The objective of the following text is to describe concisely the method of communication with mobile stations within the MORSE system. The document is divided into the following sections:
- Specific features of mobile stations — elements of the MORSE network
- The principles of the solutions for communication with mobile stations
- The principal rules for configuration of networks with mobile elements
- Methods of using mobile operation in possible applications
- The present status and the expected development of the MORSE system capabilities
The discussion in this document focuses solely on the problem of organisation of the network operation from the software point of view (i.e. the protocol). The problems concerning the level and quality of the radio signal in networks with mobile stations are not discussed here.
1. Specific Features of Mobile Stations
The main and in fact the only difficulty about incorporating mobile stations into networks is that the mobile stations change, in short intervals, their location within the topological network. For this reason, it is not possible to use static routing, which constitutes the basic method of communication in the MORSE networks, using retransmission tables defined on the basis of a specifically designed and verified network topology of each individual point. Dynamic routing is not applicable in any type of non-trivial network with mobile stations. Therefore, special rules must be used for communication with mobile stations.
2. The Principles of the Solutions for Communication with Mobile Stations
Each packet in the MORSE network carries two addresses in the network layer header of the internal protocol — the destination and the source. A packet travelling from or to a mobile station must carry three addresses — the address of the particular mobile station, the address of the base station currently within the reach of the mobile station, and the address of the fixed station of the packet destination. (Direct communication between two mobile stations is generally not possible. Should such communication be required, it will be necessary to handle it via an intermediate fixed station.)
The above three communication parties are denoted as Mobile, Base, and Center. The header of every mobile packet contains the information indicating on which of the four possible routes the packet currently is. This information is displayed also in the monitoring information in the formats: M2B, B2C, C2B, and B2M (i.e. Mobile to Base, Base to Center, etc.). The source and the destination of each partial route are written at the usual locations within the header, being displayed in the same way also in the monitoring information, while the remaining part of the address is specified in the extended header, and displayed at the end of the line in the monitoring information, after the indication of the part of the route that the packet is currently travelling along.
Each mobile station continuously tracks the communication in the radio channel and keeps in memory the information of the base station from which it can receive the strongest signal. This information is considered valid for 30 seconds. At the instant when sending a packet to any station of the network is requested, the mobile station labels the packet as M2B, adds the address of the fixed station of destination into the extended header, and sends the packet to the base station. If there is no valid record of the base station in its memory, the mobile station first sends a query (a CQ packet), to which all the fixed stations of the network which intercept it respond.
The base station, which receives an M2B packet from a mobile station, «converts» it into a B2C packet, which means that it writes the address of the fixed station of destination into the destination entry, inserts its own address into the source entry, and specifies the address of the mobile station in the extended header. The resulting packet is delivered to its destination by standard methods using the retransmission tables.
If a fixed station wants to send a packet to a mobile station, it has to specify the base station currently within the reach of the particular mobile station. It creates a C2B packet, where destination carries the address of the base station and the mobile station address is in the extended header. This packet is delivered by standard means using the retransmission tables to the base station, where it is, as in the above case converted to a B2M packet and the mobile station is called, in this case using the retransmission tables again. This reference to the retransmission tables may seem superfluous at the first sight, but it allows for communication with multiple addresses within a single mobile device without each of them having a direct access to the radio channel.
As indicated above, the base station address has to be specified whenever a packet is to be transmitted from fixed to mobile station. The standard and easiest way of handling that problem is running the Mobile Center on one of nodes in the central MCU. Addreses of mobile stations are registered automatically there and headers of packets to/from mobiles are appropriately changed.
3. Configuration of Networks with Mobile Elements
There is only one necessary configuration — each RF channel protocol, which is to operate as a mobile one, has to have a flag set indicating that it is a mobile channel. The flag is set in the Fpe menu. Any packet directed from such station «into the air» is labelled as M2B and the respective base station is called (see above). Please note that a packet has to enter the respective RF channel protocol before it can become a mobile one. All the settings in the Ne menu apply to the packet routing in a standard way, including routing tables, up to the moment when the packet enters the RF channel protocol interface. It is therefore possible to define such configuration that there are several addresses within a single mobile device that the packet runs through before it is sent «into the air» from the mobile address. However, such combination requires a corresponding setting in the retransmission tables of all base stations (because of the return path) and is therefore rather demanding from the configuration point of view.
In networks where operation of mobile stations is expected, the fixed stations should by definition be configured such that configuration of paths from all to all fixed stations be specified, or at least to all «central» stations. The base station «chosen» by a mobile station located at a particular position can often be at a very different place than what would appear obvious at the first sight.
Other special settings of the mobile station concern access to the channel and the method of solving collisions (automatic retries). Since the situation of two mobile stations calling the same base stations and ending up interfering each other is quite typical, it is appropriate to increase the maximum number of slots for CSMA and RC-CSMA of the mobile station to as much as 8, and to extend the length of one slot to the length of a typical short packet (30 ms). If transmissions of packets longer that 20 — 40 bytes is expected, RC-CSMA will probably be employed. In networks where users do not have stringent requirements on delays in packet delivery, it is recommended to greatly extend the automatic retry periods, and also to increase the number of retries. This is such that in the event of a failure, the mobile station is able to keep trying to make a connection for the entire interval set for regular position reporting of the mobile station. Naturally, it is necessary to correspondingly extend the store timeout.
Example:There is a station of the position tracking system with the position reporting interval set to 30 seconds. The appropriate setting is 5 retrials, ACK timeout fix 4000 ms, variable 1500 ms, store timeout 28 s.
The last configuration speciality: It is possible to set a particular Node (a particular address) in the Node edit menu as the Center — see item 2) at the end of Section 2. This can be done in the menu Ne, and this setting is indicated by a capital C at the end of the respective row. Setting of an Node as Center allows for communicating with mobile stations by any ordinary protocol. This means that the modem set in this way converts the incoming packets from the mobile stations into standard packets, maintaining at the same time a dynamic table of base stations, through which the individual mobile stations have to be called.
4. Applications of Mobile Operation
It has to be kept in mind that irrespective of the final requirements of the user, a more or less regular communication initiated by the mobile terminal stations must be maintained in the network. The Center must at all times know which is the base station through which any particular mobile station can be reached. Only in cases when the communication required by the user is always and without exception initialised by the terminal mobile stations, it is possible to leave the interval of position reporting of the mobile stations up to the choice of the user. If there is a possibility that any of the fixed stations will initialise a communication with a mobile station, it is necessary to introduce a regular position reporting by the mobile stations to the Center. The period between these reports has to correspond to the requirement of the user regarding the time within which he needs to make a connection with a mobile station.
From the above it is apparent that the task of tracking the position of vehicles at some control centre in regular intervals is fit for the mobile network MORSE. The operation of a circuit with a mobile and a fixed station is also very well traceable, while some mechanism of maintaining connection is necessary in the protocol (HAYES will do very well). If such a circuit should be initiated from the Center, regular position reporting of the individual mobile stations is once again necessary.
An attempt to operate a telemetry system with a single master constantly polling all slaves in a network with mobile stations would be extremely ineffective. Should such applications occur in the future, it will be necessary to «shield» the control centre of such system from the network by an additional communication server, which will accept initiative from below and simulate slave stations to the control centre.
5. The Present Status and the Future
At this time, all of the above configurations are functional in the current firmware. A fully functional Center mode is also available, however, within the framework of MR25, it allows for communication with only a limited number of mobile stations. The reason for this is the limited memory size of RAM in MR25.
For larger networks, Center can be implemented in a personal computer (communication server) where the Morce program is running. For large networks, a promising solution seems to be connecting our communication server directly into the LAN network of the user.