Designing your MOTOTRBO system: Some Ideas

Now that you've convinced your customer that MOTOTRBO is the best choice for their communication needs, the next step is to design a system that meets his/her needs and budget.

I must confess that I am neither a system engineer nor am I a Project Manager so I cannot talk from experience of implementing large projects. My role in such cases has always been as an adviser. (Though I have some formal training in Project Management and have some knowledge of TPS LEAN.) So any advice offered here is done so with a big Caveat Emptor sticker on it.

This process is divided into several steps:
  1. Establish functional requirements
  2. Determine system architecture and best approach.
  3. Prepare a draft system design.
  4. Review system design in consultation with the customer and provide a budgetary estimate.
  5. Finalize system design and obtain customer approval.
  6. Plan work activities (Project Management).
  7. Factory Acceptance Test
  8. Deploy system in agreed timeframe and at the agreed price  (Project Management).
  9. Complete deployment and get signoff from the customer.

Some things to consider while establishing the functional requirements:

- How many radios will be on the system now (and in the future)?
- What is - or will be - the usage profile of the radio users?
- What other services will be needed? (data; GPS etc.)
- Where is coverage needed?
- Is this a new system, or an upgrade from something else?
- What budget is available?

There are, of course, a multitude of other factors to consider but the above points will make the biggest impact on system design.

Choosing the right topology is critical. MOTOTRBO is available in the following topologies:
No of users
Single Site
A single repeater in digital mode. 70 users per slot. More repeaters can be installed on the site to increase the capacity but the traffic-per-slot balance is not optimized.
IP Site Connect
Multiple repeaters linked via IP to form a wide area network. IPSC does not add capacity as each connected repeater acts as a single channel. More capacity could be attained if some sites had so-called local area slots (slots which are not linked via IP). Like Single Site more repeaters can be installed on the sites busier sites to increase the capacity, but the traffic-per-slot balance would still not be optimized.
Capacity Plus
A single Capacity Plus repeater can support slightly more users because the slots are allocated more efficiently. Capacity Plus supports up to eight trunked repeaters. Additionally GPS traffic can be farmed off to GPS revert channels thus freeing up the trunked channels for voice and a few location requests.
Linked Capacity Plus
Linked Capacity Plus supports up to six trunked repeaters per site. Additionally GPS traffic can be farmed off to GPS revert channels thus freeing up the trunked channels for voice and a few location requests. More capacity can be attained by configuring the system such that group calls do not go out on sites where there are no members.
Connect Plus
Connect Plus is a 100% trunked system and supports call queuing which will allow a site to be loaded a lot more than a (Linked) Capacity Plus site. Each site supports up to 15 repeaters – one slot on one of these repeaters is needed for a Control Channel. Connect Plus also supports up to 70 sites.
The maximum number of users shown above, is merely a rough guide and is based on a certain user experience. IPSC; Cap+ and LCap+ will follow the Erlang B traffic model and Connect Plus will follow Erlang C.

The decision which topology to use is based on the answers to the above questions.

To calculate the number of repeaters required on a site, and to understand the IP link bandwidth required at these sites, Motorola has produced a System Design Tool which automates these calculations. The system design tool can be downloaded here (Motorola Online access required)**.

** If you would like a copy of this tool, contact your Motorola Dealer/Distributor/Account Manager.

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