Information Technology Reference
In-Depth Information
A viable alternative to LANE is to simply operate the workstation/server con-
nections in a native ATM mode. As the ATM technology becomes more widely
available, ATM network adapter cards with software drivers for popular operating
systems are becoming available. In the future, it may be practical to implement giga-
bit-speed networks over our structured cabling networks.
ATM-155 over fiber uses the OC-3c physical layer. This connection has been
available for some time, and can be supported by standards-based fiber cabling.
With the proper workstation network interface cards, OC-3 to the desktop is prac-
tical. In this application, the ATM network runs in native mode, alleviating the
requirement for LAN emulation.
At this point, most copper development has been done for 155 Mbps ATM. In
fact, a method now exists to support the nonreturn-to-zero (NRZ) signaling of
ATM-155 over Category 5 cabling. The ATM Forum provides a standard for a cop-
per physical layer that allows for the same link parameters as the Channel defined
by TSB-67 and other supplements.
Unlike Ethernet, the copper ATM connection uses the 1-2 pair and the 7-8 pair.
However, standard structured cabling connects all four pairs, so there is no problem
in utilizing standards-based structured cabling of the proper category. This means
that a properly installed Category 5 (or higher) cable of 90 m, 10 m or less of patch
and user cords, and a maximum of four Category 5 (or higher) connectors comply
with the requirements for ATM-155 over copper.
ATM-622 uses the OC-12c physical layer at 622 Mbps. At this time, imple-
mentations for this technology are fiber optic-based. However, techniques for oper-
ating over copper are in the testing phase. It is reasonable to assume that the 622
Mbps signaling can use the same techniques that Gigabit Ethernet uses, as the trans-
mission frequencies should be no greater. Keep in mind that the effective data rate
and the maximum signal frequency may not be in step. In all of these very high data
rate technologies, symbol encoding and signal splitting are both used to reduce the
maximum signaling frequency that must be transmitted.
ATM generally uses standard data rates that are multiples of a lower-order
data rate, Optical Carrier 1 (OC-1). As each standard rate is set, a judgment must
be made to determine which multiple is optimum. Often, the data rate chosen is a
result of a careful analysis of operating requirements and media performance. Thus,
a rather unusual multiple of 3 was chosen for the 155 Mbps specification.
4
However, this rate was well within the frequency and bandwidth capabilities of the
fiber optics and the fiber media to allow operation at the desired distances.
4
The OC-3 rate, at 155.52 Mbps, is three times the base rate of 51.84 Mbps, OC-1. The OC data
rates are frequently referred to by the number before the decimal in the exact bit rate, e.g., 155 or 622.
