This invention relates to an information transmission system.
In order to achieve, for example, a fully distributed peer protocol transmission system, i.e., a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) system in a bus-configured broadband local area network (LAN), it is necessary to ensure the collision detecting characteristic of the same degree as the collision detecting characteristic in a baseband network and the compatibility with an upper layer protocol.
A short period of time is required for detecting a signal collision in the network. If a long period of time is taken for detecting the collision then the permissible network length will have to be short, deteriorating the extention capability of the network system.
According to the present IEEE standards, a data format for achieving the CSMA/CD of the baseband network has a 56-bit (7-octet) preamble, 8-bit SFD (Start Frame Delimiter), 48-bit DA (Destination Address), 48-bit SA (Source Address) and subsequent data of plural octets. In the baseband network the collision detection is carried out by detecting direct-current components on the network. With the broadband network, on the other hand, the collision is detected by either a bit comparison method or a beat-signal level detection method. In either case, a difference in bits between colliding data is used for the collision detection. To keep compatibility with the CASMA/CD in the baseband network the data format used in the broadband network must conform to the IEEE standards.
In a bus-shaped LAN, a plurality of data terminal equipments (DTEs) are each coupled through a corresponding medium attachment unit (MAU) to an arbitrary point on a communication medium to effect communication of information with each other. In such a LAN, particularly, the broadband LAN, when information signals are transmitted from a plurality of MAUs at a certain time interval, the detection of signal collision is expected to be effected on the basis of 1-bit or more data difference as quick as possible. However, it might occur that, in the worst case, bit streams consisting of the preamble, SFD, DA and SA transmitted from MAUs coincide with each other except the final 1 bit thereof. This occurs when the DAs are equal but the final bits of the SAs are different. Accordingly, in such a case, the collision of the information is detected after 160 bit times from the start of the transmission of the preamble. In other words, the collision detection requires a long period of time.
In a communication network, an information processor transmitting an information signal must be informed of the occurrence of collision during the transmission of information signal. As described above, due to the requirement of a long period of time for detecting a collision the length of the network must be shortened. Therefore, the extension capability of the network is limited, and the network becomes costly.