Early Ethernet systems (of the 10 Mbps variety) use the CSMA/CD access method. This gives a system that operates with little delay if lightly loaded, but becomes very slow if heavily loaded. Ethernet network interface cards are relatively cheap and produced in vast quantities. Ethernet has, in fact, become the most widely used networking standard. However, CSMA/CD is a probabilistic medium access mechanism, there is no guarantee of message transfer and messages cannot be prioritized.
Modern Ethernet systems are a far cry from the original design. From 100BaseT onwards they are capable of full duplex (sending and receiving at the same time via switches, without collisions) and the Ethernet frame can be modified to make provision for prioritization and virtual LANs.
Early Ethernet was not entirely suitable for control functions as it was primarily developed for office-type environments. Ethernet technology has, however, made rapid advances over the past few years. It has gained such widespread acceptance in Industry that it is becoming the de facto field bus technology for OSI layers 1 and 2. An indication of this trend is the inclusion of Ethernet as the level 1 and 2 infrastructure for Modbus/TCP (Schneider), Ethernet/IP (Rockwell Automation and ODVA), ProfiNet (Profibus) and Foundation Fieldbus HSE
10 Mbps Ethernet The IEEE 802.3 standard (also known as ISO 8802.3) defines a range of media types that can be used for a network based on this standard such as coaxial cable, twisted pair cable and fiber optic cable. It supports various cable media and transmission rates at 10 Mbps, such as:
- 10Base2 : thin wire coaxial cable (RG-58), 10 Mbps baseband operation, bus topology
- 10Base5 : thick wire coaxial cable (RG-8), 10 Mbps baseband operation, bus topology
- 10BaseT : UTP cable (Cat3), 10 Mbps baseband operation, star topology
- 10BaseFL : optical fiber, 10 Mbps baseband operation, point-topoint topology
Other variations included 1Base5, 10BaseFB, 10BaseFP and 10Broad36, but these versions never became commercially viable.
100 Mbps Ethernet
100BaseT is the shorthand identifier for 100 Mbps Ethernet systems, viz. 100BaseTX (copper) and 100BaseFX (fiber). 100BaseT4 was designed to operate at 100 Mbps over 4 pairs of Cat3 cable, but this option never gained widespread acceptance. Yet another version, 100 BaseT2, was supposed to operate over just 2 pairs of Cat3 cable but was never implemented by any vendor.
One of the limitations of hub-based (CSMA/CD) 100BaseT systems is the size of the collision domain, which is only 250 meters or 5.12 microseconds. This is the maximum size of a network segment in which collisions can be detected, being one tenth of the maximum size of a 10 Mbps network. This effectively limits the distance between a workstation and hub to 100 m, the same as for 10BaseT. As a result, networks larger than 200 meters must be logically interconnected by store and-forward devices such as bridges, routers or switches. This is not a bad thing, since it segregates the traffic within each collision domain, reducing the number of collisions on the network. The use of bridges and routers for traffic segregation, in this manner, is often done on industrial Ethernet networks. Of course, the use of switches instead of hubs allows the construction of very large networks because of the full duplex operation.
The format of the frame has been left unchanged. The only difference is that it is transmitted 10 times faster than in 10 Mbps Ethernet, hence its length (in time) is 10 times less.
Some of the advantages :
- Increased distance
- Ability to use standard access points, routers, switches, hubs, cables and optical fiber
- Ability to have more than two nodes on link
- Better interoperability