Switched telecommunications networks, such as Local and Wide Area Data and Voice Networks, route transmission information (data and/or voice) over various transmission links connected to Data Terminal Equipments (DTEs). The transmission links transmit electrical signals between DTEs with transmission media such as two-wire open lines, twisted pair lines, coaxial cable, optical fibers, satellite systems, terrestrial microwave links, radial links, and so on.
Switched telecommunications networks may include public carrier networks, private carrier networks, or hybrid combinations of the same. Public carrier networks include networks intended primarily for voice communication, such as PSTN (Public Switch Telephone Network), and networks intended mainly for data communication such as PSDN (Packet-Switched Data Network) and ISDN (Integrated Services Digital Network). Private carrier networks have capabilities similar to those of public carrier networks, but are privately-owned and controlled.
Switched telecommunications networks in North America Europe and Japan utilize multiple channel transmission links, which transmit multiple channels of information in the form of a time-division-multiplexed (TDM) signal. In forming the TDM signal, sets (characters) of digital bits, with each set (character) corresponding to a respective channel of information, are interleaved in time by time-division-multiplexing (TDM); and the interleaved sets of bits are transmitted serially on a common bus which forms the multiple channel transmission link. Such multiple channel transmission links are known as primary rate carrier links. In North America and Japan, T1 primary rate carrier links are used in which 24 information channels are grouped together within each link; in Europe, PCM-30 primary rate carrier links are used in which 30 information channels are grouped together within each link. T1 and PCM-30 primary rate carrier links have aggregate bit rates of 1.544 Mbps and 2.048 Mbps, respectively.
T1 and PCM-30 systems transmit character (byte)interleaved serial digital bit streams, and are used to form transmission links in a switched telecommunications network. A typical switched telecommunications network includes, among several entities, a central office, a remote office, and one or more repeaters. The repeaters are disposed between the central office and the remote office, and regenerate signals passing therethrough, to thereby extend the transmission distance between the central and remote offices.
FIG. 1 illustrates a system for converting several input channels (24 channels with T1, and 30 channels with PCM-30) of channel information into a TDM signal that comprises a byte-interleaved serial digital bit stream. A plurality of coder circuits 100 are provided. Each coder circuit 100 corresponds to a particular channel of transmission information. The output of each coder circuit 100 is connected to a corresponding input of a time division multiplexer 102. Time division multiplexer 102 manipulates transmission information which is input from coder circuits 100, and provides each of a plurality of buffers 104 with a byte of data corresponding to a respective coder circuit 100. Each byte is stored in a buffer 104, and is then assigned a specific time slot within the byte-interleaved serial digital bit stream.
FIG. 2 illustrates a D4 framing structure of a T1 link. As illustrated in FIG. 2, the byte-interleaved serial digital bit stream of a T1/D4 transmission link is arranged in accordance with a framing schedule in which 24 channels 106 (each channel corresponding to a byte) are consecutively arranged in the form of a frame 108. A channel 106 is illustrated in the top portion of FIG. 2, a frame 108 is illustrated in the middle portion of FIG. 2, and a superframe 110, which includes 12 frames, is illustrated in the bottom portion of FIG. 2. The consecutively numbered bits, channels, frames and super frames of the bit stream are transmitted in time from left to right, thus resulting in a direction of transmission as indicated by the arrow at the bottom of FIG. 2.
The first through seventh bits of each channel 106 comprise transmission information, i.e., link payload information. The eighth bit (the least significant bit) of each channel 106 (called a signalling bit) is used either for supervision, or signalling (e.g. to establish a connection, or to terminate a call). The eighth bit of selected channels in every sixth and twelfth frame of superframe 110 contains signalling information. These signalling bits are inserted by "robbing" the eighth bit of each data word of each channel 106, and by replacing the "robbed" eighth bit with a signalling bit. This mechanism is referred to as "robbed bit signalling."
The 193rd bit of each frame is referred to as the multiframe alignment bit or "F" bit. The "F" bit may comprise a bit for terminal framing, designated as Ft, or a bit for multiframe synchronization (used to identify frames 6 and 12), designated as Fs.
FIG. 3 illustrates a CEPT PCM-30 transmission framing format. The first channel 106 (which corresponds to time slot zero) and the seventeenth channel 106 (time slot sixteen) facilitate the transmission of signalling information, such as on hook and off hook, call progress, dialing digits, and so on. For "even" frames, within the sixteen frame multiframe structure (superframe 110), time slot zero is utilized to indicate a frame alignment signal. During the alternate "odd" frames of the superframe 110, time slot zero contains an international Bit I, national bits N (reserved for national use by respective countries), and an alarm indication signal A. Time slots 1-15 and 17-31 are assigned to 30 telephone channels numbered 1 to 30, and all eight bits of each time slot represent transmission information.
As illustrated above, transmission links, such as the T1 and PCM-30 links, must transmit both transmission information (i.e., link payload information) and overhead information. Various enhanced T1 and PCM-30 primary rate carrier systems have been implemented in order to increase the payload bandwidth (i.e., the amount of the link's bandwidth available to the transmission information), and to minimize the extent to which the transmission of network overhead information infringes upon the payload bandwidth.
One form of network overhead information is signalling information, which is used for establishing, maintaining, terminating, and accounting for a circuit-switched connection between two end points. Conventional schemes employ an in-band approach (i.e., an approach that uses channels that contain transmission information) to transmit signalling information within the network; that is, the signalling information runs in-band, and shares the same channel as the transmission information. One in-band approach is the "robbed bit signalling" described above. With such an approach, the user is deprived of a portion of the limited payload bandwidth provided by the transmission link.
In order to address this problem, ISDN includes a D-channel which provides an out-of-band common channel signalling facility. The ISDN D-channel conveys, by means of special messages, signalling information out-of-band, and thus fully utilizes the available payload bandwidth provided by the transmission link.
Accordingly, with ISDN, a primary rate carrier system, e.g., T1, may be enhanced to include special features such as common channel signalling.
Another significant emerging telecommunications protocol is the Synchronous Optical Network (SONET), which is intended to provide a formal framework for fiber-optic transport within wide area networks on a local, national and international basis. Because of the significantly high transmission rates available with the SONET protocol, currently from 1.728 Mbps to 2.48832 Gbps, the SONET protocol has a relative abundance of bandwidth for data transmission. The SONET protocol can thus afford to provide several out-of-band overhead bytes, which facilitate overhead functions including a path trace, to ensure receiver-to-transmitter terminal connection, an error monitoring function, a path status byte, and other useful control and monitoring functions. The out-of-band overhead bytes, together, greatly enhance the network maintenance and signalling aspects of a SONET transmission link.
Accordingly, with SONET, performance monitoring can be performed on a per-frame basis. This provides significant advantages over conventional transmission link protocols that are presently implemented.
A virtual tributary (VT) structure of SONET is defined for transporting lower rate signals such as DS1, DS1C, DS2, and 2.048 Mbps signals. Accordingly, T1 links can be transmitted within a SONET fiber-optic cable of a SONET network (virtual tributary--VT), and thus can gain the benefits of the performance monitoring and embedded signalling that accompanies a SONET system. However, once data leaves the "SONET hardware" of the VT, monitoring and other out-of-band information that accompanies the SONET system is terminated. Thus, once the information goes beyond a specific point, and enters the NON-SONET (typically copper-based) T1 system, it is difficult to ascertain the performance of the system, with the same real-time (per-frame) accuracy as with a SONET system.
For out-of-band transmission of overhead information within T1 links, an extended framing format has been utilized. Several frames are transmitted in order to convey out-of-band overhead information, such as information indicative of the integrity of data being transmitted by the link. For example, with the Extended Superframe Format (ESF or Fe), in order to perform a cyclic redundancy check, a 24 frame superframe must be transmitted in order to convey six bits of information to represent the error performance of the link. While the six bit CRC (Cyclic Redundancy Check) can detect errors that occur on a DS1 signal, and can be used in applications to provide end-to-end performance monitoring, such information can only be obtained after waiting for the passage of 24 frames.
A delay is encountered in obtaining the error detection information, due to the limited bandwidth that is available to the overhead information. The ESF T1 link is unable to obtain performance information on a real-time (i.e., per-frame) basis, which can be disadvantageous. For example, one cannot individually ascertain the integrity or accuracy of each individual frame. Thus, should a short spurt of noise occur within one or two frames in a bit stream, the noise would go undetected, but nonetheless would affect the data within that frame, or group of frames. This significantly limits the ability to maintain and/or verify the integrity of the system. In addition, data indicative of the performance of the system, such as the bit error rate, can only be obtained with the use of statistical methods. On the other hand, if this information could be acquired on a per-frame basis, an accurate bit error rate (per-frame) could be provided.
The limitations in performance monitoring of links such as DS1/D4 and ESF are even more significant when troubleshooting problems with the transport of packet data. For example, with an ESF T1 link, an error check is performed across a complete superframe. If a transmission error occurs within a particular packet, the error detection technique utilized by an ESF link would be unable to localize the problem error, and any action taken to correct a detected error would affect the whole superframe.
The need for an ability to accurately monitor (in time and place) the performance of a telecommunications link is becoming increasingly significant with the integration of multiple telecommunications networks. For example, in recent years, computer-controlled private branch exchanges (PBXs) have been developed, and have been integrated with public carrier networks, to form hybrid networks. It is thus increasingly important to afford parties individually responsible for each network, within a hybrid internetwork, the ability to isolate where a problem has occurred. With this ability to isolate problems, the party responsible for correcting the problems can be accurately identified.
Several telecommunications protocols, such as those described above, transmit channels of data in the form of a character-interleaved serial digital bit stream. Such telecommunications protocols transmit overhead information regarding network maintenance and signalling. Network, maintenance, and signalling information are transmitted in accordance with a channel and/or bit arrangement scheme to implement such functions as diagnostics (e.g., loopback), performance monitoring, embedded signalling, and ancillary communication links.
Various protocols, such as SONET and ISDN, have been brought forth which increase or maintain the payload bandwidth (i.e., the bandwidth available for transport of transmission information), and provide transmission link resources (i.e., channels, frames, bits) for transporting overhead information. However, implementation of such protocols presently requires the use of special hardware in addition to, or instead of, the standard hardware being utilized for primary rate carrier systems such as T1 and PCM-30.