Florida Repeater Council
About CTCSS and the APCO25 NAC
CTCSS stands for continuous tone coded squelch system, often referred to as "PL" (Motorola's trade name). Many repeaters require the use of a PL tone to access the repeater. APCO25 (P25) use a very similar process in the digital world called a NAC, or Network Access Code that will be explained further below.
Contrary to popular belief, the requirement of a PL tone to access a repeater does NOT mean it is closed. PL is frequently used to preclude nuisance kerchunks in high RF environments as well as helping solve interference problems. Some repeaters may also generate a PL tone on the repeater output so that repeater users who are equipped with a radio capable of decoding PL will not hear other interference sources on the channel that would otherwise open the squelch on the user's radio.
It is up to the owner / trustee of the repeater to decide whether or not to make public the PL tone for a particular repeater. The FRC follows the wishes of the owner / trustee in publishing the PL code only when so requested.
The FRC strongly recommends the use of PL on repeaters' receivers. PL is a minor inconvenience when you consider how many potential problems it can eliminate. The use of PL may be required for a coordination to be granted if conditions so warrant, such as proximity to a co-channel repeater, or in an area where band openings frequently aggravate co-channel interference problems.
Conventional P25 systems don't support CTCSS tone or DCS code for access. Instead they use what is called a Network Access Code, or NAC. The NAC is a feature similar to CTCSS or DCS for analog radios. That is, radios can be programmed to only break squelch when receiving the correct NAC. NAC's are programmed as a 3 digit hexadecimal code that is broadcast along with the digital signal being transmitted. Since the NAC is 3 digit hexadecimal number (12 bits), it gives 4096 possible NAC's for programming, which far exceeds all its analog counterparts combined. It should be noted that 3 of the possible NAC's have special meaning:
The FRC has designed a PL tone and NAC plan in the hopes that repeater owners / trustees in a given area will standardize on a particular PL tone or NAC and incorporate it into their operational plans. The reason for this plan is to make it easier for users to operate the local repeaters in an area, as some older radios are only capable of a single PL tone as compared to modern radios which can have PL tones selected on a per-channel basis. A specific PL tone and NAC has been recommended for each region based upon actual usage, as listed below. Click HERE for a region map.
You can assist your users in getting PL boards for their radios. Ask any 2-way shop and they will most likely recommend Communications Specialists, 426 W Taft Ave, Orange, CA 92865 Phone: (800)854-0547. They can provide boards for a modest sum that will fit inside most radios including handhelds. They are available from Amateur Electronic Supply and the Ham Radio Outlet.
Following is a chart showing each PL tone's two-character alphanumeric designator and the corresponding tone frequency in Hertz.
Latest update: 5 January 2011