Telecommunications Management Networks (TMN)

OSI Network Management Services for the Telecommunications Industry

TMN is becoming an important topic in the telecommunication service provider's world. With the size and sophistication of the networks rapidly expanding, a clear need has been established to provide convenient services to manage the telecommunications enterprise. Fault, Configuration, Accounting, Performance, and Security services are all important elements of managing critical resources within large distributed telecommunications networks. The TMN concepts have been widely adopted to manage networks ranging from high-speed fiber-optic networks to distributed cellular and satellite based wireless communication systems. With a significant amount of the fielded equipment providing heterogeneous interfaces, a push has been on to encourage the development of a single set of protocols and service specifications to manage these networks. Requirements have been levied by the ITU to encourage the integration of TMN capable services into network equipment. For example, new protocols must now include a General Description of Managed Objects (GDMO) based Management Information Base.

This document highlights some of the differences between OSI network management and SNMP, the major concepts involved in TMN and CMIP based management architectures, and start into some of the specific areas.

CMIP/CMISE, on which TMN is based represents the major alternative to using SNMP. If you enjoy starting arguments, go into a room of network management folks and indicate a strong opinion on the great advantages of using either CMIP or SNMP to the exclusion of the other. It doesn't matter which protocol suite that you advocate, someone will be convinced that you must be from somewhere on the wrong side of the tracks to have espoused such an opinion. Fortunately, there will probably also be several people within the room that recognize that each of the approaches have there merits, you just need to understand the pros and cons of each. SNMP is simple, that's what the S stands for. CMIP is more complicated, (the C doesn't stand for complicated, but it certainly would have served the purposes here).

Why then are there these two popular but vastly different standards? The CMIP approach has been to provide a feature-rich set of services in which sophisticated services can be provided by the agents. SNMP is based on a simpler interface, in which sophisticated management services fall to either out-of-band mechanisms or to the management system.

SNMP, by virtue of being connectionless with lower overhead is a good choice for managing networks where in-bound transport mechanisms are the primary means of management traffic transport. Requests are issued once, if network problems or congestion impede their flow, re-transmissions are avoided, thereby avoiding situations in which the management protocols become part of a problem rather than part of the solution. However, SNMP is not without its limitations, operations are all atomic, often requiring several interactions between the management system and the managed agents to achieve meaningful results. Event notifications from the agents are sent in the form of unacknowledged traps, that can result in critical network problems going unnoticed should the trap be discarded. While strategies for resolving these issues can be addressed through clever constructs in the Managed Information Base (MIB), they aren't always available.

CMIP, on the other hand, sacrifices a good bit of the simplicity of SNMP in return for more sophisticated services provided by the agents. Managers can make simple requests of agents that can cause the agent to execute sophisticated sequences. Event management is managed through a set of accepted standards for event notification, discrimination, and forwarding. Relationships between Managers and Agents can operate in either connection-oriented or connectionless modes (SNMP is connectionless only). Frameworks have been specified for Alarm Management, Security Management, Fault Management, and Configuration Management. Advanced sets of services include scoping and filtering which permit managers to issue requests that involve a larger set of components of the managed system.

TMN leverages the OSI management standards, and has included provisions for the use of SNMP where necessary. Important areas include:

TMN Components and Their Interfaces

Within TMN, the management system consists of several types of components. Shown in Figure 1, these components include Operations Systems (OS), a Data Communications Network (DCN), Workstations, Network Elements, Mediation Devices, and Q Adapters. Operations Systems provide management services that provide for the surveillance, monitoring and control of the managed network, interacting with components within the network to gather information and issue commands that may affect the behavior of the network. These are roughly equivalent to the manager components of SNMP managed networks. Workstations provide network management staff an ability to monitor and interact with the network management system. The data communications network can take several form ranging from a dedicated OSI network using CLNP routing to in-band communications services. The Network Elements (NE) and Q-Adapters (QA) provide agent services in which the actual MIB objects are contained. The primary distinction between the NE and QA are that typically the NE are used to represent network communications equipment that has an embedded CMIP agent, where the QA are typically equipment that act as proxy agents between the TMN CMIP interface and legacy equipment supporting either proprietary or other interface standards. The Mediation Devices (MD) can be used to provide mid-level management services that represent aggregate behaviors of groupings of the network elements supported by the NE and QAs.

Figure 1 - TMN Functional / Interface Architecture

As in many telecommunications systems, the interconnections between the major elements of the system are represented by additional alphabet soup. Interfaces include: Of these interfaces, much of the current system developers are focused on development of equipment that supports the Q3 interface.

TMN Layers

Following the lead of the OSI layered protocol model, TMN provides an organization of management services in the form of a five layer hierarchy of services. The services provided by the TMN layers include: If you are involved in development of communications equipment such as switches, hubs and the like, you are most likely to need to focus on the Network Element layer, with occasional forays into element management. Much of the focus of developers has been on the areas of Network Element and Element Management services, building a framework on which the higher layer services can be incorporated into management systems for deployment.

Standards Galore

The standards that define TMN span a large number of ITU-T documents. The X.700 series of standards define the baseline CMIP/CMIS services as well as defining the major management functions. The X.700 series of standards were developed to support generic OSI network management services and include: Light reading if you have a week or two to spare. Not to mention the fact that these protocols and services also make fairly extensive use of external OSI application and presentation layer services including the Remote Operations Service Element (ROSE) as a request transport mechanism and the Association Control Service Element (ACSE) to establish bindings between the participating management elements. Naming and directory services can also come into play when correlating network element information to the CMIP object naming model.

Extending this information into a framework that is useful for the TMN services, the M.3000 series of standards have been developed. The TMN M.3000 series includes the following recommendations: Additional information describing the Q interface can be found in recommendations Q.811, Q.812, and G.773. Additional information and guidance can be found from the Network Management Forum.