Small Group Relay Protocol using TDMA Contention Slot

Transcription

1 Small Group Relay Protocol using TDMA Contention Slot Byoungchul Ahn 1, Sang-Ho Hwang 1, Chang-Hyeon Park 1 and Sang-Ho Moon 2, 1 Dept. of Computer Engineering, Yeungnam University, Gyungsan, Korea 2 Div. of Computer Engineering, Yeungnam College of Science & Technology, Daegu, Republic of Korea Abstract - WiFi and Bluetooth have become attractive wireless communications nowadays. But application area of Blutooth wireless connections are growing slowly. In the field of voice communications for small group, it is very difficult to apply the Bluetooth technology to the Ad-hoc communication although it has Scatternet feature. This paper presents a relay protocol to communicate among several users with Ad-hoc capability. The proposed protocol can relay data or voice to other nodes by multi-hop retransmission using TDMA wireless networks. This TDMA based approach is very efficient for the personal area networks and it easy to implement its function on FPGA. The maximum number of nodes is sixteen nodes for 64Kbps voice communications. The simulation results show that node connection set-up time is very long when the number of nodes is more than 10 nodes. The proposed network delivers very reliable voice packets since the packet loss is 7% when nodes move at the speed of 6m/sec. Keywords: WiFi, Bluetooth, TDMA, small group relay, Adhoc 1 Introduction Wireless Ad-hoc networks have become in steep research topics and significant advancements in recent times due to their important advantages. One of advantages is easy to set-up a network fast in environments where there is no existing network. For this reason, it can be utilized in emergency rescue operations, military operations, and etc. There are, however, several technical challenges in ad-hoc networks. First, ad-hoc networks are characterized by high bit error rates and path breaks due to changing network topology. If the topology change does not occur frequently, people can tolerate packet losses up to 5% according to topology changes[1]. Second is transmission delay or processing delay. In real time transmission such as voice application, there are delay problems. This delay is the transmission delay and processing delay. Processing delay is caused by a codec or any other system processing, and the transmission delay is caused by delivery. The transmission delay is related to the number of hop in the Ad-hoc network. In general, the maximum end-to-end delay bound of voice packet is assumed to 285ms by ITU-T[2]. Thirdly, ad-hoc network have small data payload. The wireless network frame must include not only network information, but also a preamble for synchronization. Therefore, the efficiency of such networks in poor for small data payloads[3]. Considering three major challenges, this paper proposes a protocol to relay data or voice up to 16 nodes to apply for small group voice communications. The routing method is described to manage the network by joining new nodes or disconnecting nodes. 2 Related works Studies for real-time speech on wireless Ad-hoc are conducted by Jarhl, Kwong and Venkat [4][5][6]. Frank Kargl et al. discuss voice transmission over Bluetooth[4]. They present a new routing protocol called Bluetooth Scatternet Routing(BSR). But they discuss the possibility, and have not implemented. Kwong et al. use multi-path routing protocol called MSDR in order to speech quality[5]. But the processing overheads are not solved. G. Venkat Raju et al. have proposed a Localized Distributed heuristic for Minimum number of Transmissions(LDMT)[6]. In order to reducing transmission delay, this algorithm minimizes voice retransmission. Several researchers have studied the problem of capacity reduction in multi-hop wireless networks[11][12]. They observe that the performance degrades quickly as the number of hops increases due to using a single radio for transmitting and receiving packets. A good way to improve the capacity of wireless is to use more network interfaces or to use speech compression in the case of voice applications. Some researches use only one network interface due to cost. Another way to improve the capacity of wireless is to use schedule transmission slots in time[13][14][15] and to use multiple non-interrupting frequency channels[16][17][18]. This paper presents a method to communicate whole group by voice. The number of nodes is limited to 16 nodes, which is used for a small group. To relay the voice to other nodes, non-interrupting frequency hopping method is used to avoid collisions.

2 3 Protocol model 3.1 Routing Protocol There are a lot of routing protocols in Ad-hoc network. However, most of these routing protocols are not appropriate to communicate voice data because they are designed to communicate data exchanges. A simple routing protocol is proposed for voice communication for a small group. The protocol does not support unicast transmission. Most packets are used to broadcast or multicast transmission Joining A node to connect to an already established network must check and try to synchronize the network by receiving a start cycle frame, which is transmitted from master node, and control frames, which are transmitted from slaves. If the received control frame is more than one, a node with the smallest hop count is chosen by the parent node. And to connect to the network, a new node sends Join_Request_Message to the parent node through the competition slot. If its parent node received the packet is not master, the node retransmits the packet to master through the control frame. If a master node is received Join_Request_Message, it assigns an empty slot number for the new node, and transmits it to the new node. If all slots are used, the master sends Join_Listen_Only_Message Path Set-up The control frame has 96-bit routing information. It contains the sequence number of parent node and descendent nodes in the network. If the parent node of a node receives the control frame, it stores the slot number of its descendent node in the routing table. Figure 1. Example of a routing table and Node-8 are transmitted through Node-3. Node-3 will ignore all routing information of the control frame coming from Node-3 s parent node, but the path of Node-3 s parent node is stored at Node-3. The information of the parent node in routing table is used to determine the relay path to the master and to check if its parent is alive or dead. Other information in the routing table is used to find the direction toward its descendent nodes Path Management The routing table managed by each node is consisted of Node_Number, Next_Node(link direction), TTL(time to live) and Sequence Number(use to prevent being updated by old routing information). When a node receives the control frame, TTL in routing table is renewed in order to check if. When a node does not receive any control frame, the TTL is reduced. If TTL becomes zero, the path is canceled. Due to frequent topology changes, there could be a possibility to receive other routing information. In this case, the routing table must be updated with new routing information which sequence number is bigger than old routing information. For synchronize with the master node, all nodes should update their hop count using the control frame of their parent node in every cycle. The master node s hop count is 1 and the hop count of slaves is incremented by one when they join the network. When received parent node s control frame with zero hop(level), the node retransmits zero hop count to its descendant nodes and deletes the route to parent node. Nodes which want to join in the network should not choose the node transmitting control frame with zero hop count for parent node. 3.2 Frame structure The Start of Cycle slot is transmitted by the master node. If neighbor nodes of the master receive the Start of Cycle frame, they send Join Request Message to the master to join the network during contention period. When the master receives it, the master assigns an empty slot number to participate in the network to that node. Each node which allocated its slot number always broadcasts its control frame in order to notice its connection to its neighbor nodes in the network. All new nodes which want to join to network must transmit Join_Reqeust Message to the master using contention period after Start of Cycle. Payload slots after the control frame are used to data transmission and they are divided into 16 small slots. For example, if Node-3 at Figure 1 sends its parent the routing information that Node-5 and Node-8 are descendent nodes, the Node-3 s parent node receives that packet and updates the Next Node in the routing table as number 2, which means Node-5 and Node-8 are connected to Node-3. Also it means that all packets that sent to Node-3, Node Contention Peiod When a new node wants to join the network, it must use contention period. It should send Join_Reqeust Message to the master using this period. If the master receives that message directly, it transmits an empty slot number through its own control frame. If other nodes except the master in the

3 network receive this message, they must relay the message to the master using their own control frame. If there are collisions during the contention period because all nodes may use this period freely, they should send messages after random back-off time in order to reduce wireless collisions Start of cycle frame and Control frame A new node assigned slot number by the master transmits its own synchronous signal named the control frame. All nodes in the network can receive this frame by the dedicated frequencies. Through this signal, new nodes recognize the established network can connect the network. This control frame includes routing information to create a routing table. Therefore, a new node to connect the network can create its own routing table. This frame also contains control command sent by nodes or the master node. Control commands are sent by broadcasting. To prevent loop, control command has sequence ID. At Figure 3, if node-8 transmits packets, depending on the routing table, node-3 receives packets using the frequency of node-8 and node-1 receives packets using the frequency of node-3, and other nodes receive packets using the frequency of parent node. If a node does not have a path in the routing table, it has to listen to the network using the frequency of the parent node Data frame Voice packets are transmitted using data slots. Data slots are consists of 16 small slots, and small slots are divided into two sections with eight channels. Nodes transmit and/or receive packets as selected order according to their hop count shown at Figure 2. If the hop count is even, it is receivetransmission mode. If the hop count is odd, it is transmissionreceive mode. 4 Simulation Figure 3. Path selection example The proposed protocol is simulated NS-2 network simulator and verified its operation [19]. Table 1 is parameters to simulate the protocol. The total nodes are 16 and mobility is human walking speed. The data bandwidth is 1Mbps and it is enough to sample 16Kbps and transmit voice data. The voice packets are generated by 20msec. Table1. Simulation parameters Field size 500m x 500m Number of nodes 16 a. Even numbered nodes transmit messages b. Odd numbered nodes transmit messages Figure 2. Packet Relay Method Mobility Bandwidth Application Transmitted period Packet size RF Range 0 10 m/s 1Mbits/s CBR 20 ms 150 bits 50m Each node has own FHT(Frequency Hopping Table). All nodes should be aware of the FHT of the other nodes. By default, all nodes listen on the frequency of its parent node. If a node is assigned a channel and transmit data, each node sets the dedicated frequency to receive packets according own routing table. When each node receives messages, it does not send ACK to the sender. Therefore a node resends Join Request again to the Master if it does not receive Join OK from the Master node in a given time. If several nodes send Join Request at the same time, the setup time is increased by collision.

4 Figure 4. Average Latency time Figure 4 shows the latency time to join the network. Longer retransmission time makes long latency time to join the network. If the network topology is shown in Figure 5 using sixteen nodes, the connection time is shown in Figure 6. The voice transmission delay of a given multi-hop connection is related to the number of hops. As the number of hops is increased, the delay is increased linearly. Figure 5. Network topology for the Join latency time estimation Figure 7. Packet loss rate for mobility of nodes 5 Conclusions A new relay protocol is presented to use a small group voice communication. The simulation results show that node connection set-up time is very long when the number of nodes is more than 10 nodes. But the set-up time can be reduced by adjusting retransmission waiting time. Data loss rate is increased as the mobility speed is increased. When nodes move at speed of 6m/sec, the packet loss rate is 7 percent. This means that the proposed network delivers stable voice transmission for human sports activities. In the future, more experiments are required for fast topology changes. The proposed protocol is planned to implement on FPGA and verify its functions late Please address any questions related to this paper to Prof. Ahn by (b.ahn@yu.ac.kr). 6 References Figure 6. Connection time for Figure 5 Figure 7 shows the packet loss rate according to node mobility. To measure the packet loss, 100 netwotk topologies are generated by randomly, 1000 messages are transmitted at each topology and loss rates are measured for each transmission. Figure 7 shows average loss rates. [1] N. Jayand and S. W. Christensen, Effects of Packet Losses in Waveform Coded Speech and Improvements Due to an Odd-Even Sample-Interpolation Procedure, IEEE Transactions on Communications, vol 29, no. 2, pp , February [2] ITU-T Recommendation G.114, One-way transmission time, May [3] Chi-hsien Lin, Hui Dong, Upamanyu Madhow, and Allen Gersho, Supporting Real-time Speech on Wireless Adhoc Networks: Inter-packet Redundancy, Path Diversity, and Multiple Description Coding in Proceedings of ACM workshop on WMASH pp , October 2004.

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