A message transmitted on a multiplexed network by an item of equipment is called a packet
The packet is made up of a sequence of impulses (called bits) which have either the value 1 or the value Ø, which means therefore presence or absence of the signal transmitted
It is the communication protocol which will define the structure of the packet
A packet (B) is made up of several blocks (A) (or fields)
- A start of packet (2)
- An identifier (3)
- A control field (4)
- The data to be transmitted (5)
- A check field (6)
- A confirmation of reception (7)
- An end of packet (8)
(2) the start of packet signals to the various items of equipment that the packet is about to be transmitted
(3) the identifier serves to indicate in particular the recipient(s) (address) of the message
(4) the control field indicates the nature of the message (transmission of data, of an order or of a command)
(5) the data is: either orders, or data to be transmitted
(6) the check field makes it possible to check that the data transmitted has not been altered (validity of the message)
If the message is not valid, the transmitter destroys the message and will re-transmit a packet later
(7) confirmation of reception is carried out by the equipment which receives the message; it makes it possible to ensure that the message was received correctly by the recipient
(8) finally, the end of packet enables the network to return to its initial status
It is possible to see, in the illustration above, that part of the data transmitted (2, 3, 4, 6, 7 and 8) is only used to ensure communication of the useful data
Priority management; non destructive packet collision
The protocol enables collision problems to be managed (when several items of equipment want to transmit a packet simultaneously)
Each transmitter reads the signals on the network during the transmission
If it notices a difference between the signal transmitted and the signal received, it then recognises that another piece of equipment is transmitting a packet of higher priority
It immediately interrupts its transmission, which enables the higher priority packet to be transmitted
This non destructive collision is controlled in accordance with a bit by bit arbitration principle: one is called the dominant bit (value = 0), the other is called the recessive bit (value = 1)
Пример
(9) a packet transmitted by equipment C
(10) a packet transmitted by equipment D
(11) сообщение передается по сети
(a): loss of arbitration
(t): время
At time (t1), equipment C (packet (9)) wishes to transmit a 1 bit and reads on the network a 0 bit (packet (11)); it interrupts its transmission due to the loss of arbitration (a)
Equipment D then continues to transmit its packet (10) of higher priority
Сообщение передается по сети (11) = a packet transmitted by equipment D (10)
The packet transmitted by equipment C (9) will be sent again later