Multimedia Networking “Technology Update”
by Bob Roman, 3Com Corporation
[Final Draft Article • Working Title • 03/11/96 • 950 Words with 1 Table]
Numerous elements must come together to make networked multimedia applications a
reality. Multimedia applications that integrate voice, video and/or data communications,
such as Intel’s ProShare for collaborative work, require certain changes in both the PC
and the local area network. This article highlights those changes and compares four
proposed desktop multimedia network alternatives.
There are two fundamental network requirements for handling multimedia: adequate
bandwidth and real-time communications in the form of predictable, timely data delivery.
Bandwidth requirements are determined by video, and can range from 128 kbps for
desktop videoconferencing to 6 Mbps for high-resolution, full-motion needs, such as
video playback or medical imaging. The real-time requirement is imposed primarily by
voice, but is also important for full-motion video. Because high bandwidth is readily
available with most LANs, the real challenge for a multimedia network is the real-time
requirement.
Two Different Multimedia Network Approaches
There are essentially two ways to provide real-time communications. One is to adopt the
traditional phone company philosophy of isochronous communications. Isochronous
networks utilize a dedicated channel or circuit that provides a fixed amount of bandwidth.
Both Asynchronous Transfer Mode (ATM) and isochronous Ethernet take this approach.
Despite the word asynchronous appearing its name, ATM can be made isochronous using
constant bit rate (CBR) communications. By contrast, the isochronous portion of
isochronous Ethernet comes from overlaying ISDN technology on to Ethernet. The
unfortunate aspect of any isochronous protocol’s use reserved bandwidth is high cost.
The second method of providing real-time communications is prioritized traffic flows.
While not truly isochronous in the strictest sense of the word, priority protocols can
achieve equivalent quality of service at a fraction of the price. Priority protocols allow
switched LANs to provide real-time service by giving maximum priority to designated
packets. Both 100VG-AnyLAN and PACE, the Priority Access Control Enabled protocol
for Ethernet, utilize the priority approach.
The Top Four Contenders
ATM is often considered the obvious choice for multimedia networking. It was
conceived to handle voice, video and data communications in the backbone, and will
ultimately fulfill that role. But there are two hurdles facing ATM as a desktop
multimedia network. First is ATM’s immaturity. The ATM Forum has focused almost
exclusively on packet-oriented communications thus far, and is only now beginning to
address isochronous support. Second is the chicken and egg dilemma facing any
revolutionary, versus evolutionary, technology. Until ATM becomes widely used, low
volumes will keep prices high. But volumes may not increase substantially until prices
drop considerably. Which will come first–and when? Expect to see ATM first in the
low-volume backbone and then at the high-volume desktop as price/performance
improves.
isochronous Ethernet, pioneered at National Semiconductor, is a hybrid solution that
combines 96 ISDN BRI channels (at 64 kbps each) and 10 Mbps Ethernet on the same
desktop connection. isochronous Ethernet offers the advantage of being compatible with
both WANs and LANs, which is achieved by implementing ISDN and Ethernet
respectively. Disadvantages include requiring new NICs in all workstations and new
switching hubs, along with installing a new ISDN infrastructure to the wiring closet.
100VG-AnyLAN uses a priority scheme, called Demand Priority Access, to achieve realtime communications. 100VG-AnyLAN is similar to the isochronous protocols in that it
requires new NICs and hubs. It also requires special internetworking equipment to
interface with other LANs, including Ethernet. In addition to upgrading the network
infrastructure, the network support staff is forced to understand a completely new
protocol and acquire new management tools. Recognizing these additional expenses,
100VG-AnyLAN is being priced aggressively at this time.
PACE is an enhancement to switched 10/100 Mbps Ethernet that was pioneered at 3Com.
PACE delivers real-time quality of service for multimedia traffic by offering a priority
scheme for Ethernet and by managing network access in a way that virtually eliminates
link contention. PACE’s biggest advantage is being the only solution that can be
implemented at the desktop by adding a new software driver to existing 10 or 100 Mbps
Ethernet NICs. This compatibility gives PACE the best price/performance of these, and
other, multimedia network alternatives.
The table offers a comparison summary of all four desktop multimedia alternatives.
Desktop Multimedia
Network Alternatives
Bandwidth (128 kbps-6 Mbps)
Real-time Communications
Keep Existing NICs
Leverage Ethernet Expertise
Cost to Implement
Vendor Support
ATM
25-155 Mbps
Isochronous
No
No
High
Good
isochronous
Ethernet
16 Mbps
Isochronous
No
Yes
High
Fair
100VGAnyLAN
100 Mbps
Priority
No
No
Medium
Poor
PACE
10-100 Mbps
Priority
Yes
Yes
Low
Growing
Table comparing the four leading contenders for desktop multimedia networking.
What’s a Network Manager To Do?
With such diverse options it is difficult for a network manager, faced with implementing
a multimedia network, to pick the right technology. Isochronous solutions, like ATM and
isochronous Ethernet, offer guaranteed bandwidth and potentially better quality of
service. But both are, at least for the time being, quite expensive. Choosing these
alternatives means replacing all NICs and hubs, and learning a new protocol. Of the two
isochronous options, ATM is clearly the better choice. ATM is infinitely scalable and
enjoys broad vendor support. For companies that can afford it today, ATM is a good
long-term multimedia network solution.
The priority protocols can offer equivalent quality of service, but at a fraction of the cost
of isochronous protocols. Of the two, PACE has the better price/performance. PACE is
the only protocol of the four that fully protects an organization’s investment in Ethernet
NICs and knowledge. For network managers uncomfortable making the strategic ATM
commitment now, PACE offers the perfect tactical solution. Of course, with its quality of
service and low cost, the best tactical option may turn out to be another strategic element
of multimedia networking.
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