Fundamentals of industrial Ethernet
Although Xerox's Bob Metcalfe sketched the original Ethernet concept on a napkin in 1973, its inspiration came even earlier. ALOHAnet, a wireless data network, was created to connect together several widely separated computer systems on Hawaiian college campuses located on different islands. The challenge was to enable several independent data radio nodes to communicate on a peer-to-peer basis without interfering with each other. ALOHAnet's solution was a version of the carrier sense multiple access with collision detection (CSMA/CD) concept. Metcalfe based his Ph.D. work on finding improvements to ALOHAnet, which led to his work on Ethernet.
Ethernet, which later became the basis for the IEEE 802.3 network standard, specifies physical and data link layers of network functionality. The physical layer specifies the types of electrical signals, signaling speeds, media and connector types and network topologies. The data link layer specifies how communications occurs over the media—using the CSMA/CD technique mentioned above—as well as the frame structure of messages transmitted and received.
Ethernet physical layer
In the early days Ethernet options were more limited than they are today. Two common options were 10Base2 and 10Base5 configurations. Both operated at 10Mbit/s and used coaxial cable with nodes connected to the cable via Tee connectors, or through 'attachment unit interfaces' (AUI) in a multi-drop bus configuration. 10Base2 networks allowed segment lengths of up to 56.39 m (185 feet) using RG 58 coaxial cable (also called Thin Ethernet). 10Base5 offered greater distances between nodes but the thick coaxial cable and 'vampire tap' connections were bulky and difficult to work with. Later, another solution in this speed category was 10Base-FL, which uses fiber optic media and provides distances greater than 609.6 m (2000 feet).
Another early 10Mbit/s physical layer option—10Base-T—quickly gained popularity because it was easier to install and used inexpensive unshielded twisted pair (UTP) Category 3 cable. Nodes (typically computers with network interface cards, or NICs) were connected in a star topology to a hub, which in turn was connected to other network segments. Each computer had to be less than 30.48 m (100 feet) from the hub. Standard RJ-45 connectors were used.
In the mid-1990s 100Mbit/s Ethernet equipment became available, increasing the data transfer rate significantly. NICs that would automatically adjust to operate at 10Mbit/s or 100Mbit/s made migration to the faster standard simple. Today, virtually all computer network interface cards implement 100Base-TX. Category 5e UTP cable is the standard cable used with 100Base-TX and cable lengths are the same as for 10Base-T networks. Coaxial-based networks are increasingly being replaced with fiber optic media, especially for point-to-point links. For example, 100Base-FX uses two optical fibers and allows full duplex point-to-point communications up to 609.6 m (2000 feet). Gigabit Ethernet (1000Mbit/s) options also are available using twisted pair and fiber optic media.
Data link layer
Ethernet's data link layer defines its media access method. Half-duplex links, such as those connected in bus or star topologies (10/100Base-T, 10Base2, 10Base5, etc.), use carrier sense, multiple access with collision detection (CSMA/CD). This method allows multiple nodes to have equal access to the network, similar to early party-line telephone systems in which users listened for ongoing conversations and waited until the line was free before accessing the line. All nodes on an Ethernet network continuously monitor for transmissions on the media. If a node needs to transmit it waits until the network is idle, then begins transmission. While transmitting, each node monitors its own transmission and compares what it 'hears' with what it is trying to send. If two nodes begin transmitting at the same time, the signals will overlap, corrupting the originals. Both nodes will see a different signal to that which they are trying to send. This is recognized as a 'collision'. If there is a collision, each node stops transmitting and only attempts to re-transmit after a preset delay, which is different for each node.
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