100VG-AnyLAN is Hewlett-Packard and IBM's answer to 100 Base-X (Fast Ethernet) technology. It is a 100 Mb/s deterministic network which uses 4-pair UTP cable. If it is used with Category 3 or 4 cable, then it can operate at a distance of up to 330 feet. With Category 5 cable, it is rated at a distance of 600 feet. It is compatible with both Ethernet and Token Ring frame types, which is a feature designed into it to allow easy migration from either type of network to the new 100VG-AnyLAN standard. The IEEE has approved a standard of 100VG-AnyLAN, and it is detailed in IEEE standard 802.12.
Since 100VG-AnyLAN was designed from the "ground up" to be a high speed technology, its developers were able to design into it some very nice features, without being saddled with older technologies (such as CSMA/CD) which work well at low speeds but are ineffecient at100 Mb/s.
The physical layout of 100VG-AnyLAN is a tree topology. Every 100VG-AnyLAN network has a central hub which can be connected either to other hubs or to workstations. Hubs may be cascaded up to three (3) deep. All 100VG-AnyLAN hubs have one special "Uplink" port which is used to connect it to any port of a higher level hub in the network. To illustrate this scheme, Figure One shows a diagram of the layout of a 100VG-AnyLAN network. Note that at any level, a hub can be connected either to workstations or one or more other hubs.
100VG-AnyLAN Network Topology
100VG-AnyLAN uses a new access scheme called DPA. DPA stands for "Demand Priority Access," and here's how it works. The hubs are intelligent devices and act as a "traffic cop" for devices wishing to transmit data to the network. Any device wishing to transmit first sends a request to its hub. All of the hubs in the network negotiate among themselves and schedule a time when they will allow the transmission to take place. When the network is available, the hub sends a "go" signal to the device wishing to transmit and the device sends its data.
This scheme may seem to be cumbersome and inefficient, but it is not. The user of the network never even knows that it is happening, and doesn't have to do any special setup to make it work. It all happens in a matter of microseconds, and since there is never a time where more than one device is transmitting data, collisions do not occur like they do in a CSMA/CD network. The end result is that the network's speed is used more efficiently, since bandwidth is not going to be taken up by collisions and retransmitions of data.
A good real-world example of DPA in action would be a formal meeting, such as a Congressional hearing. If everyone who had something to say spoke without any control from the Speaker, the meeting would be a noisy event where nothing got accomplished. Now, if someone wishes to speak, he first raises his hand and waits to be recognized. After being recognized, this person says his piece and then sits back down. No one speaks out of turn, and by the end of the day, all business is complete.
DPA also has a provision for High Priority traffic. If a device has an urgent message to send, it can request High Priority access from the hub. When the hub gets such a request, it will allow the "High Priority" traffic to be sent before any of the lower priority traffic.
Returning to our meeting example, let's say that the House is discussing a mundane topic (such as farm subsidies) and all of a sudden the Navy is attacked by aliens from Space. This is now a far more important topic, so the congressman who knows what just happened asks the Speaker for a few moments to address this new topic. After saying what happened, and answering any questions, the discussion can return to farm subsidies. A final feature of the DPA protocol which is worth noting is that after a low priority request has waited more than 300 microseconds (0.0003 second) it is automatically upgraded to a high priority request. That way, high priority traffic can't "hog" the available speed of the network.
One point worth noting is that the DPA protocol is transparent to the user of the network. Although it is completely different from Ethernet and Token Ring access protocols, the details are all taken care of in the software drivers that the customer loads on workstations and servers. These drivers ship with the NICs, and are standardized. The process the customer goes through to install them is exactly the same as if putting in Fast Ethernet, or any other networking scheme.
100VG-AnyLAN is a very good choice for networks where speed is a major concern. Nowadays, file servers and workstations built around high speed processors such as Intel's Pentium chip are creating large amounts of network traffic. Also, the current trend toward bringing full motion video to people's desktops for things such as videoconferencing creates a need for much more speed (bandwidth) than a standard 10 Mb/s Ethernet network can provide.
100VG-AnyLAN & Switched Ethernet
Another role where 100VG-AnyLan excels is as a backbone for switched Ethernet applications. In this application, multiple Ethernet switches, each serving several small Ethernet segments, are interconnected by a 100VG-AnyLAN network. This network topology is very useful in cases where individual users don't require a lot of bandwidth, but segmenting the network is needed to prevent a large number of users from causing congestion. Usually, the file servers for the network are also installed on the 100VG-AnyLAN backbone. See Figure Two for an example of how this is done. The nicest part about this type of network layout is that it is very easy to migrate the entire network to 100VG-AnyLAN by simply replacing the Ethernet switches with 100VG-AnyLAN hubs, and replacing any 10 Base-T NICs with 100VG-AnyLAN cards. This saves the customer the expense of rewiring the building - which in many cases can be far higher than the cost of the equipment used to make a 100VG-AnyLAN network.
This article looked at 100VG-AnyLAN technology, and have briefly touched on its DPA protocol. The distance limitations for the network were shown to be 300 Feet from hub to station or hub to hub over Category 3 or 4 cable. If Category 5 is used, the limits are 600 feet. We also looked at applications where it can be used to meet a specific network's requirements for high speed, including how ingegrate 100VG-AnyLAN with Ethernet swiching.
A very in depth discussion of 100VG-AnyLAN technology can be found at Richard's Unofficial 100VG AnyLAN FAQ site.