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Iso-Ethernet (Isochronous Ethernet Integrated Services)

Ratified by the IEEE in the Fall of 1995 and providing 16.144 Mbps, Iso-Ethernet adds speed to traditional 10Base-T Ethernet. It is intended to support isochronous (stream-oriented) traffic such as realtime voice and video conferencing through an Ethernet switching hub. The 809-2a standard provides 10 Mbps for data traffic and sets aside an additional 6.144 Mbps for voice and video through 96 64-Kbps ISDN B channels and 1 ISDN D channel at 64 Kbps. Multiple B channels can be linked, which most typically would be required in support of videoconferencing. In fact, the entire network can function in isochronous mode at 16.144 Mbps, providing 248 B channels at 64 Kbps. Although the products currently available limit connectivity to a single hub, Iso-Ethernet should be available on a multisegment basis in the future. LAN-WAN connectivity over ISDN links is facilitated by the adherence to ISDN standards. There are plans to develop a version for 100Base-T. Manufacturers of Iso-Ethernet hubs include Nexion Inc, Ericsson Business Networks AB, Incite, and Luxcom Inc. Costs vary widely, but are in the range of $200 to $300 per NIC and $400 to $1,700 per port for the hub [9-32], [9-33], and [9-34].

100VG-AnyLAN

100VG-AnyLan (VG=Voice Grade) is a joint development of AT&T Microelectronics, Hewlett-Packard, and IBM and is standardized by the newly created IEEE 802.12 committee. 100VG-AnyLAN supports Ethernet, Token Ring, and other LAN standards, incorporating a collisionless polling technique. It’s not quite as simple as it appears—a router upgrade is required to connect 100VG Ethernet and 100VG Token Ring [9-35].

Distances are 100 meters for Cat 3 UTP (4 pairs), 150 meters for Cat 5 UTP (2 pairs) and Type 1 STP (2 pairs), and 2000 meters for fiber. The specifications suggest that higher speeds are possible [9-35]. All pairs are used for transmission in half-duplex mode at 25 MHz. Priority access is provided through the DPMA (Demand Priority Media Access) technique. 100VG-AnyLAN can be deployed in a scaleable star topology [9-28]. As 100VG-AnyLAN has the ability to support priority access and collisionless transmission, some view it as superior to 100Base-T for multimedia LAN communications. However, the huge embedded base of Ethernet, as well as manufacturer support for it, makes 100Base-T a natural choice for most organizations [9-29].

Not surprisingly, an exception to this judgement deals with linking non-Ethernet LANs. 100VG-AnyLAN does a much better of linking Token Ring LANs, for instance. While Ethernet sends data in frames of 1,500 bytes, maximum, Token Ring can accommodate data fields up to 16,000 bytes. 100VG-AnyLAN handles this well; 100Base-T doesn’t [9-36] and [9-28].

FDDI (Fiber Distributed Data Interface)

FDDI is the standard (ANSI X3T9-5; IEEE 802.2) for a fiber optic, token-passing ring LAN. Bandwidth is at 100 Mbps, although several manufacturers offer 200 Mbps, full duplex interfaces. The excellent performance characteristics of fiber optics, in general, apply well to the LAN world. Error performance is in the range of 10-14; devices can be separated by as much as 1.2 miles (2 kilometers) over multimode fiber and 37.2 miles (62 kilometers) over single mode fiber [9-37] and [9-38]. The maximum frame size is 4500B, which accommodates the native frame sizes of all standard LAN networks [9-39].

FDDI is largely relegated to a backbone application due to the high cost of termination. In other words, the opto-electric conversion process is costly. A direct fiber interface to a PC workstation can be accomplished at an average cost of around $600, although this cost should drop over time. The advantages of FDDI, however, can be extended to the workstation through a concentrator which accomplishes the opto-electric conversion process for multiple attached devices. The connection from the concentrator to the workstations is accomplished via UTP over distances of 100 meters or less, based on a standard known variously as CDDI (Cable Distributed Data Interface) and TPDDI (Twisted-Pair Distributed Data Interface).

The fragility of the fiber is a deterrent to the application of FDDI, as well. The FDDI specifications provide for a dual counter-rotating ring, which provides a measure of redundancy. Should the primary ring fail, a Dual Attached Station (DAS) or Dual Attached Concentrator (DAC) can still communicate with any other device by transmitting in the opposite direction through the secondary ring, which typically is collocated in the same cable sheath as the primary ring (see Figure 9.12). Should there be more than one physical failure in the cable plant, however, the ring will segment and the network will fail. There are dual-homing solutions to this dilemma, although they involve considerable additional expense, with the designated stations being connected via fiber to multiple servers in order to provide redundancy [9-40].


Figure 9.12  FDDI dual counter-rotating ring.

FDDI is assured a place as a LAN backbone technology, at least until such time as ATM becomes a major force. However, its future as a desktop technology is questionable due to the high cost of FDDI NICs, which average in the range of $600 and can be as much as $2,000; CDDI cuts that cost by about 50% [9-38]. The cost of FDDI router modules is in the range of $10,000 to $30,000 [9-41]; FDDI switches are in the range of $7,000 to $18,000 per port [9-42]. 100Base-T and VG-AnyLAN offer similar performance at much lower cost.

ATM (Asynchronous Transfer Mode)

Discussed in detail in Chapter 11, ATM promises to change the face of LAN networking. ATM is based on cell switching technology which accommodates any form of information. The data is segmented into a cell of 53 octets, 48 of which are data payload and 5 of which are control overhead. Voice and video are supported as effectively as data. Isochronous data is transported and switched as easily as is asynchronous or synchronous data. Additionally, and as ATM is viewed as the future of Wide Area Networking, ATM in the world of the LAN provides a smooth path to LAN-to-LAN internetworking over the WAN. The LAN standard for ATM was originally specified a rate of 155 Mbps, although a 25-Mbps standard was later added.

At the time of this writing, ATM has yet to make a significant impact on the LAN world. ATM is not backward-compatible, as it requires a complete and expensive upgrade of equipment in the form of ATM hubs and NICs. Additionally, there are issues of hardware interoperability among the various manufacturers. Despite these drawbacks, ATM is being implemented on a limited basis in organizations with the funds and courage to step into the future [9-43].


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