A solution for proving MPT1327 radio systems
Traditional testing methods are unsuitable
Communications Service Monitors have been available for a number of years to test MPT1327 mobile radios. However, up until now there has been no easy way of checking new system installations or software upgrades to existing infrastructure. New systems can be tested in the factory by the manufacturers. They traditionally use a 'home-brewed' test system which would consist of a large number of radios connected together under computer control. This would be housed in several 19" cabinets and would be quite unsuitable for field use.
A further problem is faced when proving the correct functioning of a system when it is being upgraded. The installer faces the dilemma that it causes less disruption to users to perform the upgrade at an off-peak time, however they cannot then test the system under full loaded conditions. They have to wait until the next peak loading time, by which time a system problem could be disastrous to users.
Marconi Instruments Ltd (MI) has developed a solution to these problems. The Bulk Call Loading system developed by MI is now being used by NB3 in the UK to test rigorously, yet non-intrusively, MPT1327 base station software under heavy loading conditions to determine the reliability of operation. NB3 operates a national network for business users providing integrated voice and data communication services. MI's MICAS Instruments Applications Group have designed the system to test any MPT1327 installation. It comprises a standard MPT1327 2945A Series Communications Service Monitor and software which can be run from a standard PC over the RS232 port.
The MPT Standards
The MPT documents were written over a five year period by a committee of interested parties taken from the UK Department of Trade and Industry and UK based industry. They incorporate many features of great originality. Although the standards and specifications governing the design and implementation of analog trunking networks have since been adopted and adapted by a large number of other countries, both in Europe and around the world, they all have their root in MPT1327.
The signalling standard on MPT1327 defines the protocol rules for communicating between a trunking system controller and the mobile radio units which it serves. It was designed to be implemented in a wide variety of systems. At one extreme, the smallest of these could be a simple single site where a control channel may have to share its channel resource with the traffic channels. NB3 operates the largest network of its kind in the world, covering a large geographical area. This is a single network with a hierarchical structure consisting of a database at the top level, then Traffic Area Controllers, and then base stations. Connections can be made from this network into customers' PABXs.
The standard defines only the over-air signalling and imposes minimal constraints on system design. Additional specifications, such as MPT1343, define specific implementations. For example, they specify the facilities which must be implemented, parameter values, a channel plan, and for a network, criteria for when a radio unit should register.
System Features and Facilities
The numbering range of the MPT1327 protocol accommodates over 1 million addresses per system, over 1 thousand channel numbers, and over 30,000 system identity codes. The protocol uses signalling at 1200 bit/s with Fast Frequency Shift Keying (FFSK) sub-carrier modulation. It is designed for use by two-frequency half-duplex radio units and a duplex Trunking System Controller (TSC). The signalling for setting up calls is transmitted on a "control" channel. Broadcast messages are available to inform radio units of system information, such as the channels which the system may use for control signalling. One of the problems of mobile radio signalling systems is the clashing of messages from different radio units transmitting at the same time. The problems of clashing are controlled by an access protocol (ALOHA) which offers high efficiency, stability and flexibility. Protection against interference is provided by labelling the signalling with a system identity code and, in some messages, the channel number. If heavy interference is encountered, control can be changed to a different channel. The protocol is also designed for use by systems which queue calls that cannot be set up immediately.
Figure 1 - 2945A Communications Service Monitor Connected Off-Air
Stressing the system under test
The Bulk Call Loading software for MI reproduces the protocol stream which would be sent by a very large fleet of radio units. This is used specifically to stress the access protocol and queue handling capabilities of the system under test. Once the software has been installed on the PC, the user defined MPT1327 System type is set up on the 2945A Series Service Monitor. After downloading the Base Station Loading software from the PC, the test can be run. The Base Station can be connected to the test set either off-air (figure 1) or by cable (figure 2).
Figure 2 - 2945A Communications Service Monitor Connected by Cable
A number of parameters can be configured to change the way in which the Bulk Call Generator works: RQS, RQR, RQQ and RQC call types can be generated with varying call repeat rates and calling party identity ranges; and all common ALH, AHY, and HEAD messages can be received.
Message repeat rates can vary from every available slot to once every 999 free slots, or each call type can be disabled individually. Call generation starts at the low end of each identity and prefix range, and loops back to the first value after the highest prefix and identity set is reached. A request for a particular call type on a free slot will only be generated if it is compatible with the most recently received ALOHA slot type. The test set does not generate calls using a random access protocol but in a deterministic manner. This ensures that whilst not violating any slots which are required for system responses, the call loading takes place in a predictable and repeatable way. This level of repeatability cannot be duplicated by other systems based on multiple connected handsets.
The software can also generate short data messages, and allow the contents of the PARAMETERS field to be changed. If the system loading is very heavy, the number of pending call requests can soon mount up, but the software allows for up to 100 such RQC's or RQQ (off-hook) calls to accumulate before losing the oldest in the queue. Finally, the received calling party identity range can be limited to generate responses to a specific range only, which can be set individually for all AHY types.
The MPT1327 Bulk Call Loading system can also be used to check for responses to invalid information, e.g. it can check that the system response to invalid call IDs, etc. are correct. It can also provide a background call logging capability. This logs all messages transmitted and received back to a PC. The log files can then be used to help debug MPT1327 Base Station system installation problems, or can be analysed to evaluate the performance of the system.
The MPT1327 Bulk Call Loading system from MI's MICAS Instruments Applications Group, provides a non-intrusive means of verifying new or upgraded MPT1327 Base Station systems. Being non-intrusive, the tests can be carries out at any time. Based on the Marconi Instruments 2945A Communications Service Monitor, the solution is low-cost and portable. It is simple to use and allows manufacturers and installers to check the whole system, not just the hardware, in a repeatable fashion. The system allows full call loading of the base stations at off-peak times and can provide background call logging.
Note : Contact Marconi Instruments for individual requirements for MPT1327 Base Station loading.