Central Idaho Amateur
Radio Club
CIARC - Repeaters - 33cm Link Back-Bone

NOTE: Correction has been made to 33cm link backbone deviation to indicate the proper deviation level is 2 kHz. (29 Dec 2016)

Repeater Network

33-centimeter Repeater / Link Back-Bone

The 33cm repeater / link operates as a closed system under FCC Part 97.205 Paragraph E. The CIARC is using a 33cm repeater channel to create a linking back-bone. The link operates as a closed system so that other repeater systems are not linked in without the CIARC control operators being able to disconnect a linked repeater at the source.

The goal of implementing the link back-bone is to provide an extensible networking capability for Amateur Radio Emergency Services (ARES) and to enable expansion of repeater capabilities through infrastructure that does not need to or cannot be located at a repeater site.

CIARC may allow other repeaters to use the linking back-bone and link to the CIARC repeaters provided that those repeater operators provide control codes to the CIARC control operators that enable the CIARC control operators to disconnect those repeater systems from the link back-bone at the discretion of the CIARC control operator. This is necessary to avoid interference issues where both the linked repeater and the CIARC repeater are carrying separate traffic or in the event of a technical issue with the linked repeater.

CIARC is using a 33cm repeater pair to perform a linking function. The 33cm band-plan that was adopted in Idaho is the same plan as adopted by the Oregon Region Relay Council (source: Carl Sorensen - NB7C). The Oregon Region Relay Council band-plan adopts a 10 MHz offset for linking while adopting a 25 MHz offset for repeaters. The majority of 900 Mhz / 33cm radio equipment comes from a commercial application where a 39 MHz split is used. This equipment is easily adapted to a 25 MHz split while retaining good equipment performance. This same equipment will not support a 10 MHz split as there is insufficient isolation provided by the duplexers at a 10 MHz split in order to avoid de-sense issues. As such, the adoption of the Oregon Region Relay Council band-plan is not optimal for the available pool of surplus equipment. The ARRL 33cm band-plan suffers from the same issues, with its 12.5 MHz split, and has has largely not been adopted (except on the East Coast). Other band-plans, such as the SERA band-plan apply frequency splits that allow the majority of surplus equipement to be adapted to either a repeater or link function while avoiding interference issues with other services.

With this explanation in mind, the 33cm repeater will be referred to as a full duplex link back-bone, and not a repeater, from this point forward.

The CIARC 33cm full duplex link operates on an output frequency of 927.2250 and an input frequency of 902.2250 MHz with a 25 MHz split. The receiver uses a Digital Coded Squelch code of 023 with the transmitter encoding the same DCS code. The 33cm full duplex link operates with low power and is intended for site-to-site communication only. The 33cm full duplex link does not provide a squelch tail. This is not needed when repeaters utilizing the link back-bone are full-quieting as a squelch tail is intended to keep the transmitter from dropping when the receive signal fades. Further, a squelch tail would preclude adding remote IRLP capabilities due to the IRLP policy that prohibits transmit squelch tails or telemetry being sent over the VOIP link. Finally, a squelch tail on the link back-bone would result in double squelch tails among linked repeater systems.

DTMF tones are not muted on the link back-bone in order to enable control to be invoked on remote linked repeater sites and other remote facilities (such as internet linking).

Repeater systems that are allowed to operate on the CIARC link back-bone, under FCC Part 97.205 Paragraph E, can use a half-duplex radio that transmits on 902.2250 MHz and receives on 927.2250 MHz, using DCS code 023 and without requiring a duplexer.

Repeater systems using the CIARC link back-bone, under FCC Part 97.205 Paragraph E, are required by CIARC to not transmit a squelch tail over the link. This requirement is a violation of Internet Repeater Linking Protocol (i.e. IRLP) and could result in termination of IRLP services.

FCC Part 97.205 Paragraph E

Ancillary functions of a repeater that are available to users on the input channel are not considered remotely controlled functions of the station. Limiting the use of a repeater to only certain user stations is permissible.

CIARC intends to support internet linking over the link back-bone by placing a half-duplex radio at a location that has internet access.

Repeaters that are interfaced on the CIARC link back-bone should transmit with a deviation of 2 KHz. The deviation has been set to this level as non-linearities in the audio signal have been observed with higher deviation levels. A 2 KHz deviation signal received on the 33cm link back-bone will be transmitted on the 2m and 70cm repeaters as a 5 KHz deviation signal.

33cm Link Radio - Satellite Repeater Interface Requirements
1 Permission to use the link back-bone must be obtained from CIARC.
2 Satellite repeaters shall not transmit a squelch tail over the link channel.
3 Satellite repeaters shall not forward a squelch tail over the link channel, except when a remote base is used to connect to another repeater.
4 Satellite repeaters shall not transmit telemetry of any kind over the link channel.
5 Satellite repeaters need to coordinate control codes with CIARC control operators or technical committee to ensure that inadvertent control operations do not occur among any of the constituent repeaters.
6 Satellite repeaters shall make link enable / disable control codes available to CIARC control operators.
7 Satellite repeaters shall transmit Digital Codec Squelch code 023 over the link channel.
8 Satellite repeaters will limit deviation of the link transmitter to 2 KHz. Best practice here is to have a 2 KHz deviation tone sent over the link transmitter resulting in a 5 KHz deviation signal transmitted from the KC7MCC 2m repeater on 147.020 MHz.
9 Satellite repeaters may optionally decode Digital Coded Squelch code 023 received over the link channel.

33cm UHF Link Back-Bone - Predicted Coverage

The following image depicts the predicted UHF coverage when operating from Lookout Peak on No Business Mountain.

Audio Routing Through the Link Network

A more comprehensive document that describes CIARC linking is available here.