Ethernet at the Physical Layer

Ethernet operates at the link layer of TCP. It defines the physical media responsible for carrying data, the format of the data carried by that media and the hardware addressing between those devices. Therefore, it covers both the data link and physical layers of the OSI 7 layer model.

Ethernet defines the physical media used to carry data. The Ethernet standards are specified by the IEEE and belong to the 802.3 family of standards. They define physical properties of the cabling (for example whether it’s copper wires or a glass fibre) as well as data speeds.

Ethernet Standards

Some of the most common Ethernet varieties are summarised below. Mbps indicates a speed in megabits per second and Gbps indicates a speed in Gigabits per second. The term ‘BASE’ means that baseband signalling is used – the signal transmitted uses the full bandwidth of the media.

10BASE-T

Friendly Name: Ethernet

IEEE Standard: 802.3

Speed: 10 Mbps

Material: Copper

Maximum length: 100m

100BASE-TX

Friendly Name: Fast Ethernet

IEEE Standard: 802.3u

Speed: 100Mbps

Material: Copper

Maximum length: 100m

1000BASE-T

Friendly Name: Gigabit Ethernet

IEEE Standard: 802.3ab

Speed: 1 Gbps

Material: Copper

Maximum length: 100m

1000BASE-X

Friendly Name: Gigabit Ethernet

IEEE Standard: 802.3z

Speed: 1 Gbps

Material: Fibre

Maximum length: depends on fibre properties: 1000BASE-SX approx. 200m to 500m and 1000BASE-LX up to 5km

10GBASE-T

Friendly Name: 10 Gig Ethernet

IEEE Standard: 802.3an

Speed: 10Gbps

Material: Copper

Maximum length: 100m

Unshielded Twisted Pair (UTP) Cable

UTP is a common and inexpensive choice for ethernet cabling. It is generally capable of transmitting data up to 100m. Data is sent as ‘1s’ and ‘0s’ using electrical signals transmitted along pairs of wires which each create an electrical circuit. One wire carries the signal in one direction and the other carries it back.

Each pair of wires is twisted together so that any electromagnetic interference (EMI) is cancelled out. This interference could be ‘crosstalk’ from the other wires which make up the table.

Most modern Ethernet cables have 8 wires – 4 pairs and are terminated with RJ-45 connectors.

Shielded Twisted Pair (STP) Cable

Twisted-pair cables can also be shielded to prevent EMI. Shielding can be applied to each pair of wires, to the whole cable (not the individual pairs) or to both the pairs and the whole cable.

Ethernet Standards vs Cable Categories

Ethernet standards (the 802.3 family) are specified by the IEEE and define the physical and datalink layers from the interface of one device to another. For example, on a 1000BASE-T link, the interface of the computer/device and switch as well as the cabling used in between must adhere to the 1000BASE-T standard.

Ethernet cabling is typically sold under a category system defined by the Telecommunications Industry Association (TIA). The category defines the physical properties of the cable and will support multiple Ethernet standards (e.g. 10BASE-T, 100BASE-TX and 1000BASE-T). Cables belonging to these categories are commonly but not exclusively used for Ethernet data.

Cat 5 Ethernet Cable

Supported Ethernet Standards: 10BASE-T, 100BASE-TX, 1000BASE-T (although 5e is recommended for 1000BASE-T)

Maximum Speed: 1000 Mbps (under good conditions)

Maximum Bandwidth: 100 MHz

Cable type: UTP

Cat 5e Ethernet Cable

Offers improved mitigation of crosstalk compared to Cat 5 cable.

Supported Ethernet Standards: 10BASE-T, 100BASE-TX, 1000BASE-T

Maximum Speed: 1000 Mbps

Maximum Bandwidth: 100 MHz

Cable type: UTP

Cat 6 Ethernet Cable

Supported Ethernet Standards: 10BASE-T, 100BASE-TX, 1000BASE-T, 10GBASE-T

Maximum Speed: 10 Gbps

Maximum Bandwidth: 250 MHz

Cable type: UTP / STP

Cat 6a Ethernet Cable

Offers improved crosstalk and interference performance than Cat 6.

Supported Ethernet Standards: 10BASE-T, 100BASE-TX, 1000BASE-T, 10GBASE-T

Maximum Speed: 10 Gbps

Maximum Bandwidth: 500 MHz

Cable type: STP

Fibre Optic Cables

Fibre optic cabling uses a fine fibre core made of glass or plastic to carry ‘1s’ and ‘0s’ as light. The network devices on either end will transmit the data using an LED or a laser and receive it using a photodetector. Fibre optic cables can generally support higher data speeds than copper.

The fibre optic core is surrounded by cladding in order to create the correct refractive properties for sending light along the fibre. To protect the fibre, a ‘buffer’ surrounds the cladding to prevent damage to the delicate materials. Finally an additional plastic ‘jacket’ may be added.

There are two principal types of fibre optic cable: single-mode and multi-mode.

Single-Mode Fibre

• Very small core diameter

• Carries a single mode of light

• More expensive

• Can carry data over longer distances

Multi-Mode Fibre

• Larger core diameter

• Carries multiple modes of light

• Less expensive

• Can only be used for shorter distances