Patent Application
20120147864
This invention relates generally to communication networks for transportation safety systems, and more particularly to synchronous wireless data transmission in hybrid communication systems.
Data communications in transportation safety systems require very high reliability and very low latency. For example, the International Electronic Commission (IEC) has set stringent safety and reliability requirements on communication networks in elevator systems. Only one error is allowed in approximately 1015 safety related data packets. The latency requirement for high priority data packets can be as low as a few milliseconds.
Conventional safety systems are typically implemented with a dedicated wired communication networks. For example, to send safety data packets between a controller and a car in an elevator system, a heavy communication cable in an elevator shaft is connected to a moveable car.
Recently, wireless communication technologies have been applied to safety systems to reduce cost and increase scalability. Communication Based Train Control (CBTC) is an example. The communication network in safety systems usually includes multiple fixed nodes such as trackside nodes for CBTC systems, and multiple mobile nodes arranged in train cars. The fixed nodes are connected by a wired network such as Ethernet. Fixed nodes are also capable of transmitting and receiving (transceiving) data wirelessly. A controller for the safety system is typically connected to at least one fixed node via a wired interface. Data packets are transmitted from the controller to a fixed node via the wired interface, and relayed hop-by-hop to all other fixed nodes via the wired network. Then, the fixed nodes retransmit the data packet to the mobile nodes using the wireless network. Mobile nodes communicate data packets via the wireless network to the fixed nodes. Fixed nodes receive the data, and then relay the data to the fixed node connected to the controller via the wired network.
However, the specifications of existing CBTC systems are insufficient in some aspects. The latency is in the order of seconds due to the use of a conventional Carrier Sense Multiple Access (CSMA) for the wireless network, and the handover process at mobile nodes. Additionally, message error rates can be as high as 10−8.
Therefore, it is desired to develop a communication network for safety systems that achieves higher reliability, such as a message error rate of 10−15, and a latency of a few milliseconds.
Embodiments of the invention provide a method for synchronous transmission in a multihop hybrid communication networks to enable high reliability and low latency for transportation safety systems.
As shown in
All fixed nodes are arranged along trajectory 120 such as an elevator car moving in a shaft, or a car moving on a train track. The FNs are arranged linearly, although not necessarily a straight line. All FNs are connected via the wired backbone, such as fiber optic cable. MNs generally move along the trajectory. The underlying physical layer protocol used on the backbone is arbitrary.
Sources and sinks of data in the network include a controller 131, such as elevator controller or train controller, and an elevator or train car 132. The safety related data are transmitted as packets.
The controller is connected to a FN via a wired interface 130, not necessarily the same as the wired backbone. In the preferred embodiment, it is assumed that the controller is connected to the FN at a first end of the linearly arranged network, say FN0 as shown in
The FNs can be classified into three types of nodes. The FN that is connected to the controller 131 is called a head node. The head node is a source and sink for safety related data packets in the network. In
The FN that is located at the second end of the network is called a terminal node. In
All remaining FNs form a set of (one or more) relay nodes that pass packets to adjacent FNs. The FNs also communicates with the MNs wirelessly. Packets generated 135 in the controller and transmitted from the head node to MNs in the cars are called downlink packets. Packets generated by cars and transmitted from the MNs to the head node and the controller are called uplink packets.
The hybrid network uses Sync packets 200, time packets 300, and data packets 500. A synchronization packet (Sync) 200, see
Data packets wirelessly transmitted (broadcasted) by any mobile node are received essentially at the same time by all the fixed nodes within range of the mobile node, hence synchronization is not an issue for upward bound data packets.
In the prior art, data packets are usually transmitted asynchronously, this increases interference and latency. To minimize interference and latency, and also increase reliability, all the FNs transmit the downlink packets to the MNs synchronously via the wireless network.
Conventional CSMA and handover techniques cannot accomplish this task due to collisions and unpredictable channel access delay because of random back-off. The invention mitigates these problems. However, it cannot be guaranteed that the clocks used by the fixed nodes are synchronized with each other. Hence, the embodiments of the invention include a process and protocol to synchronously transmit data packet even if the clocks are unsynchronized.
Synchronous Wireless Transmission
The synchronous transmission of data packets 500 is achieved as follows. A data packet 500 from the controller 131 is first transmitted from the head node to the FN1 via the wired backbone. Then, a relay process over wired backbone begins. The FN1 relays the data packet to FN2, FN2 relays the packet to FN3, . . . , and FNm-1 relays the packet to FNm. All the FNs eventually receive the data packet at instants staggered in time. Then, all the FNs synchronously transmit the data packet to the all MNs via the wireless network.
To do so, each FN determines a time latency from the time the FN receives a data packet from the backbone to the time the FN transmits the packet over the wireless network, so that all fixed nodes synchronously transmit the data packet over the wireless network, even when they receive data packets asynchronously from the wired backbone.
The embodiments include a quick and a precise synchronization scheme.
The Direction_Bit indicates that the Sync packet is transmitted downwards in the direction from the head node to the terminal node, or upwards in the direction from the terminal node to the head node. To start, the head node FN0 sets the Direction_Bit to downwards in the Sync packet transmitted to the FN1. The terminal node FNm sets Direction_Bit to upwards in the Sync packet transmitted to FNm-1. Other FNs do not change Direction_Bit field.
TX_RX_Diff 212 and Wait_Time 213 are only used when the Sync packet is transmitted upwards. TX_RX_Diff 212 is the time difference between when the FN receives the downward Sync packet to the time the same node transmits the Sync packet upwards.
The Wait_Time 213 indicates the time the FN has to wait receiving the downlink packet before transmitting the packet over the wireless network. TX_RX_Diff 212 and Wait_Time 213 are set to zero in the downward Sync packet.
The Pad_Bits 214 field is set to zero. Pad_Bits 214 is used to pad Sync packet payload to a predetermined maximum payload (data) length 245. This guarantees a downlink data packet of any length can be synchronously transmitted over the wireless network by all FNs. That is, the padding bits that ensure that the length of the synchronization packet is greater than or equal to a longest data packet.
The time needed to transmit the packet down from the FN0 to FNm via wired backbone and the waiting time at each FN before the node synchronously transmits the packet wirelessly is determined as follows.
In
The upward Sync packet transmission starts from the terminal node FNm. The terminal node determines the amount of time needed to convert packet received via the wired backbone at time Rm1 into a transmission over the wireless network. The time difference Wm is the waiting time for the FNm node. In the upward Sync packet, the FNm sets the Direction_bit to upwards, TX_RX_Diff to Tm2−Rm1 and Wait_Time to Wm and transmits the Sync packet to FNm-1. After FNm-1 receives the Sync packet from FNm, FNm-1 determines the latency D(m-1)m from FNm-1 to FNm as
and its waiting time Wm-1 as
W
m-1
=D
(m-1)m
+W
m
In general, after FNk (k=0, 1, 2, . . . , m−1) receives the upward Sync packet from FNk+i, FNk determines the latency Dk(k+1) from FNk to FNk+1 as
and the waiting time Wk as
W
k
=D
k(k+1)
+W
k+1.
T(k+1)2−R(k+1)1 is received in the TX_RX_Diff field 212 in the upward Sync packet, and Wk+1 is received in the Wait_Time field 213 in the upward Sync packet.
For the head node FN0, R01 is set so that T01−R01 is the time needed by the head node to receive the packet from the controller to the time the node relays the Sync packet via the backbone.
The waiting time Wk (k=0, 1, 2, . . . , m) is
Total latency Dtotal from head node FN0 to terminal node FNm is
The above equations use “time-of-flight” to determine the delay for packets between two adjacent fixed nodes, as shown in
Noticed that time Tk2 is pre-determined because when FNk (k=m, m−1, . . . , 1) transmits the Sync packet to FNk−1, FNk needs to include time difference Tk2−Rk1 into Sync packet payload in advance.
Frame Structure Over Wireless Network
As shown in
Each frame of the wireless network is partitioned into a downlink data interval (DDI) and uplink data interval (UDI). That is, frames and associated uplink, downlink, and synchronization periods define the use of the wireless network between MNs and FNs. Communication between FNs can have a different framing as determined by the wired network.
The DDI and UDI are further partitioned into a high priority period (HPP) and a low priority period (LPP). The HPP is used to transmit high priority packets. The LPP is used to transmit low priority packets. Offsets of DDI and UDI are fixed.
Data Transmission
For downlink transmission, the data packets are transmitted from the head node, FN0, and relayed to all FNs via wired backbone. When FNk (k=0, 1, 2, . . . , m−1) receives a downlink packet from FNk−1, the node immediate relays the packet to FNk+1 via wired backbone, and duplicates the packet and places the packet into outgoing queue for the wireless network. The packet remains in the outgoing queue for Wk amount of time, and then the packet is synchronously transmitted to the MNs wirelessly in the DDI of the wireless frame structure defined in the embodiment.
For uplink transmission, the MNs transmit packets wirelessly. All FNs that receive and successfully decode the packets wirelessly relay the packets to the head node FN0 via wired backbone.
Data Retransmission
To avoid latency due to feedback, no packet acknowledgement is used. Rather, to increase reliability, each packet is transmitted multiple times over different frames as long as there is enough bandwidth, and a latency requirement is satisfied.
Alternatively, after a packet error, the sink indicates a retransmission request in the next outgoing data packet to the source. The source retransmits the failed packet as long as there is enough bandwidth and latency requirement is satisfied. The failed packet can be retransmitted separately or as part of a new data packet from the source.
Although the invention has been described with reference to certain preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the append claims to cover all such variations and modifications as come within the true spirit and scope of the invention.
