T-carrier facilities provide the network platform for several switched dedicated and packet-switched services. The switched dedicated services usually rely on the Public Switched Telephone Network (PSTN) for transport, but the channels are dedicated to a specific customer. A variety of data services are available, many of which run over separate networks.
Regardless of the type of network or the type of network, T-carrier often is used for the underlying transport medium. Even though fiber optic networks are increasingly used to consolidate traffic of all types over high-speed backbones, the traffic often enters or leaves in the T-carrier format used by PBXs, channel banks, multiplexers, cross-connect systems, and other voice-oriented systems. In addition, network devices that are data-oriented, such as routers, also have T1 interfaces.
Switched Dedicated Services
Among the switched dedicated services are NX 64 Kbps, Multirate ISDN Switched 384 Kbps, and Switched 1,536 Kbps.
Nx 64-KBPS SERVICE NX 64-Kbps Service entails the dynamic aggregation of multiple 64-Kbps channels into a high-speed switched data pipe. A network access device—which could be a T1 multiplexer, an inverse multiplexer, a PBX, or a video coder/decoder (codec)—equipped with ISDN PRI is used to implement this service. The device would transmit a message with the telephone number of the remote site and the number of 64-Kbps channels needed to support the application. The message would then be transmitted over the PRI link’s D signaling channel to the carrier’s Class 5 central office switch. The switch establishes the number of 64-Kbps channels needed to support the application. Channels could be added or dropped from the switched link as needed. With NX 64-Kbps service, the channels would almost always take different paths through the carrier’s network and be reassembled at the receiving end.
MULTIRATE ISDN A related type of service, called Multirate ISDN, enables a user to establish on-demand, switched digital links at speeds ranging from 64 Kbps to 1.544 Mbps. An ISDN-equipped network access device such as a T1 multiplexer, an inverse multiplexer, or a PBX transmits an ISDN Q931 message that contains the telephone number of the remote site and the number of 64-Kbps channels needed to support the particular application. The message is transmitted over the PRI link’s D channel to a carrier switch. The switch then establishes a single, contiguous pool of bandwidth across the user’s carrier network and signals the network access device that the link has been established. The user’s network device can also instruct the central office switch to add or delete channels if needed. However, the existing link must first be torn down. By contrast, some inverse multiplexers can add bandwidth during a transmission.
SWITCHED 384-KBPS SERVICE As its name implies, Switched 384-Kbps Service provides a channel that is equivalent to six contiguous 64-Kbps channels or DS0s. Throughout the network, the 384-Kbps channel is switched as a single channel from its point of origination to its destination. Since it is a switched service, the bandwidth is called up only when needed. With each call, the traffic may take a different path within the network to its destination.
SWITCHED 1,536-KBPS SERVICE Switched 1,536-Kbps Service is equivalent to 24 contiguous 64-Kbps channels or DS0s. Throughout the network, the entire 1,536-Kbps of bandwidth is switched as a single channel from its point of origination to its destination. Since it is a switched service, the bandwidth is called up only when needed. With each call, the traffic may take a different path within the network to its destination.
Advanced Packet Services
T-carrier also provides the foundation for several innovative advanced packet services, including frame relay, SMDS, and ATM.
FRAME RELAY Frame-relay backbones are based on T-carrier links. Frame relay is a streamlined version of X.25 that achieves throughput rates that are orders of magnitude greater than conventional X.25. Frame relay achieves high throughput by eliminating 66% of the overhead functions, including error correction, that are traditionally carried out at intervening nodes by X.25.
Frame relay offers two types of connections: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). A PVC is a predefined path between end points. An SVC provides a temporary connection that is set up for the duration of the call in accordance with instructions embedded within the frame itself. This type of connection is very useful when only occasional network access is required, since network resources can be allocated to other users after the call is completed. Where groups of user devices create a continual stream of information, as in LAN interconnection, PVCs are the preferred type of connection.
SMDS Access to SMDS is provided by T-carrier links. SMDS itself is a high-speed, packet-switched service aimed at the LAN interconnection needs of corporate locations within the same metropolitan area. SMDS is a shared service and employs a dual counter-rotating fiber-ring architecture. The two rings transmit data in opposite directions so that if one ring fails, the other is capable of handling the traffic and getting it to its proper destination, thus circumventing the fault.
CPE can access an SMDS switch using either twisted-pair wiring (at the T1 rate) or optical fiber (at the T3 rate). Each customer has private access to an SMDS switch for up 16 devices per access link. The devices are connected in a bus arrangement, just like an Ethernet LAN SMDS access may be customized to suit the individual bandwidth needs of subscribers. By means of access classes, limits can be enforced on the level of sustained information transfer and on the burstiness of the transfer. In the case of T3, the access classes are 4, 10, 16, 25, and 34 Mbps. For T3 access paths, an “ingress access class” may be applied to the information flow from the CPE to the carrier’s switch, and an “egress access class” applied to information flowing from the switch to the CPE. Both types of access classes may be selected by the subscriber. For T1 access paths, the same 1.536 Mbps access class is applied to both directions of information flow. In addition, several local carriers enable their customers to access their SMDS networks at speeds of 56/64 Kbps or lower.
ATM ATM switching, or cell relay, is designed as a general-purpose broadband switching method for voice, data, image, and video traffic. With ATM, data is switched at the packet level—53-byte cells—rather than at the bit or byte level, thus minimizing internodal processing requirements. The benefits of ATM include high speed, low latency, and increased network availability through the automatic and guaranteed assignment of network bandwidth, which supports isochronous traffic such as voice and video, as well as real-time data traffic for online, interactive applications. ATM can run over T-carrier facilities and some multiplexer vendors offer T1/E1 ATM User-Network Interfaces (UNIs) to allow users to access ATM services without requiring investments in broadband network infrastructures.
Last Word
The main driving force in the growth of T-carrier services is the Internet. Services based on T-carrier scale fairly easily to accommodate user needs, as demonstrated by the availability of fractional services and the ability of carriers to aggregate multiple T-carrier facilities to deliver additional bandwidth. For example, Sprint offers a service that aggregates four or eight T1 lines to provide, respectively, 6- or 12-Mbps dedicated links. MCI WorldCom offers multiple T1 access at rates of 3, 4.5, and 10 Mbps. UUNET Technologies offers a Double T1 service with a 3-Mbps access speed. These services are aimed at corporations that have saturated their T1 links into the Internet, but still do not need a 45-Mbps T3 connection. Other big attractions driving T-carrier demand are the ease with which the line termination equipment can be managed and the ability to consolidate traffic—both voice and data—from multiple remote sites onto a single access line into a company.
