The System Identity Code and Capacity Max

The information required for radios to identify a DMR Tier3 network is contained within the System Identity Code. This is regularly transmitted within the CACH of control channels at each RF site. ETSI TS 102 361-4 defines a 16-bit System Identity Code (C_SYScode) that is made up of the following subfields.

Bit	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1
Fn.	M	M	X	X	X	X	X	X	X	X	X	X	X	X	P	P

DMR Tier 3 networks may range from small systems consisting of maybe one or two RF sites; to nationwide systems with hundreds of RF sites.  To provide flexibility and the coexistence of many systems in the same frequency band, ETSI TS 102 361-4 defines a means for System Identity Code to be split into four network models.

Network Model	Bit 16-15 of C_Syscode₂	No. of bits for NET	No. of bits for SITE
Tiny	00	9	3
Small	01	7	5
Large	10	4	8
Huge	11	2	10

 As you can see in the above table, the Network Model selection determines how many bits are allocated to the NET and SITE subfields.

For example, if a Small network model is chosen then bits 8-14 are reserved for the NET subfield and bits 3-7 are available for the SITE subfield.

Bit	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1
Fn.	0	1	N	N	N	N	N	N	N	S	S	S	S	S	P	P

The value in the NET subfield is the System ID and is the (decimal) number you would enter into Radio Management together with the Network Model selection.

SITE contains the Site ID. In a DMR Tier3 system, each RF site will have a unique Site ID. This is the Site ID which is set up in the table located in the Site Selection set (used by the radios) of Radio Management.

You may find that certain vendors have an issue with the value of NET in their configuration software. Although the transmitted System ID is 31₁₀, you need to enter 32₁₀ in a MOTOTRBO (or any other) radio. Similarly, if their system is set up with a System ID of 32, you need to use 31 in the System Set of the radio.

The DMRLA can be used to control which RF sites a particular radio (or certain radios) may access. Depending on the vendor, this value might be shown as Length of Area in their configuration software. Capacity Max does not use Length of Area so the (full) SITE subfield must be used as the Site Number in the Site Selection set.

The Length of Area refers to the number of bits in SITE the radio will need to consider. So if the Length of Area in the configuration software is set to 3; and the network model is Small, then the radio will only consider bits 7 - 5 of the SITE subfield.

A Capacity Max system does not use Length of Area since it is better to allow a radio to roam to all sites and restrict site access at a system level (rather than in each radio configuration). This allows site-access changes to be made without ever having to change something in the radio. 

If the Small network model is chosen, then the Length of Area cannot be more than 5. The minimum is 1 though some vendors will allow this to be set to 0. If set to zero the radio will consider sites where the System ID is a match.

Bits 1 and 2 are allocated to the PAR (Partition) subfield. Some vendors will call this the Control Category. In Radio Management you will find this setting in the Site Selection set: Partition Category. Practically, there are only two choices: A and B. Some vendors will have an option to allow A and B. 

PAR	2	1
Reserved	0	0
A	0	1
B	1	0
A and B	1	1

If a radio is set up to register on sites with a control channel set to PAR=A and it finds a site which has PAR=A and B then it will consider this site.  However, if the radio is set finds a site which has PAR=B then it will ignore this site.

The idea behind PAR is to allow a site to have two control channels. This would allow the number of radios to be divided between the two control channels. Capacity Max does not need to use this as (in Advantage Mode) timeslot 2 of the control channel host repeater can be used to handle mass registration (and handle this way better too).

Before attempting to register on the site, the radio will evaluate the System Identity Code. It will also evaluate the control channel (as well as any control channels on neighbouring sites) for signal strength and bit error rate.

For those who have in-depth experience with MPT1327, a lot of this will be surprisingly familiar. A fair amount of MPT1327 terminology and concepts were reused in ETSI TS 102 361-4 (yes, Aloha and Ahoy too!).

The remaining bits in the CACH (in the diagram) are used to tell radios whether the channel is busy (AT bit); whether this is timeslot 1 or 2 (TC bit); link control (LCSS bits) and error correction (FEC bits). 

In case you didn't know or have forgotten, subscript ₁₀ means decimal (otherwise known as base 10) and subscript ₂ means binary.