Paging infrastructure: The agent, the manager, and the MIB
RF site equipment, adapted to use a network management protocol, can provide paging carriers with redundant communications paths for site monitoring and information distribution. Troubleshooting, control and programming of the equipment can be performed from a centralized location.
During the past few years, the nature of telecommunications has changed dramatically. The recent aggressive growth of nationwide paging networks and the introduction of advanced messaging technologies have resulted in dramatic changes in site management needs.
Systems engineering is required to support greatly expanded coverage areas while technical staffing is held constant-or even reduced. Simultaneously, new technology is frequently demanding more, rather than less, attention during the early stages of implementation. This situation would seem to demand an increased reliance on remote monitoring and control of site infrastructure. Indeed, a number of carriers are transferring some site monitoring responsibility to the regional network operations centers that monitor their network performance.
However, carriers are finding that while some infrastructure manufacturers offer network access via proprietory methods, much ancillary site equipment remains isolated from network access. Indeed, most monitoring equipment manufacturers historically serving the paging industry still rely upon the public switched telephone network (PSTN) or RF links for communications with the site. Existing monitoring methodology must be reevaluated.
Voltage level signals and tone control In the early days, remote site control was generally accomplished by current or voltage level signaling over dedicated lines. While this was adequate for PTT or channel-select functionality, there was no capability to report unanticipated changes of operational parameters. Control functionality was severely limited by the restricted command set that was possible.
To remedy this situation, a variety of tone control systems were introduced, and remote control by PSTN or radio link became possible. Reporting from the site was still restricted, however, because a condition could be reported but a specific parameter could not. For example, you might know that a transmitter was keyed, but you wouldn’t know that transmission line voltage standing wave ratio was excessive. Truly useful remote monitoring had to wait for digital technology.
Modem control It could be argued that true remote site management was simply not available until cost-effective modem communications over the PSTN became possible.
In the mid-to-late-1980s, several companies released devices capable of reporting the status of discrete digital or analog inputs in a reasonably user-friendly format to a remote VDT, printer or pager. Most of these devices also allowed some degree of remote control by contact closure or relay driver. Such systems were faster, friendlier and offered at least partial path redundancy because communication was no longer limited to a dedicated connection to a single control point. Such systems could call a variety of phone numbers and communicate via modem, voice synthesizer or pager.
These systems are still limited by an over-reliance on a functioning “plain old telephone service” (POTS) line at the site, but they were a tremendous advantage when communication from switch to transmitter site was primarily accomplished by leased lines or RF links.
TCP/IP protocol The modem monitoring approach is still adequate for many users, but the emerging use of nationwide Transmission Control Protocol/Internet Protocol (TCP/IP) data networks by paging carriers presents new demands for the telecommunications professional and new opportunities for remote monitoring equipment manufacturers. As paging carriers find data communications via wide-area network technically desirable, they understandably wish to integrate their site-monitoring equipment within that network.
Indeed, the Personal Communications Industry Association’s (PCIA) “Guidelines for Implementations of the TCP/IP ProtocolSuite” state that “The protocol and procedures, while not mandatory, are important tools that are applicable to paging networks, particularly larger networks.”
For this reason, there is renewed interest in the Simple Network Management Protocol (SNMP), the predominant network management tool on IP networks. PCIA’s guidelines state that “The use of the Internet network management features should be advantageous to most paging networks,” and that “SNMP has emerged as the management agent of choice.”
Setting standards A number of proprietary network management methods have evolved over the years, but the need for a management standard was early recognized as a mandatory element of TCP/IP networks. The Internet Engineering Task Force adopted SNMP in April 1989 as an interim management technique to be used until the International Standards Organization Common Management Information Protocol (CMIP) was fully implemented.
In reality, the success of the SNMP standard, and the sluggish implementation of CMIP, has resulted in SNMP remaining the most common monitoring protocol on IP networks. It is now generally accepted that SNMP will remain a useful component of IP-based monitoring for some years to come.
SNMP structure The protocol comprises three basic components: the agent, the manager and the management information base (MIB). The agent software resides in a managed element of the network where it collects data, responds to commands from the SNMP manager and sends alerts (called traps) to a specified IP address. The manager software polls the agents at specified intervals, listens for traps, retains the information for analysis and presents the resultant information to the NOC staff in a user-friendly way. The information reported by the agent is specified in the agent’s MIB, which resides within an agent that organizes those parameters that are to be monitored and/or controlled by the SNMP manager. Support for any device, including one emerging or currently non-existent, can be implemented by the development of a MIB for that device and an associated configuration file for the SNMP manager.
SNMP adoption Acceptance of SNMP among infrastructure manufacturers is growing. The protocol offers platform and vender independence, flexibility and coexistence with a variety of proprietary approaches such as Netbios, SNA, and Netware. Support is routinely available for access servers, routers, UPS, switches and other network devices. Manufacturers of related equipment are adding such support to allow easier integration within a networked environment. Still, a substantial base of current RF site equipment offers no SNMP capability and will effectively remain inaccessible by network without a suitable hardware interface to the IP network. This issue can be resolved by using a proxy, which is a device that functions as an agent for equipment that does not offer SNMP management capabilities. An SNMP proxy such as the ProTek jr+SNMP will allow IP access to such equipment.
Proxy application The SNMP proxy can be installed on one port of an access server located at the site. Other networked devices at the site can share the remaining ports. The proxy can, with suitable programming, monitor any legacy equipment outputting system status via discrete logical state, voltage or serial ASCII stream. Possible bi-state (digital) inputs include contact closure or logic levels from intrusion or fire alarms, power supplies, PTT lines, COR lines, tower light controllers and similar inputs. Analog inputs monitor voltage levels, or, by use of appropriate sensors, RF power, current, temperature or other linear inputs. The proxy also can access many transmitters, controllers, and ASCII stream devices that offer an RS-232 port for troubleshooting, control or programming. Additionally the jr+SNMP, by retaining full POTS line capability, provides redundant communications capability via an onboard modem to a remote VDT, printer or paging terminal. In the absence of a POTS line, cellular or RF-modem communications are possible. If a continuous IP connection is not available, SNMP monitoring via dialup networking to a site-local IP service provider is still possible, although at the cost of redundant communications channels. This redundancy of communication allows the support staff to retain communications with the site even if a critical IP network segment goes down.
Centralized monitoring An SNMP manager running on a workstation at the centralized NOC communicates via an IP network with the SNMP proxy at the sites to be monitored. The manager polls the agents at specified intervals and displays the information at the same management station used to monitor any other SNMP device on the network. The information can be distributed throughout the NOC by local-area networks (LAN); it can be logged for future analysis; and it can be made available to repair-order-generation and tracking software on receipt of a trap.
With the introduction of such a system, paging carriers can now gain redundant communications paths for site monitoring, facilitate distribution of such information throughout the organization, and access equipment that would otherwise be unavailable by network.
Summary Network management using agent-assisted SNMP adheres to a stable standard that should be available for years to come. The approach increases redundancy and extends access. It also integrates into existing network management systems gracefully, with little or no changes to existing NOC procedures.
Further reading All of the technical aspects of the Internet protocol and SNMP are covered by the Request for Comment (RFC) documents published by the Internet Engineering Task Force (IETF). RFCs are available for downloading at www.isi.edu.
Documents particularly pertinent to SNMP are RFC1155 (“Structure and Identification of Management Information for TCP/IP-based Internets”), RFC1157 (“A Simple Network Management Protocol”), RFC1158 (“Management Information Base for Network Management of TCP/IP-based Internets: MIB-II”) and RFC1212 (“Concise MIB Definitions”).
ASN.1 and the basic encoding rules are not described in RFCs. For information on these subjects, see “A Layman’s Guide to a Subset of ASN.1, BER, and DER,” at www.rsa.com/rsalabs/pubs/PKCS/.
For an overview of SNMP network monitoring in general, refer to SNMP, SNMPv2, and RMON: Practical Network Management, by William Stallings (Addison-Wesley, 1996).
The PCIA’s Paging Technical Committee has published guidelines for those involved in networking for telecommunications applications. “The PCIA Guidelines for Implementations of the TCP/IP Protocol Suite” directly addresses the use of IP networks in support of the TIPP protocol. The PCIA TIPP and TDP specifications address paging networking in general. The committee, at the time of the standards’ publication, comprised major infrastructure manufacturers and carriers.
Ball, a certified technician, is general manager for PageTek, Raleigh, NC. His email address is: [email protected]dspring.com/pagetek.
