3.9   GIGABIT ETHERNET

The Gigabit Ethernet (1000BASE-x), or GbE for short, is the natural evolution of
two older standards, the 100 Mbit Ethernet (100BASE-x) and the 10 Mbit Ethernet
(10BASE-T). The success of the latter two led to GbE, which was initially defined
for twisted copper cable, for which it was known as 1000BASE-T and subsequently
for optical fiber.

Ethernet defines the Logical Link Control (LLC), the Media Access Control
(MAC), the Physical Medium Independent Interface (PMI), and the Physical Layer
(PHY), which consists of the physical sublayer and the Medium Dependent Interface
sublayer (MDI). GbE also defines an intermediate layer between the PMI and
PHY known as the Medium Independent Interface (MII). The purpose of the MII is
to provide medium transparency to the layers above it and allow for a variety of media
(wired, MMF, and SMF), as previously described.

The MAC provides network access controllability during frame assembly and
disassembly of the client data and during frame transmission to lower layers. It also
provides network access compatibility of MACs, end-to-end and of all intermediate
MACs on the path.

Optionally, the MAC layer may provide full- or half-duplex capability; full-duplex
capability means that the MAC supports a transmit and a receive side. It may
also request that subsequent peer MACs inhibit further frame transmission for a pre-
determined period, and it may allow for a second logical MAC sublayer. In the latter
case, the basic frame format is maintained but the interpretation of the data fields and
the length may differ. It may also support VLAN tagging (per IEEE 802.3ac, 1998)
to prioritize packets; however, VLAN tagging requires changes to the frame format.

The 1000BASE-T is not defined at a serial Gbit/s bit rate. Instead, it is defined
over four unshielded twisted pairs (UTP), category-5, 100 ohm copper cables, 100
meter maximum length conforming to ANSI/TIA/EIA-568-A cabling requirement,
at 250 Mbit/s per pair. Conversely, the 1000BASE-CX standard defines a Gigabit
Ethernet for single-pairs but for very short copper cables.

The 1000BASE-T complies with the same topology rules of 100BASE-T, and it
supports half-duplex CSMA/CD as well as full-duplex. It also uses the same autonegotiation
protocol as 100BASE-TX. The four pairs form a parallel cable, each
keeping the symbol rate at or below 125 Mbaud (Figure 3.15).

Because 1000BASE-T is used over copper lines, the standard had to address
known issues such as echo, near-end crosstalk, far-end crosstalk, noise, attenuation,

and EMI. To keep noise, echo, and crosstalk at low levels commensurable with a
10^-10 bit error rate, the following design strategies were adopted:

• PAM-5 multilevel encoding, where each symbol represents one of five levels,
  –2, –1, 0, +1, and +2. The four levels are used for data and the fifth for FEC
  coding. This results in a reduction of the signal bandwidth by a factor of two.
• 4D 8-state trellis forward error correction (FEC) code
• Signal equalization with digital techniques
• Pulse shaping at the transmitter to match the characteristics of the transmission
  channel and increase the signal-to-noise ratio.
• Scrambling to randomize the sequence of transmitted symbols and reduce
  spectral lines in the transmitted signal.

In addition to UTP cable, the 802.3ab task force has defined standards for fiber cable.
These standards are the 1000BASE-SX for short-wavelength fiber (850 nm,
MMF) and the 1000BASE-LX for long-wavelength fiber (1300 nm, SMF).

Because of the four physical media defined in GbE (1000BASE-CX,
1000BASE-SX, 1000BASE-LX, and 1000BASE-T), it was necessary to bring
transparency to the Media Access Control (MAC) layer. Thus, a new layer under
the MAC was defined, known as the Reconciliation sublayer and the Gigabit Media
Independent Interface (GMII). Figure 3.16 illustrates the relationship of the various
Gigabit–Ethernet PHY derivatives.

The GMII provides physical layer independence to the MAC layer so that the
same MAC can be used for any wired and fiber media. It also includes signals such
as management data clock and management data input–output, and it registers, basic
and extended (which is optional). The registers are used for autonegotiations,
power down, loop-back, PHY reset, duplex/half-duplex selection, and others.

The Reconciliation sublayer in the transmit direction maps service primitives or
physical layer signaling (PLS) from the MAC to GMII, and vice versa in the receive
direction.

The PLS includes signals such as data request, transmit enable, transmit error,
transmit clock, collision detect status, data valid, receive error, signal status indicate,
carrier status indicate (collision detect, carrier sense, carrier extend), and receive
clock.

The gigabit Ethernet defines two different bit rates. Raw data is formatted at the
MAC layer and is passed via the GMII over to the physical layer at 1000 Mbit/s.
This is known as the instantaneous transmission rate for encoded MAC data. However,
at the physical layer a process takes places in which 8-bit octets are encoded to
10-bit characters, according to 8B10B coding. As a result of the 8B/10B coding, the
bit rate on the physical medium is increased by 25% to a line bit rate of 1250
Mbit/s. This is known as the instantaneous transmission rate. Thus, the user data
transmission rate over the medium, is

Based on the latter, this transmission rate is zero for as long as user data is not transmitted.
Therefore, the above relationship is not a measure of efficiency.

The Gigabit Ethernet momentum prompted the study of a new Ethernet standard
planned to match the OC-192 bit rate for local area network applications, and
thence called native 10GbE. The 10GbE supports a variety of fiber-optic physical
media, MMF and SMF, for fiber lengths up to 10 and, in certain cases, up to 20 km.
For example:

• 10GBaseLX-4 supports transmission in the 1300 nm window (SMF, O-band:
  1260–1360 nm) using the CWDM grid.*
• 10GBaseSX-4 supports transmission in the 850 nm window (MMF) using the
  CWDM grid.

However, the 10GBasex-4 does not transmit at 10 Gbit/s (or at the line rate of 12.5
Gbit/s as a result of 8B/10B coding) over a single wavelength, but at 3.125 Gbit/s
over four optical channels (wavelengths), or “lanes,” with a spectral spacing of 24.5
nm. The nominal wavelengths defined for each lane are:

Lane 0: 1275.7 nm
Lane 1: 1300.2 nm
Lane 2: 1324.7 nm
Lane 3: 1349.2 nm

There are many reasons for transmitting over four coarsely spaced wavelengths:

• The lasers can be directly modulated and without cooling, thus inexpensive.
• Dispersion for lengths of 10 to 40 km and at low bit rate becomes unimportant
  so that compensation and equalization are not required.
• Polarization effects are negligible.
• It is a low-cost and low-maintenance technology.

The 10GbE is still in evolution and proposals are under study to define new recommendations
and redefine older ones.

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*ITU-T G.694.2 specifies the CWDM grid within the range 1270–1610 nm with 20 nm channel spacing,
or 18 channels (wavelengths) equally spaced. ITU-T G.652, Table G.652.C lists the parameters of the
water-free fiber.