Ethernet physical layer

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IEEE photograph of a diagram with the original terms for describing Ethernet drawn by Robert M. Metcalfe around 1976.

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 ranges 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 auto-negotiation 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.

[edit] Physical layers

[edit] Xerox experimental Ethernet

[edit] Early implementations (10 Mb/s and 1 Mb/s)

[edit] Fast Ethernet (100 Mb/s)

[edit] Gigabit Ethernet

All of these use a star topology.

[edit] 10 gigabit Ethernet

• 10 gigabit Ethernet is still fairly new and it remains to be seen which of the standards will gain commercial acceptance.
• Note that both IEEE 802.2ae and IEEE 802.3ak have been incorporated into IEEE 802.3-2005.

[edit] Ethernet over twisted-pair cable

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. (Telephones also use echo cancellation to simultaneously transmit in both direction over a single cable pair).

[edit] Ethernet Minimum Cable Lengths

Ethernet Minimum Cable Length

other keywords: ethernet minimums length minimum length minimums

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 is normally 10-100 Mb stuff; Gigabit is covered at the end of this section.

for example: http://support.microsoft.com/kb/97550 http://www.comptechdoc.org/independent/networking/guide/nethwethernet.html

Fiber connections have minimum cable lengths too, due to received signal level requirements:

http://www.cisco.com/en/US/products/hw/modules/ps5455/products_data_sheet0900aecd801ba88e.html

Fiber ports designed for long distances (kilometers) need a signal attenuator if used within a data center, for example:

for example: from http://apps.extremenetworks.com/services/documentation/ConsolidatedHWRev2-Chapter1.asp "The transmitter output power level for the 1000BASE-SX is -5 dBm. The maximum allowable receiver input power level is -17 dBm. Therefore, there is a minimum of 12 dB loss required for the link to operate without errors. This minimum required loss can be achieved using a fiber length of 52 km (0.25 dB/km provides 8 dB loss), or by adding 10 dB of fixed optical attenuator at the receiver end."

Industrial Ethernet uses a star topology with no collisions, therefore no minimum cable length is required. Please see http://www.ccontrols.com/pdf/NWSL61.pdf

Aircraft use a special type of Ethernet also; not included in this article yet.

So here's where it gets interesting. 1000BaseT, (Gigabit over copper, 4-pair) actually supports half-duplex mode, so collisions are possible. This would definitely require a minimum cable length for CD to work, so to avoid this in Gigabit, they pad small frames in half duplex mode:

http://www.intel.com/network/connectivity/resources/doc_library/white_papers/solutions/copper_guide/gig_over_copper.htm

"When running in half-duplex mode, Gigabit Ethernet uses the CSMA/CD protocol. Half-duplex collision domains should be the same as for 100BASE-TX, although each domain can support only one half-duplex repeater. In order to make CSMA/CD work for Gigabit Ethernet, it was necessary to alter the normal timing within a collision domain. At Gigabit speeds, smaller sized data packets could literally reach their destinations before the sending station could detect a collision signal. To overcome this problem, minimum-sized packets are padded with an extension field."

[edit] See also

• Ethernet
• physical layer
• IEEE 802.3