Understanding the network.pdf - Back to Home

distances between 500m and 550m are supported. All four multimode
implementations use SC-type connectors. 1000Base-SX implementations are the
most common because they can be run on existing cabling infrastructures. One byte
1000Base-LX was developed for use primarily in long backbone transport scenarios,
such as building-to-building or floor-to-floor connectivity. 1000Base-LX utilizes a
long wavelength (1300nanometer) light source over multimode and single-mode
fiber. With 62.5/125 and 50/125 micron fiber, multimode fiber distances up to
550m are supported. With 10/125 micron single-mode fiber, distances up to
5,000m (and beyond) are supportable. The last PMD specification uses 150-ohm
twinax cabling with either the Fibre Channel Type-2 balanced connector or
D-subminiature connector.1 byteThe purpose of 1000Base-CX is to provide a more
cost-effective method for short haul (up to 25M) interconnections between
repeaters and switches. It operates by connecting the PMA interfaces of the two
connected devices, essentially extending the PMA over separate cable paths. Due to
the extremely high frequency operating rate (1,250MHz), the connecting cables and
connectors must be shielded and balanced.
1000Base-T
The 1000Base-T standard IEEE 802.3ab is expected to be ratified by the end of 1999.
The proposed standard uses four pairs of Category 5/5E cabling and the standard
RJ-45 connector. All four wire pairs are used for data transmission reception using
a technique known as dual duplex. The 1000Base-T standard specifies a new PHY
layer from the GMII down. The PCS interface uses a variation of the Pulse Amplitude
Modulation 5 (PAM5) line encoding scheme. This is a five-state encoding system.
PAM5 uses five voltage variations to represent 2 bits of data, with each wire pair
operating at a clock rate of 125MHz. This new scheme is called 4D-PAM5. To actually
interpret the simultaneous transmission and reception signals, a digital signal
processor is part of the transceiver circuitry. Although the 1000Base-T standard has
been anxiously awaited by some, its added technological complexity will
undoubtedly come at a cost greater than the current 1000Base-X standard-based
products that are already shipping. 1000Base-T also has a smaller maximum
segment distance (100M) than the currently shipping products, making it only
practical for end-station, not backbone, connectivity use.
1000Base-X CSMA/CD
Although the PHY implementation of Gigabit Ethernet is quite similar to Fast
Ethernet, there are some real differences as far as implementation topology is
concerned. Gigabit Ethernet uses the same CSMA/CD mechanism and 802.3 frame
format used by 10Mbps and 100Mbps Ethernet implementation. Minimum frame
size is 64 bytes; the maximum is 1,518 bytes.