IBM CTI 2572 manual Ieee 802.4 Token Bus, Future Technologies

The primary disadvantage of Ethernet is that the response time is not consistent under widely varying loads. As the network loading increases above 50% , collisions occur more frequently and the network response time increases. In practice, most large busy networks use bridges to reduce the number of collisions. Bridges are discussed in the next chapter.

2.4 IEEE 802.4 (Token Bus)

Token-Bus is a LAN standard developed in conjunction with the General Motors Manufacturing Automation Protocol (MAP) effort. All Token Bus stations are connected a to a common wire, in this case coaxial cable. Like Token-Ring, a station can transmit only when it has received the token. The IEEE 802.4 Token-Bus standard supports both a broadband network and a carrierband network using RF (Radio Frequency) technology. The broadband network (envisioned as the factory "backbone") supports multiple channels which could contain different data types (e.g. analog video and digital data) and operates at 10 Mb. The single channel carrierband supports up to 32 devices at a 5 Mb data rate.

Token Bus was selected by the MAP committee because it provided a bus topology (which some consider to be a better plant floor topology) and it offered the consistent, deterministic response time inherent in token passing schemes. RF technology was selected for noise immunity and because it provided the opportunity for a single factory cabling system to carry both digital data and analog video.

The big disadvantage of Token Bus is cost and availability. Not only are the adapters very costly (up to 30 times the cost of Ethernet) but RF transmission is very sensitive to factors such as cable length, amplifier alignment and termination impedances - making it expensive to install and maintain.

2.5 Future Technologies

While both Token-Ring and Ethernet are excellent for transferring typical factory data, none have the capacity to transmit high bandwidth information such as full motion video. There are a number of emerging LAN standards which promise to offer this capability. These include FDDI (Fiber Distributed Data Interface), CDDI (Copper Distributed Data Interface), ATM (Asynchronous Transfer Method), "Switched" Ethernet, and "Fast" Ethernet. These will offer signaling rates in the range of 100 Mb.

At this point in time, it is difficult to predict which technology will prevail as the new high speed standard. Many experts speculate that future networks will incorporate both current (medium speed) and high speed LAN technology.

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CTI 2572 Technical Overview Manual