Information Technology Reference
In-Depth Information
Isochronous Ethernet
One of the greatest challenges in integrating voice, data, and even video, is that the
technologies heretofore have used entirely different means of encoding and trans-
mitting information. The only practical solution has been to separate those signals
in separate cables, each routed to its respective switching or distribution equipment.
Isochronous Ethernet proposes to end that divergent situation by combining multi-
ple signals onto the same data stream, so that all three technologies can be simulta-
neously carried on the same physical medium. Indeed, the technology actually
allows quite a number of channels to be carried over the standard 100 m 4-pair
cable link. Isochronous Ethernet is standardized in IEEE 802.9, and the information
here reflects the proposed draft revisions to become IEEE 802.9a.
Isochronous Ethernet uses a clever scheme of time-division multiplexing
(TDM) and signal encoding to place a full 10 Mbps Ethernet channel plus an addi-
tional 6 Mbps or so into the same composite signal. The total data rate in the sig-
nal is 16.384 Mbps. A 4B/5B encoding scheme is used instead of the Manchester
coding of standard Ethernet. This yields a 10.24 MHz line transmission rate, thus
making the signal well within the capabilities of Category 3 cable or higher cable.
Two modes of operation are available. In the multi-service mode, the signal is
divided into three user channels, the 10 Mbps (actually 10.016) Ethernet P-channel,
a 6.144 Mbps bearer C-channel, and a 64 kbps signaling D-channel. The C-channel
is easily further divided into 96 channels of 64 kbps subchannels. Of course, the sub-
channels are exactly the same size as an ISDN bearer channel, so the network can
easily support data, digital voice, or compressed video. An alternative isochronous-
only mode divides the total signal into 248 bearer subchannels, each of 64 kbps.
A third mode of operation allows the devices to operate as standard 10BaseT,
Manchester encoded network interfaces, in full accordance with IEEE 802.3 and
with no support for the TDM channels.
AppleTalk/LocalTalk
AppleTalk has turned into a somewhat generic term that is generally used to
describe Apple LocalTalk networks. AppleTalk is actually a higher level protocol
that can be transported over several types of network topologies. The wiring for
classic LocalTalk is a low-speed two-wire bus that is usually run in a daisy-chain
fashion from computer to computer.
The unique feature of most Apple computers since the introduction of the
Macintosh is the built-in LocalTalk networking port. The provision of a built-in net-
working capability has made printer sharing and file transfer a normal part of oper-
ation with Apple computers.
