Design Feature: August 3, 1995

One of the major goals of the computer industry is finding a way to bring multimedia collaboration capabilities to the desktop. Wide-area networks (WANs), including publicly switched services, are standardizing on integrated-services digital-network (ISDN) B and D channels for deployment of interactive multimedia communication. Thus far, such implementations have been confined to specialized video-conference rooms with direct connections to the WAN. Even though such video-conference rooms have been a driving force in establishing standards for transmission and compression of voice and video, they have lacked the flexibility and accessibility needed to empower widespread, peer-to-peer, collaborative computing.

The key obstacle to bringing such full-featured interactive multimedia capabilities to the desktop has been finding an effective way to "bridge the last 100 meters." Although most corporations can connect to the outside world via ISDN-based WANs, the majority of desktop computers in large organizations are connected by LANs, with Ethernet accounting for over 70% of today's installed base of LANs. The basic problem has been that this existing Ethernet infrastructure has not had the bandwidth or the real-time capability required to effectively support symmetrical full-duplex multimedia connections between desktops.

Ethernet's "connectionless" carrier-sense-multiple-access/collision-detection (CSMA/CD) method of controlling data-packet traffic is inherently incompatible with setting up the kind of dedicated connections needed with the public switched network. CSMA/CD is based on a concept that allows all nodes constant access to the network's available bandwidth, thereby ensuring a high level of interleaving between data packets and a resultant maximized use of available bandwidth. However, the reverse is also true. Because no connection between individual nodes can continuously "own" a portion of the bandwidth, all communications are subject to response-time degradation as overall network traffic increases.

The isoEthernet standard, recently adopted as IEEE-802.9a by the IEEE LAN/MAN (metropolitan-area-network) Standards Committee, can break through the WAN/LAN barrier. It brings ISDN channels directly to individual desktops over existing Ethernet type 3 unshielded-twisted-pair (UTP) networks, with no adverse impact on the Ethernet data-packet traffic being carried over those same physical networks.

IEEE-802.9a isoEthernet uses the existing physical twisted-pair infrastructure of 10BaseT Ethernet but recodes the data to allow 16 Mbps of data to be transmitted instead of existing Ethernet's 10 Mbps. The additional 6 Mbits are used to switch a dedicated 96 ISDN B channels plus one D channel between the hub and the desktop. By combining Ethernet and ISDN onto a single wire without changing their behavior, isoEthernet gives existing LAN users seamless integration with WAN B channel-based multimedia services, while preserving all of the Ethernet capabilities.

Because isoEthernet preserves the existing LAN structure and integrates data from existing WAN services, the major investment required to initially implement isoEthernet is the installation of new hubs with no changes required to mission-critical data routers. However, because any standard 10BaseT Ethernet controller can continue to reside on the isoEthernet network, you can upgrade individual desktops on an incremental basis as the need for interactive multimedia communications expands.

Furthermore, the isoEthernet isochronous B channels are ISDN-compatible and, therefore, allow direct connection to the WAN. This close adherence to established standards lets organizations implement isoEthernet in an evolutionary fashion that preserves and extends their existing investment while greatly expanding the capabilities of the upgraded desktop machines.

The isoEthernet architecture

Following the channel definitions contained in the IEEE-802.9 standard, isoEthernet allows for simultaneous integration of isochronous and packet-based communications by assigning the 10BaseT packet data traffic to a P channel; grouping the 96 ISDN B channels into a C channel; and using a separate 64-kbps D channel for establishment, teardown, and maintenance of connections. In addition, a single 96-kbps M channel is reserved for conveying control and status information to the remote end of the link.

Above these four channels, the existing transport, session, application, and presentation layers remain unchanged from today's standard operation. IEEE-802.9a specifies the isoEthernet physical layer, which resides beneath the C, P, D, and M channels and defines their connection with the physical transport medium (Fig 1).

A time-division-multiplexed (TDM) information stream provisions these multiple channels on a pair of UTP wires. The information stream consists of a continuous sequence of 125-µsec (8-kHz) TDM frames, with each frame consisting of 256 bytes of useful information. Within each frame, specific bytes are reserved to carry information corresponding to each of the multiple channels, so that every frame is guaranteed to have the bandwidth required by each of the service channels. In addition to the service-channel information, 1 byte at the start of each frame is reserved as a start-of-frame delimiter to allow the PLLs at the remote end to synchronize the incoming frame with the 8-kHz network clock.

To allow isoEthernet to be deployable in a variety of environments, the architecture makes provisions for three different operating modes:

• Multiservice mode: combines a 10-Mbps P channel, a 6.144-Mbps C channel (consisting of 96 ISDN B channels), a 64-kbps D channel, a 96-kbps M channel, and a 64-kbps start-of-frame delimiter

• All-isochronous mode: dedicates the whole service to isochronous communications with a 15.872-Mbps C channel (consisting of 248 ISDN B channels), a 64-kbps D channel, a 96-kbps M channel, and a 64-kbps start-of-frame delimiter

• 10BaseT mode: In this mode, the isoEthernet physical layer behaves just like a 10BaseT-encoded UTP transceiver. No TDM structure is employed, and no provision for C, D, M, or start-of-frame channels is provided.

In the multiservice and all-isochronous modes of operation, the isoEthernet physical layer multiplexes the service channels into a single stream, maps the stream into a TDM frame, and performs 4B/5B symbol encoding and the line coding for serialization and NRZI (nonreturn to zero inverted). In the receive direction, it performs NRZI line decoding, 4B/5B symbol decoding, clock extraction, and the demultiplexing of service channels.

IEEE-802.9a isoEthernet codes individual octets of service-channel information using 4B/5B symbol encoding because of its inherent efficiency and ease of implementation. The encoding scheme transforms a nibble of information into a coded symbol (5-bit value) for transmission. The symbols have been specifically chosen to maintain the ac balance of the wiring and to minimize the frequency spectrum of the waveforms as they are transmitted over the wire. During normal data transmission, the dc component of the signal varies less than 10% from the nominal center. The 4B/5B, along with the NRZI line coding, provides an 80% bandwidth utilization as compared to the Manchester data encoding method used in 10BaseT, which yields only a 50% bandwidth utilization (Fig 2).

This encoding scheme is the key mechanism that lets isoEthernet gain the additional 6.384 Mbps of usable bandwidth for carrying isochronous communications. Even though both 10BaseT and isoEthernet utilize a line-transmission frequency of approximately 20 MHz, isoEthernet can achieve 16.384 Mbps of usable bandwidth vs the 10 Mbps of usable bandwidth for 10BaseT.

In a manner similar to ISDN, the isoEthernet C channel can be provisioned into any multiple of 64-kbps (B-channel) bandwidth segments up to 96, depending on your application. Signaling procedures that are transported over the D channel provision these channels. These signaling procedures, as defined in the IEEE-802.9a specification, are based on the original International Telecommunication Union (ITU) Q.931 family of protocols that have already been employed in ISDN networks. This allows for easier interoperability between ISDN- and isoEthernet-based networks.

Because of IEEE-802.9a isoEthernet's compatibility with both ISDN and asynchronous-transfer-mode (ATM), organizations can easily deploy isoEthernet as a last-100-meters LAN-based link to the desktop, which can be interconnected directly to ATM backbones, ISDN WAN services, or a combination of the two. This strict adherence to standards and interoperability allows the most cost-effective evolutionary upgrading of networks as an organization's multimedia and other communications requirements grow and change. Furthermore, because both ATM and isoEthernet are ISDN-compatible, LAN/WAN/LAN connections between remotely located desktops can be easily established via isochronous links, regardless of the intervening isoEthernet, ATM, or WAN transport mechanisms.

In addition, isoEthernet's support for Microsoft's Telephony Application Programmers Interface (TAPI) allows easy porting of user applications between isoEthernet- and ISDN-connected end stations and enables such TAPI applications to interoperate. This means that designers and developers of user applications need not acquire expertise in the underlying transport technologies to create solutions that can be deployed across the different networks.

Industry support

Successful industrywide implementation of a standard such as the IEEE-802.9a isoEthernet, which involves so many technologies, requires the resources of many types of businesses. Industry participants have formed the isochronous networking communications incAlliance; members include Apple Computer, Ascom Nexion, Dialogic, Ericsson Business Networks, IBM, ITT, Incite, Luxcom, MCI, National Semiconductor, Pacific Bell, Quicknet Technologies, Siemens, Telios, TRI Inc, VCON, and Zydacron.

The enabling technologies have matured and now allow widespread deployment of interactive desktop multimedia communications applications. Key requirements for successful adoption will be the ability to provide dedicated bandwidth isochronous connections that are configurable and allocable according to ITU, IEEE, and ISDN standards. This connection capability must be able to upgrade existing LAN installations and to coexist without disrupting existing LAN data traffic.

IsoEthernet employs 4B/5B symbol encoding; also shown are the data-and control-code symbols that are employed in this technology.

Richard Brand is strategic marketing manager for National Semiconductor Corp (Santa Clara, CA), where he has been employed for 17 years. He has a bachelor's degree from Stanford University, Stanford, CA, and is a member of the IEEE 802 LAN/MAN Committee and president of the Multimedia Communications Forum.