HK1072136A - Multicast ip zones for fast spanning tree convergence in wide-area packet network systems - Google Patents
Multicast ip zones for fast spanning tree convergence in wide-area packet network systems Download PDFInfo
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Description
Technical Field
The present invention relates generally to communication systems, and more particularly to wide area communication systems incorporating Internet Protocol (IP) multicast routing protocols.
Background
Communication systems typically include a plurality of communication devices, such as mobile or portable wireless units, scheduling consoles, and base stations (sometimes referred to as base station repeaters) geographically distributed between various base stations and control stations. The wireless units communicate wirelessly with the base stations, or with each other, using Radio Frequency (RF) communication resources, which are typically logically divided into subgroups, or talk groups. Communication systems are usually composed in the form of relay systems (trunk systems), with communication resources being distributed between a plurality of users or groups on a call-by-call basis. Wide area relay systems sometimes make up multiple "zones," where each zone includes multiple sites and a central controller or server ("zone controller") for allocating communication resources among the multiple sites.
Next generation communication systems are beginning to use Internet Protocol (IP) multicast technology to transport packet data representing voice, video, data, or control traffic between endpoints (or "hosts" in IP terminology). In such systems, where the host devices include base stations, consoles, zone controllers, (and in some cases wireless mobile or portable wireless units in different zones that desire to receive packets for a particular call), the host devices send Internet Group Management Protocol (IGMP) join messages to the routers to which they belong, which messages cause the routers in the network to create a router interface spanning tree for distributing packets for the call. Currently, there are two basic types of IP multicast routing protocols, commonly referred to as sparse mode (sparse mode) and dense mode (dense mode). In general, in sparse mode, the spanning tree of the router interface is preconfigured only to the branches of the endpoints that are already connected to the multicast address; while dense mode uses "flood and prune" operations, so the spanning tree branches initially to all endpoints of the network and then retracts (or prunes) to eliminate unnecessary paths.
In IP multicast communication systems, particularly in very large systems comprising hundreds of sites and/or areas, a problem arises in that the multicast spanning tree is so large that the time taken and the resulting traffic to build the spanning tree by the multicast routing protocol adversely affects call setup time or voice quality, since each site is competing for limited site bandwidth. What is needed is an apparatus and method for routing IP multicast packets in a multi-zone system without requiring a multicast spanning tree across multiple zones. Advantageously, the apparatus and method will provide a plurality of multicast spanning trees, each confined to a single region, so that spanning trees converge much faster than would be necessary across multiple regions. The present invention addresses these needs.
Drawings
The above and other advantages of the invention will be appreciated upon reading the following detailed description and upon reference to the drawings in which:
fig. 1 illustrates a multi-zone packet-based communication system employing a packet duplicator (duplicator) in one embodiment of the present invention;
FIG. 2 is a flowchart showing steps performed by a packet duplicator in implementing a talkgroup call in a multi-zone packet-based communication system of one embodiment of the present invention;
FIG. 3 is a flowchart showing steps performed by a zone controller implementing a talkgroup call in a multi-zone packet-based communication system of one embodiment of the present invention; and
figure 4 is a message sequence chart illustrating one example of a multi-zone talkgroup call in the present invention.
Detailed Description
Fig. 1 shows, by way of example and not limitation, a packet-based communication system 100 that includes a plurality of base stations 102 that form a plurality of zones ("zone 1" through "zone 4"). Although it should be understood that a communication system may include hundreds of sites and/or regions, for convenience only four regions are shown, with five base stations in each region. Although it should be understood that base stations and/or control stations are typically located in each area, base station 102 is shown only in area 1. The base station 102 includes a base station 106 for communicating with wireless communication units (e.g., communication unit 157-163) over RF resources within respective coverage areas, which may roam from one site to another and from one area to another. Communication system 100 may also include a console or infrastructure device not shown, as is known in the art, including, for example: dispatch console, call recorder, site controller(s), comparator(s), telephony interconnect device(s), internet protocol telephony device(s), scanner(s), or gateway(s). These devices are typically wired devices, i.e., wired to a base station or other infrastructure device, although wireless devices may be equally applicable.
The base stations 102 are logically connected together with router units 116, 118, 120, 122 ("core routers") in various regions via the router elements 104 ("base station routers"). The base station router 104 and the core routers 116, 118, 120, 122 are functional elements contained in individual physical devices or combinations of devices. The core routers are logically connected together via (inter-regional) packet network links 148, 150, 152, 154. The core routers 116, 118, 120, 122 are connected to zone controllers 124, 126, 128, 130, respectively, which perform call processing and mobility management functions for the communication units in their respective zones.
Generally, routers in a network comprise special purpose or general purpose computing devices configured to receive IP packets from a particular host in communication system 100 and relay those packets to other router(s) or host(s) in communication system 100. Thus, routers define packet networks for routing packets between host devices in communication system 100. A host device that is the source or recipient of IP packets representing control or payload messages for a particular call (or call setup), as defined herein, is the "participating device" of this call. The area in which the participating devices are located is referred to as the "participating area". Host devices may include routers, base stations 106, zone controllers 124, 126, 128, 130, consoles, or generally any wired device in communication system 100. Recent technological developments have also expanded the functionality of IP hosts into wireless devices, in which case the wireless communication unit 157, 163, or other wireless devices may constitute a host device, as defined herein. Each host device has a unique IP address. The host device includes a respective processor (which may include, for example, a microprocessor, microcontroller, digital signal processor, or combination of such devices) and memory (which may include, for example, volatile or non-volatile storage devices, or combination of such devices).
Packets are distributed from point-to-point using a unicast routing protocol, or from point-to-multipoint (i.e., a group of hosts) using a multicast routing protocol. As will be described in detail with reference to fig. 2-4, the preferred embodiment of the present invention uses a multicast routing tree that is built separately in each area to avoid the multicast spanning tree spanning multiple areas. Suitable Multicast Routing protocols may include Sparse Mode Routing protocols such as the Core Tree (CBT) Protocol or the Protocol Independent Multicast-Sparse Mode (PIM-SM) Protocol, Dense Mode Routing protocols such as the Distance Vector Multicast Routing Protocol (DVMRP), the Protocol Independent Multicast-Dense Mode (PIM-DM) or the Multicast Open Shortest Path First (MOSPF) Protocol. The multicast protocol used by each zone may be different.
In accordance with the principles of the present invention, the communication system includes a plurality of packet duplicators 132, 134, 136, 138 respectively associated with zones 1-4. The packet duplicator is a functional host that may be contained in a separate physical device or a combination of such devices. For example, the packet duplicators 132, 134, 136, 138 could be employed in one or more of the zone controllers 124, 126, 128, 130. In general, as will be described in detail with reference to fig. 2, each packet duplicator is adapted to receive any packets originating from the region in which it is located, duplicate the packets, and send the duplicated packets to the packet duplicators of the other participating regions. The packet duplicators of the participating zones are then responsible for distributing these packets in their respective zones. The packet duplicator communicates packets between them using IP unicast directed to the direction of the unique IP address of the receiving packet duplicator(s). The packet duplicators send and receive packets to and from the host device with each other within their respective zones via multicast addresses, which in a preferred embodiment are different from zone to zone. In this manner, packet communication between areas is accomplished by using IP unicast, and communication within an area (i.e., intra-area communication) is accomplished by using IP multicast with a separate multicast tree.
Figure 2 shows the steps performed by the packet duplicator to implement a talk group call in accordance with the present invention. For convenience, the steps in FIG. 2 will be described with reference to FIG. 1, where communication unit 157 (in zone 1) is the source and communication units 158-160 (in zone 3) and 161-163 (in zone 4) are the recipients of the talkgroup call; and reference will be made to figure 4 which illustrates the message sequence for a call. Initially, at least zone 1 is defined as the source zone and zones 3 and 4 are defined as the listening zones for the call. At step 202, the participating packet duplicators receive a multicast group address to be used in the respective region. In the example of fig. 1, packet duplicators 132 (in zone 1), 136 (in zone 3), and 138 (in zone 4) are participating packet duplicators of the call.
As best described in fig. 4, in one embodiment, after receiving a call request 400 for a talkgroup call (e.g., TG1) from a base station of an intended source (e.g., communication unit 157), the zone controllers of the participating zones dynamically determine the multicast group address. In the preferred embodiment, the multicast group address for each participating zone is different (e.g., zone 1 for MC1, zone 3 for MC3, zone 4 for MC4) so separate multicast trees will be established in the respective zones. In addition, the multicast group address may be statically determined and stored in the memory of the zone controller and/or the packet duplicator for each zone. As shown in fig. 4, the multicast group address is communicated to the participating packet duplicators in the form of a "start call origination" message 402 (for packet duplicators in the origination area) and a "start call listening" message 404 (for packet duplicators in the listening area).
In fig. 2, the packet duplicator of the source region and the packet duplicator(s) of the listening region are distinguished at step 204. The packet duplicator in the source region is defined as the packet duplicator in the source payload region of the call (or the intended call). Correspondingly, the packet duplicator(s) in the listening area(s) is (are) defined as the packet duplicator(s) in the area(s) that are (at least now) not the source of the payload call. For example, referring to fig. 1, the communication unit 157 is the source of the payload call, the packet duplicator 132 (at least initially) is the source zone packet duplicator of the call, and the packet duplicators 136 (in zone 3) and 138 (in zone 4) are the listening zone packet duplicators of the call. In fig. 4, the source zone packet duplicator 132 is referred to as PD1, and the listening zone packet duplicators 136 and 138 are referred to as PD3 and PD4, respectively. It should be appreciated that the source zone and listening zone packet duplicators may periodically switch when members of different talkgroups (i.e., different zones) generate payloads for calls, or when the source device moves from one zone to another.
At step 206, the source zone packet duplicator joins the multicast group address of the zone in which it is located, or the source zone multicast address. In one embodiment, this is done by the source region packet duplicator sending an IGMP join message to the core router to which it belongs. Thus, in this example, the packet duplicator 132 ("PD 1") sends an IGMP join message 406 to its core router 116 in order to join the multicast group MC 1. In response to the join message 406, the core router generates a multicast spanning tree in region 1, which allows the PD1 to receive control or payload messages addressed to the source region multicast group address MC 1.
At step 208, the source region packet duplicator receives a unicast address of the listening packet duplicator. In a preferred embodiment, as best described in fig. 4, unicast addresses are transmitted from the zone controllers ("ZCs 3", "ZCs 4") of the listening zones to the zone controllers ("ZCs 1") of the source zone in the form of respective "call response" messages 408, 412; and then transmitted to the packet duplicator ("PD 1") of the source region in the form of an "add destination" message 410, 414. For convenience, in fig. 4, the unicast addresses of the packet duplicators "PD 1", "PD 2", etc. are represented in the same form as their own names "PD 1", "PD 2", etc. Thus, in the example of fig. 4, the packet duplicator PD1 receives the unicast addresses PD3, PD4 of the snooping packet duplicators PD3, PD 4.
At step 210, the listening area packet duplicator receives the unicast addresses of all participating packet duplicators. In one embodiment, as best described in fig. 4, the unicast addresses are transmitted in the form of a "call grant" message 416 to zone controllers ("ZCs 3", "ZCs 4") of the listening area. Thus, in the example of fig. 4, the call admission message 416 includes the unicast addresses PD1, PD3, PD4 of all packet duplicators participating in the call, PD1, PD3, PD 4.
After the source communication unit (e.g., communication unit 157) generates the payload for the call, the participating devices that joined the multicast address are able to receive the payload. In one embodiment, a source communication unit (e.g., communication unit 157) sends a payload to a station (e.g., base station 106) associated therewith on a designated RF channel, and the station or infrastructure device then maps the payload address to the correct multicast address (e.g., MC 1). Alternatively, the source communication unit 157 may itself send payload traffic to the multicast address MC 1. In either case, the payload (represented by sequence number 418 in fig. 4) is sent from the base station 106 to the participating devices joined to the source region multicast address MC1 via the network's routers, thereby defining a source region multicast routing tree.
At step 212, it is determined whether the packet duplicator received a payload via the multicast address MC 1. In one embodiment, as already said, the source zone packet duplicator (e.g., PD1) joins the source zone multicast address MC1 at step 206, but listens to the zone packet duplicator PD3, PD4 will not join any multicast group address. Alternatively, the listening area packet duplicators may join the multicast addresses of their respective listening areas, but in either case, not join the source area multicast address MC 1. Thus, in this example, the PD1 is the only packet replicator capable of receiving payloads at step 212 via the source region multicast address MC 1. If at step 212 the source region packet duplicator (e.g., PD1) receives the payload via the multicast address MC1, it duplicates the packet at step 214 and forwards the payload to the listening packet duplicator (e.g., PD3, PD4) via a unicast message, if needed. For example, as described in fig. 4, the source region packet duplicator PD1 sends a payload message 420 to the unicast address PD3 and a payload message 422 to the unicast address PD4, and thus the packet duplicators PD3, PD4 will receive the payload messages 420, 422, respectively.
If at step 212 the packet duplicator does not receive a payload via the source region multicast address, a determination is made at step 216 as to whether the packet duplicator receives a payload via a unicast address. As already said, the listening packet duplicator (i.e., not joined to the source region multicast address) is able to receive the payload from the source region packet duplicator via the unicast address. If at step 216 the listening packet duplicator (e.g., PD3, PD4) receives the payload via a unicast address, it redistributes the payload to the participating devices in the area in which it is located, at step 218. In one embodiment, this is done by snooping the packet duplicators PD3, PD4 sending payloads to their core routers CR3, CR4, respectively, via their respective multicast addresses MC3, MC 4. For example, in fig. 4, packet replicator PD3 is shown sending payload message 424 (addressed to multicast address MC 3) to CR3, and packet replicator PD4 sending payload message 426 (addressed to multicast address MC4) to CR 4. The core routers CR3, CR4 then distribute the payloads to devices joining the listening area in the listening area multicast addresses MC3, MC4, respectively. In this way, the core routers distribute the payloads separately in the listening area via separate multicast routing trees.
At step 220, it is determined whether the call is over. This may occur, for example, if no call activity occurs for as long as a defined "hang time" period, as is well known in the art. If the call ends, devices previously joined to the multicast group for the call (e.g., PD1 joined to MC1) leave the respective multicast group at step 222. As is well known, this may be accomplished by the devices sending IGMP "leave" messages to the routers to which they belong. The router then tears down the corresponding multicast routing tree based on the detach message.
If the call is not ended at step 220, a determination is made at step 224 as to whether there is a new call source in a different area. This may occur, for example, if the communication unit generates payload for a call that moves to a different area, or if the communication unit in a different area begins generating the source payload for a (source) call. If there is no new source of the call in the other zone (i.e., the source zone is unchanged), the process returns to step 212 and the process continues with the same source zone packet duplicator and the same source zone controller until the call ends at step 220 or the source zone changes at step 224.
If the source region has changed, the process proceeds to step 226 and 234. At step 226, it is determined whether the packet duplicator is a previous source region packet duplicator; at step 232, a determination is made as to whether the packet duplicator is a new source region packet duplicator. For purposes of illustration, assume that communication unit 163 generates a new payload (in region 4) for the call. In this case, zone 4 instead of being the source zone of the call, the packet duplicator 138 ("PD 4") becomes the new source zone packet duplicator and the packet duplicator 132 ("PD 1") is the previous source zone packet duplicator. The previous source zone packet duplicator (e.g., PD1) disengages its associated multicast address (e.g., MC1) at step 228 and changes state to a listening zone packet duplicator at step 230. Instead, the new source region packet duplicator (e.g., PD4) joins the multicast address (e.g., MC4) associated therewith at step 234 and changes state to the source region packet duplicator at step 230. After the state of the previous source zone packet duplicator changes to a listening zone packet duplicator or after the state of the previous listening zone packet duplicator changes to a source zone packet duplicator, the process returns to step 212 with a different source zone and/or listening zone packet duplicator until the call ends at step 220 or the source zone changes again at step 224.
If the new call source does not result in a different source region and/or listening area packet duplicator being generated, the process returns to step 212 with the previous source region and/or listening area packet duplicator until the call ends at step 220, or the source region changes at step 224.
Figure 3 shows the steps performed by the zone controller to implement a talk group call in one embodiment of the invention. In general, FIG. 3 accomplishes the same steps as FIG. 2 (i.e., the steps performed by the packet duplicator), and similarly will be described with reference to FIG. 1, where communication unit 157 (in zone 1) is the source of the talkgroup call, communication units 158 and 163 (in zone 3 and zone 4) are the recipients of the talkgroup call, and FIG. 4, where the message sequence of the call is illustrated. At step 302, the participating zone controller receives a call request for a talkgroup call. As best shown in fig. 4, the call request 400 is initially sent from the base station 106 associated with the intended source (e.g., communication unit 157) to the controlling zone controller ("ZC 1"), and the controlling zone controller forwards the call request 400 to the participating zone controllers (e.g., ZC3 and ZC 4).
At step 304, the zone controllers identify multicast group addresses for their respective zones. In the preferred embodiment, the multicast group address for each participating zone is different (e.g., zone 1 is MC1, zone 3 is MC3, and zone 4 is MC4), so a separate multicast tree will be established in each zone. Alternatively, the multicast group address may be statically determined and stored in the memory of the packet duplicator and/or the zone controller of the respective zone.
In fig. 3, step 306 distinguishes between zone controllers in the source zone and zone controllers in the listening zone. The zone controller in the source zone is defined as the zone controller in the zone where the call (or call as envisioned) generates the payload. In contrast, the zone controller(s) in the listening zone(s) are defined as zone controller(s) that are not (at least now) in the zone(s) where the call generated the payload. For example, referring to fig. 1, (at least at the very beginning), the communication unit 157 generates payload for the call, the zone controller 124 (in zone 1) is the source zone controller, and the zone controllers 128 (in zone 3) and 130 (in zone 4) are the listening zone controllers for the call. In fig. 4, the source zone controller is referred to as "ZC 1" and the listening zone controllers are referred to as "ZC 3" and "ZC 4", respectively. It should be appreciated that the source zone and listening zone controllers may be periodically changed while different talkgroup members (i.e., in different zones) generate payloads for calls.
At step 308, the source region controller (in one embodiment) instructs the source region packet duplicator to begin the call source sequence by sending a "begin call source" message 402 to the source region packet duplicator. Thus, in this example, the source zone controller 124 ("ZC 1") sends the Start Call Source message 402 to the source zone packet duplicator 132 ("PD 1"). The start call source message identifies the talk group (e.g., "TG 1") and the multicast address (e.g., "MC 1") to which the source zone packet duplicator and any participating host devices in the source zone are to join.
At step 318, the listening area controller(s), in one embodiment by sending a "start call listening" message 404 to the listening area packet duplicator(s), instructs the listening area packet duplicator to start a call listening sequence. Thus, in this example, the zone controller 128 ("ZC 3") sends a Start Call listening message 404 to the packet duplicator 136 ("PD 3") and the zone controller 130 ("ZC 4") sends a Start Call listening message 404 to the packet duplicator 138 ("PD 4"). The start call snoop message(s) determine the talk group (e.g., "TG 1") and multicast address (e.g., MC3, MC4) to which the listening area packet duplicator and any participating host devices in the listening area are to join.
At step 320, the listening area controller(s) sends a call response to the origination area controller. For example, as shown in figure 4, the zone controllers ZCs 3, ZCs 4 send call response messages 408, 412 to the source zone controller ZC1, respectively. In a preferred embodiment, the call response identifies the talkgroup (e.g., "TG 1") and the unicast address (e.g., PD3, PD4) of the listening packet duplicator.
At step 310, the origination area controller receives a call response. At step 312, the source region controller instructs the source region packet duplicator to add the call destination to which the source region packet duplicator sent the payload. In a preferred embodiment, the instruction(s) to add a destination comprise instructions to send the payload to one or more packet duplicators in the listening area. For example, as shown in FIG. 4, the source zone controller ZC1 first receives a call response 408 from the ZC3 that determined the unicast address of the packet duplicator PD 3. In response to the call response 408, the ZC1 sends an "Add destination" message 410 instructing the source region packet duplicator PD1 to add the PD3 as the destination unicast address for the call. The ZC1 then receives the call response 412 from the ZC4 that determined the unicast address of the packet duplicator PD 4. In response to the call response 412, the ZC1 sends an "Add destination" message 414 that instructs the source zone packet duplicator PD1 to add the PD4 as the destination unicast address for the call. The source zone controller continues to receive call responses and instructs the source zone packet duplicator to add destinations until all participating zones have responded at step 314 or until no response is received again after an appropriate period of time has elapsed.
Once all the zones have responded, the origination zone controller sends a call admission message to each participating zone at step 316. At step 322, the listening area controller(s) receives the call grant message(s). In a preferred embodiment, the call admission message is sent to the zone controller of the participating zone via a unicast address. As already described with reference to fig. 2, the call grant message 416 includes the unicast addresses of all packet duplicators participating in the call (e.g., PD1, PD3, PD 4).
The present invention identifies a method for limiting the formation of multicast spanning trees to a single region in a multi-region system so that spanning trees converge much faster than would be possible across multiple regions. The method uses the functionality of a packet duplicator and a zone controller in each zone and is suitable for providing communication units that roam between different sites or zones for the duration of a call.
The present invention may be embodied in another specific form without departing from its spirit or essential characteristics. It is to be understood that the described embodiments are for purposes of illustration only and are not to be taken in a limiting sense. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
Claims (11)
1. In a communication system comprised of a plurality of participating areas of a call, a method comprising:
distributing one or more packets for the call within a source zone of the participating zone through a source zone multicast routing tree;
receiving, by a host associated with the source region, the packet;
transmitting, by the host associated with the source zone, the packet to one or more hosts associated with listening zones of participating zones;
receiving, by the host associated with the listening area, the packet; and
redistributing the packets within the listening areas separately through one or more separate listening area multicast routing trees.
2. The method as in claim 1 wherein the host associated with the source zone comprises a source zone packet duplicator, and wherein the host associated with the listening zone comprises a listening zone packet duplicator.
3. The method of claim 1, comprising, prior to the distributing step:
one or more receiving devices including at least the source region packet duplicator join a multicast address associated with the source region that defines a source region multicast address;
one or more network devices establishing the source region multicast routing tree logically interconnected with the receiving devices joining the source region multicast address; and
transmitting, from a communication source in the source zone, one or more packets addressed to the source zone multicast address to the one or more network devices.
4. The method as recited in claim 3, further comprising the steps of:
redefining the source region and the listening region to thereby define a previous source region packet duplicator and a new source region packet duplicator, a previous source region multicast address and a new source region multicast address;
said previous source zone packet duplicator disassociates from said previous source zone multicast address; and
the new source region packet duplicator joins the new source region multicast address.
5. The method of claim 1, wherein the transmitting step comprises:
sending, from a first host to one or more network devices, one or more packets addressed to a second host; and
routing the packet from the one or more network devices to the second host.
6. The method as recited in claim 1, comprising, prior to said step of redistributing separately:
adding different multicast addresses of the monitoring areas into receiving equipment of one monitoring area of different monitoring areas; and
one or more network devices establish a separate listening area multicast routing tree that logically interconnects devices in different listening areas that have joined different listening area multicast addresses.
7. The method of claim 6, wherein the step of separately redistributing comprises:
sending, from the second host associated with the different listening area, one or more packets addressed to a different listening area multicast address to one or more network devices, respectively; and
transmitting, by the network device to receiving devices in the different listening areas, one or more packets addressed to different listening area multicast addresses.
8. A method comprising the steps of:
receiving a request for a call from a communication source;
determining a plurality of participating regions for the call, the participating regions defining a source region and one or more listening regions;
instructing a source region packet duplicator to begin a call source sequence, whereupon the source region packet duplicator receives payload for calls placed in the source region;
instructing the source zone packet duplicator to transmit the payload to one or more listening zone packet duplicators associated with the one or more listening zones; and
instructing the one or more listening area packet duplicators to begin a call listening sequence so the listening area packet duplicators receive the payload transmitted by the source area packet duplicator and distribute the payload in their respective areas.
9. The method of claim 8, performed by one or more zone controllers associated with the participating zones.
10. The method of claim 9, further comprising said zone controller determining a source zone multicast address for distributing payload for said call within said source zone and determining one or more listening zone multicast addresses for distributing payload separately within said listening zone.
11. A communication system, comprising:
a plurality of communication devices participating in a call, the communication devices being distributed among a plurality of participating areas;
a packet network for routing one or more packets from a source zone to one or more listening zones in said participating zones via respective unicast addresses via a source zone multicast address in a source zone of said participating zones for said call, said routing being performed within said listening zones via one or more separate listening zone multicast addresses;
a source zone packet duplicator configured to receive packets within the source zone and to transmit the packets to the packet network for routing to the listening zone via the unicast address;
one or more listening area packet duplicators for receiving packets transmitted from the source area and transmitting the packets to the packet network for routing within the listening area via the separate listening area multicast address.
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Application Number | Priority Date | Filing Date | Title |
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US09/961,602 | 2001-09-24 |
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HK1072136A true HK1072136A (en) | 2005-08-12 |
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