ROAMEO uses DECT (Digital Enhanced Cordless Telecommunications) as its RF protocol. DECT is a mature radio technology that allows multiple users to occupy RF carriers by means of TDMA (Time Division Multiple Access), which means each carrier frequency is divided into multiple timeslots. Each of these timeslots carries audio and control information for the system.
In Europe, and most parts of the world, there are 10 RF carriers available. In the United States, Canada and most of North America there are 5 RF carriers available. In South America, it is a mix between 5 and 10 RF carriers.
Although there are 24 timeslots available (12 downlink and 12 uplink timeslots), the Intercom use of the DECT protocol only allows the use of a total of 20 timeslots. The other 4 timeslots are used internally by the DECT protocol. There are two types of wireless intercom DECT based products; products that use one timeslot and products that use two timeslots to convey audio and control information to a single Beltpack. Those companies that use one timeslot, typically, can support up to 10 Beltpacks per RF carrier but at the expense of some audio bandwidth or performance. The companies that use two timeslots can typically support up to 5 Beltpacks per RF carrier but with increased bandwidth for better audio performance. The ROAMEO system allows the user to decide. A user can choose more Beltpacks by selecting a specific CODEC in the configuration software, selecting narrowband operation or wideband operation. The narrowband CODEC uses one timeslot for each Beltpack and wideband CODEC uses two timeslots per Beltpack allowing for enhanced audio fidelity.
Once the CODEC configuration is known, the number of BPs supported per carrier frequency is also known; 5 BPs per carrier for systems that use two timeslots and 10 BPs per carrier for systems that use one timeslot. Given the number of RF carrier frequencies available and the CODEC selection, the maximum capacity of Beltpacks in an area is calculated by multiplying the two numbers as shown in Table 1 and 2:
While some manufacturers may tell customers that the Tables are the Beltpack capacity that they can have in an area for their system, it is actually just the mathematical maximum. In real world situations, and as proven out by our testing as well as the DECT specification, the maximum capacity of devices is reduced by interference in the local environment and by adjacent Access Points (APs) and Beltpacks (BPs) use of the RF spectrum.
At maximum area capacity, as in Tables 1 & 2, there are no timeslots open on a carrier that allow a Beltpack to change frequencies or timeslots when interference is detected.
For robust systems, timeslots on each AP should be reserved for intra-AP frequency and timeslot negotiation. This provides for a better, interference free, optimized, connection on one of the RF carriers and timeslot(s) available in the area. DECT is a make-before-break audio protocol that requires that an open timeslot is confirmed before it will disconnect from the current timeslot. For effective intra-AP frequency and timeslot negotiation in your DECT system, please see Table 3 & 4 for the RTS Maximum Beltpack Capacity Recommendation.
In order for Beltpacks to roam, DECT timeslots in a system (multiple Access Points) must be left open (unused by Beltpacks) to allow a BP to roam into an Access Point. How many DECT timeslots should remain open depends on the situation, but this should be considered as a necessary component of a successful system design.
The ROAMEO system is highly scalable. With the assertion that DECT RF frequency and timeslot management is being considered in the system design and, thus, effective RF spacing of reused DECT frequencies is being maintained for both Access Points and Beltpacks, then the effective ROAMEO system size (number of APs and number of BPs) can grow well beyond 80/40 Beltpacks using the narrowband or wideband CODEC. This will be covered in a separate RTS White Paper on DECT System Frequency Management. With careful system design, including radio frequency reuse management, the ROAMEO system is scalable beyond 100/50 Beltpacks in Europe and the United States, respectively.
When planning the number of Beltpacks that can be used in an area, do not just consider the maximum devices possible in a physical location. Consider that interference, such as DECT phone operation within the same area, which could occur. The system will need to have the ability to automatically adjust and optimize the RF paths. In order to optimize operation, the DECT protocol requires some empty timeslots to be reserved for these situations. In our recommendation, we account for these situations by reserving at least one open DECT timeslot on our Access Points for intra-AP frequency and timeslot negotiation for a system using the wideband CODEC and 2 timeslots for a system using the narrowband CODEC. In addition, at least one open DECT timeslot on an Access Point is needed for Access Point to Access Point (inter-AP) roaming. The ROAMEO system can support 100 Beltpacks in Europe and 50 in the United States. We recommend the maximum number of Beltpacks outlined above in Tables 3 and 4 to allow for effective DECT frequency and timeslot negotiation³. However, with proper frequency reuse management through effective Access Point and Beltpack spacing, the RTS ROAMEO system can easily scale to beyond 100/50 Beltpacks.
Written by Jim Andersen and Kevin Moore, Bosch / RTS 2018
¹ Maximum capacity of an area shouldn’t be construed as recommended capacity. A system with a maximum capacity of Beltpacks (any manufacturer’s DECT product) will not allow Beltpacks to roam freely between Access Points. Effective roaming of any manufacture’s Beltpacks using DECT protocol requires both free timeslots (for both Beltpack Roaming and IntraAP Access Point timeslot / frequency management) and non-overlapping frequencies. See Intra-AP timeslot section.
² The RTS AP-1800 Access Points can support up to 10 Beltpacks using one timeslot per Beltpack (narrowband mode) and 5 Beltpacks using two timeslots per Beltpack (wideband mode) but these numbers are reduced in the default setting from factory to make available frequencies and timeslots for intra-AP (within an Access Point) negotiation. The default number timeslots reserved for intra-AP frequency and timeslot negotiation per Access Points are narrowband = 2 and wideband = 1.
³ This DECT system limitation is the same for all manufactures of DECT-Based Beltpacks.