Introduction to 100VG-AnyLAN and the IEEE 802.12 local area network standard

Hewlett-Packard Journal, August, 1995 by Alan R. Albrecht, Patricia A. Thaler

100VG-AnyLAN is a new, high-speed addition to HP's AdvanceStack local area network (LAN) product group. It is an economically effective upgrade path for congested 10-Mbit/s 10Base-T Ethernet and 4/16-Mbit/s token ring networks. It provides a 100-Mbit/s data rate with guaranteed bandwidth and bounded access delay for time-critical applications, using existing building wiring. This provides high performance for traditional data transfer applications. It also provides emerging multimedia applications, such as interactive video, with the low delays they require. It delivers this performance over the most common networking medium, 4-pair unshielded twisted-pair (UTP) telephone wire.

100VG-AnyLAN uses the demand priority protocol, which was developed as a joint effort by Hewlett-Packard Laboratories in Bristol, England and the HP Roseville Networks Division in California. Now supported by over 30 companies ranging from integrated circuit vendors to systems suppliers, demand priority is well on its way to becoming the IEEE 802.12 standard.

The 100VG-AnyLAN articles in this issue look at the development of the demand priority protocol and the 100VG-AnyLAN product set.

Local Area Network Technology

Before the initial development of local area networking in the late 1970s, the telephone system was the only generally available data communications option. Bandwidth (3 kHz on a voice-grade line) was clearly a problem and a number of different types of new data networks were proposed. Two, Ethernet(1) and token ring,(2) have emerged to dominate the local area networking market.

Ethernet (IEEE 802.3). Ethernet was developed in the late 1970s as a 10-Mbit/s answer to the limitations of the telephone system. It became an IEEE standard in 1985. All nodes were connected to a single central coax bus, which proved to be somewhat inflexible as users change locations or are added to the network.

The Ethernet access policy is CSMA/CD, which stands for carrier sense, multiple access with collision detection. It allows any node to transmit a packet (with up to 1500 bytes of data) anytime it detects silence (no signal) on the network. This can lead to packet collisions if two or more nodes need to transmit and detect silence at the same time. Each involved node is required to back off (cease transmitting) immediately after a collision is detected, but time is consumed and the available bandwidth is effectively reduced during high-traffic periods.

The protocol also requires each node to monitor the network traffic and to decode (filter) the destination address of each packet to determine whether it should be received by the node. Packets with the node's individual or group address are copied into memory and packets with nonmatching addresses are ignored.

The 10Base-T star topology was proposed by Hewlett-Packard in 1987 and became part of the standard in 1990. The center of the star is a network concentrator (hub) which is typically located in a wiring closet. Each node is connected to the hub by voice-grade twisted-pair cable. 10Base-T retains the basic features and access policy of the bus network and also adds a level of fault tolerance. Link faults at individual nodes are isolated by the hub and do not take down the entire network. 10Base-T has become the most common IEEE 802.3 network configuration.

Token Ring (IEEE 802.5). Token ring was proposed as a 4/16-Mbit/s solution to the Ethernet collision problem and became an IEEE standard in late 1984. The original network structure is a ring around which both tokens and information packets (up to 4500 data bytes) are passed. The network medium is IBM type 1 shielded twisted-pair (STP) cable. Token ring networks are also now commonly installed in star configurations.

The token ring access policy is designed to be both collision-free and priority-based. It prevents any node that does not currently "own" the token from transmitting a data packet, and it provides eight priority levels to allow some classes of data to take precedence over other classes.

All data packets and tokens contain an access control field that allows the successive nodes on the network both to reserve the token and to indicate their reservation priority level. The node that currently owns the token transmits its data packet with the reservation bits in the access control field set to minimum priority. Each successive node forwards the packet as it is being received. It also interrogates the destination address field to determine whether it should copy the data frame and the access control field to determine the current reservation level. If the node needs to send a data packet and the reserved priority level is less than the node's level, the node indicates its need by changing the value of the reservation bits in the forwarded packet.

The sending node removes the packet from the network and transmits a new token with the priority bits of the access control field set to the priority level indicated in the returned packet. The token then circulates to the node that first reserved that priority. That node removes the token and transmits a data packet. The token circulates continuously at minimum priority in an idle network.