100VG AnyLAN

802.12 - Demand Priority Access

This is not Ethernet !!  It is included here, because it is so often confused with Ethernet, and also because it can co-exist with Ethernet.

100VG AnyLAN is rather rare, and you probably will never see it.  It was originally developed by Hewlett Packard along with others to be the standard for Fast Ethernet !!  But IEEE had other ideas, and they were running the show.  They gave the Fast Ethernet title to 100BASE T, and relegated 100VG AnyLAN to an entirely separate standard  .  .  .  802.12.

Again, it is not Ethernet.  But it is compatible with either 802.3 Ethernet or 802.5 Token Ring (depending on the manufacturer).

Advantage - Demand priority access method supports varied priorities for different types of signals. For example, real-time voice and video could be assigned a higher priority than other types of data.

100VG AnyLAN relies on the hub to control network access. The hub uses a form of Token Passing, a method where each station receives and passes the right to use the single shared channel on the LAN. All stations are guaranteed access to the network at regular consistent intervals. This provides support for time sensitive applications like voice, video and multimedia.

Demand Priority Access

While Ethernet uses a CSMA/CD access scheme, 100VG AnyLAN uses a Demand Priority access scheme.  Demand Priority allows one or more stations to send high priority transmissions first. If the hub receives a high priority request, it completes its current transmission and then immediately grants access to the high priority request. When all high priority requests are dealt with, normal operation resumes. Preventing time outs is simple: the hub grants temporary high priority status to normal priority requests. This creates a collision free network and is designed for higher throughput. However, 100 BaseT may be a better choice, depending on your needs.

Cabling

100VG AnyLAN can use several types of cable, depending on your needs. 4 pair Category 3 or 4 UTP can be used for a maximum segment length of 100 meters.  4 pair Category 5 can be used for a max segment length of 200 meters. Type 1 STP can be used but you must also use a balun to match impedance. Max Segment length is 100 meters. 62.5/125 micron duplex multimode fiber optic cable can be used for a maximum segment length of 2km. This allows most existing cabling to be used with the need for rewiring virtually eliminated.