Understanding the network.pdf - Back to Home

The main advantage of using FDR over HDR is the elimination of Gigabit Ethernet's
CSMA/CD implementation. Like switches, FDRs also use 802.3x flow control to
regulate the traffic flow rate between the stations and the repeater. This way, the
repeater will not drop packets in the event that its buffers overflow. FDRs also check
packet integrity (also like switches), though this function is not performed by
traditional half-duplex switches. FDR's advantage over switches is cost. FDRs are
not as complex as switches, and they do not require the additional programming
and processing to perform MAC-to-port address lookups. They also do not require
the same amount of backplane capacity.
Token Ring
Token Ring was originally developed by IBM in the 1970s to provide a
transaction-oriented communications protocol for its Systems Network Architecture
(SNA) protocols, which were used to provide data communication services for its
mainframe computers. When IBM introduced its PC product in 1981, it provided no
networking solution, because IBM had envisioned the PC primarily as consumer
product or as a front-end device for its larger computing systems, which would
communicate serially with its mainframe systems. In 1984, in response to Ethernet,
IBM released a 4Mbps version of Token Ring to provide LAN transmission services to
its PCs that would be compatible with its existing networking architecture
technologies. 4Mbps Token Ring uses STP and UTP cabling in a logical star topology
that uses active hubs (known as Multistation Access Units [MAUs]) to interconnect
end-stations. IBM did not endorse Ethernet as a networking technology when it was
released in 1981, mainly because it was not compatible with IBM's existing
technology. In 1985, the IEEE developed a Token Ring standard, IEEE 802.5, which
is largely based on IBM's Token Ring architecture. IBM Token Ring and IEEE 802.5
are essentially compatible; the major difference is that IBM's support for source
route bridging which is accommodated by the data field in the 802.5 packet and
additional network management functionality. In 1989, IBM released a 16Mbps
version of Token Ring.
In 1992, IBM licensed its proprietary Token Ring implementation to other network
equipment vendors. This move resulted in some substantial improvements to Token
Ring, particularly, Token Ring multiport bridges (switches), full-duplex Token Ring
operation, and most recently, the development of high-speed Token Ring (which is
currently in the IEEE standards process, IEEE 802.5t). High-speed Token Ring uses
the standard Token Ring MAC and operates on a variation of the Fast Ethernet PHY
standard.
Token Ring makes up about 10 percent of the installed LAN networking base. It still
enjoys a small popular following, particularly in shops where IBM is the major
vendor. The main drawbacks to Token Ring have been and will continue to be cost
and limited vendor support. Token Ring adapter cards and hubs (MAUs) traditionally