The Symbian OS (779886), страница 48
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Interestingly, the addition of telephony services did notsubstantially change the earlier assumption of the primacy of serial communications, since the initial expected use for telephony was a two-boxsolution, using a serial connection (either cable or infrared) to connect aSymbian OS device to a modem or a mobile phone.This was more or less the communications architecture of the firstreleases of the operating system and largely survived through the SymbianOS v6 and Symbian OS v7 releases. Over those releases, there weresignificant extensions, most obviously to telephony and networking toadd the required packet capabilities for 2.5G and 3G data services,as well the addition of new short-link technologies such as Bluetoothand USB. However, the general principle of the serial server as theprimary communications server (the ‘first among equals’) remained eventhough, as each release increasingly specialized the operating systemfor mobile phones, the primary communications use case was not serialcommunications but on-board telephony, with or without networking.This architecture was unsatisfactory for a number of reasons, not leastof which was the resource cost of having the serial server running all thetime to support non-serial communications services.Beginning with Symbian OS v8, therefore, some significant changes tothe communications architecture were introduced as the foundation forfurther evolution to support the increased demands for high data rates.The key change was to introduce a primary communications server,the Comms Root Server, which is designed to provide a purpose-built,lightweight server for which the dedicated communications servers (C32serial, ETel telephony and ESock sockets servers) act as service providers.The Serial Server is relieved of its privileged role and becomes just anotherdedicated service provider.ARCHITECTURE209In this architecture, the Root Server becomes a communications process server, initiating a single communications process within which itruns the servers for individual services as threads, starting and stoppingthem in response to client requests and providing process, shared resourceand common settings management including fast, low-overhead communication between the dedicated server threads.
Each server is run in itsown thread and only a single instance of any server is ever running. Anumber of supporting components implement the messaging abstractionsand communications channels which allow passing of messages betweenrunning server threads, while the Comms Database provides the sharedsettings service. (From Symbian OS v9, the CommDB API is provided forcompatibility only; the Central Repository should be used for all sharedsettings.)The Root Server is responsible for running the following dedicatedservers, which implement a common Comms Provider Module (CPM)interface defined by the framework:• C32 Serial Comms Server• ETel Telephony Server• ESock Socket Server• Resolver Server• Fax server.The individual services are described in more detail in the sectionsthat follow. Each service provides a client-side session API, encapsulatedin a single static DLL to which clients link.
The general usage pattern isthus:1. Create a client session with the appropriate server, for example theSerial Server or Socket Server; this exposes the server’s client-sideAPIs to the client.2. Create a client sub-session with an appropriate object, for example acommunications port or a socket; this exposes the object APIs to theclient.3. Use the object.4. Close the sub-session with the object when finished.5. Close the session with the server when finished.It is also worth noting that in Symbian OS communications servicesare provided user-side; in other words, communications services are notbuilt into the kernel. This protects the kernel from resource failures or210THE COMMS SERVICES BLOCKbadly behaved processes originating from communications services orclients.FrameworksAs well as implementing server functions, the principal communicationsservers also provide extensible frameworks, which are at the heart of thecommunications architecture.Frameworks provide extensibility at a number of levels, including:• at the client-interface level (for example, extending core telephonyservices to enable fax over mobile networks)• within the protocol stacks at the protocol level (for example, adding theWAP stack or extending core TCP/IP services to enable packet-levelsecurity)• at the network-interface level (for example, adding support for newtechnologies such as the Bluetooth Personal Area Networking (PAN)profile)• at the hardware-abstraction-interface level (for example, extendingthe telephony baseband interface to support CDMA).All implementations of communications framework plug-ins conformto the Plug-in Framework (i.e.
ECom), in other words they are polymorphicDLLs that implement the standard interfaces which enable the Plug-inFramework server to find and load the appropriate modules at run timeon behalf of the requesting framework, as well as the communicationsspecific interfaces required by the specific communications frameworks.The Comms Services frameworks include:• C32 Serial Server, which defines CSY virtual serial port modules• ETel Telephony Server, which defines TSY baseband interface modules• Socket Server, which defines PRT protocol modules• Network Interface Manager, which defines AGT interface agent andNIF network interface modules.In addition, the Comms Framework component defines the CPMinterface which is implemented by all of the dedicated communicationsservers (but not by the Root Server itself).9.6 Comms FrameworkThe Comms Framework components implement the infrastructure usedby all communications services:• The Comms Root Server is the primary communications server,responsible for starting and stopping the communications serversCOMMS FRAMEWORK211that provide dedicated services and for providing the process contextin which all dedicated servers are run.• The C32 Serial Server and the ESock Socket Server are, respectively,the data communications and socket servers that provide the two directclient interfaces for communications services (all communicationsservices are accessed through sockets and serial communicationsservices are also available directly through the Serial Server).• The Network Interface Manager and Network Controller are, respectively, the network interface and connection managers that find andset up appropriate network connections requested by Socket Serverclients and that are used (indirectly) by all communications services.The Comms Framework also includes common utility and frameworksupport, including the framework classes that define the Comms ProviderModule (CPM) interfaces to which all communications servers conformand specialized messaging and memory management (Comms Channels and MBufs), designed to enable fast inter-thread communicationswithin the communications process including thread-shared memory.(Communications servers run in their own threads inside the singlecommunications process managed by the root server.)Also included is the Comms Database, which supports the legacy interface used for storing shared communications settings.
(New applicationsshould use the Central Repository.) See Figure 9.8.CommsProcesses &SettingsCommsConfig. UtilsData Comms ServerComms FrameworkUtilitiesBasebandAbstractionComms FrameworkFigure 9.8Comms Framework components212THE COMMS SERVICES BLOCKDesign GoalsIt is important to remember that on a typical device based on SymbianOS (a mobile phone, for example), all communications must be virtualized over an available, and usually transient, connection. Thus Internetbrowsing, for example, typically does not take place over a direct Internetconnection (as it would on a PC) but is virtualized over telephony orshort-link services. As Wi-Fi begins to appear on phones, direct networkconnections also become possible but very much as complementaryoptions.The Comms Framework has evolved to provide a generic infrastructurethat enables the seamless interoperation of services while providingimproved performance, ready for the next generation of high-data-rateservices.ArchitectureThe Comms Framework sub-block is less a self-contained architecturalunit than the architectural glue that binds the different dedicated communications services together.
It provides the frameworks that defineessential, common communications abstractions, the Root Server thatprovides the runtime context within which all communications servicesoperate, and the shared settings database and utilities, as well as utilitiesand libraries, such as the MBuf Manager and Elements components.The Root Server and Framework UtilitiesFrom Symbian OS v8, all communications servers are implemented asComms Provider Modules and are run and managed by the Root Server,which loads, configures, runs and monitors CPMs as dedicated threadswithin the Root Server’s own process. Starting the Root Server creates thesingle communications process and starts the server as the main runningthread within in. The Root Server runs from device startup to shutdown.In Symbian OS, a process is the fundamental unit of protection, with itsown address space, while a thread is the fundamental unit of execution,running inside a process and sharing the process address space and anyother resources (file handles, for example) with other threads running inthe process.The Comms Framework is the component that provides the abstractions needed to implement Comms Provider Modules including theCPM interface, common thread management and Comms Channels, theasynchronous message queue abstraction that provides an efficient communication mechanism between active CPMs.
The CPM framework alsodefines a file-based configuration method that is used by the Comms RootServer to configure CPMs on loading.COMMS FRAMEWORK213To support implementation of new CPMs, the Comms Elements provides a reusable catalog of common design pattern implementations, forserver startup, message passing and generally useful abstractions suchas state machines. The MBuf Manager provides a memory management framework that allows direct sharing of data (for example, networkpackets) between CPMs without copying.Serial CommunicationsThe C32 Serial Server provides serial services for application and systemclients. A key component from the first Symbian OS release, it has beenre-architected and re-engineered to support platform security and the newcommunications infrastructure based on the Root Server.
From SymbianOS v8, the C32 Serial Server is a CPM, run and managed by the RootServer. The CPM and supporting mechanisms provide data sharing andefficient inter-server communications without the overhead of runningthe Serial Server to support other communications services.The Serial Server follows the standard Symbian OS server pattern,providing serialized access to shared resources. In the simplest case, andunlike other communications servers, clients can gain direct access to theserial hardware on a device by initiating a client session with the server(by making a serial service request to the communications configurator)and then from within the session loading, opening and configuring a(virtualized) serial port.
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