Browse Prior Art Database

Establishing the SDLC Primary/Secondary Relationship on Switched Communication Links

IP.com Disclosure Number: IPCOM000046143D
Original Publication Date: 1983-Jun-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Bergman, WC: AUTHOR [+6]

Abstract

The synchronous data link control (SDLC) protocol is widely used in data communication. When using SDLC protocol in normal response mode on switched links, a primary/secondary relationship must be established between the two terminals. In the typical host-to-terminal SDLC connection, the host is customarily designated the primary, and the terminal is secondary. However, advanced function terminals are now able to talk to other non-host terminals. In this peer-to-peer environment, a determination of the primary/secondary relationship must be established for the two terminals. In order to prevent endless contention as to which is the secondary and which is the primary, the hardware of the present expedient utilizes a random number generator. This permits a quick determination of primary and secondary relationships.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 52% of the total text.

Page 1 of 2

Establishing the SDLC Primary/Secondary Relationship on Switched Communication Links

The synchronous data link control (SDLC) protocol is widely used in data communication. When using SDLC protocol in normal response mode on switched links, a primary/secondary relationship must be established between the two terminals. In the typical host-to-terminal SDLC connection, the host is customarily designated the primary, and the terminal is secondary. However, advanced function terminals are now able to talk to other non-host terminals. In this peer-to-peer environment, a determination of the primary/secondary relationship must be established for the two terminals. In order to prevent endless contention as to which is the secondary and which is the primary, the hardware of the present expedient utilizes a random number generator. This permits a quick determination of primary and secondary relationships.

On switched links, the problem of establishing the primary/secondary relationship is based on two requirements. The first is to avoid simultaneous transmission by the two stations. This situation occurs easily if both stations assume they are primary and are primed to transmit a frame when Data Set Ready (DSR) turns on. In an auto-answer situation DSR may turn on at both ends simultaneously, resulting in a collision of the frame such that neither terminal receives the other's frame. In a manual answer situation, the same collision can occur if the operators go to data mode at the same time. The requirement is to get the two stations' transmissions synchronized so that each can properly receive the other's transmissions. Achieving synchronization does not, in itself, guarantee that there is only one primary station. Thus, the second requirement is an exchange of command and response frames indicating one station is primary and the other is secondary.

The solution to establishing the SDLC primary/secondary relationship between two stations on a switched link involves a negotiation procedure that has two parts. The first part resolves the possible collisions in transmitting the first frame on the link when both terminals attempt to be primary. The second part determines which station is to become the primary link station and, consequently, which is to become the secondary. The solution accommodates stations that insist on being primary (e.g., some hosts) and stations that are secondary only (e.g., s...