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Networks must guarantee bandwidth for multimedia traffic and must
control end-to-end delay and delay jitter (fluctuation in the arrival time of
packets). The new campus network, 100VG-AnyLAN, can meet these
requirements in many circumstances through the basic operation of the
protocol. More flexibility can be obtained through the use of bandwidth
allocators and the target transmission time protocol. Until either the
Broadband Integrated Services Digital Network (B-ISDN) or reliable
Internet protocols become available, the use of dial-up remote bridges
with existing WANs can accommodate multimedia traffic in the near term.
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API
Applications
Internetwork
MMCF, Microsoft
IETF
Bridges
API
Applications
Internetwork
Routers
Links
Links
100VG-AnyLAN
+ Bridges
WAN Services
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Table I
Delays in a Remote Collaboration System
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1(&'3
To illustrate the operation of a dial-up remote bridge, Fig. 1 shows two sites, each
with a backbone FDDI network linking 100VG-AnyLAN hubs.
Users connected to end nodes 1 and 2 want to communicate by video and audio
between the two sites. The backbone networks on each site are connected to the
wide area network (WAN) through dial-up remote bridges specifically designed to
carry multimedia traffic. We assume that site A and site B can also communicate
through existing routes, such as some kind of corporate internet.
When the call between node 1 and node 2 is initialized, an isochronous link with
the appropriate bit rate is set up between the two dial-up bridges. Bridge A is told
the 48-bit MAC address of node 2. Any subsequent packets that appear on the
backbone FDDI bearing that destination address will be forwarded across the
WAN. Bridge B puts them on the backbone FDDI at site B, from which they reach
node 2.
Any normal-priority data packets destined for node 2 will not be picked up by
bridge A because they will carry the destination address of the router connecting
site A to the corporate internet, not that of node 2.
Site A
Site B
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Dial-Up
Remote
Bridge A
Backbone
FDDI
Dial-Up
Remote
Bridge B
100VG
Hub
End-Node
1
End-Node
2
Fig. 1. Remote sites connected by dial-up remote bridges.
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&# !
The introduction of new LAN technologies (such as 100VG-AnyLAN) is only one
element in the development of multimedia networking. Higher-level protocols are
also needed that can control and transfer stream data over multiple network hops.
The Internet Engineering Task Force (IETF) and the International Telecommunications Union (ITU-T) have been working in this area.
IETF
The IETF has been developing protocols for a multiservice, packet-based network
spanning the world (see Fig. 1), such as the Internet Stream Protocol (ST-II). This
is a network-layer protocol roughly equivalent to Internet Protocol (IP), but specifically developed to support stream-based traffic. ST-II is being used in the Multimedia Teleservices section of the BERKOM-II program (see “Related Projects” on
page 38).
The IETF has a number of groups looking at new protocols for multimedia that can
be introduced into the worldwide Internet.
User’s
System
Routers
IETF’s Area of Interest
100VG-AnyLAN
Internet, Spanning the World
Fig. 1. IETF involvement in worldwide networks.
ITU-T Narrowband ISDN Standards
The ITU-T works towards developing standards for data transmission over an
enhanced telephone network, such as ISDN or other digital circuits with fixed bit
rates up to 2 Mbits/s (Fig. 2). 100VG-AnyLAN could provide desktop connections
to these services. ITU-T Recommendation H.320 includes the following:
• H.261: Video CODEC for audiovisual services at p × 64 kbits/s
• H.242: A system for establishing communication between audiovisual terminals
• H.221: A frame structure for a 64-to-1920-kbit/s channel in audiovisual teleservices
• Q.931/2: A D-channel signaling prot col.
ITU-T’s Area of Interest
Private
User’s
System
Public
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100VG-AnyLAN
Fig. 2. ITU-T involvement in worldwide networks.
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Suppose that all the 100VGĆAnyLAN hubs are 32Ćport or
smaller. The protocol guarantees that in the event of all
nodes simultaneously wishing to transmit normalĆpriority
data, they will each have a bandwidth of a little over 3
Mbits/s. The allocator then only has to ensure that the sum
of the highĆpriority synchronous multimedia traffic across the
FDDI does not exceed 80 Mbits/s. A system such as this can
typically support up to 80 video connections each with a
peak bandwidth of 1 Mbits/s, or up to 200 364Ćkbit/s conĆ
nections.
Video, Audio and Real-Time Data
Applications on New or Modified Protocol Stacks
Target Transmission Time (TTT)
Guarantees Bandwidth and Low
Bounded Delay for New Applications
Data Applications on Existing
Protocol Stacks
High Priority
Normal Priority
Demand Priority MAC
In 100VGĆAnyLAN, the proposed mechanism for allocating
bandwidth and setting delay bounds is the target transmisĆ
sion time.
100VGĆAnyLAN end node stacks.
A third property of 100VGĆAnyLAN facilitates the allocation
of bandwidth on networks: spare bandwidth utilization. Any
bandwidth not used by highĆpriority traffic is immediately
available for normalĆpriority data.
Example of Target Transmission Time Allocation. A nominal time
value, the target transmission time (TTT) is set, and nodes
request permission from the allocator to transmit a maximum
amount of data within a TTT period. Provided that the sum
of all such requests is less than a preset maximum, say 80%
of the network capacity, the allocator grants the request. The
end node is then guaranteed that its data will be transmitted
during the TTT period.
This property is particularly relevant if highĆpriority traffic is
bursty. It also ensures that normalĆpriority traffic does not
need to participate in the allocation process.
Fig. 4 illustrates how the protocol stacks for high and normal
priority are independent. This is important because resource
allocation protocols need only be introduced on end systems
that actually run multimedia applications. End systems that
only use normal priority do not need any new software.
Note that the use of a bandwidth allocator requires higherĆ
level interfaces and services to be defined, such as those on
which the IETF is working (see HigherĆLevel Protocols" on
page 36.
Fig. 5 shows a simple example of how this could work. In
the example, there are four stations operating at high priorĆ
ity. Nodes 1 and 2 are each generating 30 Mbits/s, node 3 is
generating 7.5 Mbits/s, and node 4 1.5 Mbits/s.
Node 4 generates one 1500Ćbyte packet every 8 ms and
requires this to be transmitted onto the network within 8
ms. If this is the lowest access delay, the TTT would be set
at 8 ms. In this time period, the other nodes will generĆ
ate 30kB (20 1500Ćbyte packets), 30 kbytes, and 7.5 kbytes
(5 1500Ćbyte packets) of data, respectively.
Example of Bandwidth Allocators. Consider the following simĆ
ple scenario illustrating the use of a bandwidth allocator.
Several 100VGĆAnyLAN hubs are connected by an FDDI
ring. The allocator ensures that the sum of the highĆpriority
synchronous multimedia connections across the FDDI does
not exceed 80 Mbits/s, and that the sum of the highĆpriority
multimedia connections across any individual 100VGĆAnyĆ
LAN network does not exceed 80 Mbits/s. This ensures that
the 100VGĆAnyLAN networks and the FDDI backbone are
not overloaded with highĆpriority traffic.
1
2
3
Fig. 5 shows how the data in each of the end nodes' transĆ
mission queues is transmitted on the network.
Bandwidth Monitoring. HighĆpriority traffic can be monitored
centrally at the hub or through a separate device attached to
a promiscuous port. From packet lengths and interpacket
gap times, the monitor can easily calculate the aggregate
Node 4 TTT
4
Node 3 TTT
Node 2 TTT
Node 1 TTT
HighPriority
Transmit
Queues
Node 4
Node 3
Node 2
Node 1
Normal Priority
ÏÏ
ÏÏ
Time
Hub
ÏÏ
ÏÏ
ÏÏ
ÏÏ
Example of the use of the
target transmission time (TTT).
August 1995 HewlettĆPackard Journal
Two projects are under way that have a bearing on the development of networks
to carry multimedia traffic.
Teleservices Project
The Teleservices project is part of the Deutsches Bundespost (DB) Telekom BERKOM technology and demonstrator program. The program covers wide area network and campus broadband communication studies, using broadband switches
and local optical-fiber links installed in Berlin to form a large test bed. It now forms
part of the German Broadband Integrated Services Digital Network (B-ISDN)
asynchronous transfer mode (ATM) pilot network, with switching centers in Cologne and Hamburg.
• Identification of particular issues and problems associated with multimedia applications related to HP products and commercial interests
• Performance measurement and behavior of a transport protocol designed for
multimedia applications running over different network technologies.
A demonstration was given in August 1993 using the first version of the conference application with software video compression and relatively simple control
options. In March 1994, an enhanced version was demonstrated at the CeBIT
exhibition in Hannover. This had a range of management and control options, an
audio server, and full-motion hardware compression (M-JPEG). It also included
multivendor terminal interworking at the show over the ATM demonstration network and conferencing over ATM link connections back to a terminal in Berlin.
The Teleservices Project is investigating the provision of multimedia services over
wide area, broadband networks. HP is involved in two areas: multimedia collaboration (MMC), supporting conferencing and shared applications, and multimedia
transport (MMT), developing high-speed networking protocols for multicast operation.
A demonstration of the Teleservices package between HP Laboratories in Bristol
and the TUB, using ISDN at about 300 kbits/s showed that a minimum transfer
rate of 1 Mbit/s is required for successful operation. Wideband operation over
ISDN connections is also being evaluated.
HP joined in April 1993, with collaboration from HP Germany, HP Laboratories in
Bristol, and TUB (Technical University of Berlin, as an HP subcontractor). The
main HP/TUB task is the development of high-speed network protocols and the
overall implementation and integration of the complete collaboration application
under the multimedia collaboration task for the demonstration of interoperability.
Five HP 9000 Model 755 computers have been installed at the TUB for the multimedia implementation, together with a range of audiovisual equipment, communications interfaces, and associated software packages. Prototype HP ATM
switches have also been installed.
Distance Learning Seminar
In October 1994, a pan-European seminar on distance learning was held on a
network linking three European universities (the Royal Institute of Technology,
Stockholm, the Technical University of Berlin, and University College London) and
various other European sites. The seminar was sponsored by HP and aimed to
promote the use of distance learning and to explore the future of the technology
and the role of multimedia.
The program will produce a limited HP multimedia communications and management product for use on HP 9000 Model 7x5 workstations in the German broadband trials. It will also provide measurement and performance evaluation results,
including:
• Observations of the interactions between video and audio compression schemes,
transport protocols, the different network technologies used, and their impact on
perceived performance
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links between Berlin and London and Berlin and Stockholm. Other links used
inverse multiplexers at each site to provide contiguous bandwidth over ISDN of
384 kbits/s. This provided adequate bandwidth for 25-frames-per-second video
using hardware compression based on the H.261 standard, plus good-quality
audio.
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