Ethernet physical layer

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The Ethernet physical layer is the physical layer component of the Ethernet standard.

The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed. The speed ranges from 3 Mbit/s to 10 Gbit/s in speed while the physical medium can range from bulky coaxial cable to twisted pair to optical fiber. In general, network protocol stack software will work identically on most of the following types.

The following sections provide a brief summary of all the official Ethernet media types (section numbers from the IEEE 802.3-2002 standard are parenthesized). In addition to these official standards, many vendors have implemented proprietary media types for various reasons—often to support longer distances over fiber optic cabling.

Many Ethernet adapters and switch ports support multiple speeds, using autonegotiation to set the speed and duplex for the best values supported by both connected devices. If auto-negotiation fails, a multiple speed device will sense the speed used by its partner, but will assume half-duplex. A 10/100 Ethernet port supports 10BASE-T and 100BASE-TX. A 10/100/1000 Ethernet port supports 10BASE-T, 100BASE-TX, and 1000BASE-T.

Physical layers

Xerox experimental Ethernet

Early implementations (10 Mbit/s and 1 Mbit/s)

Fast Ethernet (100 Mbit/s)

For more details on this topic, see Fast Ethernet.

Gigabit Ethernet

For more details on this topic, see Gigabit Ethernet.

All of these use a star topology.

10 Gigabit Ethernet

For more details on this topic, see 10 Gigabit Ethernet.

• 10 gigabit Ethernet is still fairly new and it remains to be seen which of the standards will gain commercial acceptance in consumer markets. 10GBASE-LR/ER are the most common usage in the Carrier/ISP market.
• Note that both IEEE 802.2ae and IEEE 802.3ak have been incorporated into IEEE 802.3-2005.

40 Gigabit Ethernet

This article or section contains information about scheduled or expected future events. It may contain tentative information; the content may change as the event approaches and more information becomes available.

For more details on this topic, see 100 Gigabit Ethernet.

This is the future version of ethernet and is not expected to be standardized until 2010. This is the proposed nomenclature:^[2]

100 Gigabit Ethernet

This article or section contains information about scheduled or expected future events. It may contain tentative information; the content may change as the event approaches and more information becomes available.

For more details on this topic, see 100 Gigabit Ethernet.

This is the future version of ethernet and is not expected to be standardized until 2010. This is the proposed nomenclature:

Ethernet over twisted-pair cable

Main article: Ethernet over twisted pair

Several varieties of Ethernet were specifically designed to run over 4-pair copper structured cabling already installed in many locations. ANSI recommends using Category 6 cable for new installations^{[citation needed]}.

RJ-45 Wiring (TIA/EIA-568-B T568A)

Pin	Pair	Color	telephone	10BASE-T	100BASE-TX	1000BASE-T	PoE mode A	PoE mode B
1	3	white/green	-	TX+	z	bidi	48V out	-
2	3	green	-	TX-	z	bidi	48V out	-
3	2	white/orange	-	RX+	z	bidi	48V return	-
4	1	blue	ring	-	-	bidi	-	48V out
5	1	white/blue	tip	-	-	bidi	-	48V out
6	2	orange	-	RX-	z	bidi	48V return	-
7	4	white/brown	-	-	-	bidi	-	48V return
8	4	brown	-	-	-	bidi	-	48V return

Combining 10Base-T (or 100BASE-TX) with "IEEE 802.3af mode A" allows a hub to transmit both power and data over only two pairs. This was designed to leave the other two pairs free for analog telephone signals^{[citation needed]}[1].

The pins used in "IEEE 802.3af Mode B" supplies power over the "spare" pairs not used by 10BASE-T and 100BASE-TX.

In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of echo cancellation. (Dial-up modems also use echo cancellation to simultaneously transmit data in both directions over a single cable pair).

Ethernet Minimum Cable Lengths

All copper Ethernet segments that run the Collision Detect (CD) portion of CSMA/CD have a minimum cable length to function properly because of reflections. This applies only to 10BASE-T and 100BASE-TX standards; The 1000BASE-TX standard is covered at the end of this section.

Fiber connections have minimum cable lengths due to level requirements on received signals^[3].

Fiber ports designed for long-haul wavelengths require a signal attenuator if used within a building.

Industrial Ethernet applications use a star topology with no collisions so that no minimum cable length is required.

1000BASE-TX supports half-duplex mode, making collisions possible. Consequently, the 1000BASE-TX standard requires a minimum cable length for collision detection to function properly; to avoid this in Gigabit Ethernet, small frames are padded into the transmission in half-duplex mode^[4].

See also

• Ethernet
• Physical layer
• IEEE 802.3

References

External links

• Get IEEE 802.3
• IEEE 802.3
• How to make an Ethernet cable