Trunking the EDACS way
Part 2-Part 1 of this series described some basic terms and capabilities of the Ericsson EDACS trunking system. Fundamental system components of basic and full featured configurations were discussed, in addition to general operation. This final article examines the channel assignment process and includes some basic simulcast considerations.
Before a user can obtain access to a channel, several supervisory functions must first be accomplished. (The following example presumes the presence of a site controller, with the user placing a group call in the analog voice mode.)
The process of trunking a call In an EDACS system, two data signals are transmitted over-the-air to provide signaling functions: “low-speed” (LS) and “high-speed” (HS) data. With 25kHz channel spacing, the HS data rate is 9,600bps, while with 12.5kHz spacing, it is either 4,800bps or 9,600bps (900MHz). In either case, it is a non-return-to-zero (NRZ) direct FM signal. The LS data rate is 150bps.
When idle, the radio units in the field monitor the continuous stream of HS data transmitted from the trunking site control channel transmitter (“outbound” data). When attempting to initiate a radio call, a user must first “key” the radio’s microphone (PTT). The radio then transmits a HS data burst on the control channel (“inbound” data) that is recovered by the trunking site control channel receiver and passed on to the site controller.
The calling unit’s data tell the site controller who is calling (the user’s logical ID or LID). The data also relate the type of call (group call, individual call, telephone interconnect), what mode is to be used (analog voice, digital voice or digital data), and who is being called (the group ID [GID], individual ID [LID] or data terminal).
After receiving the data from the caller, the site controller checks the LID to ensure it is a valid user. If it is not, the caller will not receive a channel assignment. If the LID is valid, the controller checks to see which channels in the system are available and then picks one for use. The site controller keys up the transmitter of the working channel that was chosen.
The controller then puts information on the outbound control channel monitored by the units in the field. This alerts all radios assigned to the call to move off the control channel and onto the assigned working channel frequency. The assigned working channel starts transmitting a constant HS data confirmation signal, referred to as “dotting.”
When the units in the field switch to the assigned working channel and hear the HS signal, they know their transition was successful. The initiating caller’s radio transmits a short HS data burst informing the working channel that it is a valid EDACS unit and not merely on-channel interference. The working channel then drops its outbound HS data signal, switches into the analog mode (so that it will pass voice audio) and applies a continuous outbound LS data signal. The receivers of the units being called unmute to hear the upcoming voice transmission.
The radio of the initiating caller produces a “grant tone” or “go ahead” beep in its speaker and the microphone is enabled, allowing voice transmission in addition to a continuous 75Hz sub-audible tone. All this can take place in less than 0.25s.
The initiating party then transmits a voice message over the trunked radio system, accompanied by the 75Hz tone. The tone serves as a “keep alive” signal to let the working channel know the user is still transmitting.
For the duration of the call, the working channel transmitter sends outbound LS data to the assigned units in the field. This data contain the LID of the initiating caller. When the units in the field receive this information, the LID shows up on the display of their radios to let them know who is talking. The LS data also contain information relating to the priority scan function and helps the units in the field maintain their proper channel assignment.
When the initiating party releases the radio PTT, the radio drops the sub-audible tone, sends a HS “channel drop” data burst and then unkeys.
This HS signal returns the working channel to the digital mode, disabling audio transmissions and drops the outbound LS data signal. The working channel then sends an outbound HS “channel drop” message telling the assigned users to revert back to the control channel. Finally, the working channel unkeys and is freed up for future assignments by the site controller. The mobiles switch back to the Control Channel and monitor for future instructions.
The channel assignment process should seem instantaneous to the user: The microphone is keyed, the “go ahead” beep is heard in the radio’s speaker and the user begins talking. However, if there is a problem and a channel cannot be assigned, then the user may hear one of two other sounds. The first sound is a low-pitched “boop” sound that tells the user that channel access could not be accomplished. This might be for a variety of reasons: Perhaps the calling unit was out of range, or the control channel data was being improperly decoded in the user’s receiver or maybe the LID was determined invalid by the site controller. The second sound is a high-pitched “beep” that indicates all channels in the system are busy. This condition is referred to as queuing and means the calling party will have to “wait in line” until a channel becomes available. Units can be ordered in the queue according to priority levels. However, if all users have the same priority, calls will be assigned on a first-come, first-served basis. Emergency calls are given top priority.
Simulcast applications Often, multiple radio sites are necessary to provide adequate radio coverage over a large area. In such applications, the site controller may be placed at a central location and connected to multiple hilltop sites via microwave and multiplex (mux) equipment (as shown in Figure 1 on page 28).
Mux modems carry the transmit and receive HS data from the centrally located site to the remote site station GE trunking cards (GETCs). Digital mux modems allow for direct RS-232 connection to the site EDACS equipment. Mux channels also carry transmit and receive audio in addition to E-Lead signaling. In simulcast applications, two E-Leads are used for each radio channel: one for the base station PTT function and the other to switch the station GETCs between the analog and digital modes (A/D lines).
The centrally located site provides a common source of LS data to multiple hilltops. HS data timing is critical in a simulcast environment and requires frequency and timing reference signals (2,400Hz and 300Hz, respectively) to be sent out to the remote sites. These are processed and used to clock the HS data signals out in a synchronized manner. The 300Hz, 2,400Hz, LS and HS data processing and distribution is carried out in what Ericsson calls its Universal Sync Shelf. The common LS data is sent to the remote sites in the form of a FSK signal. It is decoded within the Universal Sync Shelf and distributed to each radio channel’s GETC as an RS-232 signal. HS signals in the outbound direction must be synchronized while inbound data is sent back to the central controlling point in an asynchronous manner.
Automated testing Often, simulcast EDACS systems employ an automated test sub-system that provides “background” testing of the channels at each site. Tests are accomplished at programmable intervals. The channels are tested only when idle and released for use if the trunkingtest passes. If the test fails, an alarm indication is registered at the central controlling site where maintenance personnel can recognize the failure. The site controller can then remove “failed” channels from service, disallowing users from being assigned “bad” channels.
Conclusion Without a detailed understanding of each step in the trunking process, one will have great difficulty detecting and isolating system faults within an EDACS system. Hopefully, this two-part series will prove valuable to those without extensive training in EDACS methodologies.
References EDACS System Guide, Ericsson GE Mobile Communications, August 1992, ECR-4581A. LBI-38495, Ericsson GE Mobile Communications. LBI-38496, Ericsson GE Mobile Communications. LBI-38498B, Ericsson GE Mobile Communications.