100VG-AnyLAN

Now considered somewhat obscure, 100VG-AnyLAN was conceptualized and introduced in the early 1990's as a 100 Megabit-per-second networking technology for use in Ethernet and Token Ring computer network topologies. Initially introduced by Hewlett Packard, 100VG-AnyLAN (the "VG" stands for "Voice Grade", referring to the cable quality) eventually got reviewed, refined, and ratified by the IEEE 802.12 committee as a part of project 802. Much of the aim was to design a flexible 100 Mbps networking standard as an alternative to 100BaseT/[802.3 that would provide greater reliability under high network utilization.

The 802.12 specification approved by the IEEE 802.12 committee includes:

1. Provide a minimum data rate of 100 Mb/s,
2. Provide smooth migration from ISO/IEC 8802-3 (Ethernet) and ISO/IEC 8802-5 (Token Ring) LANs,
3. Support either ISO/IEC 8802-3 (Ethernet) or ISO/IEC 8802-5 (Token Ring) frame formats and MAC service interface to the LLC,
4. Support a cascaded star topology over twisted pair Category 3, 4, and 5 and fiber optic wiring,
5. Allow topologies of 2.5 km and greater with 3 levels of cascading,
6. Provide a physical layer bit error rate of less than 10-8,
7. Provide fair access and bounded latency,
8. Provide two priority levels, normal and high priority,
9. Provide a low latency service through high priority for support of multimedia applications over extended networks,
10. Support an option for filtering individually addressed packets at the repeater to enhance privacy,
11. Support network management to monitor network performance, isolate faults, and control network configuration,
12. Enable low cost implementation and high levels of integration, and
13. Provide for robust operation by testing the Physical Layer connection before allowing an end node to enter the network and by removing disruptive nodes.

From my understanding, the primary difference between 100BaseT and VG is the media access control that is in place. CSMA/CD (Collision Sense Multiple Access/Collision Detection) is the method used by 802.3, whereas 100VG-AnyLAN uses Demand Priority, a tokenless, collisionless, centralized media access control method which relies on the hubs to determine when each node may transmit. The individual client stations/nodes send a transmit request (which can be tagged as normal or high priority to the hub. The hub cycles through each port in order examining only ports with transmit requests. When the hub determines the node may transmit, it notifies the node and allows it to transmit a packet.

The link setup training, which may occur both at initial connection/power-up or at other determined intervals, includes transmission of device type, frame type, and normal/promiscuous mode setting. Before a link is finished with setup, at least 24 consecutive packets are transmitted successfully by the hub, or the link isn't activated. This provides reasonable assurance of Layer 1 quality.

Max. Cable Lengths:

| Cat 3/4- 100 meters
| Cat 5- 200 meters
| STP- 100 meters
| Fiber (800nm)- 500 meters
| Fiber (1300nm)- 2 kilometers

Maximum copper distance between any two nodes: 2 kilometers (if combining with fiber, the maximum total cable distance is 2.5 kilometers, due to latency issues).