ARCHITECTURE OF COMPUTER CONTROLLED EXCHANGES

The architecture ofcomputer controlled exchanges is influ­enced to a large extent by the architecture and technology ofboth the switching system and the (central) control sys­tem and how they are related via decentralized (regional) control systems (see Fig. 4).

A modular architecture can lower the costs of system handling and make it easier to adapt the system to the changing world of telecommunications. In a truly modular system each module is fully decoupled and independent of the internal structure of other modules. There are different forms of modularity, for example:

• Application modularity. Make it easier to combine several larger applications in one node.

• Functional modularity. The system defined in terms of functions rather than implementation units. Functions should be possible to add, delete, and change without disturbing the operation of the sys­tem.

• Software modularity. The software modules should be programmed independently of each other, and they should interact only through defined interfaces and protocols. In this way, new or changed modules can be added without changing existing software.

• Hardware modularity. Supports that new hard­ware can be added or changed without affecting other parts of the exchange.

On the highest level the system architecture of the exchange can be divided into various application mod­ules in analogy to how telecommunications nodes inter­act and communicate, using protocols enabling modules to be added or changed without affecting the other modules.

Typically the implementation of a telecommunications ex­change can be divided into:

• Application modules. These implement various telecommunication applications much like virtual nodes using standardized interfaces to other appli­cation modules. Application modules act as clients to resource modules.

• Resource modules. These modules coordinate the use of common resources available to applications by means of well-defined interfaces to the users. Re­source modules act as servers to application modules.

• Control modules. These modules are responsible for the operating system functions, input-out-put func­tions, basic call service functions, and so on.

Application Modules

The application modules implement various telecommu­nication applications and have standardized interfaces to resource modules. In general an application consists of ac­cess and services. Examples of application modules are:

• Analog access module

• Digital access module

• Mobile access module

• PSTN user services module

• ISDN user services module

• MSC user service module

• Home location register (HLR) module

Resource Modules

The resource modules typically handle and coordinate the use of common resources and may contain both soft­ware and hardware. The most important part is the group switch. Trunks and remote and central subscriber switches (RSS and CSS respectively) are connected to the group switch. The trunks are used to connect the switch to other switches, to data networks, mobile base stations, etc. The subscriber switch handles the subscriber calls and concen­trates the traffic (see Fig. 5).

Group Switch. The main function of the group switch is selection, connection, and disconnection of concentrated speech or signal paths. The group switch often has a gen­eral structure.

The overall control of the group switch is performed by the central processor system. The regional processors take care of simpler and more routine tasks, such as periodic scanning of the hardware, whereas the central control sys­tem handles the more complex functions. Associated func­tions included in the group switching resource module are network synchronization devices and devices to create mul­tiparty calls.

Subscriber Switch. The subscriber switch handles selec­tion and concentration of the subscriber lines, its main functions are as follows:

Figure 4. Typical architecture of stored program controlled exchange.

Figure 5. Switch architecture.

• Transmission and reception of speech and signaling data to and from the subscriber equipment (for exam­ple, on-and off-hook detection).

• Multiplexing and concentration of the subscriber lines, to save hardware and make more efficient use of the communication links between the subscriber stage and the group switch.

The architecture should be modular and enable to com­bine PSTN and ISDN access in the subscriber stage. The subscriber switch can be colocated with the group switch in the exchange (central subscriber switch, CSS) or located at a distance from the exchange (remote subscriber switch, RSS).

Remote Subscriber Multiplexer. The remote subscriber multiplexer (RSM) is an add-on subscriber access node, used in the access network, which can cater small groups of subscribers. It provides both mobile and standard tele­phony connections. The RSM multiplexes and concentrates the traffic to the central or remote subscriber switch but does not carry out traffic switching functions.

Trunk and Signaling. This resource module includes the circuits for connecting trunks and signaling devices to the group switch. The module should handle the adaptation to different signaling systems, namely common channel sig­naling as well as various register and line signaling sys­tems.

Traffic Control. This resource module contains the traffic handling and the traffic control functions of the exchange. This module is responsible for finding the most suitable route between calling and called subscribers and of verify­ing that call establishment is allowed.

Operation and Maintenance. This resource module en­ables tasks such as supervision of traffic, testing of the transmission accessibility and quality, and diagnostics and fault localization of devices or trunks.

Common Channel Signaling. This resource module in­cludes the signaling terminals and the message transfer part (MTP) functions for common channel signaling sys­tems such as SS7.

Charging. This resource module is used in exchanges that act as charging points. Both pulse metering and spec­ified billing (toll ticketing) can be offered. It should be pos­sible to charge both calls and services, and the charging should be based on:

• Usage

• Provision/withdrawal of subscriber services and sup­plementary services

• Activation/deactivation of subscriber services and supplementary services

Control Modules

The primary function of the control module is to provide the real-time processing and execution environment re­quired to execute software in application modules and re­source modules used to perform traffic-handling functions and call services. The processing can be centralized where one processor takes care of all tasks, or distributed where the processing of information is distributed over several processors.

Execution of telecom software imposes stringent real­time requirements on the control system. Calls appear stochastically, short response times are needed, and over­load situations must be handled. The main control modules are: the central processor(s); the data store to store call data; and the program store to store the actual programs.

In order to achieve an efficient overall control system, it can be divided into:

• Central control. One or several processors that per­form the non-routine, complex program control and data-handling tasks such as execution of subscriber services, collection of statistics and charging data, and updating exchange data and exchange configuration.

• Regional control. A set of distributed processors that perform routine, simple and repetitive tasks but

also some protocol handling. They are of different types optimized for their main tasks, for example, input/output processing. They often have strict real­time and throughput requirements, for instance for protocol handling, and may have customized hard­ware support in the form of ASIC, FPGA and DSP circuits for this purpose.

Switching Techniques

Until about 1970 most switches were analog and based on electromechanical rotor switches and crossbar switches. Since then the utilization of digital techniques has be­come dominant. Digital switches have been based on syn­chronous time and space multiplexed circuit switch tech­nology. For pure data communication, packet switching technology is often used. In order to be able to support voice and data sharing a common infrastructure, an asyn­chronous transfer mode (ATM) has been developed. Syn­chronous time and space multiplexed circuit switch tech­nology is based on synchronous transfer mode (STM) car­rier and synchronization technology (STM transport tech­nique is also often used as a carrier for ATM).

A typical STM-based switch architecture is made up of a combination of time and space switches (T and S, respec­tively). A space switch connects physical lines by changing of positions in the space domain, see Fig 6a and a time switch changes the ordering sequence of data (voice) sam­ples by changing of positions in the time domain, illus­trated in Fig. 6b.

The elements T and S can be combined in several ways to realize a switch and make it configurable in many ways. Usually time switching is used in input and output stages and space switching is often used in central parts of a switch. This basic time-space-time (TST) switch structure can be used both in subscriber and in group switching stages. The first part of a TST switch is a time switch, which interchanges time slots between the external incoming dig­ital paths and the space switch. The space switch connects the time switches at the input and the output. The last part of the TST switch is a time switch, which connects the time slots between the external outgoing digital paths and the space switch (see Fig. 7).

The time switch moves data contained in each time slot from an incoming bit stream to an outgoing bit stream but with a different time slot sequence. To accomplish this, the time slot needs to be stored in memory (write) and read out of the data store (DS) memory and be placed in a new posi­tion (read). The reads and the writes need to be controlled, and the control information needs to be stored in a control store (CS) memory as well. The timing of DS and CS is controlled by a time controller (TC). Examples of control actions are time slot busy or time slot idle.

A typical space switch consists of a cross-point matrix made up of logic gates that realize the switching of time slots in space. The matrix can be divided into a number of inputs and a number of outputs and is synchronized with the time switching stages via a common clock and a control store.

Figure 8. Typical ATM switch structure with input and output ports and control space switch.

Switch architectures based on asynchronous transfer mode (ATM) handle small packets called cells with a given
fixed size (53 bytes) divided into header (5 bytes) and pay­load (48 bytes). The header contains virtual path identifiers (VPIs) and virtual channel identifiers (VCIs), where a vir­tual path (VP) is a bundle of virtual channels (VC). Traffic can be switched at the VP or VC level cell by cell. Associated with a VP or a VC is a quality of service (QoS) contract. In order to be able to guarantee the switching of cells accord­ing to the contract without unacceptable cell loss a number of queues are used at the input ports and output ports of the switch. Between input and output ports (including bufferqueues and cell multiplexers and demultiplexers) a space matrix often is used, as in Fig. 8. However, other types of central switching structures sometimes are used, such as fast fiber buses and Banyan networks.

Other switching techniques are used in a telecommu­nication exchange in addition to STM and ATM. These include Ethernet and Rapid IO switches as well as IP routers, each with their own characteristics. RapidIO is highly efficient internally in nodes with hard latency and real-time requirements and for small packets, while Eth­ernet switches show good enough performance for high throughput for large packets. Embedded IP routers, for IP forwarding, are useful for nodes that border IP networks or are part of these. Sometimes, several switches can be needed inside a node.

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