System size is also a consideration when defining backhaul. While all modern P25 networks are IP based, each vendor has its own calculations to determine the capacity of the links connecting your network sites. A simple (and preliminary) rule of thumb is 64Kbps per channel (working or control).Depending on your vendor’s technology implementation, that may be either excessive or insufficient.
Microwave, fiber, or…?
Beyond the important work of analyzing and selecting the best combination of backhaul solutions, it is important that the backhaul network must be “mission critical” – meet agency standard redundancy and resiliency requirements. Eliminating single points of failure, installing fail-over back-up units, and multi-path routing capability will ensure redundancy. Ensuring power requirements are available with sufficient capacity to ensure reliable power for an extended period (hours, days, weeks as your standards or local circumstances require) in case of primary power failure will guarantee power resiliency and alternate traffic routing capability.
None of the backhaul technologies is universally superior. Fiber is favored due to practically unlimited bandwidth, but many believe that operating your own microwave network provides higher level of reliability. This is because IT crews typically in charge of fiber networks are generally unaware of Public Safety radio specific network needs and sometimes nonchalant about shutting a link down for a few hours for maintenance.
Today, there are more backhaul options than ever. At the most basic level the choices are: copper, fiber and micro-wave, with multiple variants of specific technologies within each topology. Each has band-width capacity and quality of service limits which help define where it is best used.
Start by discussing backhaul requirements with your IT department addressing the following topics:
- Characteristics of the proposed new LMR network. Vendors and consultants can provide transport characteristics and requirements impacting backhaul choices.
- Review all existing backhaul networks – owned and leased.
- Discuss future network requirements at sites which may impact backhaul.
- Anticipate timeline of LMR and other network deployments at sites.
You can ask questions and solicit information and opinions from commercial carriers, backhaul infrastructure manufacturers and vendors, LMR manufacturers and
vendors. At the minimum, establish working groups to research:
- which backhaul networks can we reuse or repurpose and which networks require new backhaul?
- what are the priority and latency requirements of each voice, data, and administration application requiring backhaul, and which can we group together?
- how can we converge voice and data backhaul requirements into least number of separate networks?
- which network topologies can meet our needs,
- what are their hardware/software components and requirements, what do they cost?
- own or lease – what are the total costs of ownership over next ten years?
Minimizing cost is always a worthwhile objective but the ever-multiplying growth in mobile communication capacity requirements support our recommendation to deploy a backhaul network with many times the capacity you need today.
|Copper||Low||High||High||Low||Medium||Carriers have much in place|
|Microwave||High||Low||High||Medium||Medium||Typically LoS although some NLoS available|
Fiber is most often the preferred choice, since applications that we have not yet thought of, will be best served by backhaul that is high in technical adaptability and capacity. However, fiber is terrain-limited so doesn’t work everywhere.
To decide which variant(s) of the three major platforms are most viable for you, you need to consider:
- user group size,
- technical expertise needed to manage/maintain,
- budget, local availability,
- existing backbone available,
- capacity for future expansion.
For most agencies, a combination of topologies will deliver the best user experience. If you are a rural agency you will likely have access to fiber in the metro areas, but will need microwave for county-wide TX/RX locations. If you are only concerned about voice, then existing copper T circuits to the sites where you will place your LMR equipment will likely work fine with modern variants such as VDLS2.
Redundancy to ensure network resilience is probably the best argument for a mixed topology solution which would typically be fiber and microwave.
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