LMR systems are deployed by organizations requiring instant communication between geographically dispersed and mobile personnel. Typical users of LMR systems include police departments, fire departments, medical personnel, EMS and the military.
Current LMR systems can be configured to provide for radio communications between a site and subscriber units in the field. A subscriber unit may be a mobile unit or a portable unit. LMR systems can be as simple as two subscriber units communicating between themselves and a site over preset channels, or they can be complex consisting of hundreds of subscriber units and multiple sites.
LMR systems may be configured to cover a large geographical area by providing hundred of sites. Communication among the sites is necessary in various situations. At present, sites generally cannot communicate directly with each other, but require the assistance of a bridge device to facilitate communication among the sites. As will be explained below, the requirement of a bridge device to facilitate communication among the sites has numerous disadvantages.
FIGS. 1-4 illustrate communication among sites 104, 108 and 112 via a bridge device 116. The sites 104, 108, 112 and the bridge device 116 are interconnected by a data network (not shown in FIGS. 1-4). The data network can be an IP network. However, the data network may also be any other type of network (e.g., packet switched network, ATM network).
Consider, for example, that the site 104 desires to exchange voice messages with the sites 108 and 112. Prior to the exchange of any voice messages, the bridge device 116 must setup the call. Referring now to FIG. 1, the site 104 first sends a request message to the bridge device 116 that it intends to communicate with the sites 108 and 112. Next, the bridge device 116 forwards the request message to each of the sites 108 and 112 inquiring if they would like to participate in a call. Note that the bridge device 116 must forward the request message to each of the sites 108 and 112 separately since the bridge device 116 can only send one request message at a time. Next, the bridge device 116 receives response messages from the sites 108 and 112 confirming or acknowledging that they would participate in the call. Again, note that the bridge device 116 can only receive one response message at a time. The call setup concludes when the bridge 116 notifies the site 104 that the sites 108 and 112 are willing to participate in the call. As can be seen, the call setup involves at least six hops. Also the bridge introduces additional latency due to the increased hop count resulting in increased call setup time for every setup message that is processed by the bridge device 116. Further, the bridge device 116 is a single point of failure because a failure of the bridge device 116 will shut down communication among the sites, thus making the system less reliable. The signals exchanged among the bridge device 116 and the sites 104, 108 and 112 during the call setup are sometimes referred to as “control plane” signals.
After the call setup, communication among the sites 104, 108 and 112 may proceed. FIG. 2 illustrates the transmission of a voice message from the site 104 to the sites 108 and 112. A voice message originating from the site 104 is first transmitted to the bridge device 116. The bridge device then separately forwards the voice message to each of the sites 108 and 112.
Consider that the site 108 desires to respond back to the site 104 and also communicate with the site 112. Referring now to FIG. 3, the site 108 sends a request message to the bridge device 116 that it wants to communicate with the sites 104 and 112. Next, the bridge device 116 forwards the request message to each of the sites 104 and 112 inquiring if they would like to participate in a call. Next, the bridge device 116 receives response messages from the sites 104 and 112 confirming or acknowledging that they would participate in the call. The call setup concludes when the bridge 116 notifies the site 108 that the sites 104 and 112 are willing to participate in the call. As can be seen, the call setup involves at least six hops.
After the call setup process described above, communication among the sites 104, 108 and 112 may proceed. FIG. 4 illustrates the transmission of voice messages from the site 108 to the site 104 and 112. Again, the bridge device 116 first receives a voice message from the site 108, which is forwarded to the sites 104 and 112.
In the control plane, the bridge device 116 introduces undesired latency during the call setup due to increased hop count. In the bearer plane, voice messages must go through the bridge device 116, introducing an extra hop and latency. Also, since all control plane and bearer plane traffic must go through the bridge device 116, a single point of failure is created at the bridge device 116 for both call setup and voice communications. Furthermore, the bridge device 116 is typically heavily loaded as all control plane and bearer plane traffic must go through the bridge device 116. This often results in a requirement that the bridge device 116 be high capacity and extremely robust, and, hence, very expensive.
Also since the bridge device forwards the voice messages to the other sites, the bridge device requires additional bandwidth for each additional site that it needs to forward voice messages. Moreover, the bridge device must be aware of all sites in a particular deployment. As a result, when a new site is deployed in a system, the bridge device needs to be notified about the site.