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Self-Definition of New Terminals to Controllers on Multi-Drop Lines

IP.com Disclosure Number: IPCOM000044465D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 3 page(s) / 26K

Publishing Venue

IBM

Related People

Baratz, AE: AUTHOR [+4]

Abstract

A mechanism is provided whereby a new terminal may be non-disruptively added to a multi-drop line and incorporated into the line controllers definitions. Due to the requirement to dynamically add resources in computer communication networks, it is important to have mechanisms that define terminals to adjacent nodes. This is made difficult because of the fact that terminals are connected on multi-drop lines that are controlled by terminal controllers. There is a "chicken and egg" problem whereby the terminals are not polled if the controllers do not know they are there, but the controller does not know they are there unless they define themselves to the controller. Today, the cumbersome technique of static-definition of terminals is generally used. This will be inadequate in future, more dynamic environments.

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Self-Definition of New Terminals to Controllers on Multi-Drop Lines

A mechanism is provided whereby a new terminal may be non-disruptively added to a multi-drop line and incorporated into the line controllers definitions. Due to the requirement to dynamically add resources in computer communication networks, it is important to have mechanisms that define terminals to adjacent nodes. This is made difficult because of the fact that terminals are connected on multi-drop lines that are controlled by terminal controllers. There is a "chicken and egg" problem whereby the terminals are not polled if the controllers do not know they are there, but the controller does not know they are there unless they define themselves to the controller. Today, the cumbersome technique of static- definition of terminals is generally used. This will be inadequate in future, more dynamic environments. This problem is addressed below. The fundamental SDLC (synchronous data link control) enhancement required by this mechanism is the introduction of a new generic "all new stations" poll (ANS_POLL) that elicits responses from any newly added terminal which has not already been recognized by the primary (i.e., line controller). This new poll may be either a currently unused but already defined SDLC frame (such as an unnumbered poll with the No-station address) or a newly defined SDLC frame.

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At periodic intervals the primary issues this generic ANS_POLL. Any terminal that has newly attached itself to the link will respond with an unnumbered XID message that identifies it. The figure gives a typical example of the flow of messages necessary for resource identification. The assignment of a link address will permit subsequent communication to take place. This technique works if no more than one new terminal appears in the interval between two ANS_POLL's. However, if more than one new terminal appears, they will both send responses to the ANS_POLL. The two messages will "collide", and what the primary receives will be garbled, the frame check sequence will indicate an error, and both messages will be discarded. A mechanism is required to resolve these collisions. As the new terminals are independent entities which cannot communicate directly with one another, the collision resolution mechanism cannot rely on any kind of coordination between the terminals. However, using some form of randomness in the ANS_POLL response is a mechanism whereby terminals can resolve collision. Such a random mechanism is set forth below. In this mechanism, a new terminal does not always respond upon receipt of an ANS_POLL. Instead, it responds to only a fraction of the ANS_POLL's that it receives. Whether or not to answer a particular ANS_POLL is determined by the output of a random number generator in the terminal. Based upon the resulting random number, with some proba- bility p, the terminal will decide to answer, and with probability 1-p it will decide to remain silen...