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back to the source, together with the confirm of the reception, a set of data octets, if these latter were previously prepared with the DL-REPLAY-UPDATE service. The PDU of LLC is shown in Fig. 3. The field Control, when the PDU is used for connectionless services, is limited to one octet. In detail, for the DLUNITDATA service, the PDU carrying the data transmitted after the issuing of a request primitive is called Unnumbered Information, UI. For the DL-REPLY service, the PDUs involved are AC0 or AC1. After a request primitive either AC0 or AC1 is transmitted. The remote user will respond with a PDU of the same type and with the complementary number (i.e. when AC0 is transmitted, AC1 must be received and vice versa). 3.2. The IEEE 802.3 CSMA/CD
The CSMA/CD technique specifies that every station continuously senses the network and acquires the frames directed to itself. A station wishing to transmit simply verifies if the network is idle and then sends the message. Obviously, there is the possibility of having a collision between two or more frames transmitted contemporaneously. In this case the stations involved have to retransmit their frames after a random time calculated according to a rule known as the truncated binary back off exponential algorithm.
This time is expressed as a multiple, R, of a network parameter called slot time, tSL. R is randomly chosen in the interval 0–2k, where k is the minimum between 10 and the number of attempts to retransmit the frame. After a user-defined number of attempts without success, the MAC reports an error to the LLC. The occurrence of collisions makes the behaviour of Ethernet networks non deterministic. This is the principal reason for what Ethernet, in the past, was not considered suitable for real time applications, such those requested at the device level of factory automation systems.
However, the situation is significantly changing because, with the introduction of switches, collisions can be practically eliminated. An Ethernet switch is an ‘intelligent’ device equipped with a number of ports to which either the network segments or the single stations can be connected. The peculiarity of a switch is the ability of addressing frames only towards their destinations. As a consequence, with these devices, many transmissions can take place simultaneously without colliding, contrarily to what happens in the ‘traditional’ networks called shared Ethernet implementations. If a switch is used, the only possibility of having a collision is verified when two or more stations transmit contemporaneously towards the same station. Also in
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this case, however, the collision can be avoided by means of a buffering functionality, which is implemented in many switches. Another important advantage of Ethernet is the high transmission speeds: from the initial speed of 10 Mbit/s it is now possible to 、 implement networks operating at 100 Mbit/s and 1 Gbit/s. The PDU of the IEEE 802.3
MAC is shown in Fig. 4. The PDU has a minimum length of 512 bits, necessary to ensure the correct operation of the collision detection technique. For this reason the field ‘Pad’ might be filled with the necessary number of octets. The minimum length is computed starting from the ‘Destination address’ field, hence the size of the shortest frame is 576 bits.
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