not have power consumption issues. In , the author shows that reducing the transmission power can increase the carrying capacity of the network. The work in  concludes that variable-range transmission power can improve the overall network performance. Some researches address the power-controlled problem on the network layer [6, 7]. The COMPOW protocol  relies on the DSDV routing protocol to discover the smallest common power level at which the entire network is still connected. However, it suffers when some nodes can only use high power to be connected. In , the proposed CLUSTERPOW protocol performs the routing protocol several times with different power levels at each run and independently builds a routing table at each power level. A node consults the routing table with the lowest power to forward packets to the next hop. This consumes too much network resource.
Several researches [8, 14, 15] introduce the interference tolerance in their transmission power control architecture. The interference tolerance represents how much interference a node can allow the potential transmission of its neighbors. Nodes transmit the packets with the power level that does not disturb the ongoing receptions of its neighbors. In , the authors proposed a power controlled multiple access wireless MAC protocol (PCMA) within the collision avoidance framework. In PCMA, each receiver sends busy tone pulses to announce its interference tolerance. If the trasmitter has data to send to the receiver, it will determine its power bound according to the interference tolerance declared by the receiver's busy tone. It then sends Request-Power-To-Send (RPTS) with appropriate power level setting accordingly. The receiver will calculate its tolerable power level and send acceptable-power-to-send (APTS) back to the sender. In other words, the protocol design is based on CSMA/CA and modifies the RTS/CTS to RPTS/APTS (Request-Power-To-Send/Acceptable-Power-To-Send) to support potential interfering transmissions to transmit concurrently rather than to silence it. However, PCMA uses the additional separate control channel to send the busy tone pulses. PCDC (Power Controlled Dual Channel)  also uses dual channels for data and control packets. A single channel solution for transmission power control (POWMAC) is used in . In POWMAC, the interference tolerance is inserted into the CTS packet and an additional control packet DTS (Decide-To-Send) is used by transmitter to confirm the transmission. Furthermore, the DTS is utilized to inform the neighbors of transmitter about the power level that the transmitter will use for its data transmission. The neighbors of the transmitter can determine whether or not they can receive the data packets form other nodes simultaneously through DTS.
From the comparisons of [8, 14, 15], we finally adopt the interference tolerance concept to be integrated into our cross-layer routing protocol design. We will facilitate concurrent interference-tolerable transmissions in the same vicinity of the receiver to enhance the WMN backbone capacity. Furthermore, we piggyback the interference tolerance information in probe packets which are integrated in the network layer routing protocol. It is not using a separate control channel to alert the neighboring nodes.
2.2. Routing Metrics in WMNs
Most of the routing protocols use "hop count" as the routing metric. The minimum hop-count routing is not suitable for wireless networks because of dynamic wireless link quality characteristics. The work in [4, 13] proposes new routing metrics considering the link quality dynamics. MiRii proposed in  further considers the intra-/interflow interferences in multi-interface routing path selections.
The work in  proposed the concept of the expected transmission count (ETX) as the routing metric. ETX is calculated by measuring the delivery ratios for probe packets in bidirectional transmissions of each link. It predicts the number of data transmissions required to send a packet and get a successful acknowledgment. Therefore, the ETX accounts for interference among the successive links of a path. Although ETX does well in single-radio wireless ad hoc network, it does not perform well in multiradio and multichannel wireless mesh networks. Reference  presents a new routing metric for multiradio, multihop wireless mesh networks, called WCETT (Weighted Cumulative Expected Transmission Time). WCETT assigns weights to individual links based on the Expected Transmission Time (ETT) of a packet over the link. As a result, the WCETT of a route with hops can be the sum of the ETTs of all hops along the path. Further, WCETT assumes that the network has a total of channels in an -hop path. However, is the sum of transmission time of hops that uses channel along the path. The total path throughput will be dominated by the bottleneck channel, which has the highest . WCETT takes both link quality (ETT) and channel diversity ( ) into considerations. Thus, WCETT combines these two features by taking their weighted average as follows
In fact, the WCETT metric takes "intraflow" (means the same flow, but between different hops) interference into consideration, but does not capture "interflow" (means between different neighboring flows) interference. In our previous work , we propose a new routing metric, MiRii, that considers both intraflow and interflow interference in the multi-interface WMNs. In order to capture the interflow interference, we calculate the nodal activity and intertraffic flow interference. We introduce Little's Result into the routing metric design that makes the interflow interference unit cost compatible with WCETT. To this end, we assume node and node 's neighboring nodes as a closed system and the Activity Time (AT) of node is shown in (2). Also, and are the average queue length and average packet receiving rate of node , respectively. The sum of average queue length of 's neighboring nodes is the second parameter of numerator, and the sum of average packet receiving rate of 's neighboring nodes is the second parameter of denominator in (2). The Activity Time regarding to is the total average queue length divided by total average packet receiving rate of the system
Therefore, by combining both intraflow and interflow interferences, the MiRii routing cost is defined as (3)
where are the constant weights subject to . Also, means the total link quality considerations and end-to-end delay over an -hops path. The is the sum of transmission times of hops using channel , and represents the channel-diversity in multiradio WMNs. Finally, is the interflow interference, and represents the load-balanced routing cost.
3. Cross-Layer Routing Protocol Design
In the WMNs, the traffic loading changes dynamically due to leaving or entering the network of traffic flows. From the above observation, when the traffic loading is low, the traffic flows should select the higher transmission power to enhance the throughput and reduce delay. As the traffic loading is increased, the high transmission power imposes more interference that may disturb the ongoing transmission. The new traffic flow needs to choose the lower power level to transmit the data to alleviate interference. Further, the packet transmission at each hop on the routing path suffers propagation, handling, and queuing delays. When the traffic loading is low, the queuing day may be insignificant. It is better to use high transmission power to reduce hop count and also reduce the handling delay. However, under high traffic loading, the low transmission power reduces the queuing delay because the queue length will grow due to more neighbors' interference or collisions. Therefore, it is a good policy that we should adapt the appropriate transmission power level according to the surrounding interference constraints. This is one of the basic motivations of our work.
3.1. Overview of Protocol
3.2. iTolerance Calculation
means the transmit power for node and is the received power at node , where is the propagation gain for the direct transmission from node to . Also, is the thermal noise at node , and is the sum of interferences that transmit concurrently with node .
However, iTolerance is defined as the interference tolerance that a receiver node can tolerate a new joining neighboring interference without destroying its existing ongoing receptions. So, the radio interface at node , regarding to the flow from node , can allow its iTolerance as follows
In the protocol design, we measure the actual propagation gain based on the received power of the probe packet at the receiver side. Nodes can locally measure their interference tolerance according to the sum of the strengths of all the interfering signals. Each radio interface on the node will advertise its interference tolerance to its one hop neighbors.
In the network layer, each node updates its neighbors' iTolerance through the probe packet. When the route discovery starts, the node looks up its neighbors' iTolerance and chooses the appropriate TxPower (transmission power) level to send RREQ or forward RREQ messages. From the (6), the routing protocol will select the highest TxPower when the iTolerance constraints can be satisfied
We also apply for the MiRii routing metric which is referred in (3) in our . Hence, each traffic flow select the appropriate transmission power to send its route request messages and use the MiRii routing metric to choose a routing path that has the smallest MiRii cost. In , we use the probe packet to measure the link quality and piggyback the iTolerance information with the neighbors.
3.3. Perflow-Based Transmission Power Control and Routing
The original AODV is a destination-based routing protocol, and suffers the route flapping problem. For example, we assume that both flows 1 and 2 route through node to the same destination node . Some time later, the route entry of 's routing table to destination changes for some reason. It will affect both flow 1 and switch their routing paths simultaneously. The destination-based routing protocol cannot balance the traffic loading. In this case, perflow routing will be a good choice to solve the problem. In order to achieve the idea of perflow-based transmission power control and routing, the routing table should keep records for not only the destination (Dst) of the route but also flow id (Fid) of pertraffic flow. Further, the routing table records each traffic flows TxPower level that it uses to reach next hop. Hence, each interface on the node looks up the routing table according to the parameters (Dst, Fid) and adjusts the transmission power level for this traffic flow to forward to the next hop.
In reality, the channel fading and interference change dynamically. It is difficult to calculate exact tolerable power level or link quality. There are some papers [16, 17] that proposed different approaches to deal with these. However, in concern with complexity of the algortihm, we are not dealing with the fading channel problem here. Instead, in our design, we use the "moving average" estimation to find the path in the sense of "statistically approximation". Thus, it may combat the slow fading but not fast fading channels. Our proposed can find the path with considering both existing and tolerable adding intereference. In this way, the throughput is increased with reducing the failed transmissions due to suffering too much intereference. Therefore, energy consumption from the system view is reduced, and efficiency is increased.
4. Simulation Results and Analysis
In this section, we evaluate the throughput and delay for using NS-2 and contrast it with the flow-based MiRii and AODV routing protocol. The radio propagation model adopts Two-Ray Ground model in NS-2. Each node is equipped with two NICs. The off-the-shelf Cisco Aironet 350 series client adapters or access points allow different transmit power setting for one of 1, 5, 20, 30, 50, and 100 mW. We adopt the 30 mW and 100 mW in our NS-2 simulation. The SINR threshold is setting to 6.02 dB and the noise floor at each node is −120 dBm. The traffic flow type is CBR (Constant Bit Rate) during the ON period of an equally ON-OFF model and the packet sizes are 1000 Bytes.
Delay-Throughput with different power levels.
Number of flows
MiRii-30 mW delay (ms)
MiRii-30 mW throughput (Kbps)
MiRii-100 mW delay (ms)
MiRii-100 mW throughput (Kbps)
5. Conclusion and Future Work
In the paper, we proposed routing protocol for multi-interface WMNs. The main purpose is to coordinate the physical layer and the network layer for a cross-layer routing protocol development. Previous researches show that variable transmit power level control can improve network performance but they still use minimum hop counts as the routing metric. We introduce the iTolerance to constrain the transmit power level and incorporate it to the route discovery. The MiRii routing metric is utilized to evaluate the routing path with consideration of both intraflow and interflow interferences. Furthermore, the power control is designed on perflow, perhop basis. We thoroughly observe the performance of at different traffic loadings. When the traffic loading is high, the newly traffic flow chooses the appropriate transmission power level along less interference path to transmit the data packets in order not to create intolerable interference to the existing transmissions. Through the simulation results, we have demonstrated that our routing protocol can enhance both network throughput and end-to-end delay.
In the current version of routing protocol, the traffic flow selects the lowest power level even if it would violate the interference tolerance constraint. In the future, we may incorporate with the traffic flow admission control and extends the to more stability and even better performance.
This work is granted by NSC-97-2221-E-004-004-MY2.
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