Since the introduction of T-carriers in the 1960s, T-carrier and E-carrier networks have come a long way. Today, designers will see more and more functions being integrated into T1/E1/J1 single-chip transceivers. (SCT). The introduction of a large number of features can easily confuse designers. This article describes the important features that need to be addressed in design. 1 T, E, J carrier network The T1 operating at 1.544 Mbps is the lowest rate of the North American Public Switched Telephone Network (PSTN) digital multiplex transmission T carrier layer, which was originally designed for the transmission of digitized voice signals. T1 uses pulse code modulation and time division multiplexing to transmit 24 carrier-level speech signals, called DS0. Each DS0 or time slot can carry 64kbps of information. E1 is a European digital transmission format that can carry 32 channels of voice with a multiplexed clock rate of 2.048 Mbps. Now, T1 and E1 can not only transmit voice signals, but also transmit data or a combination of the two. T1 and E1 transmission lines have been widely used in cellular base stations, business access routers, and personal branch exchanges. Figure 1 shows a typical functional block diagram of the T1/E1/J1 SCT. J1 is a variant of T1 dedicated to Japan. 2.1 Line interface unit and framer The SCT's Line Interface Unit (LIU) is the physical layer interface to the copper loop. The framer can synchronize with the data received from the LIU and format the LIU data for transmission. The advantage of some SCTs is that the LIU can provide both short-range and long-range operation, which enables the SCT to be used simultaneously for the first layer of digital signal cross-connect (DSX-1) of the CSU (Channel Service Unit) / DSU (Data Service Unit). In the traditional T1/E1/J1 design, LIU and framer are independent ICs. With the development of ultra-large-scale integration technology, they can now be integrated into a single IC, and one of them needs to be checked in SCT. The function is to be able to directly access the signal between the LIU and the framer to allow modification of the data stream or insertion of code from an external source. Another benefit of this feature is the ability to use bidirectional code streams for monitoring purposes. 2.2 Software Programmable Line Termination Software-programmable copper loop termination is a very useful feature in a given T1/E1/J1 device internal feature. Dallas Semiconductor began with the DS2155 SCT, and later introduced ICs with this capability, engineers can design A complete T1/E1/J1 board to change the line termination matching impedance required by the copper loop through software without adding any external components. At present, this function has become a standard of design due to the increasingly demanding cost requirements of T1/E1/J1 devices. The T/E carrier loop sometimes has an instantaneous out-of-synchronization. This phenomenon is often caused by jitter. The jitter is defined as the amplitude of the phase change when the reference clock frequency changes by more than 10 Hz. To ensure system reliability and clock repeatability, it is best to choose an SCT that provides jitter suppression on the transmit and receive channels. In addition, some devices offer two jitter suppression buffers for applications that require large jitter, require high reliability, or require low response times. 2.4 Advanced Data Link Control Advanced Data Link Control (HDLC) is the Layer 2 processing type of the ISO 7-layer model. The maintenance and quality monitoring of physical layer links is a typical application of HDLC controllers. In order to improve the adaptability of the system, you can choose to be able to allocate single or multiple. HDLC controller for DSO or FDL bits. 2.5 BER Test Circuit (BERT) Communication channels generally require diagnostic tools for testing link quality. To this end, most SCTs include transmit and receive BERTs. The pseudo-random data template can be injected from the transmitting end into the copper loop by design, and then the injected signal is synchronized by the receiver BERT. BERTs can attenuate signals or inject noise into copper loops while working, which helps describe the performance of SCTs, copper loops, and higher software layers. However, if the BERT detects an error condition, it may be necessary to further study or test the communication channel. 2.6 protection switch Considering that the T/E transmission line provides critical data, many designs require a backup solution when the copper loop is unreliable or the line card is terminated. Since the protection switch typically allows the alternate SCT to switch to the line without losing frame synchronization to replace the failed SCT, many chips can provide this functionality without the need for external relays or switches. In fact, receivers with high-impedance inputs and LIU transmitters with three-state outputs are suitable for this type of application. 2.7 multi-port density One of the reasons why most T/E carrier line cards use multiple ports is to satisfy the 1.544 Mbps or 2.048 Mbps bandwidth. In this case, T1 and E1 are usually bound together or logically combined into one unit. According to the user's needs, the new SCT typically provides multiple port densities in a single package. 2.8 Low voltage power supply Traditional framers and LIU products typically require +5V power. However, in order to ensure low power consumption, most of the new products are powered by a single 3.3V supply, but also allow 5V input/output interfaces.
2 Design points
figure 1
2.3 jitter attenuator