The Emergence of Multimedia Applications
Data only transmission is losing the spotlight to more dynamic and richer forms of communication such as full-duplex voice, still photographs, and interactive video communications because these media are approximations of the real thing; face-to-face communication. Furthermore, sound and images enhance presentations, training, project management, and product demonstrations.

Corporate executives realize the benefits these media can bring to organizational communications. As a result, they are pushing for ways to add network capacity without a major retooling investment. Network Managers would do well to investigate and jump on any viable new bandwidth-expanding opportunities that come along.

What is Isochronous Ethernet (or isoEthernet)?

IsoEthernet uses multi-channel protocols.  When an isoEthernet -equipped PC sends Ethernet data traffic, the data goes from the IsoHub/Switch to he Ethernet hub and is handled like all other Ethernet transmissions. When an iso-PC sends isochronous traffic (e.g. video), however, the isoHub/Switch sends that data to either a PBX or to a Time-division Multiplexer (TDM) via a T-1, E-1, or ISDN Primary Rate Interface/Access (PRI/PRA) trunk and then out over the WAN.  The bandwidth, required for a specific video/audio call, is allocated end-to-end.

With isoEthernet we can run multimedia applications on a single wiring plant without encountering a proprietary technology. Typical Ethernet traffic for file services, print services and client'/server application can be kept completely separate from the channels carrying voice, T.120 the data (application sharing), videoconferencing, graphical interfaces to the Internet, and any other ISDN applications.

The importance of ISDN as the basis for Isochronous Ethernet lies in the fact that ISDN is the predominant network infrastructure that supports Application Sharing (T.120), videoconferencing (H.320) and voice (G.711) which are the standards set by the ITU-TSS to ensure interoperability among videoconferencing systems.

Why isoEthernet?

Many of the vendors have white papers describing the benefits of isoEthernet. We summarize the benefits by emphasizing the fact that isoEthernet enables compatible communication in the global enterprise.

Most people would agree that Ethernet is the standard for enterprise packet data transfer. Packet data is connectionless and can be transmitted easily over a variety of media, such as the various flavors of switched/fast Ethernet, ATM, Frame Relay on T1/E1, etc. Upgrading a packet data network to an isoNetwork requires the enterprise to purchase new equipment hubs and adapter cards, and perhaps lease a higher bandwidth Wide Area Network (WAN) data interface. This is relatively easy, and under the full control of the enterprise. The rapid growth of 100M Ethernet is proof of this concept.

The worldwide standard for digital isochronous communications is ISDN, at a wide range of data rates. The worldwide analog phone system was a marvel of compatibility, and now the conversion to a digital infrastructure is now largely complete. Over the last 17 years, ISDN (in its various flavors) has become the standard for global end-to-end connection-oriented digital communication. No other system will allow two people anywhere in the (developed) world to communicate as if they were talking face-to-face. The promise of other technologies, such as IP over ADSL and cable modems does not replace ISDN or Broadband ISDN over ATM.  Vendors of VoIP solutions are struggling with the fact that isochronous traffic, like voice and video, cannot be 

Upgrading is not an enterprise-level decision, it is an infrastructure and governmental decision. The reality of non-packet ATM deployments (and internet phone systems) will continue to prove this.

isoEthernet provides a way to bring both Ethernet and ISDN directly to the enterprise desktop. In order for an enterprise to keep pace with changing environments, it must own its internal network infrastructure. isoEthernet is the only way to bring the digital telecommunications network transparently to the desktop computer, in a manner which integrates it seamlessly with the existing data network, under the full control of the enterprise.

Follow the links on video conferencing and CTI above for examples of applications which can use these capabilities. It is also the fastest and cheapest way for the telecom carriers to expand the services currently available on single channels to wide-bandwidth communications within their infrastructure.

The internetworked computer is the engine of information age. It will become the endpoint for wideband telecommunications. isoEthernet is the solution that can deliver the necessary connection to the enterprise desktop today.

Cost Benefits

Even skeptics acknowledge that isochronous Ethernet can overcome these problems (for an overview of competing technologies, including ATM, see "Networking Alternatives"). Isochronous Ethernet glues together channels that handle isochronous and packet data separately and transports the two data types over existing Ethernet 10Base-T LANs that use Category 3 UTP (unshielded twisted pair) or higher. Thus 10Base-T-network administrators avoid rewiring costs.

Isochronous Ethernet is closely related to ISDN in that they use multichannel protocols and are based on TDM (time division multip lexing), a technique for multiplexing multiple channels over a single physical medium. TDM allocates to each channel a specific amount of time on the physical network wire. This is done independently of the other network protocols running on the same cable pairs, so existing LAN applications operate on isochronous Ethernet facilities unaffected by the isochronous traffic. By time slicing, isochronous Ethernet transmits isochronous information over the Ethernet LAN at a constant rate over time.

 Isochronous Ethernet uses a P+96B+D format that can mix video, audio, and Ethernet into one LAN bundle. By comparison, in North America, BRI (Basic Rate Interface) ISDN is 2B+D (two bearer and one delta channel), and 23B+D for PRI (Primary Rate Interface). As the figure "A Division of Labor" illustrates, isochronous Ethernet's 10-Mbps P (packet data) channel supports standard IEEE 802.3 Ethernet packet data. A 64-Kbps D channel is used for signaling, much as it is for ISDN service. The M (ma intenance) channel is a 96-Kbps channel that provides physical-layer status and control information. The 6.144-Mbps C (circuit mode) channel handles multimedia applications. Isochronous Ethernet allows you to split the C channel into as many as 96 ISDN B narrowband channels (of 64 Kbps each) as desired, or it can combine the channels to add 64-Kbps bandwidth or make a single 6.144-Mbps wideband channel.

Unlike traditional 10Base-T Ethernet, which uses the Manchester encoding scheme to send data over the physical connection, the isochronous Ethernet standard specifies 4B:5B encoding. The latter is also used in FDDI networks and benefits isochronous Ethernet by supporting the 10-Mbps packet channels and 6.144-Mbps isochronous bandwidth, as well as 10 Mbps of Ethernet over 100 meters of UTP cabling.

Isochronous Ethernet's bandwidth-on-demand lets you request whatever video quality you need for each connection. Thus, participants in a video conference, for example, can reserve a large portion of bandwidth for continuous and immediate full-motion video. By contrast, someone who wants to send a short video clip without audio needs to select only a small segment of the isochronous-Ethernet pipe. An isochronous Ethernet hub, using bandwidth management, acts as the mediator that awards or refuses the requested WAN bandwidth based on availability, or it can tell you what lower-bandwidth service is available. Isochronous Ethernet uses a separate 6.144-Mbps channel between each user and the hub.

Seamless Networking

The figure "Multimedia Network" shows a prototypical isochronous Ethernet-hub configuration. According to specification 802.9a, isochronous Ethernet hubs will support standard 10Base-T Ethernet, so only those workstations that require isochronous capabilities have to be upgraded with isochronous Ethernet NICs; workstations that need only access to text and numerical data can plug into the hubs using traditional NICs.

The figure also shows that the PSTN (Public Switched Telephone Network) can be connected directly to the hub (both the intelligent telephone and the remote video workstation shown in the figure connect to the hub through an ISDN connection). This allows remote users to participate in multimedia applications on the LAN. The hubs support switched digital services, such as ISDN (BRI or PRI) and T1. This also means that for the first time 10Base-T hubs will have mixed RJ-45 connectors: Some will be used for 10Base-T, and some will be used for ISDN. Because ISDN and the 10Base-T RJ-45 specification use different pins for each service, users should be relatively safe if they accidentally plug the wrong service into the wrong jack.

While important, WAN connectivity has its limitations. Don't assume that an ISDN connection to an isochronous Ethernet hub offers remote LAN access. This is not the case unless the hub supports delivery of both the standard Ethernet and isochronous channels. Isochronous Ethernet hubs will support P and C channel connections to their Ethernet ports. However, only some of those hubs will support P and C channel connections to remote users over the ISDN ports. Other, presumably less expensive, hubs will support only the C channel for remote connections. It is also a function of the remote device's ability to handle the P and C channels.

For wide-area isochronous Ethernet, networks will typically need independent ISDN or T1 connections among isochronous Ethernet hubs. This establishes a parallel path to handle the C channel, and the traditional bridge or router deals with the P channel.

Through special software included in the isochronous Ethernet hub or through an MCU (multipoint control unit), three or more users can participate in the video sessions. Because isochronous Ethernet sets up private channels among individual users, it operates closer to switched digital circuits; thus, it's more likely to be kept private than standard LAN video or audio that is multicast over the network.

As shown in Figure 1 below Isochronous Ethernet uses different channels for the different services it provides. It can also operate in different modes, from 100% compatible to 10Base-T to 100% multimedia traffic.

1.2 Overview of the Involved Standards

Isochronous Ethernet complies to a lot of standards from different organizations. This section is just a small overview of the involved standards. Figure 1 shows how all this works together.

Figure 1: An Isochronous Ethernet based H.322 MMDC application architecture.

2.1 Framing

The multiple channels are provided through the use of Time Division Multiplexing (TDM). The information stream is comprised of a continuous sequence of 125 sec (8 KHz) TDM frames. Each frame consists of up to 256 bytes of information. Within each frame, bytes are reserved to carry information for the different channels. As this is a point-to-point connection between an Isochronous Ethernet Hub and a computer, each frame, and therefore each channel, is guaranteed to have the required bandwidth.

Depending on the Mode of Operation, more or less of these channels are used. Every 125 sec a TDM frame like the one shown in Figure 2 is sent.

Figure 2: TDM Framing of Multiple Channels.
 

2.2 Modes of Operation

Each Isochronous Ethernet connection from a Hub to a computer can operate in one out of three modes of operation. These modes are configures automatically using the Auto-Negotiation link pulse signalling scheme that I describe in the next chapter. The different modes allow for interoperatibilty with a variety of end-stations and support for a mixed environment network. The three modes are defined as follows:

Table 2 illustrates the capabilities and bandwidth allocation for these tree modes of operation.

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