Legacy and Less Common Cabling Systems - LAN Wiring - page 479

Other Network Types
Some other types of networks can operate in the office environment. Some of these
other technologies have similarities to standard Ethernet and Token-Ring, but are
not truly a part of those networking methods.
We will cover a few of the other networks here. As Ethernet twisted-pair tech-
nology has become dominant, these network systems are truly deserving of being
called “other.” In some cases, many technologies, such as isochronous Ethernet and
100VG-AnyLAN, are offered as alternatives to standard Ethernet, but are not
widely used. The Apple networking technologies and protocols have given way to
Ethernet and Internet protocol, for the most part.
100VG-AnyLAN
A new technology called 100VG-AnyLAN overcomes some of the distance and
repeater limitations of the 100BaseT and allows operation on 4-pair Category 3
cable. The standard specification for 100VG-AnyLAN is contained in IEEE
802.12.
The 100VG-AnyLAN technology is an interesting combination of asynchro-
nous communication, such as that employed by Ethernet, and round-robin message
scheduling, such as that of Token-Ring. The 100VG-AnyLAN topology is an arbi-
trated star arrangement, that is, a star with transmission control in the hub.
Workstations and servers are on the legs of the star, with an intelligent hub at the
center. The hub performs the functions of an arbitrated gateway and a switch. A
workstation that wants to transmit a message makes a request to the hub. If the
network is idle, the request is granted. The workstation transmits its message and
the hub switches it to the proper outgoing port. If more than one station needs to
transmit, the hub acknowledges each one in turn, according to a predetermined
priority scheme.
The wiring systems supported by a 100VG-AnyLAN include 4-pair Category
3 or higher UTP, 2-pair STP-A, and single or multimode optical fiber. The signaling
rate on 4-pair UTP is kept low by splitting the signal into four data streams, one per
cable pair. The bandwidth requirement of approximately 15 MHz is within the
capability of standard Category 3 cabling. The wiring pattern for 100VG-AnyLAN
is the same as for any standard TIA-568-C cabling system.
Because 100VG-AnyLAN is not dependent on a collision detection mechanism,
it is not subject to the distance limitations of unswitched Fast Ethernet. Some
100VG-AnyLAN adapter implementations offer dual speed capabilities, as with the
100BaseT products.