The following picture is used to give an initial idea about the MORSE network. Information transmitted in the form of data packets comes from the connected device A along the wire link to “radio” CU1. This “radio” is a complicated piece of equipment that checks the packet, modifies it and supplements it with records necessary for further transmission over the radio channel. This is why it is labelled as a communication unit (CU).
Amongst information added to the packet is the first section of the route which the packet will travel through via the net environment to the destination CU3. In our example CU3 is out of reach of direct audibility of CU1, therefore the packet is routed through CU2. This communication unit, as for all the others, is equipped with intelligence, which is used to take care of the packet along the next section of its route. CU2 thus receives the packet from CU1 and sends it on to CU3. This method of transmitting packets can be called store and forward relaying. At the destination CU3 the packet is sent along the wire link to the external device B.
A radio network is generally more branched. Connection is made between individual points in the direction of lines marked in the drawing.
Besides these routes it is often possible to achieve connection by further combinations, for example between CU3 and CU6. Whichever links will be used during operation is determined by the quality of the connection, according to loading of the network and according to the structure of addresses used in the network. Each CU has information about the choice of routes in its routing table (also store and forward table) and according to these it then decides about routing obtained packets.
The network can transmit data in two ways. For connection-oriented transmission a route is created in advance by connecting individual sections of the network and the route created is then available for bi-directional transmission of data. An example is the telephone network. This method is suitable for the transmission of a large amount of data in one transmission. Other participants must wait during this period.
The second method is connection-less transmission which is also used in the MORSE network. Packets with data are sent to the network here. Each packet is provided with the destination address and travels through the network by itself. The length of the packet is limited to 1500 bytes, so transmission between two CU’s runs from 0.1 to 1 second depending on the length of the packet and other conditions. Another packet can then pass through the same section of the route to another destination. The suitability of non-linked transmission for frequent routing of short messages to a large number of participants comes from this.
Non-linked transmission including addressing can be likened to the postal service. In this case information is sent with the address of the network of post offices which gradually regulate its progression to the addressee. Let’s try and compare analogical terms:
|Postal address||Packet header with address|
|Registered letter||Acknowledged receipt of packet|
|Letters can overtake each other||The same order of registered packets is not guaranteed|
|Sorting of letters before delivery||Routing of packets|
The MORSE address contains 4 levels for determining the destination in its 4 bytes. They are termed:
global - net - wide -
An example of an address is given below:
It contains 4 bytes with each created from two hexadecimal symbols:
– global (country specification, 69=CR, 6A=Slovakia…)
0F– net (area in the country or other closer definition of the network)
0A– wide (subnetwork, further specification)
11– local (specific CU user)
Each byte can acquire a value of 0 to 0xFF (0 to 255 dec), the overall theoretical number of addresses is thus 2564 which is more than 4 billion. The address architecture is important for the accessibility of individual networks, especially at the remote supervision of the network functionality. If not stated otherwise the MORSE address is recorded using eight hexadecimal symbols.
Work with the address is described in paragraph routing.
The internal layout of the modem from the point of view of configuration is contained in paragraph Communication unit.
A packet is a sequence of bytes ordered according to precise rules.
At the front of this sequence is a group of bytes which contains basic information about the packet (type, length, who determined for, etc) and this is called the head.
Behind this other bytes follow which carry necessary information (data monitored by measuring devices, GPS position of vehicle, text message, command for remotely operated pump etc.). This part is labelled as data.
A packet is generally ended with a CRC which serves for data integrity checking.
The particular form of the packet depends from the communication protocol used.
The range or audibility of the radio signal in the 400 MHz band is influenced by the shape of the terrain and other obstacles. Under unfavourable conditions (city, valley) the range of the MR400 is several hundred metres or several kilometres. Under favourable conditions (direct visibility, good antenna) good quality operation is possible to distances over 100 km.
The medium used for packet transmitting enables such broadcasting of messages where the transmitted message can be picked up by more receivers. Therefore the message must carry information about which of the receivers must process it further. In our case it is the medium of radio signals or the Ethernet network medium.
The other media type only allow signals to be transmitted between two participants and can be called a link media. An example is the wire communication line protocol RS232 which is used for the connection between the CU and external devices.
Operation in the network medium can be controlled in principle in two ways:
The network can only contain one centre which controls routing of messages, for example a telephone exchange. Participants then only request the centre to mediate the routing of messages and the centre undertakes everything necessary.
The second possibility is that the network is composed of elements from which each is able to process the message in the necessary way. This means deciding where the message should be routed and route it to the appropriate channel (which here is the radio channel, Ethernet, wire communication line, output port, service channel) and check the data integrity. The MORSE system is composed of a CU equipped in this way. All CU are the same and differ only in the setting of configuration parameters. Therefore the MORSE network is easily expandable or adjustable.