Trunking the EDACS way
Part 1- A discussion of the basic configurations, terminology and operations associated with Enhanced Digital Access Communications System technology. Ericsson GE has maintained its standing in the trunked radio arena for many years with a system they called EDACS (Enhanced Digital Access Communications System). This marriage of RF and digital technology produced a system that has grown in popularity. Installations can now be found around the globe for use in both the private and public sectors. This article is the first of a two-part series that will discuss basic EDACS configurations, terminology and operations.
Basic EDACS – trunked failsoft Depending on system requirements, EDACS system configurations can be quite different. The simplest is referred to as “basic EDACS,” where the system operates in the trunked failsoft mode. This means the trunking operations are controlled by individual processing units, one corresponding to each channel in the system (See Figure 1 below.) These units are called GE trunking cards (GETCs) and form an architecture of distributed processing where when one GETC fails, the rest continue handling the trunking operations for the remaining functional channels. These GETCs are connected through a 19.2kbps serial data bus called the back-up serial link (BSL). The cards are capable of communicating with one another rapidly over this link so that each GETC is kept up-to-date on what the others are doing.
Full-featured trunking In contrast to the distributed processing of the trunked failsoft mode, an EDACS system can also be configured to dedicate the normal trunking processes to a single, centralized computer called a site controller. This site controll er, in turn, is connected to another computer called a system manager. The system manager is the interface between operators and the system and is capable of (among other things) generating reports that help evaluate the operational performance of the trunked system. Its reports include database information for the various units, groups and sites, system activity, site and channel usage statistics and alarms. (See Figure 2 on page 30.)
This EDACS configuration is referred to as full-featured trunking. There are several levels in a full-featured EDACS system, each with different capabilities.
For example, in contrast with basic EDACS, full-featured trunking (level one) offers call validation, basic diagnostics, activity logging, automated background testing, telephone interconnect, call priority, management reports and more.
With a full-featured system, radios in the field can also be disabled and enabled remotely. This is of value when a radio is lost or stolen and subject to use by unauthorized personnel. Talk groups can be reconfigured dynamically. New talk groups can be created and radios in the field can be reprogrammed over-the-air at the rate of 30 per second to accomplish alternate grouping in the case of emergency, or for any other reason.
In the full-featured configuration, if the site controller fails, the individual base station GETCs can still handle normal trunking operations in the trunked failsoft mode (through distributed processing). In this event however, any additional functionality normally provided by the site controller and system manager will be lost.
Working channel/control channel In an EDACS system, the channel assigned to handle the unit-to-unit traffic is referred to as a working channel.
However, prior to the actual unit-to-unit traffic, supervisory data transmissions between the user radio and the controlling processor (the station GETC, in basic EDACS, or the site controller in full-featured EDACS) must be accomplished in order to direct the user (and the units being called) to the proper channel in the system. This supervisory control is handled over a single, separate radio channel called the control channel.
The control channel acts as a conduit for the commands and requests being sent back and forth between the units in the field and the processor controlling system operations. In an EDACS system, one base station at the trunking site operates as the control channel while the others function as working channels. The control channel receiver acts as the system processor’s “ears,” listening for requests from field units. The control channel transmitter acts as the “mouth,” sending channel assignments out to the units on the system.
When idle, the radios in the field stay tuned to the control channel transmitter frequency, monitoring the steady stream of supervisory data, waiting for instructions from the controlling processor that may apply to them. If a valid unit in the field transmits (keys up), the processor should recognize the unit and choose an available working channel for it, as well as for the unit(s) being called. Through supervisory instructions sent over the control channel, it will route these units to the assigned working channel, and communications through the system may begin.
When the calling party unkeys, the working channel is released and all involved parties revert back to the control channel to monitor the supervisory data. If a user radio is turned on after a call for its group is already under way, it will have missed the original channel assignment information. One might assume that it would, consequently, miss the call. EDACS, however, is able to route “late arrivals” to the proper working channel because the supervisory data on the outbound control channel is repeated continuously. When a radio is first turned “on,” it monitors the control channel. The repeating working channel assignment for that group will be received and will route the radio to the same channel as the other members of its group. This same method applies when a user has gone out of range of the trunking system. When back in range, the first thing the radio does is listen to the control channel information. Since channel assignments are repeated continuously, it will be routed to the proper working channel and pick up a call already in progress.
Normally, when a channel has been assigned and communications has begun, the initiating party has control of the channel. The other units within the same talk group cannot transmit during this period-they can only listen. This is called transmitter-busy lockout, and it eliminates malicious or unintentional interference to the initiating party by other units. The only way another group member can be heard during this period is to activate the radio’s emergency button, triggering an emergency channel request transmission. The site controller will then assign a different working channel to that group, dropping the original call and giving channel control to the user who initiated the emergency call.
Control channel protection At an EDACS site, any base station in the multichannel system can function as the control channel. Typically, channel one is used as the control channel, however, if this radio or its associated GETC fails, the control channel duties can switch to another. Units in the field have the ability to scan all channels in the system to find the newly assigned control channel.
Trunking: transmission vs. message In an EDACS system, when an assigned channel is released from one user, it becomes available to the next. Even if the same unit transmits again immediately after releasing a channel, the whole assignment process will repeat, starting “from scratch.” This means the same group of users over a period of several transmissions are assigned a different channel in the system each time. This is referred to as transmission trunking.
Message trunking is when a delay or hang time is introduced after a user unkeys. The channel assignment can be maintained as long as users in the group continue to use the channel before the pre-programmed delay period expires. This is not an efficient mode of operation and can significantly impact the overall traffic handling capacity of a trunking radio system. Under certain conditions, however (such as an important dispatch or emergency call), it may be desirable to dedicate a channel to a particular group for a short period of time.
Call modes In an EDACS system, there are three call modes: analog voice, digital voice and digital data. The analog voice mode uses a combination of high- and low-speed data (signaling) along with a standard analog voice transmission. In the digital voice mode, the analog voice is digitized and coded. Three coding schemes are available: Aegis, Aegis encrypted or Voice Guard. The last mode is the digital data mode where MDTs or laptops communicate through the system.
Call types Group calls allow one individual within a group to contact other members of that same group. When the initiating party is assigned a channel, all units in the same group will be directed to that channel to hear the transmission. Other groups will not hear the call, only the members of the initiating party’s group. An individual call is when one individual wants a private conversation with another individual. In this type of conversation, no one can hear the conversation except the two parties involved.
Emergency calls can be made by pressing a special emergency button on the user radio. Although group calls are typically transmission-trunked, emergency calls are message trunked. When an emergency call is initiated, an audible alert signal is sent over-the-air to notify all units in that group as well as the dispatcher. Only the dispatcher or a radio equipped with supervisory functions can clear the emergency call.
A system all-call is where a specially equipped radio can communicate with all users on the system at one time. When the system call is placed, all active calls are dropped and assigned to a single channel. The specially equipped radio now broadcasts to everyone on the system. The conclusion of this series will detail the step-by-step channel assignment process within an EDACS system and will reference basic simulcast considerations.
References EDACS System Guide, ECR-4581A, Ericsson GE Mobile Communications, August, 1992.