100VG AnyLAN designed to handle speed with safety

A traffic engineer, trying to relieve congestion on the streets of Hong Kong or any other major city in Asia, can usually apply several techniques to the problem. Trying to get the vehicles to speed up without widening the roads or improving traffic management is obviously not one of them as this would lead to an increase in collisions, resulting in even worse congestion.

The 'just speed up the cars' approach, however, is a perfect analogy of what happens to a network based on 100Base-T Fast Ethernet technology.

100VG AnyLAN, on the other hand, has been engineered to handle increased network speeds in safety. In any discussion of the two high-speed LAN technologies there are several other comparisons to be made, but the collision factor is the critical one.

To better understand the collision factor you need to look at the core technology used by the original Ethernet specification and 100Base-T. The Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol is a'blind driving' system under which a network node can send data packets down the wire without reference to what the other nodes are up to.

At 10 megabits per second (Mbps) this simple system works reasonably well. The time taken to detect, clear up and re-send after a packet is not too much of a problem because network traffic moves relatively slowly.

At 100 Mbps, the window of opportunity for doing this is one-tenth the size, and, as a result, the network has to be physically smaller.

In practice what happens with 100Base-T is that as more nodes are added to a network segment the number of collisions rises rapidly, effectively limiting the throughput of 100Base-T. This phenomenon is not hard to demonstrate.

Foreseeing this problem, the designers of 100VG AnyLAN decided to institute a more intelligent traffic management system, the Demand Priority Protocol (DPP). This is a round-robin system in which every node gets a turn at sending data packets on to the network. Collisions are avoided because a clear road is guaranteed.

Another difference between the two technologies is the width of the road, or the physical capacity of the cabling. 10Base-T uses two pairs of wires to send and receive data running at a clock rate of 16MHz; 100Base-TX also uses two pairs but clocks the system at 125Mhz, which is actually 25 per cent more than the Electronics Industry Association's recommended upper limit for Category 5 unshielded twisted pair cabling.

100VG AnyLAN, on the other hand, sticks with the 16MHz clock rate and uses four pairs of wires for transmission.

Advocates of 100Base-T will, of course, tell you that its is the true successor to Ethernet because it retains CSMA/CD and was promulgated, as a standard, by IEEE 802.3, the original Ethernet standards committee. It is true that 100VG AnyLAN makes a break from Ethernet's 20 year-old technology, which is why IEEE formed a new standards committee, 802.12, to deal with it.

What users care about, however, is whether their applications will work and here the critical factor is the data format. 100VG AnyLAN supports all Ethernet frame types and supports Token Ring frames as well.

The Fast Ethernet Alliance claims that, as it includes a large number of vendors, 100Base-T is the de facto market standard for 100 Mbps Ethernet. Far from being a proprietary technology, 100VG AnyLAN is supported by over 30 network system vendors, including such industry luminaries as Cabletron, Cisco, Compaq, IBM, Lucent Technologies, Newbridge Networks and UB Networks.

Market research also indicates that 100VG AnyLAN has wide acceptanace as as a high-speed link to the desktop.

In its 1995 Worldwide PC Network Interface Card Vendor Market Share report IDC found that although three times as many 100Base-T cards were shipped last year, 100VG AnyLAN user were most likely to be running at 100Mbps, while many of the 100Base-T users were using their cards in 10Mbps mode.

The Fast Ethernet Alliance also claims 100Base-T has a performance edge because it doesn't have Dpp's overheads.

This kind of benchmark is analogous to a drag racing strip and under such circumstances 100Base-T is the faster means of transmitting data between two points - because there is no other network traffic to collide with.