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Main article: "Ethernet hub

For signal degradation and timing reasons, coaxial "Ethernet segments have a restricted size. Somewhat larger networks can be built by using an "Ethernet repeater. Early repeaters had only two ports, allowing, at most, a doubling of network size. Once repeaters with more than two ports became available, it was possible to wire the network in a "star topology. Early experiments with star topologies (called "Fibernet") using "optical fiber were published by 1978.[35]

Shared cable Ethernet is always hard to install in offices because its bus topology is in conflict with the star topology cable plans designed into buildings for telephony. Modifying Ethernet to conform to "twisted pair telephone wiring already installed in commercial buildings provided another opportunity to lower costs, expand the installed base, and leverage building design, and, thus, twisted-pair Ethernet was the next logical development in the mid-1980s.

Ethernet on unshielded twisted-pair cables (UTP) began with "StarLAN at 1 Mbit/s in the mid-1980s. In 1987 "SynOptics introduced the first twisted-pair Ethernet at 10 Mbit/s in a star-wired cabling topology with a central hub, later called "LattisNet.[14][36][37] These evolved into 10BASE-T, which was designed for point-to-point links only, and all termination was built into the device. This changed repeaters from a specialist device used at the center of large networks to a device that every twisted pair-based network with more than two machines had to use. The tree structure that resulted from this made Ethernet networks easier to maintain by preventing most faults with one peer or its associated cable from affecting other devices on the network.

Despite the physical star topology and the presence of separate transmit and receive channels in the twisted pair and fiber media, repeater-based Ethernet networks still use half-duplex and CSMA/CD, with only minimal activity by the repeater, primarily generation of the "jam signal in dealing with packet collisions. Every packet is sent to every other port on the repeater, so bandwidth and security problems are not addressed. The total throughput of the repeater is limited to that of a single link, and all links must operate at the same speed.

Bridging and switching[edit]

"Patch cables with "patch fields of two Ethernet switches
Ethernet switch and "Bridging (networking)

While repeaters can isolate some aspects of "Ethernet segments, such as cable breakages, they still forward all traffic to all Ethernet devices. This creates practical limits on how many machines can communicate on an Ethernet network. The entire network is one "collision domain, and all hosts have to be able to detect collisions anywhere on the network. This limits the number of repeaters between the farthest nodes. Segments joined by repeaters have to all operate at the same speed, making phased-in upgrades impossible.

To alleviate these problems, bridging was created to communicate at the data link layer while isolating the physical layer. With bridging, only well-formed Ethernet packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. At initial startup, Ethernet bridges (and switches) work somewhat like Ethernet repeaters, passing all traffic between segments. By observing the source addresses of incoming frames, the bridge then builds an address table associating addresses to segments. Once an address is learned, the bridge forwards network traffic destined for that address only to the associated segment, improving overall performance. "Broadcast traffic is still forwarded to all network segments. Bridges also overcome the limits on total segments between two hosts and allow the mixing of speeds, both of which are critical to deployment of "Fast Ethernet.

In 1989, the networking company "Kalpana (acquired by "Cisco Systems, Inc. in 1994) introduced their EtherSwitch, the first Ethernet switch.[h] This works somewhat differently from an Ethernet bridge, where only the header of the incoming packet is examined before it is either dropped or forwarded to another segment. This greatly reduces the forwarding latency and the processing load on the network device. One drawback of this "cut-through switching method is that packets that have been corrupted are still propagated through the network, so a jabbering station can continue to disrupt the entire network. The eventual remedy for this was a return to the original "store and forward approach of bridging, where the packet would be read into a buffer on the switch in its entirety, verified against its checksum and then forwarded, but using more powerful "application-specific integrated circuits. Hence, the bridging is then done in hardware, allowing packets to be forwarded at full wire speed.

When a twisted pair or fiber link segment is used and neither end is connected to a repeater, "full-duplex Ethernet becomes possible over that segment. In full-duplex mode, both devices can transmit and receive to and from each other at the same time, and there is no collision domain. This doubles the aggregate bandwidth of the link and is sometimes advertised as double the link speed (for example, 200 Mbit/s).[i] The elimination of the collision domain for these connections also means that all the link's bandwidth can be used by the two devices on that segment and that segment length is not limited by the need for correct collision detection.

Since packets are typically delivered only to the port they are intended for, traffic on a switched Ethernet is less public than on shared-medium Ethernet. Despite this, switched Ethernet should still be regarded as an insecure network technology, because it is easy to subvert switched Ethernet systems by means such as "ARP spoofing and "MAC flooding.

The bandwidth advantages, the improved isolation of devices from each other, the ability to easily mix different speeds of devices and the elimination of the chaining limits inherent in non-switched Ethernet have made switched Ethernet the dominant network technology.[38]

Advanced networking[edit]

A core Ethernet switch

Simple switched Ethernet networks, while a great improvement over repeater-based Ethernet, suffer from single points of failure, attacks that trick switches or hosts into sending data to a machine even if it is not intended for it, scalability and security issues with regard to "switching loops, "broadcast radiation and "multicast traffic, and bandwidth choke points where a lot of traffic is forced down a single link.["citation needed]

Advanced networking features in switches use "shortest path bridging (SPB) or the "spanning-tree protocol (STP) to maintain a loop-free, meshed network, allowing physical loops for redundancy (STP) or load-balancing (SPB). Advanced networking features also ensure port security, provide protection features such as "MAC lockdown and broadcast radiation filtering, use "virtual LANs to keep different classes of users separate while using the same physical infrastructure, employ "multilayer switching to route between different classes, and use "link aggregation to add bandwidth to overloaded links and to provide some redundancy.

"IEEE 802.1aq ("shortest path bridging) includes the use of the "link-state routing protocol "IS-IS to allow larger networks with shortest path routes between devices. In 2012, it was stated by David Allan and Nigel Bragg, in 802.1aq Shortest Path Bridging Design and Evolution: The Architect's Perspective that shortest path bridging is one of the most significant enhancements in Ethernet's history.[39]

Ethernet has replaced "InfiniBand as the most popular system interconnect of "TOP500 supercomputers.[40]

Error conditions[edit]


A node that is sending longer than the maximum transmission window for an Ethernet packet is considered to be jabbering. Depending on the physical topology, jabber detection and remedy differ somewhat.

Runt frames[edit]

Varieties of Ethernet[edit]

Ethernet physical layer

The Ethernet physical layer evolved over a considerable time span and encompasses coaxial, twisted pair and fiber-optic physical media interfaces, with speeds from 10 Mbit/s to 100 Gbit/s, with "400 Gbit/s expected by 2018.[49] The first introduction of twisted-pair CSMA/CD was "StarLAN, standardized as 802.3 1BASE5;[50] while 1BASE5 had little market penetration, it defined the physical apparatus (wire, plug/jack, pin-out, and wiring plan) that would be carried over to 10BASE-T.

The most common forms used are "10BASE-T, 100BASE-TX, and 1000BASE-T. All three utilize twisted pair cables and "8P8C modular connectors. They run at 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s, respectively. "Fiber optic variants of Ethernet are also very common in larger networks, offering high performance, better electrical isolation and longer distance (tens of kilometers with some versions). In general, network "protocol stack software will work similarly on all varieties.

Frame structure[edit]

A close-up of the SMSC LAN91C110 (SMSC 91x) chip, an embedded Ethernet chip.
Ethernet frame

In IEEE 802.3, a "datagram is called a packet or frame. Packet is used to describe the overall transmission unit and includes the "preamble, "start frame delimiter (SFD) and carrier extension (if present).[j] The frame begins after the start frame delimiter with a frame header featuring source and destination MAC addresses and "a field giving either the protocol type for the payload protocol or the length of the payload. The middle section of the frame consists of payload data including any headers for other protocols (for example, "Internet Protocol) carried in the frame. The frame ends with a 32-bit "cyclic redundancy check, which is used to detect corruption of "data in transit.[51]:sections 3.1.1 and 3.2 Notably, Ethernet packets have no "time-to-live field, leading to possible problems in the presence of a "switching loop.



Autonegotiation is the procedure by which two connected devices choose common transmission parameters, e.g. speed and duplex mode. Autonegotiation is an optional feature, first introduced with 100BASE-TX, while it is also backward compatible with 10BASE-T. Autonegotiation is mandatory for 1000BASE-T.

See also[edit]


  1. ^ The experimental Ethernet described in the 1976 paper ran at 2.94 Mbit/s and has eight-bit destination and source address fields, so the original Ethernet addresses are not the "MAC addresses they are today.[13] By software convention, the 16 bits after the destination and source address fields specify a "packet type", but, as the paper says, "different protocols use disjoint sets of packet types". Thus the original packet types could vary within each different protocol. This is in contrast to the "EtherType in the IEEE Ethernet standard, which specifies the protocol being used.
  2. ^ In some cases, the factory-assigned address can be overridden, either to avoid an address change when an adapter is replaced or to use locally administered addresses.
  3. ^ There are fundamental differences between wireless and wired shared-medium communication, such as the fact that it is much easier to detect collisions in a wired system than a wireless system.
  4. ^ In a CSMA/CD system packets must be large enough to guarantee that the leading edge of the propagating wave of a message gets to all parts of the medium and back again before the transmitter stops transmitting, guaranteeing that "collisions (two or more packets initiated within a window of time that forced them to overlap) are discovered. As a result, the minimum packet size and the physical medium's total length are closely linked.
  5. ^ Multipoint systems are also prone to strange failure modes when an electrical discontinuity reflects the signal in such a manner that some nodes would work properly, while others work slowly because of excessive retries or not at all. See "standing wave for an explanation. These could be much more difficult to diagnose than a complete failure of the segment.
  6. ^ This "one speaks, all listen" property is a security weakness of shared-medium Ethernet, since a node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses.
  7. ^ Unless it is put into "promiscuous mode.
  8. ^ The term switch was invented by device manufacturers and does not appear in the 802.3 standard.
  9. ^ This is misleading, as performance will double only if traffic patterns are symmetrical.
  10. ^ The carrier extension is defined to assist collision detection on shared-media gigabit Ethernet.


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Further reading[edit]

External links[edit]

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