Abstract:
Systems and methods are provided that facilitate distributed multichannel wireless communication and provide the highest level quality of service (“QoS”) guarantee and support extremely high bandwidth applications such as voice over internet protocol (“VOIP”) streaming audio and video content (including high definition), and multicast applications and also supports convergent networks, ad hoc networks, and the like. A modular MAC architecture provides a group of nodes with the ability to simultaneously communicate with each other using multiple separate communication channels during the same timeslots. The additional throughput gained by employing multiple communication channels is amplified by dynamically mapping the communication channels and timeslots in a network so that multiple channels can be reused simultaneously throughout the network during the same timeslot in a fashion that does not create collisions.

Description:
RELATED APPLICATION  
       [0001]     The present application is a continuation-in-part of U.S. patent application Ser. No. 11/076,738 filed on Mar. 9, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 10/816,481 filed on Apr. 1, 2004, which is a continuation-in-part of U.S. patent application Ser. Nos. 10/437,128 and 10/437,129 filed on May 13, 2003 that each claim the benefit of provisional application 60/380,425 filed on May, 13, 2002, all of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to wireless communications and more specifically relates to a software implemented media access control (“MAC”) layer and implementation of distributed multichannel wireless communication over a wireless network medium.  
         [0004]     2. Related Art  
         [0005]     Time division multiple access (“TDMA”) is a digital transmission technology that allows a number of users to access a single radio-frequency (“RF”) channel without interference by allocating unique timeslots to each user within each channel. To implement TDMA in a wireless communication network, a centralized controller is required that broadcasts a timing beacon for wireless devices to synchronize with and assigns timeslots to the various wireless devices in the network. The centralized controller requirement for conventional TDMA is a drawback of TDMA for next generation wireless communications.  
         [0006]     TDMA additionally suffers from wasted bandwidth. In some TDMA systems such as the global system for mobile communications (“GSM”), the central controller assigns each wireless device one or more timeslots for transmission and if a particular device has no pending data to transmit, the timeslot will go unused. In aggregate, unused slots can be very burdensome on a TDMA communication system, in particular a communication system adapted for data communications.  
         [0007]     In some TDMA systems such as the general packet radio service (“GPRS”) or the third generation of mobile phone technologies covered by the ITU IMT-2000 family (“3G”) cellular networks, dynamic timeslot allocation is allowed. However, this has to be performed by a central controller, e.g., base station. In order to send the dynamic information, e.g., queue length, traffic load, etc., of network nodes to the central controller, frequent message exchange is needed between network nodes and the central controller, which causes a high signaling overhead.  
         [0008]     Additionally, for wireless communications based on IEEE 802.11, 802.15, 802.16, ultra wide band (“UWB”) and other wireless technologies, scalability and quality of service (“QoS”) are two critical concerns. One significant problem is that the conventional 802.11 MAC is not scalable because the throughput dramatically drops as the number of hops increases and/or as the number of nodes increases. This lack of throughput causes an unacceptable QoS level. Some improvements have been identified such as 802.11e, but even this proposal suffers because it is dependent on the carrier sense multiple access/collision avoidance (“CSMA/CA”) protocol that does not guarantee any level of traffic flow (i.e., throughput) for single or multihop networks.  
         [0009]     For example, enhanced distributed channel access (“EDCA”) only enforces traffic prioritization of different flows on the same node or client. It does not provide prioritization between flows on different nodes or clients. Additionally, while the hybrid coordination function (“HCF”) allows the allocation of a collision free period (“CFP”) the CFP cannot coexist with a collision period (“CP”) because retransmission of packets at the end of the CP will encroach on the CFP and thus cause interference between the CP and CFP periods. The same problem exists when retransmitted packets of CFP fall into the CP. Furthermore, IEEE 802.11e does not have a method to ensure that HCFs of different nodes in a wireless network do not overlap. Accordingly, overlapping HCFs result in collisions and severely limited QoS. Excellent QoS is extremely important for QoS sensitive applications such as voice over internet protocol (“VOIP”), streaming video, and other high bandwidth applications that require high fidelity.  
         [0010]     Finally, using alternative techniques such as frequency division multiple access (“FDMA”) or code division multiple access (“CDMA”) as a basis for the wireless communication fails on 802.11 because such techniques require a non 802.11 physical layer. Therefore, these and other significant problems found in the conventional systems have created a need in the industry for a system and method that provides efficient high bandwidth communication over a wireless network medium and overcomes these significant problems found in the conventional systems as described above.  
       SUMMARY  
       [0011]     Accordingly, the problems found in the conventional systems are overcome by the systems and methods described herein that combine the advantages of both TDMA and multichannel operation. The system operates in a distributed fashion and therefore does not require a centralized operation. Additionally, the system provides a MAC that is compatible with commercially available IEEE 802.11 chipsets, which makes it more economical to provide and maintain compatibility with evolving standards.  
         [0012]     The system employs a TDMA based MAC protocol implemented in software that uses multiple channels that are available in the IEEE 802.11 (and other wireless protocols such as UWB) frequency bands. The system operations are distributed amongst the nodes in a wireless network so that the system can maintain communications as nodes travel in and out of local networks. Advantageously, the combined advantages of multichannel and TDMA communication resolve the scalability issues found in the conventional systems. The TDMA based MAC protocol may also be implemented in hardware or a combination of hardware and software.  
         [0013]     Furthermore, systems and methods are provided to allow for distributed multichannel wireless communication that provides the highest level quality of service (“QoS”) guarantee and supports extremely high bandwidth applications such as VoIP, streaming audio and video content (including high definition), multicast applications, and also supports convergent networks, ad hoc networks, and the like. This is achieved by a modular MAC architecture that provides a cluster of nodes or clients with the ability to simultaneously communicate with each other using separate communication channels during the same timeslots. This additional throughput capability is amplified by dynamically mapping communication channels and timeslots in a network so that multiple channels can be reused simultaneously throughout the network during the same timeslot in a fashion that does not create collisions.  
         [0014]     For example, a node or client first determines the network topology of its wireless communication network in order to identifying its neighboring nodes or clients and the discrete link the node or client has with each of its neighbors. Timeslots for transmission of packets between the node and its neighbors are then identified and a separate channel is assigned for communication between the node and each neighbor. The various timeslots and communication channels are dynamically mapped across the network to maximize throughput and then queued up packets are transmitted over the various channels during the various timeslots in accordance with the dynamically changing mapping. The dynamically changing mapping allows maximum flexibility for spikes in throughput by individual nodes and also accounts for efficient ingress and egress of nodes in the wireless network. Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The details of the present invention, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts, and in which:  
         [0016]      FIG. 1  is a network diagram illustrating an example wireless communication network according to an embodiment of the present invention;  
         [0017]      FIG. 2  is a network diagram illustrating an example wireless communication network according to an embodiment of the present invention;  
         [0018]      FIG. 3  is a network diagram illustrating an example wireless communication network according to an embodiment of the present invention;  
         [0019]      FIG. 4  is a block diagram illustrating an example wireless communication device for distributed multichannel communication according to an embodiment of the present invention;  
         [0020]      FIG. 5  is a block diagram illustrating an example MAC architecture for distributed multichannel communication according to an embodiment of the present invention;  
         [0021]      FIG. 6  is a flow diagram illustrating an example process for buffering packets for transmission according to an embodiment of the present invention;  
         [0022]      FIG. 7  is a flow diagram illustrating an example process for multichannel packet transmission in a single timeslot according to an embodiment of the present invention;  
         [0023]      FIG. 8  is a flow diagram illustrating an example process for processing in incoming packets according to an embodiment of the present invention;  
         [0024]      FIG. 9  is a flow diagram illustrating an example process for software retransmission of packets according to an embodiment of the present invention;  
         [0025]      FIG. 10  is a flow diagram illustrating an example process for multichannel transmission rate control according to an embodiment of the present invention;  
         [0026]      FIG. 11  is a flow diagram illustrating an example process for fast channel switching according to an embodiment of the present invention;  
         [0027]      FIG. 12  is a block diagram illustrating an example TDMA frame according to an embodiment of the present invention;  
         [0028]      FIG. 13  is a block diagram illustrating an example portion of a TDMA frame according to an embodiment of the present invention;  
         [0029]      FIG. 14  is a flow diagram illustrating a high level process for distributed multichannel wireless communication according to an embodiment of the present invention;  
         [0030]      FIG. 15  is a block diagram illustrating an example wireless communication device that may be used in connection with various embodiments described herein; and  
         [0031]      FIG. 16  is a block diagram illustrating an example computer system that may be used in connection with various embodiments described herein. 
     
    
     DETAILED DESCRIPTION  
       [0032]     Certain embodiments disclosed herein provide for systems and methods for distributed TDMA communication between nodes of a wireless communication network. For example, one system as disclosed herein provides an enhanced MAC layer on a network device that is configured to identify its neighboring nodes and create a mapping of communication channels and timeslots for multichannel TDMA communication between the network device and its neighbors.  
         [0033]     After reading this description it will become apparent to one skilled in the art how to implement the invention in various alternative embodiments and alternative applications. To facilitate a direct explanation of the invention, the present description will focus on an embodiment where communication is carried out over an IEEE 802.11 network, although the invention may be applied in alternative networks including 802.15, 802.16, worldwide interoperability for microwave access (“WiMAX”) network, wireless fidelity (“WiFi”) network, wireless cellular network (e.g., wireless wide area network (“WAN”), UWB network, ZigBee, and/or any other wireless communication network topology or protocol. Additionally, the described embodiment will also focus on a single radio embodiment although multi-radio embodiments and other multiple input multiple output (“MIMO”) embodiments are certainly contemplated by the broad scope of the present invention. Therefore, it should be understood that the single radio, 802.11 network embodiment described herein is presented by way of example only, and not limitation. As such, this detailed description should not be construed to limit the scope or breadth of the present invention as set forth in the appended claims.  
         [0034]      FIG. 1  is a network diagram illustrating an example wireless communication network  10  according to an embodiment of the present invention. In the illustrated embodiment, the wireless network  10  comprises a plurality of devices including device  22 , device  24 , device  26 , device  28 , device  30 , and device  32 . Each of these devices has also been provided with an alphabetical label (A-F, respectively) to be used in the context of a later discussion.  
         [0035]     Each of the wireless communication devices  22 - 32  can be any of a variety of wireless communication devices, including a cell phone, personal digital assistant (“PDA”), personal computer (“PC”), laptop computer, PC card, special purpose equipment, access point, router, switch, radio frequency identification (“RFID”) reader, base station controller, WiFi phone, security camera, set top box, or any combination of these and other devices capable of establishing a wireless communication link over the network  10 . Throughout this detailed description a wireless communication device such as those shown in  FIG. 1  may be referred to as a wireless device, device, wireless node, node, wireless client, client, mesh client, mesh node, or the like.  
         [0036]     In the illustrated embodiment, the network  10  is a wireless mesh network, but as described above, the network  10  could be any of a variety of network types and topologies and employ any of a variety of types of protocols. For the sake of providing a straightforward description, the illustrated embodiment will be described as an IEEE 802.11 network  10  that is a wireless mesh network and has a cluster topology.  
         [0037]     As will be understood by those having skill in the art, alternative topologies may also be present as a wireless mesh network shifts over time with ingress and egress of the various client devices. Accordingly,  FIG. 2  is a network diagram illustrating an example wireless communication network  12  according to an embodiment of the present invention. In the illustrated embodiment, the wireless network  12  comprises wireless nodes A-F and has a linear topology. For example, terminal node A is within communication range of node B, node B is in communication range of nodes A and C, and so on through terminal node G that is in communication range of node F. The linear topology shown in the embodiment of  FIG. 2  will be used in the context of a later discussion. In summary of  FIGS. 1 and 2 , although only the cluster and linear topologies are illustrated, various additional wireless network topologies can also be present in a network as will be understood by those having skill in the art.  
         [0038]     For example,  FIG. 3  is a network diagram illustrating an example wireless communication network  18  according to an embodiment of the present invention. In the illustrated embodiment, the network  18  comprises a mesh network  14  and two nodes  50  and  52  that are communicatively coupled to each other and also to other portions of network  18  via a wireless mesh network backbone  16 .  
         [0039]     The mesh backbone  16  comprises a plurality of router, gateway, and bridge devices including device  62 , device  64 , device  66 , device  68 , device  70 , and device  72 . These devices cooperatively communication with each other and various nodes to provide a dynamic wireless communication network that enables each node to communicate directly or indirectly with each other and also with the network  18 .  
         [0040]     For example, nodes  40 ,  42 ,  44 ,  46 , and  48  are in mesh network  14  and can communicate directly with each other. Additionally, these nodes can communicate with nodes  50  and  52  through the mesh backbone  16 . In one embodiment, node  40  communicates with node  50  via the router devices  72  and  66 . Furthermore, if node  50  were to move into proximity with node  48 , for example, then node  50  may join mesh network  14 , after which communications between node  40  and node  50  may travel along the multi-hop path including nodes  44  and  48 .  
         [0041]     The network  18  may comprise any of a variety of network types and topologies and any combination of such types and topologies. For example, the network  18  may comprise a plurality of networks including private, public, circuit switched, packet switched, personal area networks (“PAN”), local area networks (“LAN”), wide area networks (“WAN”), metropolitan area networks (“MAN”), satellite network, roaming networks, or any combination of the these. Network  18  may also include that particular combination of networks ubiquitously known as the Internet.  
         [0042]      FIG. 4  is a block diagram illustrating an example wireless communication device  32  for distributed multichannel TDMA communication according to an embodiment of the present invention. In the illustrated embodiment, the node  32  comprises a topology module  100 , a packet queuing module  105 , a packet transmit (“Tx”) and receive (“Rx”) module  110 , a transmission rate control module  115 , a channel switching module  120 , a hardware abstraction layer (“HAL”) module  125 , a signaling module  130 , and a timeslot and channel allocation module  135 . Additionally, in the illustrated embodiment the node  32  implements distributed multichannel TDMA communication in the MAC layer, thus providing the node  32  with a distributed multichannel TDMA MAC (“DMT-MAC”).  
         [0043]     The topology module  100  is configured to interrogate the surroundings of the node and communicate with other nodes that are discovered to determine the topology of the wireless network that the node is part of. For example, a node may only find a single device and learn from that device that other nodes are present in the network in accordance with a linear topology, such as the topology illustrated in  FIG. 2 . Alternatively, the node may find multiple devices in communication range and learn from those devices that together they all form a wireless mesh network cluster, such as the topology illustrated in  FIG. 1 .  
         [0044]     The packet queuing module  105  is configured to receive packets from a higher layer in the communication stack and buffer those packets for later transmission over the physical medium. In one embodiment, when a packet is received from a higher layer such as the logical link control (“LLC”) layer, its header is checked to determine the type of the packet. In one embodiment, the packet queuing module  105  recognizes certain packet types including: (1) unicast packets; and (2) broadcast packets. The different types of packets are processed differently before they are buffered in a transmission queue. For example, the processing may include operations such as configuration of transmit power level, security encryption, and many HAL related configurations.  
         [0045]     In one embodiment, broadcast packets are buffered in a single broadcast queue, while unicast packets are buffered in different queues as determined by the destination MAC header and whether a channel has been assigned for communication with the destination node. For example, if no channel has been assigned for communication with the destination node, the packet can be buffered in a special slot queue for later transmission on a common channel. In one embodiment, if a node has N neighbors, the number of transmission queues is N+2, which accounts for one broadcast queue, one special slot queue, and N neighbor queues. Since each neighbor has a queue, the special slot queue may be used for new nodes that join the network and do not yet have an assigned communication channel.  
         [0046]     Accordingly, in one embodiment an node may have three different types of packet buffer queues, including (1) a broadcast queue used for all broadcast packets, where packet transmission from the broadcast queue only occurs in the broadcast timeslot on a common channel; (2) a special slot data queue used for unicast packets when a timeslot or channel has not assigned to a destination node, where packet transmission from the special slot queue only occurs in special timeslot on a common channel; and (3) a regular data queue used for sending unicast packets to destination nodes, where packet transmission from a regular data queue occurs during a regular data timeslot on an assigned channel. Advantageously, each queue corresponds to a particular link from one node to another. In addition, among all neighbor nodes, each timeslot allows one queue to send packets on a certain channel.  
         [0047]     In an alternative embodiment, the special slot queue can be eliminated by creating a new regular data queue to buffer packets destined for any node that does not yet have an assigned channel for communication, although transmission of those packets would take place after a channel and timeslot were allocated for communication with that destination node.  
         [0048]     The packet Tx/Rx module  110  is configured to send and receive packets. Packets are sent from the various transmission queues and received packets are validated and then passed along to a higher layer. Because there is typically more than one transmission queue, the queue from which packets are transmitted is determined by transmission queue selection. The transmission queue is selected according to (timeslot→queue) mapping, as assigned by a timeslot and channel allocation algorithm. Once a transmission queue is selected, the packet Tx/Rx module  110  causes the packets in this queue to be sent to the destination node over a predetermined channel identified for communication with the particular destination node. The predetermined channel is also assigned by the timeslot and channel allocation algorithm and given in the (timeslot→channel) mapping. Accordingly, the packet Tx/Rx module  110  sends packets in accordance with the (timeslot→(queue, channel)) mapping.  
         [0049]     In one embodiment, for a given timeslot, when a transmission queue is selected and the channel is determined, packets in this queue are sent using the determined channel. Packet transmission actually occurs in the HAL and physical layer hardware, however, the transmission status is provided back to the packet Tx/Rx module  110 . The transmission status is used by packet Tx/Rx module  110  to manage software retransmission and facilitate transmission rate control in cooperation with the rate control module  115 .  
         [0050]     Software retransmission allows the DMT-MAC to control retransmission of packets. Thus, when a packet is sent successfully, as acknowledged by the HAL, the packet Tx/Rx module  110  removes the packet from the queue and starts to send the next packet. If the packet is not sent successfully, the packet is sent again by the packet Tx/Rx module  110 . To implement robust software retransmission, the packet Tx/Rx module  110  tracks the number of transmissions so that if a packet cannot be successfully delivered before the number of retries reaches zero, the packet is removed from the queue, and the next packet is sent. As will be understood by those skilled in the art, removal of a packet from the transmission queue causes packet loss due to exhaustion of retransmission. This typically occurs when the channel quality is poor.  
         [0051]     In one embodiment, to facilitate software retransmission, the standard IEEE 802.11 hardware retransmission is disabled by the packet Tx/Rx module  110  so that the DMT-MAC itself solely manages retransmission of packets. Implementation of software retransmission provides the following advantages: (1) timeslot boundaries are not crossed by retransmitted packets; (2) the packet Tx/Rx module  110  is aware of when to send the next new packet.  
         [0052]     In one implementation of software retransmission, the default retransmission by the HAL is disabled and the HAL is configured to acknowledge when a packet is successfully sent and the number of retries in the HAL registers is set to one. With these configurations, the HAL transmission status registers can advantageously be read by the packet Tx/Rx module  110  to determine when packet transmission is successful. One significant advantage of software retransmission is that TDMA retransmission relies upon the acknowledgment (“ACK”) mechanism of 802.11 MAC and therefore there is no need to develop a separate SEND/ACK procedure for software retransmission.  
         [0053]     An additional advantage of software retransmission is that the transmission and retransmission statistics are collected by the packet Tx/Rx module  110  and used by the rate control module  115  to facilitate selection of an appropriate transmission rate for new packets.  
         [0054]     In one embodiment, for new packet transmissions there are three scenarios that initial the process new packet transmission: (1) a new packet arrives from a higher layer (e.g., LLC layer); (2) channel switching is complete and a new packet transmission is initiated; and (3) the transmission of the current packet is successfully completed and the current timeslot still belongs to the selected queue.  
         [0055]     Notably, channel switching disables the transmit interrupt and as a result, when a new timeslot arrives there may be no event that can trigger packet transmission. Although a new arrival packet may trigger new packet transmission in the new timeslot, this is not always the case, which results in low utilization of a timeslot and low throughput. Accordingly, the packet Tx/Rx module  110  starts a new packet transmission when channel switching is done. The queue that is selected for new packet transmission is determined based on the current timeslot and the (timeslot→queue) mapping.  
         [0056]     In the case where a transmit interrupt is received, the packet Tx/Rx module  110  identifies which queue the interrupt belongs to. Once the queue is identified the packet Tx/Rx module  110  begins retransmission of a packet or a new packet transmission. In one embodiment, identification of the transmission queue is achieved by indexing each packet transmission with a packet type and its destination MAC address. For example, when a packet in a unicast queue (e.g., queue N) is sent to a target node, the packet Tx/Rx module  110  maintains two variables for the queue: (1) the queue type; and (2) the MAC address of the target node. When this packet is sent by the HAL and a Tx interrupt is received by the packet Tx/Rx module  110 , the two variables help to identify queue N so that transmission of a new packet from queue N or retransmission can be started immediately.  
         [0057]     It should be noted that a MAC address alone is insufficient to uniquely identify a transmission queue because, for example, the special slot data queue and a regular data queue can have the same destination MAC address.  
         [0058]     The packet Tx/Rx module  110  also performs packet receiving, which is much simpler than packet transmission. For example, given a particular node, as long as a channel is selected according to the (timeslot→channel) mapping, any packets with this node as the destination will be received. When a packet is received, it will be processed according to its packet type. For example if the packet is a beacon, then conventional beacon related operation as specified in IEEE 802.11 MAC is executed. Additionally, DMT-MAC specific network topology information may also be sent in a beacon packet.  
         [0059]     Alternatively, if a received packet is a DMT-MAC signaling packet, it is processed by the timeslot and channel allocation module  135 . If the received packet is a MAC layer control or management packet such as probe-request and probe-response, the corresponding function in standard IEEE 802.11 MAC is executed. If the received packet is a regular data packet, it is processed by checking its integrity and if the packet is valid, it is sent to a higher layer further processing.  
         [0060]     The transmission rate control module  115  is configured to maintain an appropriate transmission rate for packets sent to each destination node. In one embodiment, the rate control module  115  determines the transmission rate of each packet based on the historical transmission status of previous packets. Rate control is performed separately for each destination, since the link qualities to different destinations may be different. A variety of rate control schemes may be used by the rate control module  115  to determine the optimal transmission rate.  
         [0061]     In multichannel operation, there can be more than one rate control process, for example because different channels may be used to the same destination from time to time. Additionally channel characteristics across different channels can vary widely. Thus, when a channel is changed, even if the destination is still the same a different rate control is advantageously invoked to determine the transmission rate of the next packet.  
         [0062]     Furthermore, because transmission rate control is dependent on the historical transmission status to determine the transmission rate of the next packet, the rate control module  115  is configured to collect such information. For example, the rate control module  115  collects combined information from both the HAL and software retransmission as transmission status information. Advantageously, feedback from the HAL tells whether or not a packet is sent successfully, while feedback from software retransmission gives information about how many retries were sent until packet transmission was successful.  
         [0063]     The channel switching module  120  is configured to quickly change between available 802.11 channels so that multichannel communications can be efficiently carried out. In one embodiment, at the beginning of each timeslot, the channel switching module  120  determines if channel switching is needed. If the channel for the new timeslot is the same as the previous timeslot, channel switching is not needed. However, in order to keep synchronization among all nodes, a certain amount of delay can be employed before new packet transmission starts.  
         [0064]     If the channel for the new timeslot is not the same channel, channel switching is needed. Accordingly, to switch to a new channel, the channel switching module  120  first backs up all of the HAL registers that are needed for TDMA operation. Next, the channel switching module  120  disables all hardware interrupts. The hardware is then reset to the new channel, the hardware interrupts are re-enabled, the registers in the hardware are reconfigured and the HAL registers are restored for continued TDMA operation. Then the new packet transmission starts.  
         [0065]     Advantageously, certain efficiencies can be employed to speed up channel switching. For example, because channel switching is only geared to switch to another channel, many hardware registers do not need to be reset and reconfigured for the new channel. Thus, only selected registers that are closely related to a physical channel needs are reset and reconfigured. By doing this partial hardware reset, the overall channel switching time spent on the hardware reset can be significantly reduced.  
         [0066]     Additionally, before switching channels both the receiving and transmitting hardware must be clear of outstanding packets to avoid packet loss and allow a proper hardware cannot. In order to avoid such errors, a guard time is provided before channel switching, which is implemented by determining whether a packet transmission will cross the boundary of a timeslot. Due to variable transmission rates, the guard times advantageously also vary as the transmission rate changes.  
         [0067]     Furthermore, channel switching by different nodes may not take exactly the same amount of time. Thus, when channel switching is completed at a first node, a second node may be still in the process of channel switching. This may occur even if channel switching at all nodes begins at the same time. Such different channel switching times can cause problems when a new packet transmission is initiated after channel switching. For example, a packet may be lost or hardware errors can happen. Accordingly, a guard time is provided to compensate for the different channel switching times among different nodes. Advantageously, the length of the guard time is variable so that the delay from the start of a timeslot to the initiation of a new packet transmission is fixed and the same for all nodes.  
         [0068]     The HAL module  125  is configured to manage the HAL in accordance with the needs of the DMT-MAC. For example, the HAL module  125  is configured to disable retransmission in the HAL. In one embodiment, certain operational parameters for the DMT-MAC are provided in the initialization function of the DMT-MAC to allow the HAL module  125  to modify or disable the functions of the HAL. For example, certain hardware registers in the HAL are set by the HAL module  125  to provide real-time interrupts or events that mark the start of each timeslot and each TDMA frame. Additionally, the HAL module  125  may configure the HAL to send beacons in a common channel in a common slot for all nodes in order to avoid network partition when in multichannel operation. Furthermore, the HAL configuration can be optimized by the HAL module  125  so that channel switching is also optimized.  
         [0069]     The signaling module  130  is configured to send and receive signaling messages needed by the DMT-MAC to implement distributed multichannel TDMA communications. In one embodiment, the signaling module  130  uses both beacon-based signaling and explicit signaling. Beacon-based signaling is accomplished by adding extra information into a beacon and signaling is carried out by the standard procedure of beacon transmission and receiving. Although this method is simple and does not impact the regular operation of beaconing, signaling based on beaconing has several disadvantages.  
         [0070]     For example, beacon-based signaling messages are sent without any guarantees of being successfully received, since a beacon is sent as a broadcast packet. Additionally, a beacon-based signaling message may take long time to receive its destination. This is in part because under the procedures of beaconing, whenever a node successfully sends out a beacon, all neighboring nodes will not send beacons until the next beacon interval. Thus, if there are N nodes in a neighborhood, the average chance of sending out a beacon is 1/N per node per beacon interval. Thus, if a node has information to send, it may not be able to send that information for a long time which makes information relay by beacons very slow, especially in a cluster mesh or other multi-hop topology.  
         [0071]     Due to such disadvantages, beacon-based signaling is advantageously used for non-time-critical signaling message. For example, the information about network topology and network degree can be sent via beacon-based signaling.  
         [0072]     Explicit signaling, on the other hand, provides signaling messages between nodes by explicitly sending signaling packets. In one embodiment a plurality of packet types are defined for MAC signaling. For example, each of these packet types can be identified by a signaling message identifier and perhaps other identification information to indicate that they are for signaling purposes only.  
         [0073]     In one embodiment, explicit signaling is accomplished by guaranteed unicast transmission in a signaling timeslot. Advantageously, because the transmission is unicast, the procedure of sending a signaling message is the same as that for a data packet. Additionally, the transmission is guaranteed by configuring the HAL such that the transmission status of unicast signaling messages is tracked and those messages can be retransmitted if necessary. The guaranteed unicast explicit signaling messages are also sequence number controlled to provide further robustness in signaling so that redundant signaling messages can be removed in the case where a message is inadvertently retransmitted and successfully delivered twice.  
         [0074]     In one embodiment, each signaling message comprises a message identification field, a sequence number field, and a data payload (i.e., content). The message ID field is used to identify the signaling message type, while the sequence number field helps to remove redundant signaling messages. The content of a signaling message advantageously varies for the different types of signaling messages.  
         [0075]     A significant advantage of explicit signaling is the guaranteed transmission in the limited period of signaling timeslot. In a wireless network, when channel quality is poor, such a period may not be enough to guarantee the successful reception of a signaling message, even though retransmission is applied. Thus, a single signaling message may be sent across multiple TDMA frames until it is finally received by a destination node. Since the transmit interrupt of the current TDMA frame cannot be kept valid and processed in the next TDMA frame, transmission of new MAC signaling messages are triggered whenever each MAC signaling timeslot starts. Thus, transmission of MAC signaling messages can be driven by either the transmit interrupt (in the same MAC signaling timeslot) or the arrival of a new MAC signaling timeslot.  
         [0076]     The timeslot and channel allocation module  135  is configured to identify and propagate an optimal timeslot and channel allocation scheme for multichannel TDMA communications. In one embodiment, the timeslot and channel allocation module  135  employs a wave-propagation algorithm that begins at one node in the network and proceeds to each other node in the network until the network topology is determined. Once the various nodal pair links are identified according to the network topology, the links are assigned to non-overlapping channels and then each channel is assigned to a timeslot to maximize throughput during multichannel TDMA communication. In one embodiment, the timeslot and channel allocation module  135  is configured to use explicit signaling to dynamically capture the network topology and resource allocation in a distributed manner.  
         [0077]     In one embodiment, the timeslot and channel allocation module  135  operates in two phases, a distributed allocation phase (“DAP”) and an allocation adjustment phase (“AAP”). Both phases apply the wave propagation principle, i.e., allocation starts at a first node and proceeds to the last node in the network and then the process is reflected back from the last node to the first node. The wave-propagation path touches each node in the network and allows each node to initiate resource allocation between itself and its neighbor nodes. Nodes that do not reside on the wave-propagation path do not initiate resource allocation but do participate in resource allocation initiated by their neighboring nodes.  
         [0078]     In DAP, timeslots and channels are allocated node-by-node as the wave-propagation path is set up. When the wave reaches the last node, it is reflected by traversing the wave-propagation path back to the first node. Once the wave reaches the first node, DAP is complete and AAP starts. In AAP, remaining timeslots and appropriate channels are assigned to nodes one-by-one along the wave-propagation path. Once the wave reaches the last node, it returns back to the initial node along the wave-propagation path. Once all timeslots have been allocated, the dynamic process of timeslot and channel allocation is accomplished and can be repeated.  
         [0079]      FIG. 5  is a block diagram illustrating an example MAC architecture on a device  32  configured for distributed multichannel communication according to an embodiment of the present invention. In the illustrated embodiment, the architecture depicts the functions of the device  32  in both the management plane and data plane. In the management plane, a key function is MAC signaling, which is used to send and receive MAC layer signaling messages. Another function that also helps the operation of the device  32  in the management plane is beaconing. Advantageously, standard beaconing defined by IEEE 802.11 can be employed. Because the functions of MAC signaling and beaconing are combined, the network topology and resource allocation are updated dynamically. Based on such real time information, the distributed timeslot and channel allocation scheme is able to determine the most appropriate timeslots and channels for each node.  
         [0080]     In the data plane, at the transmitter side data packets from a higher layer, e.g., the LLC layer, are processed and sent according to TDMA with different channels in different timeslots. At the receiver side, bit streams from the physical layer are converted back to a packet in the HAL, and are decoded and processed in the MAC Rx. In order to keep the multichannel operations working without network partition, all time periods except for the regular data slots work on a common channel that is the same for all nodes. Advantageously, the channels for the various regular data slots vary according to the needs of multichannel operation and are accordingly determined by a timeslot and channel allocation algorithm.  
         [0081]      FIG. 6  is a flow diagram illustrating an example process for buffering packets for transmission according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  100  the node receives a packet. The packet is examined to determine if it is a broadcast packet in step  105 . If it is a broadcast packet, then it is put into the broadcast transmission queue in step  110  for immediate or deferred transmission, depending on the current timeslot and the presence of earlier buffered packets. If not a broadcast packet, then in step  115  the node determines if the packet is a special unicast packet. As previously described, special unicast packets are those packets destined for a node that does not yet have an allocated channel. If the packet is a special unicast packet then it is put into the special unicast transmission queue in step  110  for immediate or deferred transmission, depending on the current timeslot and the presence of earlier buffered packets. If not a broadcast or special unicast packet, then the packet is a regular data packet and in step  120  the MAC header information is examined to determine the destination for the packet and then in step  110  the packet is put into the appropriate transmission queue for immediate or deferred transmission, depending on the current timeslot and the presence of earlier buffered packets.  
         [0082]      FIG. 7  is a flow diagram illustrating an example process for multichannel packet transmission in a single timeslot according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  130  a new timeslot arrives. The arrival may be indicated by a software beacon alert event as explained below with respect to  FIG. 12 . Upon the arrival of the timeslot, the node next identifies the channel to be used to send packets, as shown in step  135 . In one embodiment, this may be a simple lookup process that examines the results of a timeslot and channel allocation process carried by the node. Next, in step  140  the node identifies the transmission queue from which packets are to be transmitted. This can similarly be a simple lookup of mappings of channels and queues.  
         [0083]     In step  145  the node then sends the packets in the queue to the HAL, one by one. In step  155  the node determines if the packet needs to be retransmitted. This determination can be made by examining certain registers maintained by the HAL or by the lack of an acknowledgement of successful transmission from the HAL before a timeout period expires. If the packet does not need to be retransmitted, in step  160  the next packet is obtained from the transmission queue and the process of transmitting new packets continues. If the packet needs to be retransmitted, it is resent in step  165 .  
         [0084]      FIG. 8  is a flow diagram illustrating an example process for processing in incoming packets according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  175  the packet is received. Next, in step  180  the packet is examined to determine if it is a beacon message. If it is not, then in step  185  the packet is examined to determine if it is a signal message. If it is not, then the packet is examined in step  190  to determine if it is a control message. If the examination of the packet in steps  180 - 190  determines that the packet is a beacon message, a signal message, or a control message, then in step  195  the packet is processed accordingly by the DMT-MAC.  
         [0085]     If the packet is not a beacon message, a signal message, or a control message, then it is a data message and the integrity of the packet is determined in step  200 . If the packet has been received intact and is authenticated, in step  205  the packet is sent to a higher layer for further processing. If the packet failed the integrity check in step  200  and is therefore not authenticated, it is discarded, as shown in step  210 .  
         [0086]      FIG. 9  is a flow diagram illustrating an example process for software retransmission of packets according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  220  the node disables the hardware retransmit capability in the standard IEEE 802.11 HAL. Next, in step  225  the node configures the HAL to acknowledge when a packet has been successfully transmitted. Then in step  230  the node configures the HAL so that its retry level is set to one. Once the HAL has been configured in steps  220 - 230 , the node sends the new packet in step  235 .  
         [0087]     After sending the new packet, the node determines in step  240  if the packet was transmitted successfully. This can be accomplished by receiving (or not receiving) and acknowledgment from the HAL of success. This can also be accomplished by examining certain registers maintained by the HAL. If the packet was successfully sent as determined in step  240 , then the process loops back to step  235  and a new packet is sent. If the packet was not successfully sent as determined in step  240 , then the node checks in step  245  to determine if the maximum number of retries has been met. The maximum number of retries advantageously reflects the number of software retransmission retries and is not related to the number of retries previously configured for the HAL.  
         [0088]     If the maximum number of retries have not been met, then the packet is resent in step  250  and the process loops back to step  240  to determine whether the resent packet transmission was successful. If the maximum number of retries has not been met then the process loops back to step  235  where a new packet is sent.  
         [0089]      FIG. 10  is a flow diagram illustrating an example process for multichannel transmission rate control according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  260  the node receives a packet for transmission. The packet is next examined in step  265  to identify the appropriate communication channel over which the packet is to be sent. In step  270 , the transmission rate for the particular channel is updated based on historical transmission information and then in step  275  the packet is sent. Advantageously, the transmission rate for a given channel may not always be the same. For example, in different timeslots a node may use the same channel for communication with different nodes. Because the signal strength between nodes may vary, the transmission rate may also vary, even though the same channel is used.  
         [0090]      FIG. 11  is a flow diagram illustrating an example process for fast channel switching according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  280  a new timeslot arrives. Next, in step  285  the channel for communication in the new timeslot is examined to determine if it is the same as the previous channel that was used by the node. If the channel for the new timeslot is the same as the previous channel, then the process ends in step  290  and the node does not switch channels.  
         [0091]     If the channel for the new timeslot is not the same as the previous channel, as determined in step  285 , then the node proceeds to switch channels. In switching channels, the node first backs up the HAL registers in step  295 . For example, certain HAL registers may be important to maintain TDMA communications. Additionally other HAL registers may be ignored. Advantageously, only backing up those registers that are important for continued robust operation can save time in the channel switching process. Next, in step  300  the hardware interrupts are disabled so that the hardware can be reset in step  305 . Once the hardware is reset, the hardware interrupts are enabled in step  310  and the HAL registers are reconfigured in step  315  and then restored in step  320  with the backed up information. Finally, in step  325  the packet(s) are transmitted on the new channel.  
         [0092]      FIG. 12  is a block diagram illustrating an example TDMA frame  340  according to an embodiment of the present invention. In the illustrated embodiment the frame  340  is identified as frame number N and is preceded by a frame number N−1 and followed by a frame number N+1. As will be understood by those having skill in the art, the frames follow each other sequentially. Advantageously, the DMT-MAC works cooperatively with the existing IEEE 802.11 MAC and is configured to control the IEEE 802.11 HAL so that packets do not follow CSMA/CA and that packets are sent according to TDMA in a TDMA frame. In one embodiment, synchronization in a distributed multichannel TDMA communication network is enforced by using the timing synchronization function (“TSF”) in the conventional 802.11 MAC.  
         [0093]     As shown in the illustrated embodiment, the length of the TDMA frame  340  is equal to a beacon interval and comprises a broadcast transmission common slot during which broadcast messages from higher layers and the MAC layer are sent. The frame  340  also comprises an ad hoc traffic indication message (“ATIM”) window during which only ATIM related messages as defined in IEEE 802.11 standard are sent. For example, beacons are sent at the beginning of this period. The frame  340  also comprises a MAC explicit signaling common slot during which MAC layer explicit signaling messages are sent. The frame  340  also comprises a special slot for unicast data transmission during which regular unicast packets are sent when no channel has yet been allocated. Additionally, traffic flows during this period will also help detection of link activities. Without packets being sent during this period, regular data transmission cannot be initiated to activate idle links. The frame  340  also comprises regular slots for unicast data transmission during which regular unicast packets are sent. Because each node may need to send packets to multiple destination nodes, different timeslots are needed for different destinations. Thus, this period can be further divided into multiple timeslots. The number of timeslots is determined dynamically by the timeslot and channel allocation algorithm.  
         [0094]     In one embodiment, in order to keep the multichannel communications operational without partitioning the network, all time periods except for the regular slots are implemented over a common channel that is the same for all nodes. The channels for different regular slots vary according to the needs of multichannel operation and are determined by the timeslot and channel allocation algorithm.  
         [0095]     To build the structure of the TDMA frame  340 , timing events are generated that indicate the beginning of each period or timeslot. Advantageously, this is accomplished by configuring the HAL to generate a timing event at the beginning of each timeslot. The timing events need to be accurate and therefore cannot be interfered by any other processes. Thus, in one embodiment timing events are generated by hardware. For example, the HAL is configured so that the software beacon alert (“SWBA”) timing is controlled for the purpose of generate timing events. In the default IEEE 802.11 MAC chipsets, the SWBA in the HAL was designed to generate a hardware interrupt so that the 802.11 MAC driver knows when a beacon needs to be prepared.  
         [0096]     This hardware interrupt mechanism is advantageously modified by configuring the HAL so that the SWBA interrupt arrives at the beginning of each new timeslot. As shown, the beginning of each time period corresponds to the arrival time of SWBA interrupt. Although the SWBA interrupt is controlled to generate timing events of the TDMA frame, it also needs to keep the integrity of beacon generation. Accordingly, the beacon generation events are aligned with the timing events of the TDMA frame. Thus, the target beacon transmission time (“TBTT”) event that starts preparation of beacons is aligned with the starting time of ATIM window inside the TDMA frame. Also, the SWBA timing is aligned with the starting time of the broadcast transmission timeslot. And the beacon response time, which is equal to the length of broadcast transmission time, is large enough for beacon related processing to be completed before the TBTT event arrives.  
         [0097]     In one embodiment, the DMT-MAC is configured to determine the current time period and/or determine out how much time is left in the current time period by indexing the timing event for each time period. As shown in TDMA frame  340 , when the starting point of the special timeslot arrives, the count for time periods is reset to zero. Thus, the regular timeslots are indexed from 1 to N−1, the broadcast timeslot is indexed by N, and the ATIM window is indexed by N+1, with the total number of SWBA events equal to N+2.  
         [0098]     Also, the time period can be determined according to the length of each time period and the timing synchronization function. Once a time period is determined, the DMT-MAC can find out how much time of a timeslot has elapsed and how much time in the timeslot still remains.  
         [0099]     Turning to  FIG. 13  an example portion of a TDMA frame according to an embodiment of the present invention is shown. As shown in the portion of the TDMA frame  345 , each timeslot also includes a small period of guard time. Guard time is designed to account for timing shifts that may occur even though synchronization is performed among all nodes. The guard time therefore helps to avoid possible transmission collisions. Guard time is also provided to avoid hardware error due to packet receipt by a node before channel switching is done. This is possible because channel switching does not always end at the same time across all nodes, as described above. Guard time is also provided to avoid the change of a timeslot when a packet is still being transmitted in the hardware and thereby ensure that packet transmission can be completed before the end of a timeslot. For example, when a packet is sent during a timeslot, it takes some time for the hardware to complete the transmission. This time is variable depending on the transmission rate.  
         [0100]      FIG. 14  is a flow diagram illustrating a high level process for distributed multichannel wireless communication according to an embodiment of the present invention. This process may be implemented by a node such as that previously described with respect to  FIGS. 4-5 . Initially, in step  350 , packets are received. Next, in step  355  the DMT-MAC checks if the queue already exists. If not, in step  360  a new first-in-first-out (“FIFO”) queue is created. Otherwise, the packet is stored at the tail of the queue. As shown by steps  365  and  370 , the arrival of a new packet does not always trigger an event of packet transmission in the following situations. For example, in step  365  if the queue for the new packet is not assigned to the current timeslot then the new packet will be buffered rather than transmitted. Also, in step  370 , if another packet in the same queue as the new packet is still in the process of transmission, then the new packet will be buffered rather than transmitted.  
         [0101]     Otherwise stated, the arrival of a new packet triggers a new packet transmission when the current timeslot is assigned to the queue of the new packet and the queue of the new packet does not have packets in the process of being transmitted. Because the packet transmission process for a queue in a timeslot continuously runs unless the queue is empty, this condition is equivalent to the queue being empty before the new packet arrives.  
         [0102]     Once the conditions are met, in step  375  the new packet transmission begins and proceeds until complete or until a transmit interrupt is received, as determined in step  380 . After an interrupt is received, in step  385  it is determined if the transmit queue is empty and if it is then the transmission stops. If it is not, then the next new packet transmission starts by looping back to step  375 . Additionally, at various times during transmission, the transmission channel may be switched by the DMT-MAC, as shown in step  395 . If such a case, the channel switching process continues until it is complete as determined in step  400 . Once the channel switching process is complete, the DMT-MAC proceeds to step  375  where the start of new packet transmission ensues in order to optimize the avoidance of wasted timeslots.  
         [0103]      FIG. 15  is a block diagram illustrating an example wireless communication device  450  that may be used in connection with various embodiments described herein. For example, the wireless communication device  450  may be used in conjunction with a node in a distributed multichannel wireless communication network such as those previously described with respect to  FIGS. 1-5 . However, other wireless communication devices and/or architectures may also be used, as will be clear to those skilled in the art.  
         [0104]     In the illustrated embodiment, wireless communication device  450  comprises an antenna system  455 , a radio system  460 , a baseband system  465 , a speaker  464 , a microphone  470 , a central processing unit (“CPU”)  485 , a data storage area  490 , and a hardware interface  495 . In the wireless communication device  450 , radio frequency (“RF”) signals are transmitted and received over the air by the antenna system  455  under the management of the radio system  460 .  
         [0105]     In one embodiment, the antenna system  455  may comprise one or more antennae and one or more multiplexors (not shown) that perform a switching function to provide the antenna system  455  with transmit and receive signal paths. In the receive path, received RF signals can be coupled from a multiplexor to a low noise amplifier (not shown) that amplifies the received RF signal and sends the amplified signal to the radio system  460 .  
         [0106]     In alternative embodiments, the radio system  460  may comprise one or more radios that are configured to communication over various frequencies. In one embodiment, the radio system  460  may combine a demodulator (not shown) and modulator (not shown) in one integrated circuit (“IC”). The demodulator and modulator can also be separate components. In the incoming path, the demodulator strips away the RF carrier signal leaving a baseband receive audio signal, which is sent from the radio system  460  to the baseband system  465 .  
         [0107]     If the received signal contains audio information, then baseband system  465  decodes the signal and converts it to an analog signal. Then the signal is amplified and sent to the speaker  470 . The baseband system  465  also receives analog audio signals from the microphone  480 . These analog audio signals are converted to digital signals and encoded by the baseband system  465 . The baseband system  465  also codes the digital signals for transmission and generates a baseband transmit audio signal that is routed to the modulator portion of the radio system  460 . The modulator mixes the baseband transmit audio signal with an RF carrier signal generating an RF transmit signal that is routed to the antenna system and may pass through a power amplifier (not shown). The power amplifier amplifies the RF transmit signal and routes it to the antenna system  455  where the signal is switched to the antenna port for transmission.  
         [0108]     The baseband system  465  is also communicatively coupled with the central processing unit  485 . The central processing unit  485  has access to a data storage area  490 . The central processing unit  485  is preferably configured to execute instructions (i.e., computer programs or software) that can be stored in the data storage area  490 . Computer programs can also be received from the baseband processor  465  and stored in the data storage area  490  or executed upon receipt. Such computer programs, when executed, enable the wireless communication device  450  to perform the various functions of the present invention as previously described. For example, data storage area  490  may include various software modules (not shown) that were previously described with respect to  FIGS. 4 and 5 .  
         [0109]     In this description, the term “computer readable medium” is used to refer to any media used to provide executable instructions (e.g., software and computer programs) to the wireless communication device  450  for execution by the central processing unit  485 . Examples of these media include the data storage area  490 , microphone  470  (via the baseband system  465 ), antenna system  455  (also via the baseband system  465 ), and hardware interface  495 . These computer readable mediums are means for providing executable code, programming instructions, and software to the wireless communication device  450 . The executable code, programming instructions, and software, when executed by the central processing unit  485 , preferably cause the central processing unit  485  to perform the inventive features and functions previously described herein.  
         [0110]     The central processing unit  485  is also preferably configured to receive notifications from the hardware interface  495  when new devices are detected by the hardware interface. Hardware interface  495  can be a combination electromechanical detector with controlling software that communicates with the CPU  485  and interacts with new devices. The hardware interface  495  may be a firewire port, a USB port, a Bluetooth or infrared wireless unit, or any of a variety of wired or wireless access mechanisms. Examples of hardware that may be linked with the device  450  include data storage devices, computing devices, headphones, microphones, and the like.  
         [0111]      FIG. 16  is a block diagram illustrating an example computer system  550  that may be used in connection with various embodiments described herein. For example, the computer system  550  may be combined with a wireless communication device such as that described with respect to  FIG. 15  to provide a network node or device such as those previously described in  FIGS. 1-5 . However, other computer systems and/or architectures may be used, as will be clear to those skilled in the art.  
         [0112]     The computer system  550  preferably includes one or more processors, such as processor  552 . Additional processors may be provided, such as an auxiliary processor to manage input/output, an auxiliary processor to perform floating point mathematical operations, a special-purpose microprocessor having an architecture suitable for fast execution of signal processing algorithms (e.g., digital signal processor), a slave processor subordinate to the main processing system (e.g., back-end processor), an additional microprocessor or controller for dual or multiple processor systems, or a coprocessor. Such auxiliary processors may be discrete processors or may be integrated with the processor  552 .  
         [0113]     The processor  552  is preferably connected to a communication bus  554 . The communication bus  554  may include a data channel for facilitating information transfer between storage and other peripheral components of the computer system  550 . The communication bus  554  further may provide a set of signals used for communication with the processor  552 , including a data bus, address bus, and control bus (not shown). The communication bus  554  may comprise any standard or non-standard bus architecture such as, for example, bus architectures compliant with industry standard architecture (“ISA”), extended industry standard architecture (“EISA”), Micro Channel Architecture (“MCA”), peripheral component interconnect (“PCI”) local bus, or standards promulgated by the Institute of Electrical and Electronics Engineers (“IEEE”) including IEEE 488 general-purpose interface bus (“GPIB”), IEEE 696/S-100, and the like.  
         [0114]     Computer system  550  preferably includes a main memory  556  and may also include a secondary memory  558 . The main memory  556  provides storage of instructions and data for programs executing on the processor  552 . The main memory  556  is typically semiconductor-based memory such as dynamic random access memory (“DRAM”) and/or static random access memory (“SRAM”). Other semiconductor-based memory types include, for example, synchronous dynamic random access memory (“SDRAM”), Rambus dynamic random access memory (“RDRAM”), ferroelectric random access memory (“FRAM”), and the like, including read only memory (“ROM”).  
         [0115]     The secondary memory  558  may optionally include a hard disk drive  560  and/or a removable storage drive  562 , for example a floppy disk drive, a magnetic tape drive, a compact disc (“CD”) drive, a digital versatile disc (“DVD”) drive, etc. The removable storage drive  562  reads from and/or writes to a removable storage medium  564  in a well-known manner. Removable storage medium  564  may be, for example, a floppy disk, magnetic tape, CD, DVD, etc.  
         [0116]     The removable storage medium  564  is preferably a computer readable medium having stored thereon computer executable code (i.e., software) and/or data. The computer software or data stored on the removable storage medium  564  is read into the computer system  550  as electrical communication signals  578 .  
         [0117]     In alternative embodiments, secondary memory  558  may include other similar means for allowing computer programs or other data or instructions to be loaded into the computer system  550 . Such means may include, for example, an external storage medium  572  and an interface  570 . Examples of external storage medium  572  may include an external hard disk drive or an external optical drive, or and external magneto-optical drive.  
         [0118]     Other examples of secondary memory  558  may include semiconductor-based memory such as programmable read-only memory (“PROM”), erasable programmable read-only memory (“EPROM”), electrically erasable read-only memory (“EEPROM”), or flash memory (block oriented memory similar to EEPROM). Also included are any other removable storage units  572  and interfaces  570 , which allow software and data to be transferred from the removable storage unit  572  to the computer system  550 .  
         [0119]     Computer system  550  may also include a communication interface  574 . The communication interface  574  allows software and data to be transferred between computer system  550  and external devices (e.g. printers), networks, or information sources. For example, computer software or executable code may be transferred to computer system  550  from a network server via communication interface  574 . Examples of communication interface  574  include a modem, a network interface card (“NIC”), a communications port, a PCMCIA slot and card, an infrared interface, and an IEEE 1394 fire-wire, just to name a few.  
         [0120]     Communication interface  574  preferably implements industry promulgated protocol standards, such as Ethernet IEEE 802 standards, Fiber Channel, digital subscriber line (“DSL”), asynchronous digital subscriber line (“ADSL”), frame relay, asynchronous transfer mode (“ATM”), integrated digital services network (“ISDN”), personal communications services (“PCS”), transmission control protocol/Internet protocol (“TCP/IP”), serial line Internet protocol/point to point protocol (“SLIP/PPP”), and so on, but may also implement customized or non-standard interface protocols as well.  
         [0121]     Software and data transferred via communication interface  574  are generally in the form of electrical communication signals  578 . These signals  578  are preferably provided to communication interface  574  via a communication channel  576 . Communication channel  576  carries signals  578  and can be implemented using a variety of wired or wireless communication means including wire or cable, fiber optics, conventional phone line, cellular phone link, wireless data communication link, radio frequency (RF) link, or infrared link, just to name a few.  
         [0122]     Computer executable code (i.e., computer programs or software) is stored in the main memory  556  and/or the secondary memory  558 . Computer programs can also be received via communication interface  574  and stored in the main memory  556  and/or the secondary memory  558 . Such computer programs, when executed, enable the computer system  550  to perform the various functions of the present invention as previously described.  
         [0123]     In this description, the term “computer readable medium” is used to refer to any media used to provide computer executable code (e.g., software and computer programs) to the computer system  550 . Examples of these media include main memory  556 , secondary memory  558  (including hard disk drive  560 , removable storage medium  564 , and external storage medium  572 ), and any peripheral device communicatively coupled with communication interface  574  (including a network information server or other network device). These computer readable mediums are means for providing executable code, programming instructions, and software to the computer system  550 .  
         [0124]     In an embodiment that is implemented using software, the software may be stored on a computer readable medium and loaded into computer system  550  by way of removable storage drive  562 , interface  570 , or communication interface  574 . In such an embodiment, the software is loaded into the computer system  550  in the form of electrical communication signals  578 . The software, when executed by the processor  552 , preferably causes the processor  552  to perform the inventive features and functions previously described herein.  
         [0125]     Various embodiments may also be implemented primarily in hardware using, for example, components such as application specific integrated circuits (“ASICs”), or field programmable gate arrays (“FPGAs”). Implementation of a hardware state machine capable of performing the functions described herein will also be apparent to those skilled in the relevant art. Various embodiments may also be implemented using a combination of both hardware and software.  
         [0126]     Furthermore, those of skill in the art will appreciate that the various illustrative logical blocks, modules, circuits, and method steps described in connection with the above described figures and the embodiments disclosed herein can often be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled persons can implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the invention. In addition, the grouping of functions within a module, block, circuit or step is for ease of description. Specific functions or steps can be moved from one module, block or circuit to another without departing from the invention.  
         [0127]     Moreover, the various illustrative logical blocks, modules, and methods described in connection with the embodiments disclosed herein can be implemented or performed with a general purpose processor, a digital signal processor (“DSP”), an ASIC, FPGA or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor can be a microprocessor, but in the alternative, the processor can be any processor, controller, microcontroller, or state machine. A processor can also be implemented as a combination of computing devices, for example, a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.  
         [0128]     Additionally, the steps of a method or algorithm described in connection with the embodiments disclosed herein can be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module can reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium including a network storage medium. An exemplary storage medium can be coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium can be integral to the processor. The processor and the storage medium can also reside in an ASIC.  
         [0129]     The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly limited by nothing other than the appended claims.