The advantage of MPT 1327 over TETRA is the increased availability, lower cost of equipment, the ease of installation, the familiarity with the equipment, and many believe that MPT 1327 is superior to TETRA, due to its uncompressed FM audio, and greater receiver sensitivity. MPT1327 control channel signalling is more resilient, since the TETRA protocol uses a complex
modulation scheme that requires a far higher Signal to Noise ratio to function than 1,200 bit/s
FFSK signalling. Systems based on MPT 1327 only require one, but usually use two or more radio channels per site. Channels can be 12.5 or 25 kHz bandwidth, and can be any variety of channel spacings, with 6.25 kHz or 12.5 kHz being typical. At least one of these channels is defined as the control channel (CCH) and all other channels are traffic channels (TCs) used for speech calls. A typical installation will have around 6–10 channels. A 7-channel trunk, correctly engineered, is capable of handling in excess of 3,000 mobile units. The capacity of the system increases dramatically with the number of traffic channels. For example, 1 traffic channel with queuing can not handle many customers, perhaps 2
minicabs with 20 mobiles. In effect this would be a CBS with queuing. However, a 7 channel trunked system can handle 40 minicabs with 20 mobiles with ease. The
Erlang formulas are typically used for calculating system capacity.
Spectrum efficiency Whilst MPT 1327 systems, unlike DMR or dPMR, do not employ digital speech compression to gain any Spectral Efficiency (voice channels per 6.25 kHz), there are several methods used that increase the Spectrum Efficiency (Erlangs per square kilometre, per 6.25 kHz). A spectrum efficiency advantage over a 4-slot TDMA system like TETRA is in areas where low-bandwidth channels are required. The absolute minimum TETRA installation would require a 25 kHz bandwidth in order to carry a control slot and three traffic slots. The absolute minimum MPT1327 assignment is a single non-dedicated control channel, utilising 12.5 kHz, in most cases. A non-dedicated control channel can be used as a traffic channel when all the other traffic channels are busy. This can be useful if the site is part of a multi-site network and has a very low traffic profile as the site could have a single channel rather than at least two freeing up one channel for use elsewhere. The disadvantage is loss of queuing, and data cannot be sent on the control channel whilst it is in traffic mode. A non-dedicated CCH should not be used as a "reserve tank" for a busy site as the lack of signalling will seriously affect the operation of the site.
Time-shared control channels and channel pooling Some MPT 1327 networks can also time-share control channels, which can be useful if the network has limited frequency availability, as it frees up channels for use as traffic channels, which can also be pooled across sites so the network capacity follows the traffic. This is another advantage of MPT 1327 (and dPMR) over TDMA-based systems such as TETRA and DMR, which cannot pool traffic channels so efficiently (if at all). The disadvantage of using a time-shared CCH is that it slows down registration and calls and requires some customization of the registration process, so is only useful if the network has a patient user community!
Speech and data Speech is sent as
narrowband frequency modulation. Data messages between mobiles and the network are exchanged on the control channel at 1,200 bits per second using
FFSK signalling, or a specific "
modem call", known as "non-prescribed data" can be established, whereby free-form 1,200 baud data can be exchanged on a traffic channel without tying up the control channel. With the use of special modems, the speed can be 19,200 bit/s. This, along with Short Data Messaging and Status Messaging via the control channel makes an MPT1327 network ideal for managing
AVL for asset management,
meter reading and
SCADA networks, the advantage being that the network can be used for this sort of application whilst still carrying voice traffic.
Numbering Each subscriber in an MPT-1327 trunked radio network has a unique call number. This call number (address) is a compound number consisting of a prefix (three digits), the fleet number and the subscriber's call number within the fleet. Different numbering schemes work differently, for example
Zetron uses the first two digits of the Ident as the fleet number, and the last two digits as the unit number. Idents in the 6,000–6,999 range are typically used to establish group calls. After it has been entered the call number will be converted in the mobile to a 20-bit address. The numbering rules are clarified and expanded by supplementary standards such as MPT-1343. For the duration of his call a subscriber is exclusively allocated a traffic channel from the available pool. If all channels are occupied, the call will be queued. If the control channel has become a traffic channel, like in the case of a non-dedicated control channel, the call will be queued in the radio, although radio queuing loses the first come, first served effect, so if there are seven units queuing, the last unit to queue may get a traffic channel first. ==The different types of communications on an MPT-1327 network and their definitions==