For successful communication over a shared link, you need to have some mechanism in place to ensure that only one device can transmit at a time. Ethernet accomplishes this through an algorithm called Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) .
As even the abbreviation is a mouthful, let’s break it down further. “Carrier Sense” means to check the wire first. If another transmission is in progress, wait until the wire is idle before transmitting. “Multiple Access” means that more than one device is sharing the bus—collisions are possible. “Collision Detect” describes how to detect and react to a col- lision. As a collision involves multiple simultaneous signals, collisions can be spotted by looking for higher-than-expected signal amplitudes. When these are detected, the trans- mitting stations send a further jamming signal to ensure that all stations are aware of the collision, and then employ a back-off algorithm for each station to wait a random amount of time before attempting to retransmit.
Ethernet Standards and Cable Types
Ethernet is available in a number of speeds and form-factors. These days, in a modern data center, you are most likely to encounter gigabit and 10 gigabit Ethernet, carried over either copper or fiber. Outside of the data center, you might be dealing with 100Mbps connections, or even wireless, but we’re going to focus the discussion on connectivity within the data center, where the magic really happens.
Fiber Versus Fibre
In the United States, discussions about fiber in the data center can get confusing quickly. Americans use “fiber” to refer to the fiber optic cables themselves. Sometimes, that fiber is used to plumb the storage area network (SAN), over which the Fibre Channel Protocol is used. So “fiber” is the medium and “Fibre Channel” is a protocol that can be run over that medium. Make sense? To complicate things further, Fibre Channel Protocol can be run over unshielded twisted pair (UTP) cable, too.
Outside of the United States, “fibre” is the preferred term for the medium as well, leading to all sorts of spellcheck frustration.
For more information on Fibre Channel, we direct you to Storage Implementation in
vSphere 5.0 by Mostafa Khalil.
Gigabit Ethernet over copper wire, and its 10 and 100 megabit ancestors, uses UTP cabling. These cables consist of four pairs of wires, twisted together down the length of the cable, terminating in RJ45 connectors on each end.
20 CHAPTER 3 Ethernet Networks
Everything You Know About Connectors Is a Lie
You might dismiss this as pedantry, but we just can’t be part of the lie anymore. You know that thing at the end of your Cat5 cable? It’s not an RJ45 connector . An RJ45 connector is keyed such that it wouldn’t fit in a standard Ethernet NIC port. The connector used on standard UTP cables is an 8P8C (or 8-position, 8-contact) connector. The real RJ45 connector is an 8P2C (8-position, 2-contact) type. The standard for which the real RJ45 plug and socket were designed for never really took off, and the connectors for 8P8C and RJ45 look similar enough (minus the keying) that the name RJ45 stuck for both.
With Gigabit Ethernet over copper wire, all four pairs are used to transmit and receive simultaneously. This differs from the earlier 10 and 100 megabit standards which defined separate send and receive pairs.
Gigabit and 10 gigabit Ethernet over fiber involve two strands of fiber optic cabling, a transmit strand and a receive strand. The fiber can be multi-mode for relatively short distances, or single-mode for longer distances. Single-mode fiber carries only a single frequency of not-safe-for-the-eye laser-driven light, while multi-mode carries multiple LED-driven frequencies which are harmless if you happen to look at them. In data center applications, fiber cables typically terminate in either SC or LC connectors. SC connectors are squarish and use a push-on, pull-off attachment mechanism, with each transmit/receive pair typically held together with a plastic clip. LC connectors are a smaller form-factor option, and use a retaining tab attachment mechanism similar to an RJ45 connector. Upstream fiber connections typically involve hot-pluggable transceivers. Gigabit interface converters (GBICs) or small form-factor pluggable transceivers (SFPs) are used to sup- port gigabit Ethernet connections, and enhanced small form-factor pluggable transceivers (SPF+) are used for 10 gigabit connections.
Ten gigabit Ethernet over copper is most commonly found in an SFP+ direct attach form- factor, in which twinaxial copper is terminated by SFP+ housings attached to the end of the cable. Some vendors refer to these as Direct Attach Copper (DAC) cables . These are used for fairly short runs, 1 to 7m for passive cables or up to 15m for active cables, with the latter drawing transmission power from the connected device. Ten gigabit copper over UTP (10GBase-T) is also available, but is less common at the moment, as upgrading infra- structure to support it tends to cost more than using existing SFP+ ports.
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Ethernet
Table 3.1 Common Ethernet Standards
Common Name Speed IEEE Standard Cable Type and Max Length Ethernet 10BASE5 10BASE2 10BASE-T 10 Mbps 802.3 802.3 802.3 Copper coaxial, 500m Copper coaxial, 185m Copper UTP, 100m Fast Ethernet 100BASE-TX 100BASE-FX 100 Mbps 802.3u 802.3u Copper UTP, 100m Fiber, 2km Gigabit Ethernet 1000BASE-LX 1000BASE-SX 1000BASE-T 1000 Mbps 802.3z 802.3z 802.3ab Fiber, 5km Fiber, 500m Copper UTP, 100m 10 Gigabit Ethernet 10GBASE-SR 10GBASE-LR 10GBASE-CR 10GBASE-T 10 Gbps 802.3ae 802.3ae Pending 802.3an Fiber, 400m Fiber 10km Copper twinaxial, 15m Copper UTP, 100m
Table 3.2 shows a number of common cable connectors and types.
Table 3.2 Common Ethernet Cable Connectors and Types
Name Image
22 CHAPTER 3 Ethernet Networks Name Image Fibre LC Connector Fiber SC Connector GBIC Module SFP/SFP+ Module
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