Bearer services are typically categorized by their information transfer characteristics, methods of accessing the service, interworking requirements (to other networks), and other general attributes. Information characteristics include data transfer rate, direction(s) of data flow, type of data transfer (circuit or packet) and other physical characteristics. The access methods determine what parts of the system control could be affected by the bearer service. Some bearer services must cross different types of networks (e.g., wireless and wired) and the data and control information may need to be adjusted depending on the type of network. Other general attributes might specify a minimum quality level for the service or special conditional procedures such as automatic re-establishment of a bearer service after the service has been disconnected due to interference. Some categories of bearer services available via the telephone system include synchronous and asynchronous data, packet data, and alternate speech and data.
Figure below shows a typical bearer service. In this diagram, a customer decides to send a data file to a computer that is connected to a public telephone network (at the office). In this example, the bearer service is circuit-switched data. The customer uses a modem to adapt their portable computer to the telephone network. The portable dials the office computer telephone number via the modem. The telephone system routes this call to a modem that connects the office computer to the telephone network. When the office computer modem accepts the call, the customer’s modem begins to send data directly on the telephone line at channel at 28 kbps. All the telephone system provides is a standard voice communications path between the portable computer modem and the office computer modem. The key bearer service attribute is the transfer of real-time information in the audio frequency range of 300 Hz to 3300 Hz. This is the bearer service.
Constant Bit-rate (CBR)
Constant bit-rate (CBR) service is a class of telecommunications service that provides an end-user with constant bit data transfer rate. CBR service is often used when real-time data transfer rate is required such as for voice service.
Variable Bit-rate (VBR)
Variable bit-rate (VBR) is a category of telecommunications service that provides the users with a data transmission rate of service that can vary. Applications that use VBR services usually require some real-time interactivity with bursts of data transmission. An example of a VBR application is videoconferencing.
Available Bit-rate (ABR)
Available bit-rate (ABR) is a communications service category that provides the user with a data transmission rate that varies dependent on the availability of the network resources. ABR service may provide the user with feedback as to the changed data transfer rate and may have established minimum and maximum levels of data transmission rates.
Unspecified Bit-rate (UBR)
Unspecified bit-rate (UBR) is a category of telecommunications service that provide an unspecified data transmission rate of service to end-user applications. Applications that use UBR services do not require real-time interactivity nor do they require a minimum data transfer rate. UBR applications may not require the pre-establishment of connections. An example of a UBR application is Internet web browsing.
Committed Information Rate (CIR)
Committed information rate (CIR) is a guaranteed minimum data transmission rate of service that will be available to the user through a network. Applications that use CIR services include voice and real-time data applications. CIR can be measured in bits per second, burst size, and burst interval.
Some service providers allow users to transmit data above the CIR level. However, when data is transmitted above the CIR level, some of the data may be selectively discarded if the network becomes congested.
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