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This page last updated 4 August, 1999.
This article provides a basic explanation of the TBOS protocol and defines its specialized terms. It has been reformatted for on-line reading. For a printable document, click here to download the PDF version.
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E-telemetry systems were developed by the pre-divestiture AT&T organization as a method to monitor and control diverse network elements from a remote, centralized location. Status and command remote (SAC) units were employed in the central office to convert discrete (contact closure) alarms from monitored equipment into E-telemetry for efficient transfer of data to and from the operations center. With the advent of microprocessor technology the alarm processing remote (APR, alarm remote or simply Remote) was developed, allowing discrete alarm links with network elements to be supplanted by a more efficient serial link. TBOS (Telemetry Byte Oriented Serial) is the communications protocol defined for this link.
TBOS points, displays and ports . . . . . . . . . . . .
TBOS provides for three types of information: alarm points, status points, and remote control points. Alarm and status points are collected together, while command points stand alone. All points (individual binary bits) are arranged in a strict hierarchy of characters, displays, and ports as shown in Figure 1. Eight points are grouped to form a character, eight characters are grouped into a 64-point display, and eight displays are grouped into a 512-point port. (Some older E2A equipment may support only two displays per port). A port corresponds to one physical interface on a Remote. The AT&T DAS unit is an example of an early APR.
A number of synonyms for these basic TBOS terms have come into common usage. Points are variously called bits, alarms, or scan points. Characters may also be referred to as bytes, rows, or lines. Displays are also known as addresses or E2A pages. Occasionally, port may refer to the TBOS interface on the monitored equipment, not the APR.
Physical interface . . . . . . . . . . . .
The serial data interface between terminal equipment and an APR is defined in Bellcore PUB 49001 and in PUB 43804, Sections 3.54 through 3.59. Additional requirements are imposed in CB-149 Section B-2. The interface is a four-wire serial data link using shield-grounded 26-gauge 100 twisted-pair conforming to the EIA RS-422 standard. Transmission is limited to 2400 baud over 4000 feet of cable. All serial TBOS communication is asynchronous half-duplex ASCII (one start bit, eight data bits, two stop bits, and odd parity). Both the APR and the monitored equipment are capable of receiving and generating characters in this format (Figure 2).
Note that TBOS is used only between the APR and the monitored equipment. A more complex protocol (such as E2A, TL1, or a proprietary protocol) is required for transactions between the APR and the central monitoring computer because more information must be conveyed within available bandwidth. TBOS and E2A are often confused, perhaps because the original Bell System documents referred to the TBOS port as the "E2A APR port" or because TBOS was universally associated with E2A in its early years, and often called "serial E-telemetry".
Alarm polling . . . . . . . . . . . .
Monitored equipment in a serial TBOS environment is very polite; the network elements do not speak until they are spoken to. Alarm and status information is scanned in a strict sequence, and a particular alarm point is not reported until the APR requests it. This could potentially lead to long delays in reporting a new alarm. Therefore alarm remotes are required to scan through all alarm and status points in no more than two seconds, repeating indefinitely. Monitored equipment is required to answer scan requests in less than 200 milliseconds, otherwise the APR declares a time-out error and moves on to scan the next port. Typically the response time is much shorter than 200 milliseconds.
Changes in status are reported to the central alarm facility by the APR (using a protocol other than TBOS). Central systems popular after AT&T's divestiture include TCAS (T-Carrier Administration System), TASC (Telecommunications Alarm, Surveillance, and Control), and DFMS (Digital Facility Maintenance System).
In normal operation the APR rotates through each character (or line) of each display in each port, requesting each reply character (one line of 8 points per character) in turn from the appropriate network element (Figure 3).
To ensure accuracy each scan request is made twice in succession and the results compared by the APR. If the replies from the monitored equipment are different a transmission error is assumed to have taken place; scanning is interrupted and a mismatch error is declared.
Serial commands . . . . . . . . . . . .
The alarm polling rotation may be interrupted by remote command requests, which are generated by the Central computer and relayed to the monitored equipment by the APR. Unlike alarm scanning poll/response pairs, serial commands take the form of a three-character set. The network element echoes each character of the request in sequence to acknowledge receipt of the command (Figure 4).
Originally only one type of command could be sent. Called momentary commands, these were point specific so that only one function could be exercised per command point. However, since many command functions involve toggling between two states (on and off, for example), latch and unlatch commands (sometimes called operate and release commands) were added to conserve command points.
The first character of the command sequence specifies the address number (1 through 8) and the command type. The second character designates the point number (01 through 64) of that address for which the command is intended. The third character is a fixed sequence of bits, used for security, which must accompany each command request.
As with scan requests, the monitored equipment may take up to 200 milliseconds to answer each command request character. However, the APR must send the second and third command characters within 80 milliseconds of receiving a command character acknowledgment from the monitored equipment.
How TBOS alarms are defined . . . . . . . . . . . .
With the large number of alarm, status, and command bits associated with modern network elements, careful organization is essential to maintain order. Displays are often depicted as 8 x 8 "checkerboards" (called TCAS maps) as shown in Figure 5.
Of the 64 points in the checkerboard, 63 are available for the vendor's use while the 64th is reserved for the central surveillance system. (Should the Remote lose contact with a particular piece of monitored equipment it sets Point 64 ON for that address, signalling the Central that the link is down to that network element and that the absence of reported alarms on other points of that address should be discounted).
The meaning of the 63 alarm and status points (as well as that of the command points) is defined by the monitored equipment vendor and varies from device to device. TBOS operates with different equipment types because messages only communicate the alarm state of (or direct commands to) particular points at particular addresses of particular ports, and do not explicitly convey the meaning of those points. Vendor-supplied documentation is therefore required to decode TBOS displays, a task performed by the monitoring system computer. The documentation for a TBOS compatible network element should, therefore, include alarm/status and command point definitions.
The two types of displays (alarm/status displays and command displays) are assigned together. Generally DIP switches or software options will need to be set during installation to configure the network element to use its assigned display address; the APR must be programmed or otherwise configured to treat the assigned display as active, scanning it for alarms and relaying commands from the Central.
Each monitored network element requires at least one complete display; sharing of display addresses by multiple devices is a departure from protocol. Depending on system complexity, an individual network element may consume more than one display. (Some relatively complex equipment such as OC-48 SONET muldems have multiple TBOS ports, requiring 32 displays to report all of their alarms!) Although command displays are allocated along with alarm/status displays, serial command processing is optional for equipment which has other user-control capability.
Other TBOS Resources . . . . . . . . . . . .
Please visit the TBOS.net home page for links to other information about and resources for users of TBOS.
This article was provided by Telephony Software Associates, Inc. , makers of TBOS Tester . Your suggestions and comments would be appreciated.
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This page last updated 4 August, 1999.