by Gerald Boerner
[Continued… ] Ethernet has become the standard for Local Area Networking. But is was not always so; in the early days of networking, mainframe computers would use this technology, but it was expensive, slow and not supported by a major vendor. IBM was pushing its Token Ring technology while office networks often used ARCNet because it was less expensive. But 3Com developed this technology into a viable player, who has become the dominant technology in this area.
The old saying when dealing with networks is this: “In a network, you need to consider speed, cost, and reliability. But you can only have two!” Ethernet provides an inexpensive network that transfers data reliably. In recent specifications, it has also achieved speeds into the Gbit/second range.
Be aware that this technology was the enabling technology for Local Area Networks which today pervades our offices and homes. The topic is so large that this is the second installment of four parts. GLB
“The cookie-cutter approach to metro Ethernet switches is still a little bit of time away.”
— Martin Lund
“You’re working with the giants of the industry, and they certainly have their own ideas. I’m not going to tell you we’re going to preempt these people who have been working on Ethernet all their lives.”
— Rex Naden
“In addition to providing instant group access to networks, email and the Internet the 3G/UMTS Router allows additional devices, such as printers, to be connected using Ethernet or 802.11g wireless connections. It is designed to be quick and easy to set up, just plug in and go.”
— Peter Bamford
“Circuit emulation, though advancing in sophistication, continues to concern operators and limit their enthusiasm for transitioning fully to Ethernet optical transport. Most operators believe Ethernet will in fact catch up soon, but standards work must be completed, performance demonstrated, and interoperability proven.”
— Scott Clavenna
“The wave of new applications for Ethernet is immense, as our customers increasingly look to connect people and machines in new and unique environments. National’s PHYTER family enables this wave, bringing innovative, application-targeted capabilities to the Ethernet physical layer and allowing system developers to connect devices they never thought possible.”
— Jeff Waters
“We’re pleased to have Foundry Networks as a founding member of the Ethernet Alliance. Foundry was a very active member in the 10 Gigabit Ethernet Alliance, successfully promoting this important technology from its inception over five years ago. We look forward to their participation in the new Ethernet Alliance as we promote the acceptance and application of current and emerging Ethernet standards in a broad range of environments.”
— Brad Booth
“Switch and Data is investing in additional space in key markets, as well as implementing advanced platforms in our core and in our interconnection of facilities with 10 Gigabit Ethernet and optical networking to deliver the reliability, scalability and stability these companies need for their businesses. The investment in Cisco ONS optical routers and switches provide us the flexibility and scalability to meet the diverse needs of our customers.”
— Ali Marashi
“Rollouts of triple-play access infrastructure and business Ethernet services infrastructure will drive demand for reconfigurable optical network systems. Ethernet and triple play will push a lot of traffic onto carrier interoffice facilities and metro core facilities, requiring more than simple, incremental bandwidth additions. The unique transport requirements of these services will force operators to move to a new generation of optical systems and architectures.”
— Scott Clavenna
Wizards of the Internet: Ethernet, Part 2
Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model, through means of network access at the Media Access Control protocol (a sub-layer of Data Link Layer), and a common addressing format.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980 to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET.
Network Devices
Generally, current implementations of Ethernet in Local Area Networks (LANs) involve more than a set of protocols (Ethernet) and a computer circuit (either a LAN card or chipset on the motherboard). We will briefly examine some of the other types of devices which are commonly employed in an Ethernet LAN. The most common of these devices are the hubs and switches, which are discussed below.
Repeaters and Hubs
For signal degradation and timing reasons, coaxial Ethernet segments had a restricted size which depended on the medium used. For example, 10BASE5 coax cables had a maximum length of 500 meters (1,640 ft). Also, as was the case with most other high-speed buses, Ethernet segments had to be terminated with a resistor at each end. For coaxial-cable-based Ethernet, each end of the cable had a 50 ohm (Ω) resistor attached. Typically this resistor was built into a male BNC or N connector and attached to the last device on the bus, or, if vampire taps were in use, to the end of the cable just past the last device. If termination was not done, or if there was a break in the cable, the AC signal on the bus was reflected, rather than dissipated, when it reached the end. This reflected signal was indistinguishable from a collision, and so no communication would be able to take place.
A greater length could be obtained by an Ethernet repeater, which took the signal from one Ethernet cable and repeated it onto another cable. If a collision was detected, the repeater transmitted a jam signal onto all ports to ensure collision detection. Repeaters could be used to connect segments such that there were up to five Ethernet segments between any two hosts, three of which could have attached devices. Repeaters could detect an improperly terminated link from the continuous collisions and stop forwarding data from it. Hence they alleviated the problem of cable breakages: when an Ethernet coax segment broke, while all devices on that segment were unable to communicate, repeaters allowed the other segments to continue working – although depending on which segment was broken and the layout of the network the partitioning that resulted may have made other segments unable to reach important servers and thus effectively useless.
People recognized the advantages of cabling in a star topology, primarily that only faults at the star point will result in a badly partitioned network, and network vendors began creating repeaters having multiple ports, thus reducing the number of repeaters required at the star point. Multiport Ethernet repeaters became known as "Ethernet hubs". Network vendors such as DEC and SynOptics sold hubs that connected many 10BASE2 thin coaxial segments. There were also "multi-port transceivers" or "fan-outs". These could be connected to each other and/or a coax backbone. A well-known early example was DEC’s DELNI. These devices allowed multiple hosts with AUI connections to share a single transceiver. They also allowed creation of a small standalone Ethernet segment without using a coaxial cable.
Ethernet on unshielded twisted-pair cables (UTP), beginning with StarLAN and continuing with 10BASE-T, was designed for point-to-point links only and all termination was built into the device. This changed hubs 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 more reliable by preventing faults with (but not deliberate misbehavior of) one peer or its associated cable from affecting other devices on the network, although a failure of a hub or an inter-hub link could still affect lots of users. Also, since twisted pair Ethernet is point-to-point and terminated inside the hardware, the total empty panel space required around a port is much reduced, making it easier to design hubs with lots of ports and to integrate Ethernet onto computer motherboards.
Despite the physical star topology, hubbed Ethernet networks still use half-duplex and CSMA/CD, with only minimal activity by the hub, primarily the Collision Enforcement signal, in dealing with packet collisions. Every packet is sent to every port on the hub, so bandwidth and security problems aren’t addressed. The total throughput of the hub is limited to that of a single link and all links must operate at the same speed.
Collisions reduce throughput by their very nature. In the worst case, when there are lots of hosts with long cables that attempt to transmit many short frames, excessive collisions can reduce throughput dramatically. However, a Xerox report in 1980 summarized the results of having 20 fast nodes attempting to transmit packets of various sizes as quickly as possible on the same Ethernet segment.[4] The results showed that, even for the smallest Ethernet frames (64B), 90% throughput on the LAN was the norm. This is in comparison with token passing LANs (token ring, token bus), all of which suffer throughput degradation as each new node comes into the LAN, due to token waits.
This report was controversial, as modeling showed that collision-based networks became unstable under loads as low as 40% of nominal capacity. Many early researchers failed to understand the subtleties of the CSMA/CD protocol and how important it was to get the details right, and were really modeling somewhat different networks (usually not as good as real Ethernet).
Bridging and Switching
While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forwarded all traffic to all Ethernet devices. This created practical limits on how many machines could communicate on an Ethernet network. Also as the entire network was one collision domain and all hosts had to be able to detect collisions anywhere on the network, the number of repeaters between the farthest nodes was limited. Finally segments joined by repeaters had 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. Bridges learn where devices are, by watching MAC addresses, and do not forward packets across segments when they know the destination address is not located in that direction.
Prior to discovery of network devices on the different segments, Ethernet bridges (and switches) work somewhat like Ethernet hubs, passing all traffic between segments. However, as the bridge discovers the addresses associated with each port, it only forwards network traffic to the necessary segments, improving overall performance. Broadcast traffic is still forwarded to all network segments. Bridges also overcame the limits on total segments between two hosts and allowed the mixing of speeds, both of which became very important with the introduction of Fast Ethernet.
Early bridges examined each packet one by one using software on a CPU, and some of them were significantly slower than hubs (multi-port repeaters) at forwarding traffic, especially when handling many ports at the same time. This was in part due to the fact that the entire Ethernet packet would be read into a buffer, the destination address compared with an internal table of known MAC addresses and a decision made as to whether to drop the packet or forward it to another or all segments.
In 1989 the networking company Kalpana introduced their EtherSwitch, the first Ethernet switch. This worked somewhat differently from an Ethernet bridge, in that only the header of the incoming packet would be examined before it was either dropped or forwarded to another segment. This greatly reduced the forwarding latency and the processing load on the network device. One drawback of this cut-through switching method was that packets that had been corrupted at a point beyond the header could still be propagated through the network, so a jabbering station could continue to disrupt the entire network. The remedy for this was to make available store-and-forward switching, where the packet would be read into a buffer on the switch in its entirety, verified against its checksum and then forwarded. This was essentially a return to the original approach of bridging, but with the advantage of more powerful, application-specific processors being used. Hence the bridging is then done in hardware, allowing packets to be forwarded at full wire speed. It is important to remember that the term switch was invented by device manufacturers and does not appear in the 802.3 standard.
Since packets are typically only delivered to the port they are intended for, traffic on a switched Ethernet is slightly 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 slightly better 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.
When a twisted pair or fiber link segment is used and neither end is connected to a hub, full-duplex Ethernet becomes possible over that segment. In full duplex mode both devices can transmit and receive to/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 (e.g. 200 Mbit/s) to account for this. However, this is misleading as performance will only double if traffic patterns are symmetrical (which in reality they rarely are). The elimination of the collision domain also means that all the link’s bandwidth can be used and that segment length is not limited by the need for correct collision detection (this is most significant with some of the fiber variants of Ethernet).
Dual Speed Hubs
In the early days of Fast Ethernet, Ethernet switches were relatively expensive devices. Hubs suffered from the problem that if there were any 10BASE-T devices connected then the whole network needed to run at 10 Mbit/s. Therefore a compromise between a hub and a switch was developed, known as a dual speed hub. These devices consisted of an internal two-port switch, dividing the 10BASE-T (10 Mbit/s) and 100BASE-T (100 Mbit/s) segments. The device would typically consist of more than two physical ports. When a network device becomes active on any of the physical ports, the device attaches it to either the 10BASE-T segment or the 100BASE-T segment, as appropriate. This prevented the need for an all-or-nothing migration from 10BASE-T to 100BASE-T networks. These devices are hubs because the traffic between devices connected at the same speed is not switched.
Autonegotiation and Duplex Mismatch
Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex, …) exist for Ethernet over twisted pair cable using 8P8C modular connectors (not to be confused with FCC’s RJ45), and most devices are capable of different modes of operations. In 1995, IEEE standard 802.3u (100baseTX) was released, allowing two network interfaces connected to each other to autonegotiate the best possible shared mode of operation. This works well for a network in which every device being set to autonegotiate.
The autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of an Ethernet peer that did not use autonegotiation. An autonegotiating device defaults to half duplex, when the remote does not negotiate, as the remote peer is assumed to be a hub (which always has autonegotiation disabled and supports only half duplex mode). If the remote is operating in half duplex mode this works. But if remote is in full duplex mode, this generates a duplex mismatch. When two interfaces are connected and set to different "duplex" modes, the effect of the duplex mismatch is a network that works, but is much slower than its nominal speed, and generates more collisions. The primary rule for avoiding this is to never set one end of a connection to a forced full duplex setting and the other end to autonegotiation.
Interoperability problems lead some network administrators to manually fix the mode of operation of interfaces on network devices. What would happen is that some device would fail to autonegotiate and therefore had to be set into one setting or another. This often led to duplex setting mismatches. In particular, when two interfaces are connected to each other with one set to autonegotiation and one set to full duplex mode, a duplex mismatch results because the autonegotiation process fails and half duplex is assumed. The interface in full duplex mode then transmits at the same time as receiving, and the interface in half duplex mode then gives up on transmitting a frame. The interface in half duplex mode is not ready to receive a frame, so it signals a collision, and transmissions are halted, for amounts of time based on backoff (random wait times) algorithms. When both packets start trying to transmit again, they interfere again and the backoff strategy may result in a longer and longer wait time before attempting to transmit again; eventually a transmission succeeds but this then causes the flood and collisions to resume.
Because of the wait times, the effect of a duplex mismatch is a network that is not completely ‘broken’ but is incredibly slow. This bad behaviour can be tolerated on low traffic link, but is really dramatic under heavy bandwidth transfer attempt, and can lead to a complete stop of the traffic.
While autonegotiation is not required for 10/100 Mbit/s, it is recommended as default behaviour by IEEE 802.3u. However, 1000baseT devices require autonegotiation to be active to elect the clock master (source of timing). Enabling autonegotiation on every node eases transition from 10/100Mbit/s to 1000baseT switch and LAN.
There are no disadvantages of keeping autonegotiation active on all devices, because complete physical link behaviours are controlled through autonegotiation (speed, duplex, clock master and flow control). For example, to force a single speed link you can keep negotiation on, but negotiate only one speed. So the old method with autonegotiation off is deprecated everywhere, on switch and LAN cards.
[ End of Part 2 of 4 ]
References:
Katie Hafner & Matthew Lyon. (1998) Where Wizards Stay Up Late: The Origins of the Internet. Simon & Schuster
Background and biographical information is from Wikipedia articles on:
Wikipedia: ARPANet…
http://en.wikipedia.org/wiki/ARPAnet
Wikipedia: The Internet…
http://en.wikipedia.org/wiki/The_Internet
Wikipedia: Ethernet…
http://en.wikipedia.org/wiki/Ethernet
Wikipedia: Robert Metcalfe…
http://en.wikipedia.org/wiki/Robert_Metcalfe
Web Sites and Blogs:
About.com: Inventors of the Modern Computer — The Invention of Ethernet and Local Area Networks…
http://inventors.about.com/library/weekly/aa111598.htm
IdeaFinder.com: Ethernet…
http://www.ideafinder.com/history/inventions/ethernet.htm
ThinkExist.com: Ethernet Quotes…
http://thinkexist.com/search/searchQuotation.asp?search=Ethernet
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