1. Field of the Invention
The present invention relates generally to local area networks (LAN), and more particularly, to a system and method for ensuring time synchronization between two or more computer systems communicating via a wireless LAN. Still more particularly, the present invention relates to a distributed time synchronization system and method for a frequency hopping spread spectrum LAN, which utilizes a plurality of logical frequency channels within a frequency band for communication.
2. Description of the Background Art
A wireless LAN system does not have any hardwired couplings between nodes. This allows nodes to easily join and exit a given LAN, and eliminates the need for routing LAN cabling to each node site. Communication between nodes in a wireless LAN is accomplished by transferring information using electromagnetic signals having a specified frequency range. Such a frequency range has a finite bandwidth, which limits the amount of information that can be transferred. In situations having many nodes or groups of nodes utilizing the same frequency range, bandwidth limitations severely restrict the efficient transfer of information. Spread spectrum (SS) communication techniques have been used to improve bandwidth availability in situations involving many communicating systems. One such spread spectrum technique is known as frequency hopping spread spectrum (FHSS). In a FHSS LAN, communication between nodes is accomplished by transferring information using electromagnetic signals within a plurality of logical frequency channels spanning a frequency band. Each logical frequency channel comprises one or more physical frequency channels, with each physical frequency channel having a designated bandwidth. In a FHSS LAN, information is preferably transmitted sequentially and redundantly on each physical frequency channel within a logical frequency channel, thereby making the logical frequency channel bandwidth the same as that of any single physical frequency channel. A group of nodes communicating in a FHSS LAN utilize one logical frequency channel within the frequency band for a designated amount of time. After the designated amount of time, all the nodes in the group must move to a new logical frequency channel within the frequency band, hence the term frequency hopping spread spectrum. Since nodes within a given group transfer information on a specific logical frequency channel only for a limited time, that channel's bandwidth becomes available after the group's frequency hop. Another group of nodes may then communicate on this logical frequency channel for the designated amount of time. Since each node within a group is required to move to a new logical frequency channel at a prescribed time, the importance of synchronization between each node's clock becomes apparent.
Any logical frequency channel within a band can be subject to interference or noise; thus, moving to a new logical frequency channel also aids in improving signal reception if such effects are present within a portion of the frequency band. FCC regulations are structured in a manner which reflects these considerations, and have mandated specific FHSS rules for frequency bands at 900 and 2400 MHz.
Within a single FHSS LAN group, the lack of perfect synchronization between nodes requires the use of a "dead-time" interval that includes time periods before and after a frequency hop. During this dead time, no information is transmitted by any of the nodes. Providing such dead time ensures that all nodes have completed all sending and receiving of information prior to a frequency hop, and that all nodes have moved to the new channel and are ready to communicate again after the frequency hop, before transmission of information begins. Thus, time synchronization between the clocks of each node included within a FHSS LAN group is essential in order to minimize the dead-time, and thereby maximize the time available for sending and receiving information.
In the prior art, many have attempted to solve LAN time synchronization problems by designating one or more nodes as masters. In a single-master LAN, one node is designated as the master node and the clock of the designated node is utilized as the timing standard for every other node connected to the network. This requires the master node periodically send a special time synchronization message to all other nodes within the network. Thus, all nodes in a single-master system must be capable of receiving messages from the master.
On a FHSS LAN, and in general on any wireless LAN, electrical noise and interference, coupled with signal attenuation effects corresponding to node separation distances, signal path obstacles, or multipath fading arising from signal reflections preclude continuous reliable communication between a node and another specified node. It is therefore desirable to maintain a wireless LAN environment in which a node is considered to be a member of a designated wireless LAN group if it can communicate with at least one other node within the group. Since each node within a single-master LAN system must be able to communicate with the master node, a single-master LAN system does not allow the flexibility desired in a wireless LAN environment.
If, within a single-master LAN system, the master's clock fails, or if the node serving as the master leaves the network, provisions must exist for the removal of the current master, and for the selection of another node as a new master. Other nodes within the LAN require a certain amount of time in order to determine if communication with the master node cannot take place. Time is also required to select a new master node. Thus, in this situation the nodes have no synchronization source until a new master is selected. This is undesirable in any type of LAN since this increases the likelihood that synchronization cannot be maintained between nodes. Particularly in the case of a FHSS LAN, close synchronization must be maintained in order to maximize the amount of time available for information transfer between nodes prior to moving to a new logical frequency channel. Moreover, close synchronization helps to minimize any drift away from a synchronized state in the event that one or more nodes cannot communicate within the FHSS LAN for a limited time.
Another problem associated with single-master LAN systems pertains to the master node recovering from a temporary communication problem. If a new master node has been selected during a time interval in which the original master node could not communicate, the LAN will have two master nodes after the original master node's recovery. Synchronization differences between the two master nodes results in conflicting synchronization messages being sent over the LAN. This situation is obviously undesirable on a FHSS LAN, since conflicting synchronization messages would cause nodes to move to a new logical frequency channel at different times.
In general, if information is being transferred on a LAN, a node wishing to send a message to another node must wait until the current information transfer has been completed before the message can be sent. In like manner, a node within a FHSS LAN is able to transmit information to other nodes only when the current frequency channel is not being used for information transmission by another node. On a FHSS LAN, the information transfer capacity of any given logical frequency channel is limited by the logical frequency channel bandwidth, and by the limited amount of time the nodes in a LAN group remain on the channel. The limited information transfer capacity on any given channel dramatically increases the importance of minimizing unnecessary information transfers. A FHSS LAN must therefore minimize the number of information transfers pertaining to LAN maintenance, thereby maximizing the amount of time available for other information transfer. The presence of special time synchronization messages and master-related LAN maintenance messages on a single-master LAN would adversely affect the performance of a FHSS LAN, and are therefore not suitable for use in such a system. Node synchronization on a FHSS LAN must be achieved without such messages in order to maximize communication efficiency.
In a multiple-master LAN, two or more nodes serve as masters for time synchronization purposes. Nodes within this type of LAN collect synchronization messages from each master at regular intervals, and typically calculate a corresponding average correct time. This requires each node to have the capability to communicate with at least one master node. As discussed above, in a wireless LAN environment reliable communication between two specific nodes is not always possible. As a result, a multiple-master system cannot be used as the basis for a FHSS LAN.
As in the case of a single-master LAN, a multiple-master LAN must have provisions for the selection and removal of masters. Such provisions would greatly increase the message traffic on a wireless LAN, adversely affecting communication efficiency as discussed for the single-master LAN.
On the multiple-master LAN, if the difference between a node's current clock value and the calculated average obtained from master node clock values is within a predetermined tolerance, the node can update its clock by setting its clock to be the same as the average. If the tolerance limit is exceeded, the node sends an argument message comprising its clock value to all other nodes within the LAN. After receiving the argument message, each of the other nodes treats the argument message as a synchronization request message, and in turn send their clock values to the node which originally sent the argument message. This node then calculates a new average clock value for use in updating its clock. The number of messages transferred during the argument procedure can become quite large, flooding a FHSS LAN with messages and consuming a channel's information capacity. Synchronization discrepancies between each node's clock and the degradation of network performance because of flooding makes such prior art methods undesirable in a wireless LAN context.
Thus, both single-master and multiple-master systems are incapable of providing an adequate solution to time synchronization issues on FHSS LANs. Therefore, there is a need for a system and method for ensuring time synchronization between nodes on a FHSS LAN that maintains synchronization without the use of one or more master nodes; requires no special messages dedicated to time synchronization, arguments, or master-related LAN maintenance; and functions in an incompletely connected environment.