Physical layer collision avoidance (PLCA) coordinator redundancy

Abstract

To provide methods and network interfaces to equip network nodes with PLCA (Physical Layer Collision Avoidance) coordinator redundancy.SOLUTION: A physical layer (PHY) network interface 110, 111 includes: a physical medium dependent (PMD) sublayer 112 that is a transceiver circuit configured to send and receive network data packets via a communications medium 102; and a PLCA controller 118 configured to provide redundancy by causing the transceiver circuit to transmit a beacon signal upon detecting a transmit opportunity for a node ID that exceeds a total node count by an amount equal to a local node ID even when the local node ID is greater than zero.SELECTED DRAWING: Figure 1B

Description

[Technical Field]

[0001] The present disclosure relates to access arbitration in multi-drop networks, and more particularly to a network interface and method that provides transparent and backward-compatible redundancy for PLCA coordinators. [Background technology]

[0002] Carrier Sense Multiple Access with Collision Detection (CSMA / CD) is a technique that various networking standards specify for network nodes to use when accessing a shared communications medium, and is a fallback option provided in the recent IEEE standard IEEE 802.3cg for 10 Mbit / s Ethernet over a single twisted wire pair (10BASE-T1S), which shows great promise for use in automotive and industrial networking environments. CSMA / CD techniques undesirably suffer from the well-known channel capture effect, where a shared medium can be dominated by a single node, potentially causing bandwidth starvation for other nodes.

[0003] More importantly, however, in "noisy" environments such as automotive and industrial networks, the channel's low signal-to-noise ratio (SNR) makes it difficult for nodes to distinguish packet collisions from the ambient channel noise. This difficulty can cause nodes to prematurely terminate transmissions that could have been safely received in the absence of actual collisions. The authors have observed situations in which functional 10TBAS-T1S networks cease to operate when CSMA / CD fallback occurs in low-SNR channels. It is desirable to make such networks more robust against such impairments. Summary of the Invention

[0004] Accordingly, disclosed herein are methods and network interfaces for providing PLCA (Physical Layer Collision Avoidance) coordinator redundancy for a network node. One exemplary network interface includes a transceiver configured to transmit and receive network data packets over a communication medium, and a PLCA controller configured to provide redundancy by causing the transceiver to transmit a beacon signal upon detecting a transmission opportunity for a node ID that exceeds the total node count by an amount equal to the local node ID, even if the local node ID is greater than 0.

[0005] An exemplary network interface method includes transmitting and receiving network data packets over a communication medium, and providing PLCA coordinator redundancy by transmitting a beacon signal upon detecting a transmission opportunity for a node ID that exceeds a total node count by an amount equal to the local node ID, even if the local node ID is greater than 0.

[0006] An exemplary network node includes a network interface coupled to a host device, the network interface configured to transmit and receive network data packets over a communication medium, detect a missed beacon condition, and in response, provide PLCA coordinator redundancy by transmitting a replacement beacon signal before a CSMA / CD fallback condition occurs.

[0007] Each of the foregoing network interfaces and methods can be used in any suitable combination with one or more of the following optional features: 1. The PLCA controller is configured to determine a node count by timing beacon signals transmitted by a remote node. 2. The PLCA controller is configured to use a default value for the total node count unless a programmed value is provided with the local node ID assignment. 3. The PLCA controller is configured to detect transmission opportunities by counting symbol intervals after each beacon signal with a pause in any packet transmitted by the remote node. 4. The communication medium is a single wire pair. 5. The PCS (Physical Coding Sublayer) module, PMA (Physical Medium Attachment) module, and Physical Medium Dependent (PMD) sublayer module are collectively configured to implement the IEEE 802.3cg 10BASE-T1S standard protocol. 6. The network interface detects a missed beacon condition by detecting transmission opportunities for node IDs that exceed the total node count by at least one. [Brief explanation of the drawings]

[0008] [Figure 1A] FIG. 2 is a block diagram of an exemplary network interface. [Figure 1B] FIG. 1 is a block diagram of an exemplary network node. [Figure 2A] 1 is a timeline of two exemplary PLCA cycles. [Figure 2B] 1 is a timeline of an exemplary PLCA cycle leading to CSMA / CD fallback. [Figure 2C] 1 is a PLCA timeline for an 8-node network segment. [Figure 2D] 1 is a PLCA timeline of a network segment with a failed PLCA coordinator. [Figure 2E]1 is a PLCA timeline for a network segment with a failed PLCA coordinator and a failed backup coordinator. [Figure 3] FIG. 1 is a state machine diagram of an exemplary PLCA controller that provides redundancy. DETAILED DESCRIPTION OF THE INVENTION

[0009] It should be understood that the following description and the accompanying drawings are provided for purposes of explanation, not limitation, of the present disclosure, and in other words, provide a basis for those skilled in the art to recognize and understand all modifications, equivalents, and alternatives falling within the scope of the claims.

[0010] FIG. 1A illustrates an exemplary network having multiple network nodes (Node A through Node D) interconnected by a communication medium 102, such as an electrical bus or cable, optical fiber, or radio frequency signaling. Examples of network nodes include computers, network routers, network switches, network bridges, network storage, network-attached sensors, controllers, and IoT devices. These network nodes include one or more network interfaces coupled to a host device, such as a processor, microcontroller, or application-specific integrated circuit. The network nodes may use a physical layer interface and communication protocol that conforms to one of the IEEE 802 (network) standards, such as IEEE 802.3 (Ethernet), 802.11 (wireless local area network), or 802.15 (wireless personal area network). While the IEEE 802.3cg standard is used herein to illustrate network interfaces and methods that provide PLCA redundancy, the disclosure provided herein is readily applicable to other communication media, network interfaces, and communication protocols that employ beacon-synchronized access to the communication medium 102.

[0011] The illustrated communication medium 102 can carry network packets from one of the nodes to the remaining nodes. In certain preferred implementations, the communication medium 102 supports only one sender at a time. Multiple transmitters attempting to operate simultaneously may prevent reception of any packets or cause network collisions that are resolved according to the standards of the relevant network protocol. Each network packet generally includes a data frame having a preamble, a header, a payload, a checksum, and an inter-packet gap. The header may include a starting delimiter, a destination field with the MAC address of the packet's desired destination, a source field with the MAC address of the packet's source node, and other fields for information such as an optional tag, packet type, and payload length. In some implementations, the header further includes a field specifying the message type. When the destination field specifies the MAC address of a given node, the packet is a unicast message. For broadcast messages, network protocols may provide a reserved MAC address to be used in the destination field to indicate that the packet is intended for all nodes on the communication medium 102.

[0012] 1B provides additional details about exemplary network nodes that may employ communication medium 102. A first node includes a physical layer (PHY) network interface 110 that couples a first host device 120 to communication medium 102. A second node includes a similar PHY network interface 111 that couples a second host device 121 to communication medium 102. The first and second host devices are shown here as central processing unit 120 and sensor 121, respectively. PHY network interfaces 110, 111 may be integrated circuits packaged as dedicated chips or may be design blocks embedded in a chip or multi-chip module that includes the circuitry of the host devices.

[0013] The IEEE 802.3cg standard adopts a modified form of the ISO / IEC Model for Open Systems Interconnection (see ISO / IEC 7498-1:1994.1) architecture to specify operational details of hardware and software components used to implement networking protocols. Figure 1B shows relevant portions of the architecture as blocks within exemplary network node PHY network interfaces 110, 111 and host devices 120, 121. The illustrated blocks include a physical medium dependent (PMD) sublayer 112, a physical medium attachment (PMA) sublayer 114, a physical coordination sublayer (PCS) 116, a physical layer collision avoidance (PLCA) controller 118, a coordination sublayer 122, and a medium access control (MAC) sublayer 124. Figure 1B also shows a microcontroller block 125 as part of host device 120, and an operating system (OS) driver 126 and application software 128 as part of host device 121.

[0014] The PMD sublayer 112 represents the transceiver circuitry and connector contacts that convert incoming channel signals into receive bit streams (or receive streams of digital symbols, if larger signal constellations are supported) and convert transmit bit streams (or transmit streams of digital symbols) into outgoing channel signals that can traverse the communications medium 102. The PMA sublayer 114 represents circuitry that provides packet framing and channel symbol encoding for transmit streams, and symbol / word synchronization, channel symbol decoding, and payload extraction for receive streams. The PCS module 116 represents circuitry that applies scrambling masks and provides channel coding (to enable clock recovery and error detection), symbol redistribution, and PCS alignment marker insertion for transmit streams. For receive streams, the PCS circuitry removes the PCS alignment markers and reverses the symbol redistribution, channel coding, and scrambling mask operations. To enable bit error rate estimation by the PHY network interface 110, 111, the PCS alignment marker may include a bit interleaved parity (BIP) value derived from the preceding bits in the stream up to and including the preceding PCS alignment marker 1.

[0015] The PLCA controller 118 circuitry controls packet transmission timing to implement coordinated access of the communication medium 102, as described further below. In at least some implementations, the PLCA controller 118 invokes the CSMA / CD functions of the MAC sublayer 124 (i.e., CRS carrier sense, COL collision detect) to control the host device's transmission timing. The PLCA controller 118 also monitors the receive stream for beacon signals that provide synchronization for coordinated access and / or periodically generates such beacon signals for the transmit stream.

[0016] The MAC sublayer 124 block represents circuitry that provides flow control and multiplexing for transmit streams, encapsulating transmit data streams into packets with a preamble, start delimiter, address information, padding (if necessary), frame checksums, and space for interframe gaps. For receive streams, the circuitry uses checksums to detect and reject corrupted frames. Data payloads are extracted from valid frames, and the multiplexed receive stream is demultiplexed. The MAC sublayer functions to isolate higher-level software and firmware from the complexities of controlling the communication medium 102 and associated PHY network interface, allowing the host device to access the network interface in a media-independent manner. Thus, the microcontroller 125 can use the MAC sublayer 124 to send data (e.g., acquired sensor data) and receive data (e.g., configuration data and command or actuator control information) over the communication medium. The OS driver 126 represents software functions that can similarly be invoked by application software 128 to send and receive data over the communication medium.

[0017] The PLCA coordination process can be best understood with reference to a timeline such as that shown in FIG. 2A. The timeline is for transmissions on a communication medium 102 shared by N+1 nodes, each with a node ID ranging from 0 to N (inclusive). Node IDs may be assigned when the bus is initially configured or may be dynamically assigned as needed. Each node ID is unique. By default, the PLCA controller 118 in the node with node ID 0 is the "PLCA coordinator" responsible for maintaining coordination among the nodes sharing the communication medium 102, a responsibility it assumes by periodically transmitting beacon signals 201. Each beacon signal initiates a PLCA cycle, which provides a sequence of N+1 transmission opportunities. There is one transmission opportunity for each node, beginning with transmission opportunity 210 for node 0 and ending with transmission opportunity 217 for node N. Transmission opportunities have a predetermined duration. The default is 32 bit or 8 symbol intervals, but the transmission opportunity duration is a configuration parameter that can be adjusted to other values. In PLCA cycle 202, no node wins its assigned transmit opportunity, and the cycle completes after transmit opportunity 217. Node 0 then transmits another beacon signal 201 to begin the next cycle 204.

[0018] In the example of FIG. 2A, two nodes earn their transmission opportunities in cycle 204. The first transmit opportunity 210 is assigned to node 0, which does not earn it. The next transmit opportunity is assigned to node 1, which earns it by transmitting network packet 221. Earning the transmit opportunity delays subsequent transmit opportunities. Network packet 221 ends with an end-of-sequence delimiter, after which a transmit opportunity is provided to node 2, which does not earn the transmit opportunity. The next transmit opportunity is assigned to node 3, which earns it by transmitting commit signal 233 and network packet 223. The subsequent transmit opportunity is delayed until packet 223 ends with an end-of-sequence delimiter. Cycle 204 is not complete until each node has been given a transmit opportunity. When transmit opportunity 217 is provided to the last node N, the PLCA controller of node 0 begins the next cycle by transmitting beacon signal 201.

[0019] The examples in Figures 2B-2E assume that the total number of nodes is eight and that no node receives a transmission opportunity during the cycle shown. These assumptions are made to conserve space and clarify the illustrations, but are merely for convenience. It should be understood that the number of nodes may be greater or less, and transmission opportunities are provided to allow nodes to use the communication medium 102 when they have network packets to transmit. The number of network packets a given node can transmit in a given cycle is limited to prevent one node from dominating the communication medium. Further details regarding the PLCA coordination process are available in the IEEE 802.3cg standard, the current version of which is incorporated herein by reference.

[0020] Node failure is a frequently encountered problem in networks, especially in industrial or automotive environments where power signals are noisy and power transients are common. Power fluctuations, operational disturbances, or equipment defects can cause nodes to lock up, reboot, or otherwise cease normal operation for at least a short time. If node 0 experiences such a failure, the standard provides an option for CSMA / CD fallback. Figure 2B shows a timeline that may be associated with the fallback process. Node 0 transmits a beacon signal 201 to begin a PLCA cycle 202, but then experiences node failure, preventing it from starting the next cycle. Due to the absence of a missed beacon signal 205 after the last node transmission opportunity 217, the remaining nodes continue to count transmission opportunities 218-219 from nonexistent node ID 8 to the maximum node ID value 255. A 13-millisecond timeout period 240 precedes the fallback to CSMA / CD operation 242, where node access is uncoordinated and subject to collisions. In industrial or automotive settings or other environments where the communication medium has a relatively low signal-to-noise ratio, nodes may not be able to reliably detect collision events, hindering network operation when PLCA coordination is lost.

[0021] To provide PLCA coordinator redundancy, one or more of the nodes (e.g., node 1, node 2) may be configured with a PLCA controller that provides a replacement beacon signal if node 0 fails. During normal operation, node 0 provides beacon signal 201 as shown in Figure 2C. Nodes 1 and 2 are ready but do not need to provide a beacon signal while node 0 is functioning normally.

[0022] 2D shows a timeline that begins with a beacon signal 201 from node 0 but includes a transmission opportunity for node 8, which is absent due to a missing beacon signal 205 from node 0. Node 1, configured for redundancy, transmits a replacement beacon signal 251 after the first transmission opportunity for the absent node, beginning a subsequent cycle 206. When the subsequent cycle again includes a transmission opportunity for a node that is absent due to a missing beacon signal 205, node 1 again provides the replacement beacon signal 251. In this way, the network continues to operate in a coordinated PLCA manner, even if node 0 fails.

[0023] This principle can be extended to enable PLCA operation even in the presence of multiple node failures. Figure 2E shows a timeline beginning with a beacon signal 201 from node 0, but including two transmission opportunities for absent nodes, node 8 and node 9. Because there is a missing beacon 205 from node 0 and no replacement beacon 255 from node 1, node 2 provides a replacement beacon signal 252 after the second transmission opportunity for the absent node.

[0024] A general rule may be expressed in terms of total node number N+1 and local node ID L: transmit a beacon signal when the current transmit opportunity is for node ID n=L+N+1. This rule ensures that when multiple PLCA controllers are configured for redundancy, their replacement beacon signals do not collide with each other. Current standards do not guarantee that any node other than node 0 is configured with the total node number N+1; therefore, a node configured to provide redundancy may need to determine the total node number itself, and / or the configuration process may be modified to provide each node with the total node number when configuring its node ID. The total node count can be easily determined by any node during normal operation of node 0 by observing the node ID for the last transmit opportunity preceding its beacon signal. Its node ID is N, one less than the total node number. In other implementations, node 0 may send a network packet with configuration information to each of the nodes. Other suitable methods may alternatively be used while still allowing robust PLCA operation.

[0025] Figure 3 shows a state machine diagram for an exemplary PLCA controller configured to provide redundancy. It is based on the PLCA control state diagram provided in Figures 148-3 and 148-4 of Section 148.4.4.6 of the referenced standard IEEE Std 802.3cg-2019, which is incorporated herein by reference, but includes modifications for specific transitions to and from the resync state. The modifications incorporate a coordinator redundancy enable signal, cr_en. The coordinator redundancy signal may be a configuration register setting or a state variable set by the controller after it determines the total number of nodes, N+1.

[0026] As is common with state machines, the diagram shown can be implemented using registers and digital logic gates. The state machine diagram of Figure 3 includes 14 states: Disable, Recover, Resync, Beacon, Sync, Wait_TO, Yield, Early_RX, Receive, Next_TO, Commit, Transmit, Burst, and Abort. These are listed in the following order:

[0027] The disabled state is the initial state at boot-up and is the state the PLCA controller returns to when the PLCA_reset signal is asserted, the PLCA_en enable signal is deasserted, or the local node ID is set to 255. Certain variables are initialized in the disabled state, including clearing the transmit command buffer tx_cmd, setting the committed state to false, setting the node ID for the current transmit opportunity to 0, and setting the PLCA_active state to false. The committed state variable may be used to track whether the local node has won its transmit opportunity. The PLCA_active state variable may be used to track whether a PLCA cycle is active, i.e., whether it has started and not finished. If the PLCA_en enable signal is asserted and the local node ID is set to 0, the PLCA controller transitions to the recovery state. If the PLCA_en enable signal is asserted and the local node ID has a value other than 0 or 255, the controller transitions to the resynchronization state.

[0028] The Recovery state is the state used by the PLCA coordinator (PLCA controller at node 0) before coordination is established or after it is lost. The PLCA_active state variable is set to false. From this state, the controller immediately transitions to the Wait_TO state.

[0029] The Resync state is the state used by the PLCA controller for all nodes at the end of a PLCA cycle. It can also be reached when the invalid beacon timer ib_timer from the Sync state expires. The PLCA_active state variable is set to false. From this state, the PLCA coordinator transitions to the Beacon state when the carrier sense signal CRS is deasserted (indicating the communication medium is free). Controllers for other nodes can also follow this transition to the Beacon state if the coordinator redundancy enable cr_en signal is asserted when the CRS signal is deasserted. The timing of this transition is governed by the transmit data clock PMCD signal. Controllers for nodes with non-zero node IDs can alternatively transition from the Resync state to the Early_RX state when the CRS signal is asserted (indicating the communication medium is carrying a signal).

[0030] The Beacon state is the state used by the PLCA controller for node 0, or by a redundancy-enabled PLCA controller, to transmit a beacon signal on the communication medium. The transmit command buffer is loaded with the Beacon BCN signal to implement the transmission, the Beacon timer bcn_timer is started, and the PLCA_active state variable is set to true. When the Beacon timer expires, the Beacon transmission is complete and the controller transitions to the Synchronized state.

[0031] The Sync state is the state reached by a PLCA controller immediately after transmitting or receiving a beacon signal over the communication medium. The current ID counter, curID, is set to 0 for the first transmit opportunity (TO), the transmit command buffer, tx_cmd, is cleared, and the PLCA_active state variable is set to true. For controllers with a non-zero node ID, the controller starts the invalid beacon timer, ib_timer, if the receive command buffer, rx_cmd, does not contain a beacon signal. From the Sync state, the controller transitions to the Wait_TO state when the carrier sense signal, CRS, is deasserted.

[0032] The Wait_TO state is the state reached by the PLCA controller at the start of each transmit opportunity. The controller sets the transmit opportunity timer to _timer, which corresponds to the predetermined duration of the transmit opportunity. If the local node ID nodeID matches the current ID counter of the current transmit opportunity, the PLCA_active state variable is true, and the packet pending signal pkt is asserted while the carrier sense signal CRS is deasserted (indicating the communication medium is free), the controller transitions to the Commit state. If the current ID counter matches and the carrier sense signal CRS is deasserted, but the packet pending signal pkt is deasserted or the PLCA_active state variable is false, the controller transitions to the Yield state. If the current ID counter does not match the local node ID and the transmit opportunity timer to_timer has elapsed while the carrier sense signal is deasserted, the controller transitions to the Next_TO state. If the carrier sense signal CRS is asserted at any time during the transmit opportunity, the controller transitions to the Early_RX state.

[0033] The Yield state is the state reached by the PLCA controller when the node ID of the current transmit opportunity matches the local node ID but the controller is yielding the transmit opportunity either because there is no pending data packet or because the PLCA_active state variable is deasserted. If the carrier sense signal CRS is asserted while the controller is in this state, the controller transitions to the Early_RX state. Otherwise, the controller exits this state and transitions to the Next_TO state when the transmit opportunity timer, to_timer, has elapsed.

[0034] The Early_RX state is the state reached by the PLCA controller when remote transmission activity is detected on the communication medium. In this state, the controller stops the transmit opportunity timer, to_timer, and starts the beacon detect timer, bd_timer. When the transceiver asserts the receive signal, rcv, (e.g., indicating that a data packet is being received rather than a beacon signal), the controller transitions to the receive state. If the controller is part of node 0 and the carrier sense signal, CRS, is deasserted along with the assertion of the receive signal, rcv, the controller transitions to the recover state. For other nodes, if the receive signal, rcv, is deasserted before the beacon detect timer, bd_timer, expires and the receive command buffer, rx_cmd, contains a beacon signal, the controller transitions to the synchronize state. If the beacon detect timer, bd_timer, expires without the presence of a beacon signal in the receive command buffer, rx_cmd, the controller transitions to the resynchronize state.

[0035] The Receive state is the state reached by the PLCA controller when a network packet is being received. The controller transitions to the Next_TO state when the carrier sense signal CRS is deasserted, indicating that the communication medium is free.

[0036] The Next_TO state is the state reached by the PLCA controller when the current transmit opportunity expires. The current ID counter, curID, is incremented for the current transmit opportunity, and the committed state variable is set to false. If the controller is part of node 0 and the current ID counter, curID, is greater than or equal to the total node count, N+1, or if the current ID counter reaches 255, or if the coordinator redundancy enable signal, cr_en, is asserted and the current ID counter, curID, is equal to the sum of the total node count and the local node ID, nodeID, then the controller transitions to the Resync state. Otherwise, the controller immediately transitions to the Wait_TO state.

[0037] The committed state is the state reached by the PCLA controller when the local transmitting node has a pending network packet and the transmit opportunity's current ID counter matches the local node ID, nodeID. The controller acquires the transmit opportunity by setting the transmit command buffer and sending a commit signal, CMT, setting the committed state variable to true, stopping the transmit opportunity timer to timer, and initializing the burst count variable, bc, to 0. If the host device asserts the transmit enable signal, tx_en, the controller transitions to the transmit state. If the transmit enable signal is not asserted and the packet pending signal, pkt, is deasserted, the controller transitions to the abort state.

[0038] The Transmit state is the state reached by the PLCA controller when the transceiver is transmitting a network packet over the communications medium. The transmit command buffer is cleared. The committed state variable is set to false if the burst counter is greater than or equal to the burst count limit max_bc. The controller transitions to the Burst state if the transmit enable signal tx_en is deasserted by the host device while the burst count bc is less than the burst count limit max_bc. If the burst count bc is greater than or equal to the burst count limit max_bc, the controller transitions to the Next_TO state when the transmit enable signal tx_en is deasserted and the carrier sense signal CRS indicates the medium is free.

[0039] The burst state is the state reached by the PLCA controller when the host device pauses after a network packet but remains eligible to send another network packet during the current transmit opportunity. The controller increments the burst count bc, asserts a commit signal CMT to the transmit command buffer, and starts the burst timer brst_timer. When the transmit enable signal tx_en is asserted, the controller returns to the transmit state. If the transmit enable signal remains deasserted and the burst timer brst_timer expires, the controller transitions to the abort state.

[0040] The Abort state is reached by the PLCA controller when the host device is unable to deliver a pending packet in time for the current transmission opportunity. The controller clears the transmit command buffer tx_cmd. When the carrier sense signal CRS indicates that the communications medium is idle again, the controller transitions to the next TO state.

[0041] Upon reviewing the above disclosure, one of ordinary skill in the art will recognize various modifications, equivalents, and alternatives that are within the scope of the present disclosure, and it is intended that the following claims be interpreted to include all such modifications, equivalents, and alternatives, as applicable.

Claims

1. A network interface, a transceiver configured to transmit and receive network data packets over a communications medium; and a PLCA (Physical Layer Collision Avoidance) controller configured to provide redundancy by causing the transceiver to transmit a beacon signal when detecting a transmission opportunity for a node ID that exceeds a total node count by an amount equal to the local node ID, even if the local node ID is greater than 0.

2. The network interface of claim 1 , wherein the PLCA controller is configured to determine the node count by timing beacon signals transmitted by remote nodes.

3. 2. The network interface of claim 1, wherein the PLCA controller is configured to detect transmission opportunities by counting symbol intervals after each beacon signal with a pause between any packets transmitted by a remote node.

4. 1. A network node comprising a network interface coupled to a host device, the network interface comprising: Sending and receiving network data packets over a communications medium; A network node configured to provide PLCA coordinator redundancy by detecting a missing beacon condition and, in response, transmitting a replacement beacon signal before a CSMA / CD fallback condition occurs.

5. The network node of claim 4 , wherein the network interface detects the missed beacon condition by detecting a transmission opportunity for a node ID that exceeds a total node count by at least one.

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