Abstract:
A network connection device bridges a first network that uses a negotiated packet delivery scheduling scheme and a second network that uses an empirically determined packet delivery scheduling scheme. The network connection device translates a request to communicate over the first network into a request to communicate over the second network, thus bridging the two networks. The negotiated packet delivery scheduling scheme permits endpoints to negotiate scheduled delivery times for packets, while the empirically determined packet delivery scheme tests various time interval locations in a network to determine favorable time locations for transmission. The two protocols are bridged by finding compatible overlaps between time interval locations in the two networks. This can provide error-free delivery with low jitter among packets.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to a device and method for facilitating virtual connections between two different types of networks, each of which uses a different packet delivery scheme.  
         [0002]     As is generally known, Ethernet and Internet Protocol (IP) are systems for transmitting packets between different points on a communications network. These switching systems are known as “contention-based” systems. That is, all transmitters contend for network resources. All transmitters may transmit simultaneously. If they do, then network resources may be oversubscribed. When this happens, data may be delayed or lost, resulting in network impairment.  
         [0003]     As illustrated in  FIG. 1 , four streams of packets are input to a packet switch  112 , which routes the packets to one or more outputs based on addressing information contained in each packet. Packets may arrive at the switch at unpredictable times, leading to bursts of inputs that must be handled. The switch typically maintains a packet queue  114  that is able to store a small number of packets. (In some devices, one queue is provided for each output port, such that if there are  8  ports, there may be  8  queues provided). Each queue may comprise multiple queues arranged by packet priority level, such that priority  3  packets, for example, take precedence over priority  1  packets. If the inputs are too bursty, the queues fill up and some packets may be discarded. The higher-priority queues are typically emptied before the lower-priority queues, such that the lower-priority queues are more likely to lose data first.  
         [0004]     IP systems suffer from impairments such as packet loss and jitter. This happens because there is no control over how many such packets reach a router at any given instant. If two packets arrive at a router at the same time, destined for the same port, one will have to be delayed. Both cannot be transmitted simultaneously. One of the packets will be saved in the queue until the first packet is completely transmitted.  
         [0005]      FIG. 2  shows a computer network comprising endpoints  100 ,  101 ,  102 , and  103 . The network includes routers  104  through  107 . As shown in the figure, if endpoints  100  and  101  communicate with endpoints  102  and  103  at the same time, a bottleneck may develop between routers  105  and  106 . This may occur because too many packets may be simultaneously transmitted between the routers, causing the routers to discard overflow packets. This can happen even at low levels of average network utilization.  
         [0006]     Various methods have been developed to overcome data loss on Ethernet and IP networks. The primary approach has been to use additional protocols to replace lost data. This is an after-the-fact solution. An example is the well-known Transmission Control Protocol (TCP). TCP is able to detect data loss and it causes retransmission of the data, until a perfect copy of the complete data file is delivered to the recipient device.  
         [0007]     One approach for providing reliable first-time delivery is to empirically determine the optimal scheduling of packets in the network by first transmitting test packets during different time periods and, after evaluating latency and/or dropped packet rates for each time period, selecting one or more time periods having favorable transmission characteristics. Packets that are dropped or delayed due to overloaded router queues will indicate unfavorable scheduling conditions, and the transmitting node can select a more favorable schedule for transmitting future packets to minimize the likelihood of packet loss. This approach is described in the previously-filed U.S. patent application Ser. No. 10/663,378 (filed on Aug. 17, 2003), which is described in detail below in conjunction with the principles of the present invention (see the heading entitled Empirical Scheduling of Network Packets below). Using this approach, a virtual connection can be established between two nodes in the network, such as a WAN comprising routers.  
         [0008]     Another type of packet congestion problem may occur in a local area network (LAN) comprising LAN switches. As shown in  FIG. 3 , for example, a conventional network comprises a plurality of Local Area Network (LAN) endpoints, such as computers connected to an Ethernet LAN. The endpoints are coupled to one or more LAN switches  302 , which connect through another part of the network to one or more additional LAN endpoints  303 . When endpoint  301  sends packets to endpoint  303 , the packets are sent through LAN switch  302 , which also handles packets from other LAN endpoints. If too many packets are simultaneously transmitted by the other endpoints to  303 , LAN switch  302  may have a queue overflow, causing packets to be lost. (The word “packets” will be used to refer to datagrams in a LAN or Wide Area Network (WAN) environment. In a LAN environment, packets are sometimes called “frames.” In a packet-switched WAN environment, packet-switching devices are normally referred to as “routers.”).  
         [0009]      FIG. 4  illustrates the nature of the problem of dropped packets in the network of  FIG. 3 , which can occur in a LAN environment as well as a WAN environment. During periods where multiple endpoints are simultaneously transmitting packets on the network, the LAN switch  302  may become overloaded, such that some packets are discarded. This is typically caused by an internal queue in the LAN switch becoming full and thus becoming unable to accept new packets until the outgoing packets have been removed from the queue. This creates a problem in that transmitting endpoints cannot be guaranteed that their packets will arrive, necessitating other solutions such as the use of guaranteed-delivery protocols such as Transmission Control Protocol (TCP). Such solutions may be inappropriate for streaming video or other real-time applications, which cannot wait for retransmission of packets.  
         [0010]     Another approach for providing reliable first-time delivery in a network of the type shown in  FIG. 3  is to have the transmitting node and receiving node in a network agree on a transmission schedule that is compatible with both nodes. For example, when a transmitting node needs to establish a virtual connection with another node in the network, the transmitting node can send a proposed schedule to the receiving node indicating time interval locations during which it proposes to transmit future packets. The receiving node can compare this proposed “transmission map” with other scheduled deliveries at the receiving node and either accept the proposed transmission schedule or propose a different transmission schedule that will minimize packet congestion in the network. This approach is described in U.S. patent application Ser. No. 10/697,103, filed on Oct. 31, 2003, and described below in conjunction with the principles of the invention (see the heading entitled Negotiated Packet Delivery). Using this approach, a virtual connection can be established between two nodes in the network, such as a LAN comprising LAN switches.  
         [0011]     It may be necessary to establish virtual connections between different types of networks having incompatible packet scheduling algorithms. For example, as shown in  FIG. 5 , a first endpoint  501  may be associated with a LAN  504  that employs the negotiated packet scheduling technique discussed above (i.e., endpoints in LAN  504  negotiate “transmission maps” in order to avoid congestion on network  504 ). A second endpoint  502  may be located on another LAN  508  that must be accessed through WAN  503  that employs the empirical packet scheduling technique discussed above (i.e., endpoints in WAN  503  transmit test packets to each other in order to empirically determine the most advantageous time periods for transmitting packets). If endpoint  501  needs to transmit packets to endpoint  502 , difficulties may arise due to incompatible protocols being used for transmitting packets on the networks.  
       SUMMARY OF THE INVENTION  
       [0012]     According to one aspect of the invention, a device and method are provided that allow endpoints on incompatible networks (e.g., an empirically scheduled network and a negotiated scheduled network) to communicate with each other. In one variation, a network connection device (NCD) acts as a proxy device which establishes a virtual connection between the networks. The device may perform handshaking functions to set up and tear down connections between the networks, and may optionally include firewall features and network address translation (NAT) functions. A method according to the invention includes steps of establishing a connection for packet transmission between the incompatible networks and for routing packets according to the differing network protocols. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  shows the problem of bursty packets creating an overflow condition at a packet switch, leading to packet loss.  
         [0014]      FIG. 2  shows how network congestion can lead to a bottleneck where two sets of endpoints share a common network resource under bursty conditions.  
         [0015]      FIG. 3  shows four LAN endpoints communicating with another LAN endpoint through a LAN switch, potentially leading to congestion in the switch.  
         [0016]      FIG. 4  shows a problem of packet (frame) loss due to congestion in the LAN switch of  FIG. 3 .  
         [0017]      FIG. 5  shows two interconnected networks including WAN  503 , which uses an empirically scheduled packet scheduling technique, and LAN  504 , which uses a negotiated packet scheduling technique.  
         [0018]      FIG. 6  shows one approach for assigning different priority levels to scheduled data (realtime level); test packets (discovery level); and other network traffic (data level).  
         [0019]      FIG. 7  shows a frame structure in which a delivery schedule can be decomposed into a master frame; subframes; and secondary subframes.  
         [0020]      FIG. 8  shows a flow chart having steps for performing empirical scheduling over a network such as a WAN.  
         [0021]      FIG. 9  shows a system using a delivery schedule for test packets from a first endpoint to a second endpoint.  
         [0022]      FIG. 10  shows a system wherein queues for realtime traffic (priority  3 ) are nearly full at one packet switch and yet the traffic still gets through the network.  
         [0023]      FIG. 11  shows one method for coordinating a negotiated delivery schedule for transmissions between a transmitting node and an intended recipient node on a network such as a LAN.  
         [0024]      FIG. 12  shows a second method for coordinating a delivery schedule for transmissions between a transmitting node and an intended recipient node.  
         [0025]      FIG. 13  shows a third method for coordinating a delivery schedule for transmissions between a transmitting node and an intended recipient node.  
         [0026]      FIG. 14  shows one possible reception map for a given transmission interval.  
         [0027]      FIG. 15  shows a scheme for synchronizing delivery schedules among network nodes.  
         [0028]      FIG. 16  shows how network congestion is avoided through the use of the inventive principles, leading to more efficient scheduling of packets in the network.  
         [0029]      FIG. 17  shows a system including a network connection device (NCD) according to one variation of the invention.  
         [0030]      FIG. 18  shows steps of a method for initiating a connection between an endpoint in a negotiated packet delivery network with an endpoint in an empirically scheduled packet delivery network.  
         [0031]      FIG. 19  shows one possible configuration for a network connection device (NCD).  
         [0032]      FIG. 20  shows how two endpoints can refer to a time interval specified with reference to frames that have a different phase but which are referenced to a common clock. 
     
    
     DETAILED DESCRIPTION  
       [0033]     Before describing various principles of the present invention, the following explains how packets can be transmitted in networks according to either an empirically determined scheduling technique or a negotiated scheduling technique. It should be clear that both techniques can be used on any type of network.  
         [0034]     A. Empirical Scheduling of Network Packets  
         [0035]     For networks that schedule packets using an empirical approach, a priority scheme can be used to assign priority levels to data packets in a network such that delivery of packets intended for real-time or near real-time delivery (e.g., phone calls, video frames, or TDM data packets converted into IP packets) are assigned the highest priority in the network. A second-highest priority level is assigned to data packets that are used for testing purposes (i.e. the so-called test packets). A third-highest priority level is assigned to remaining data packets in the system, such as TCP data used by web browsers.  FIG. 6  illustrates this scheme. These priority levels can be assigned by enabling the packet priority scheme already available in many routers.  
         [0036]     Other priority levels above and below these three levels can be accommodated as well. For example, a priority level above the real-time level can be assigned for emergency purposes, or for network-level messages (e.g., messages that instruct routers or other devices to perform different functions).  
         [0037]      FIG. 7  shows how an arbitrary delivery time period of one second (a master frame) can be decomposed into subframes each of 100 millisecond duration, and how each subframe can be further decomposed into secondary subframes each of 10 millisecond duration. Each secondary subframe is in turn divided into time intervals of 1 millisecond duration, and this decomposition can be continued to any desired level of granularity. According to one variation, the delivery time period for each second of transmission bandwidth is decomposed using a scheme such as that shown in  FIG. 7  and packets are assigned to one or more time interval locations according to this schedule for purposes of transmitting test packets and for delivering data using the inventive principles. In this sense, the scheme resembles conventional TDM systems. However, unlike TDM systems, no endpoint can be guaranteed to have a particular timeslot or timeslots. Instead, nodes on the network transmit using interval locations that are empirically determined to be favorable based on the prior transmission of test packets between the two endpoints. (Note: the term “interval location” or “time interval location” will be used rather than “time slot” in order to distinguish TDM systems).  
         [0038]      FIG. 8  shows method steps that can be used to schedule packets in a network, such as a WAN, using an empirically determined packet delivery scheme. Beginning in step  801 , a determination is made that two endpoints on the network (e.g., an Ethernet network or an IP network) desire to communicate. This determination may be the result of a telephone receiver being picked up and a telephone number being dialed, indicating that two nodes need to initiate a voice-over-IP connection. Alternatively, a one-way connection may need to be established between a node that is transmitting video data and a receiving node. Each of these connection types can be expected to impose a certain amount of data packet traffic on the network. For example, a voice-over-IP connection may require 64 kilobits per second transfer rate using 80-byte packet payloads with packets being sent every 10 milliseconds (not including packet headers). A video stream would typically impose higher bandwidth requirements on the network.  
         [0039]     Note that for two-way communication, two separate connections must be established: one for node A transmitting to node B, and another connection for node B transmitting to node A. Although the inventive principles will be described with respect to a one-way transmission, it should be understood that the same steps would be repeated at the other endpoint where a two-way connection is desired.  
         [0040]     In step  802 , a delivery schedule is partitioned into interval locations according to a scheme such as that illustrated in  FIG. 7 . (This step can be done in advance and need not be repeated every time a connection is established between two endpoints). The delivery schedule can be derived from a clock such as provided by a Global Positioning System (GPS). As one example, an arbitrary time period of one second can be established for a master frame, which can be successively decomposed into subframes and secondary subframes, wherein each subframe is composed of 10 intervals each of 10 milliseconds in duration and each secondary subframe is compose of 10 intervals each of 1 millisecond in duration. Therefore, a period of one second would comprise 1,000 intervals of 1 millisecond duration. Other time periods could of course be used, and the invention is not intended to be limited to any particular timing scheme.  
         [0041]     In step  803 , the required bandwidth between the two endpoints is determined. For example, for a single voice-over-IP connection, a bandwidth of 64 kilobits per second might be needed. Assuming a packet size of 80 bytes or 640 bits (ignoring packet overhead for the moment), this would mean that 100 packets per second must be transmitted, which works out to (on average) a packet every 10 milliseconds. Returning to the example shown in  FIG. 7 , this would mean transmitting a packet during at least one of the time intervals in the secondary subframe at the bottom of the figure. (Each interval corresponds to one millisecond).  
         [0042]     In step  804 , a plurality of test packets are transmitted during different time intervals at a rate needed to support the desired bandwidth. Each test packet is transmitted using a “discovery” level priority (see  FIG. 6 ) that is higher than that accorded to normal data packets (e.g., TCP packets) but lower than that assigned to realtime data traffic (to be discussed below). For example, turning briefly to  FIG. 9 , suppose that the schedule has been partitioned into one millisecond time intervals. The test packets might be transmitted during time intervals  1 ,  3 ,  5 ,  7 ,  9 ,  11 , and  12  as shown. Each test packet preferably contains the “discovery” level priority; a timestamp to indicate when the packet was sent; a unique sequence number from which the packet can be identified after it has been transmitted; and some means of identifying what time interval was used to transmit the packet. (The time interval might be inferred from the sequence number). The receiving endpoint upon receiving the test packets may return the packets to the sender, which allows the sender to (a) confirm how many of the sent packets were actually received; and (b) determine the latency of each packet. Instead of returning the packets, the receiving endpoint can send a summary packet summarizing statistics for the test packets. Other approaches for determining latency can of course be used. The evaluation can be done by the sender, the recipient, or a combination of the two.  
         [0043]     In step  806 , the sender evaluates the test packets to determine which time interval or intervals are most favorable for carrying out the connection. For example, if it is determined that packets transmitted using time interval # 1  suffered a lower average dropped packet rate than the other intervals, that interval would be preferred. Similarly, the time interval that resulted in the lowest packet latency (round-trip from the sender) could be preferred over other time intervals that had higher latencies. The theory is that packet switches that are beginning to be stressed would have queues that are beginning to fill up, causing increases in latency, jitter, and dropped packets. Accordingly, according to the inventive principles other time intervals could be used to avoid transmitting packets during periods that are likely to increase queue lengths in those switches. In one variation, the time intervals can be “overstressed” to stretch the system a bit. For example, if only 80-byte packets are actually needed, 160-byte packets could be transmitted during the test phase to represent an overloaded condition. The overloaded condition might reveal bottlenecks where the normal 80-byte packets might not.  
         [0044]     Rather than the recipient sending back time-stamped packets, the recipient could instead perform statistics on collected test packets and send back a report identifying the latencies and dropped packet rates associated with each time interval.  
         [0045]     As explained above, packet header overhead has been ignored but would typically be included in the evaluation process (i.e., 80-byte packets would increase by the size of the packet header). Interval selection for the test packets could be determined randomly (i.e., a random selection of time intervals could be selected for the test packets), or they could be determined based on previously used time intervals. For example, if a transmitting node is already transmitting during time interval  3 , it would know in advance that such a time interval might not be a desirable choice for a second connection. As another example, if the transmitting node is already transmitting during interval  3 , the test packets could be transmitted in a time interval location that is furthest away from interval location  3 , in order to spread out as much as possible the packet distribution.  
         [0046]     In step  806 , a connection is established between the two endpoints and packets are transmitted using the higher “realtime” priority level and using the interval or intervals that were determined to be more favorable for transmission. Because the higher priority level is used, the connections are not affected by test packets transmitted across the network, which are at a lower priority level. In one variation, the IP precedence field in IP packet headers can be used to establish the different priority levels.  
         [0047]      FIG. 9  shows a system employing various principles of the invention. As shown in  FIG. 9 , two endpoints each rely on a GPS receiver for accurate time clock synchronization (e.g., for timestamping and latency determination purposes). The IP network may be comprised of a plurality of routers and/or other network devices that are able to ultimately route packets (e.g., IP or Ethernet packets) from one endpoint to the other. It is assumed that the organization configuring the network has the ability to control priority levels used on the network, in order to prevent other nodes from using the discovery priority level and realtime priority level.  
         [0048]     It should be appreciated that rather than transmitting test packets simultaneously during different time interval locations, a single time interval location can be tested, then another one, and so on, until an appropriate interval location is found for transmission. This would increase the time required to establish a connection. Also, as described above, for a two-way connection, both endpoints would carry out the steps to establish the connection.  
         [0049]     The scheme will also work with “early discard” settings in router queues since the empirical method would detect that a discard condition is approaching. In other words, it would be able to detect situations where discards could occur, such as might happen if more traffic were to be added at that point in time.  
         [0050]     In another variation, packet latencies, jitter, and packet dropped rates can be monitored during a connection between endpoints and, based on detecting a downward trend in either parameter, additional test packets can be transmitted to find a better time location in which to move the connection.  
         [0051]      FIG. 10  shows a system in which a first endpoint  1001  communicates with a second endpoint  1006  through a plurality of packet switches  1003  through  1005 . Each packet switch maintains a plurality of packet queues. (As pointed out above, in some devices separate queues may be maintained for each output port). For illustrative purposes, four different priority levels are shown, wherein  4  is the highest level, and level  1  is the lowest level. Assume that endpoint  1001  attempts to initiate a connection with endpoint  106  through the network. Endpoint  1001  transmits a plurality of “test” packets using priority level  2 . As can be seen, packet switch  1003  is lightly loaded and the queues have no difficulty keeping up with the traffic.  
         [0052]     Packet switch  1004 , however, is heavily loaded. In that switch, the queue for priority level  1  traffic is full, leading to dropped packets and latencies. Similarly, the test packets transmitted by endpoint  1001  at priority level  2  cause that queue to overflow, causing dropped packets and longer latencies. However, the priority level  3  queue (existing realtime traffic) is not yet full, so those packets are transported through the network unaffected at a given instant in time. In accordance with one variation of the invention, upon detecting that test packets sent during certain time interval locations are dropped and/or suffer from high latencies, endpoint  1001  selects those time locations having either the lowest drop rate and/or the lowest latencies, and uses those locations to schedule the packets (which are then transmitted using level  3  priority).  
         [0053]     It is assumed that each endpoint in  FIG. 10  comprises a node (i.e., a computer having a network interface) including computer-executable instructions for carrying out one or more of the above-described functions.  
         [0054]     B. Negotiated Packet Delivery  
         [0055]     In networks that transmit packets using negotiated packet delivery, a transmitting node transmits a query to the intended receiving node. The receiving node responds with a reception map indicating what transmission time interval locations have already been allocated by other transmitting nodes (or, alternatively, what transmission time interval locations are available). The transmitting node then proposes a transmission map to the receiving node, taking into account any time locations previously allocated. The receiving node either accepts the proposed transmission map or proposes an alternate transmission map. Upon agreement between the nodes, the transmitting node begins transmitting according to the proposed transmission map, and the receiving node incorporates the proposed transmission map into its allocation tables. Because the proposed delivery schedule has been agreed to between the two endpoints, uncoordinated contention that might otherwise overflow network switches near the endpoints is avoided. (Because, in some devices, each port has its own queue or queues, traffic on different queues would not conflict). Because the schedule is determined by the two endpoints, no network arbiter is needed to coordinate among network resources. In one embodiment, negotiation occurs only between single LAN switches.  
         [0056]     In one embodiment, a transmitting node transmits a bandwidth requirement to an intended recipient node, indicating the bandwidth it requires to support a proposed transmission (e.g., streaming video packets). The intended recipient node, after evaluating time interval locations previously allocated to other transmitters, responds with a proposed delivery schedule indicating time locations during which the transmitter should transmit packets in order to avoid contention with other previously scheduled packets while maintaining the necessary bandwidth for the transmitter. The transmitter thereafter transmits packets according to the proposed delivery schedule.  
         [0057]     In yet another variation, a transmitting node transmits a proposed delivery schedule to an intended recipient, indicating time interval locations corresponding to times during which it proposes to transmit packets. The intended recipient either agrees to the proposed delivery schedule, or proposes an alternate delivery schedule that takes into account the transmitter&#39;s bandwidth requirements. Upon agreement between the nodes, transmission occurs according to the agreed-upon delivery schedule. The schedule can be released at the end of the transmission.  
         [0058]     Returning briefly to  FIG. 7 , a transmission interval can be partitioned into units and (optionally) subunits of time during which data packets can be transmitted. In the example of  FIG. 7 , an arbitrary transmission interval one second (a master frame) can be decomposed into subframes each of 100 millisecond duration, and each subframe can be further decomposed into secondary subframes each of 10 milliseconds duration. Each secondary subframe is in turn divided into time interval locations of 1 millisecond duration. (As described previously, the time decomposition could be carried out to any desired level of granularity, and the description is not intended to be limiting in this respect.)  
         [0059]     According to one variation, the scheduled delivery scheme applies to prioritized packets in the network; other non-prioritized packets are not included in this scheme. Therefore, in a system that supports only priority traffic and non-priority traffic, the scheduled delivery scheme would be applied to all priority traffic, and ad-hoc network traffic would continue to be delivered on a nonpriority basis. In other words, all priority traffic would be delivered before any nonpriority traffic is delivered.  
         [0060]     Returning to  FIG. 11 , in step  1101 , a transmitting node sends a query to an intended receiving node in the network for a reception map. A reception map (see  FIG. 14 ) is a data structure indicating time interval locations that have already been allocated to other transmitters for reception by the receiving node (or, alternatively, time locations that have not yet been allocated, or, alternatively, time locations that are candidates for transmission). More generally, a reception map is a data structure that indicates—in one form or another—time interval locations during which transmission to the intended receiving node would not conflict with other transmitters. Although there are many ways of representing such a map, one approach is to use a bitmap wherein each bit corresponds to one time interval location, and a “1” indicates that the time location has been allocated to a transmitting node, and a “0” indicates that the time location has not yet been allocated.  FIG. 11  thus represents  25  time locations of a delivery schedule, and certain time interval locations (indicated by an “x” in  FIG. 14 ) have already been allocated to other transmitters.  
         [0061]     In step  1102 , the intended receiving node responds with a reception map such as that shown in  FIG. 14 , indicating which time locations have already been allocated to other transmitters. If this were the first transmitter to transmit to that receiving node, the reception map would be empty. It is also possible that time locations could have been previously allocated to the same transmitter to support an earlier transmission (i.e., the same transmitter needs to establish a second connection to the same recipient).  
         [0062]     In step  1103 , the transmitter sends a proposed transmission map to the intended receiving node. The proposed transmission map preferably takes into account the allocated time locations received from the intended receiving node, so that previously allocated time locations are avoided. The transmitter allocates enough time locations to support the required bandwidth of the transmission while avoiding previously allocated time interval locations.  
         [0063]     Suppose that a virtual connection is to be established between two nodes on the network to support a telephone voice connection. A voice-over-IP connection may require 64 kilobits per second transfer rate using 80-byte packet payloads every 10 milliseconds (not including packet headers). A video stream would typically impose higher bandwidth requirements on the network. On an Ethernet LAN, each packet would comprise up to 1,500 bytes.  
         [0064]     In step  1104 , the intended recipient reviews the proposed transmission map and agrees to it, or proposes an alternate transmission map. For example, if the intended recipient had allocated some of the proposed time locations to another transmitter during the time that the transmitter was negotiating for bandwidth, the newly proposed delivery schedule might present a conflict. In that situation, the intended recipient might propose an alternate map that maintained the bandwidth requirements of the transmitter.  
         [0065]     In step  1105 , the transmitter repeatedly transmits to the intended recipient according to the agreed delivery schedule. To support a voice-over-IP connection, for example, the transmitter could transmit an 80-byte packet every 10 milliseconds. For a video connection, the transmitter could transmit at a more frequent rate. Finally, in step  1106  the receiver&#39;s map is deallocated when the transmitter no longer continues to transmit. Deallocation could instead be performed implicitly by noticing that traffic is no longer being transmitted.  
         [0066]     Note that for two-way communication, two separate connections must be established: one for node A transmitting to node B, and another connection for node B transmitting to node A. Although the inventive principles will be described with respect to a one-way transmission, it should be understood that the same steps would be repeated at the other endpoint where a two-way connection is desired.  
         [0067]      FIG. 12  shows an alternative method for negotiating packet delivery times. Beginning in step  1201 , the transmitter sends a bandwidth requirement to the intended recipient. For example, the transmitter may dictate a packet size and/or bandwidth, and the intended recipient could determine which time locations should be allocated to support that bandwidth. In step  1202 , the intended recipient responds with a proposed transmission map that takes into account previously allocated time locations.  
         [0068]     In step  1203 , the transmitter agrees to the proposed transmission map, causing the intended receiver to “lock in” the agreed time locations (this step could be omitted), and in step  1204  the transmitter transmits packets according to the agreed-upon schedule. Finally, in step  1205  the transmission map is deallocated upon termination of the connection.  
         [0069]      FIG. 13  shows another variation for negotiated packet delivery. In step  1301 , the transmitting node sends a proposed transmission map to the intended recipient. In step  1302 , the intended recipient either agrees to the proposed transmission map (if it is compatible with any previously-allocated maps) or proposes an alternative map that meets the transmitter&#39;s bandwidth requirements, which can be inferred from the proposed transmission map. For example, if the transmitter had proposed transmitting in time locations  1 ,  11 ,  21 ,  31 ,  41 , and so forth, it would be evident that the transmitter needed to transmit once every tenth time interval. If the requested locations were not available, the intended recipient could instead propose time locations  2 ,  12 ,  22 ,  32 , and so forth.  
         [0070]     In step  1303 , the transmitter transmits packets according to the agreed-upon delivery schedule, and in step  1304  the transmission map is deallocated upon termination of the transmission.  
         [0071]     In another variation, a transmitter may request bandwidth (e.g., one 1000-byte packet every 10 milliseconds) and the receiver responds with a placement message (e.g., start it at the 75th time location). The receiver could also respond with multiple alternatives (e.g., start it at the 75th, the 111th, or the 376th time location). The transmitter would respond with the time interval location that it intended to use (e.g., the 111th), and begin transmission. This variation is intended to be within the scope of sending “transmission maps” and “reception maps” as those terms are used herein.  
         [0072]     In order for each transmitter and receiver to agree on a delivery schedule, it is desirable to develop and maintain some time synchronization between the nodes.  FIG. 15  shows one possible approach for synchronizing delivery schedules among nodes in a network.  
         [0073]     As shown in  FIG. 15 , the network comprises various endpoints connected through a switch  1502 . According to one variation of the invention, a clock source  1504  (e.g., a GPS-derived clock) is coupled through an electrical wire  1505  to each network node participating in the scheduled delivery scheme. The clock source generates pulses that are transmitted to each node and used as the basis for deriving the delivery schedule. Each node may comprise a timer card or other mechanism (e.g., an interrupt-driven operating system) that is able to use the timing signals to establish a common reference frame. This means for synchronizing may therefore comprise a physical wire (separate and apart from the network) over which a synchronization signal is transmitted to each node. It may further comprise a hardware card and/or software in each node to detect and decode the synchronization signal.  
         [0074]     The clock pulses may comprise a pulse according to an agreed-upon interval (e.g., one second) that is used by each node to generate time locations that are synchronized to the beginning of the pulses. Alternatively, the clock source may generate a high-frequency signal that is then divided down into time locations by each node. Other approaches are of course possible. As yet another alternative, each node may contain its own clock source that is synchronized (via GPS or other means) to a common reference signal, such as a radio signal transmitted by the U.S. Government. Wire  1505  may comprise a coaxial cable or other means of connecting the clock source to the nodes. In one variation, the connection is of a short enough distance (hundreds of feet) so that transmission effects and delays are avoided. Any of these means for synchronizing may be used independently of the others.  
         [0075]     Another way or means of synchronizing time locations and delivery schedules among the nodes is to have one node periodically transmit (e.g., via multicast) a synchronization packet on the node on the network. Each node would receive the packet and use it to synchronize an internal clock for reference purposes. As an alternative to the multicast approach, one network node can be configured to individually send synchronization packets to each participating network node, taking into account the stagger delay involved in such transmission. For example, a synchronization node would transmit a synchronization packet to a first node on the network, then send the same packet to a second node on the network, which would be received later by the second node. The difference in time could be quantified and used to correct back to a common reference point. Other approaches are of course possible.  
         [0076]      FIG. 16  illustrates how the negotiated packet delivery scheme can reduce congestion by more efficiently scheduling data packets between transmitters and receivers. As shown in  FIG. 16 , because each transmitting node schedules packets for delivery during times that do not conflict with those transmitted by other nodes, no packets are lost.  
         [0077]     C. Network Connection Device  
         [0078]     Having reviewed the principles of a network that uses an empirically scheduled packet delivery scheme and a network that uses a negotiated packet delivery scheme, reference will again be made to  FIG. 5 , which illustrates the problem where two endpoints  501  and  502  must communicate over different types of networks. According to one variation of the invention, a network connection device (NCD) is provided for facilitating virtual connections between endpoints across the different networks.  
         [0079]      FIG. 17  shows a system including one or more network connection devices (NCDs)  1707 ,  1708 , and  1710  according to one variation of the invention. Network connection device  1708  bridges first network  1704  (in this case, a LAN that uses a negotiated packet scheduling technique for transmitting packets to other nodes) and second network  1703  (in this case, a WAN using an empirical scheduling technique). In this respect, NCD  1708  acts as a proxy between network  1704  and network  1703 , translating packet delivery protocols in order to allow communication between different packet scheduling techniques. NCD  1710  also connects LAN  1704  to WAN  1703  through routers  1701  and  1706 . Similarly, NCD  1707  bridges LAN  1709  and WAN  1703 .  
         [0080]     Although each NCD is shown as a separate device, the functions of each NCD as described below can be implemented using software and/or hardware (e.g., PLDs, PALs, etc.) added to existing network devices, such as routers or other elements. The NCD functions can also be implemented in access aggregation devices, such as a DSLAM (DSL Access Multiplexer), which is a device that aggregates multiple DSL lines into a larger network structure. Accordingly, the term “network connection device” or NCD should be understood to connote a device that performs NCD-like functions, regardless of the specific hardware, software, or network element in which those functions are implemented.  
         [0081]     When endpoint  1701  needs to communicate with endpoint  1702 , it attempts to initiate a negotiated packet delivery schedule with endpoint  1702 . The delivery protocol in LAN  1704  can determine, based on the IP address of requested endpoint  1702 , that endpoint  1702  does not reside within LAN  1704 , and thus determines that it must be accessed by routing the request through NCD  1708 . Therefore, the protocol routes the request to NCD  1708 , which determines that endpoint  1702  must be accessed through NCD  1707 , which is over an empirically scheduled WAN  1703 . Consequently, NCD  1708  initiates an empirically determined protocol (see section A above) with NCD  1707  over WAN  1703 . In other words, it sends test packets to NCD  1707  and empirically determines which time locations would be most advantageous to avoid overloading routers in the path leading to NCD  1707 . NCD  1708  then returns these empirically determined time locations to endpoint  1701  as an alternate transmission map according to the negotiated packet delivery protocol (see section B above). Similarly, NCD  1707  establishes a negotiated time location delivery schedule with endpoint  1702  to complete the path over LAN  1709 .  
         [0082]     In one embodiment, the endpoints must conform to the empirically determined time interval locations, since it may be difficult to force the WAN to accept time intervals that are not empirically desirable. However, it may not be necessary to follow this restriction in all systems.  
         [0083]     In addition to acting as a proxy between the networks, each NCD may perform other optional functions, such as (1) protecting each network from unauthorized higher priority traffic that has been improperly introduced into the network; (2) encryption/decryption of packets; (3) network address translation (NAT); (4) proxy IP addressing; (5) firewall protection; and (6) controlling total and individual flows by bandwidth, type, etc. in order to prevent overloading of network choke points. In some embodiments, a router can be placed on either side of the NCD  1708 , or the NCD functions can instead be incorporated into a router. Some or all of these functions, described in more detail below, can be implemented in software executing on a general-purpose computer.  
         [0084]      FIG. 18  shows steps of a method for initiating a connection between an endpoint in a negotiated packet delivery network with an endpoint in another network that must traverse an empirically scheduled packet network, such as WAN  1703 .  
         [0085]     In step  1801 , a negotiated delivery endpoint (e.g., endpoint  1701  in LAN  1704 ) requests a virtual connection (e.g., a voice-over-IP circuit) to an endpoint which is in a network that can only be reached via an empirically scheduled network (e.g., endpoint  1702  via WAN  1703 ). Based on the IP address of the requested endpoint, the network protocol in LAN  1704  is able to determine that the endpoint is not located in LAN  1704 , but instead must be accessed through NCD  1708 . Consequently, it sends the request to NCD  1708  (e.g., through a router not shown in  FIG. 17 ).  
         [0086]     In step  1802 , NCD  1708  receives the request, which as described in section B above may include a proposed transmission map, or a bandwidth request for an associated transmission map to be provided by the intended recipient. Thereafter, NCD  1708  initiates a connection with NCD  1707  using the empirically scheduled delivery scheme described above in section A. In other words, NCD  1708  sends test packets in various time locations to NCD  1707  and empirically determines which time locations would be advantageous for transmission. Furthermore, NCD  1707  initiates a negotiated delivery schedule with endpoint  1702  that is consistent with the empirically determined time locations between NCD  1707  and  1708 .  
         [0087]     In step  1803 , NCD  1708  obtains the empirical data and converts it into a negotiated transmission map (see, e.g.,  FIG. 14 ) and returns it to endpoint  1701 . If endpoint  1701  had proposed a certain transmission map that coincided with favorable empirical data, then NCD could indicate that the proposed transmission map was accepted. If endpoint  1701  had proposed a certain transmission map that was not consistent with the empirically determined advantageous time locations, then NCD  1708  would return an alternate transmission map consistent with the empirical data. If endpoint  1701  had instead requested a bandwidth, NCD could propose a transmission map that met the bandwidth requirement and that was consistent with empirical scheduling determination.  
         [0088]     In step  1804 , the negotiated endpoint thereafter transmits to endpoint  1702  according to the transmission map received from NCD  1708 .  
         [0089]     In step  1805 , the connection is torn down by NCD  1708  when one or the other endpoint requests termination of the circuit.  
         [0090]     For a two-way connection, an endpoint in an empirically scheduled network may need to set up a connection to an endpoint in a negotiated delivery network. Therefore, the steps shown in  FIG. 18  will be repeated in reverse (from endpoint  1702  to endpoint  1701 ) in order to establish two-way communication, such as might be required for a voice-over-IP connection.  
         [0091]      FIG. 19  shows one possible configuration for an NCD. The unit  1901  may include a memory, a CPU, and I/O circuitry. The CPU is programmed with software to carry out the functions described above and shown in  FIG. 18 . The I/O circuitry may be coupled to a switch  1903  which is in turn coupled to various devices such as IP phones which can be used to establish two-way connections using the principles outlined above. The device  1901  may receive timing from an uplink and/or an external clock source, such as from a GPS or cellular CDMA source.  
         [0092]     It should also be understood that the phase of all frames may be independent from one another; they need only be derived from or aligned with a common clock. Different endpoints need not have frames synchronized in phase with each other. In other words, each time interval need not be uniquely identified among different endpoints, as long as the time intervals remain in relative synchronicity. This principle is shown with reference to  FIG. 20 , which shows how two endpoints can refer to a time interval specified with reference to frames that have a different phase but which are referenced to a common clock. (It is not necessary that the endpoints actually be synchronized to a common clock, although  FIG. 20  shows this for each of understanding).  
         [0093]     As shown in  FIG. 20 , suppose that endpoint A (bottom of  FIG. 20 ) needs to communicate with endpoint B (top of  FIG. 20 ) through a WAN that introduces a packet delay. Each endpoint has an associated NCD that handles the connection with the WAN. Suppose also that the timeline across the top of  FIG. 20  and the timeline across the bottom of  FIG. 20  represent “absolute” time; i.e., time interval  1  at the top of  FIG. 20  appears at the same instant in absolute time as time interval  1  at the bottom of  FIG. 20 . Suppose further that NCD A transmits a first test packet X across the network during interval  1  and a second test packet Y across the network during interval  3 . Due to the packet delay introduced by the WAN, test packet X will not arrive at endpoint B until what endpoint B perceives to be time interval  4 . Similarly, test packet Y will not arrive at endpoint B until what endpoint B perceives to be time interval  6 . Yet endpoints A and B (through their respective network connection devices NCD A and NCD B) need to agree on what time interval future packets will be transmitted.  
         [0094]     In short, when NCD B determines that test packet X was received with minimal delay, it informs NCD A that the test packet identified as “packet X” was empirically favorable for future transmissions. Thus, NCD A identifies the relevant time interval as interval  1 , whereas NCD B identifies the relevant time interval as interval  4 . Similarly, NCD A identifies the relevant time interval for packet Y as interval  3 , whereas NCD B identifies the relevant time interval for packet Y as interval  6 . As long as the timeline at the top of  FIG. 20  and the timeline at the bottom of  FIG. 20  do not move relative to each other, the system can accommodate packet, delays and can agree on what time interval locations should be used to transmit packets. Other approaches can of course be used.  
         [0095]     Although not explicitly shown above, the networks may include one or more soft phone switches (essentially a small computer coupled to the network) that maintains a database of phone numbers and maps them to IP addresses. To make a phone call to an intended recipient, the phone switch is contacted to determine the IP address corresponding to the recipient&#39;s telephone number. The inventive system and method may also be employed with video terminals to transmit video-grade data across networks; computer terminals that transmit computer data; or any other type of data.  
         [0096]     Any of the method steps described herein can be implemented in computer software and stored on computer-readable medium for execution in a general-purpose or special-purpose hardware or processor (PLDs, PGAs, routers, switches, etc.) or computer, and such computer-readable media is included within the scope of the intended invention. The term “processor” as used herein should be understood to include any of these various types of devices. Numbering associated with method or process steps in the claims is for convenience only and should not be read to require a particular ordering or sequence