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Deterministic Access

Deterministic access is a media access control convention that allows both the centralized master station and each slaved station to determine the maximum length of time which will pass before access is provided the network. In other words, each station can be guaranteed the right to communicate within a certain time frame. Additionally, access priorities can be assigned by the system administrator. Deterministic access is also known as noncontentious, because the devices do not contend for access; rather, access is controlled on a centralized basis.

Deterministic access employs token passing. The token, which consists of a specific bit pattern, indicates the status of the network—available or unavailable. The token is generated by a centralized master control station, as described above, and transmitted across the network. The station in possession of the token is in control of access to the network. It may transmit or may require other stations to respond. Transmission is in the form of a data packet of a pre-determined maximum size, determined by the number of nodes on the ring and the traffic to be supported; oversized transmissions are segmented. After transmitting, the station will pass the token to a successor station in a predetermined sequence. While the process is complex and overhead-intensive, it yields careful control over the network.

Deterministic access is especially effective in high-traffic environments where a lack of control would cause chaos in the form of frequent data collisions. It also finds application in environments such as process control, where it is critical that each station be able to access the network on a periodic basis. For example, oil refineries employ deterministic media access control through proprietary token-passing LANs. The refining process requires that the nature of the specifics of the crude oil (raw material) be considered in terms of a number of characteristics, including sulfur content, paraffin content, and viscosity. With these factors in mind, the refining process is tailored to act on the raw material in such a way as to ensure that the end product (e.g, 97 octane unleaded gasoline) is consistent from one batch to another. In a hypothetical scenario, the master control station would address tokens to individual devices in the form of various sensors. The individual sensors monitor pressure and temperature conditions as well as the rate of flow of various chemicals through valves, throughout the entire process. In order to control the process effectively, each sensor must be addressed on a regular basis so that it can transmit data to the master controller. Additionally, some devices at certain times during the process may require more frequent, priority access. In consideration of that data, the master controller will command other devices to increase or decrease temperature, open or close valves, and so on. Such a network also is highly redundant in order that a network or device failure will not compromise the integrity of the process.

An extension of this concept is being applied in field trials by CATV (Community Antenna TV) providers in a convergence scenario. Internet access is provided to large numbers of residences, businesses, and schools over a two-way coaxial cable system terminating in cable modems at each premise. As much as 30 Mbps is reserved for such applications, with as much as 10 Mbps available to an individual user. In order to manage contention over such a network, which extends over fairly significant distances, a token-passing media access control technique is employed, with the master station located at the CATV provider’s head-end.

General characteristics of token-based networks include a high level of access control, which is centralized. Access delay is measured and assured, with priority access being supported. Throughput is very close to raw bandwidth, as data collisions are avoided; throughput also improves under load, although absolute overhead is higher than is the case with non-deterministic access techniques. Deterministic access standards include Token-Passing Ring, IBM Token Ring, and Token-Passing Bus.

Token-based LAN technologies are somewhat overhead-intensive, due to the token passing and management processes. However, they can more than compensate for that fact by virtue of the avoidance of data collisions. Token Ring, for instance comes in 4, 16, and 20 Mbps. In each case bandwidth utilization is virtually 100% [9-11].


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