Mixed Media
Multiple media types may be used on the same network. In larger installations it is very common for distributed 10baseT hubs to be interconnected via a fiber optic hub. The fiber optic "backbone" provides extended distance and additional noise immunity while the 10BaseT cabling
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provides a simple connection to nodes within a local area.
The flexibility of cabling schemes and interconnection options is one of the strengths of the Ethernet physical
Workstation Workstation | PLC |
Workstation
10 BaseT Hub
PLC
Workstation PLC Workstation
PLC
layer. You may wish to consult your network vendor for additional details on the numerous options available.
3.3 Ethernet Protocol - Data Link Layer
Figure 9 Mixed Ethernet Media
Media Access
As previously presented, Ethernet uses a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) as a media access scheme. When a station has a message to transmit, it monitors the line for a carrier signal (which indicates that another station is transmitting). If there is no carrier signal, the station transmits the data. It is possible that more than one station will attempt to transmit at the same time; when this occurs the signals will "collide". When transmitting Ethernet stations detect a collision, each waits a random amount of time, then re- transmits. Should a second collision occur, the stations back off exponentially until the transmissions are successfully completed. In actual practice, 3 or more collisions are a rare occurrence. The fact that stations delay a random amount of time when a collision occurs results in the network being
Ethernet Version 2 and IEEE 802.3 Packet
Ethernet Version 2 and IEEE 802.3 transmit data as "packets" which can vary from 64 to 1518 bytes in length. Each packet contains the source address, destination addresses, the application data, and a checksum. At this point the Ethernet II and 802.3 specifications diverge.
CTI 2572 Technical Overview Manual | 13 |
