Ethernet Physical Standards

Up to this point we’ve mainly been looking at the Data Link layer elements of Ethernet. However, Ethernet standards also define Physical layer characteristics such as cable distances, media types, and just about anything to do with physical connectivity, including connectors. You may already be familiar with some of the different varieties of Ethernet. They’re usually represented in a format such as 10BaseT or similar. Understanding the designations is critical, so we’ll look at these first.

When you see Ethernet defined as 10BaseT, you’re actually being provided with 3 pieces of information. In this example:

“10” defines the maximum speed of transmission in Megabits per second.
“Base” specifies that baseband transmission is used. Baseband transmission provides a single channel for digital transmission. In contrast, broadband transmission is analog and separates the cable into different frequency ranges or channels.
“T” defines that this type of Ethernet runs over twisted pair wiring. On a 10BaseT network, the minimum cable standard is Category 3.

A variety of different Ethernet standards exist, a cross section of which are outlined below.

10Base2. 10Mbps Ethernet that runs over ThinNet coaxial cable. Maximum segment length of 185 meters and a maximum of 30 connected systems per segment.
10Base5. 10Mbps Ethernet that runs over ThickNet coaxial cable. Maximum segment length of 500 meters and a maximum of 100 connected nodes per segment.
10BaseF. 10Mbps Ethernet that runs over fiber optic cabling for distances up to 2 kilometers in full duplex.
100BaseTX. Fast Ethernet (100Mbps) that runs over Cat5 twisted pair wiring. Maximum cable length is 100 meters.
100BaseFX. Fast Ethernet that runs over fiber optic cabling.
1000BaseT. Gigabit Ethernet (1000Mbps) that runs over Cat5 twisted pair wiring. Maximum cable length is 100 meters.
1000BaseLX. Long wave Gigabit Ethernet over fiber. If using multimode fiber, maximum distance is 550 meters. If single mode fiber, maximum distance of approximately 5 kilometers.
1000BaseSX. Short wave Gigabit Ethernet over fiber. Uses multimode fiber to span distances up to 550 meters.

Ethernet also makes use of features at the Physical layer by auto-negotiating elements such as link speed and duplex type when a network card is plugged into a switch or hub. Originally defined in the IEEE 802.3u specification (Fast Ethernet), this is accomplished using something called Fast Link Pulses (FLPs), which are sent between the system and the connected port. For example, you may have a network card that supports both 10 and 100 Mbps speeds. However, if the hub only supports 10 Mbps, they will negotiate the connection to the common setting (in this case 10 Mbps). The same is true for negotiation of the duplex type used. When using half duplex, a system can be either sending or receiving data, but not both concurrently. In full duplex, systems can send at receive at the same time.

Note that when plugged into a hub, systems will always communicate using half duplex, since they share the media and only one system can communicate at any given time. When a system is plugged directly into a switch port, full duplex becomes possible. To that end, it is worth noting that when you connect a hub to a switch, all computers plugged into that hub will automatically use half-duplex, since they’ll again be part of the same collision domain.

Posted By Dan DiNicolo