Since the introduction of T carrier in the 1960s, T carrier and E carrier networks have gone through a long development path. Today, designers will see more and more functions being integrated into T1 / E1 / J1 monolithic transceivers (SCT). The introduction of a large number of functions is easy to confuse designers. This article introduces the important performance that needs to be paid attention to in the design. 1 T, E, J carrier network T1, which works at 1.544 Mbps, is the lowest rate of the T carrier layer of the North American Public Switched Telephone Network (PSTN) digital multiplex transmission. It was originally designed for the transmission of digitized voice signals. T1 uses pulse code modulation and time division multiplexing technology to transmit 24 carrier-level voice signals, called DS0. Each channel of DS0 or time slot can carry the information volume of 64kbps. E1 is a European digital transmission format that can carry 32 channels of voice, and the multiplexed clock rate is 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 the typical functional block diagram of T1 / E1 / J1 SCT. J1 is a variant of T1 dedicated to Japan. 2.1 Line interface unit and framer The line interface unit (LIU) of SCT is the physical layer interface to the copper cable 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 LIU can provide both short-range and long-range operations, which allows SCT to be used for the first layer of digital signal cross-connection (DSX-1) of CSU (channel service unit) / DSU (data service unit). In the traditional T1 / E1 / J1 design, the LIU and the framer are independent ICs. With the development of ultra-large-scale integration technology, they can now be integrated into a single-chip IC. One that needs to be checked in SCT The function is whether the signal can be directly connected between the LIU and the framer to allow modification of the data stream or insertion of code from an external signal source. Another advantage of this function is the ability to use bidirectional code streams for monitoring purposes. 2.2 Software programmable line termination In the given T1 / E1 / J1 device internal characteristics, software programmable copper cable loop termination is a very useful function. Dallas Semiconductor started with DS2155 SCT. ICs introduced later have this function. Engineers can design A complete T1 / E1 / J1 board, so as to change the line terminal matching impedance required by the copper cable loop through software without adding any external components. At present, because T1 / E1 / J1 equipment's cost requirements are becoming more and more stringent, this function has become a design standard. The T / E carrier loop sometimes has instantaneous out-of-synchronization. This phenomenon is often caused by jitter, which is defined as the amplitude of the phase change when the reference clock frequency changes by more than 10 Hz. In order to ensure the reliability of the system and the repeatability of the clock, it is best to choose a SCT that can provide jitter suppression in the transmit and receive channels. In addition, some devices also provide two-way jitter suppression buffers, which are suitable for applications with large jitter, high reliability, or low response time. 2.4 Advanced Data Link Control Advanced Data Link Control (HDLC) is the layer 2 processing type of the ISO 7-layer model. Physical layer link maintenance and quality monitoring are typical applications of HDLC controllers. In order to improve the system's adaptability, you can choose to assign a single or multiple A HDLC controller with a DSO or FDL bit. 2.5 Bit error rate test circuit (BERT) Communication channels generally require diagnostic tools to test link quality. For this reason, most SCTs include sending and receiving BERT. During design, the pseudo-random data template can be injected from the sending end to the copper cable loop, and then the receiver BERT can synchronize the injected signal. When BERT works, it attenuates the signal or injects noise into the copper cable loop, which helps to describe the performance of the SCT, copper cable loop, and higher software layers. But if BERT detects an error condition, it may require further research or testing of the communication channel. 2.6 Protection switch Considering that the T / E transmission line provides vital data, many designs require a backup solution when the copper cable loop is unreliable or the line card terminates. Since protection switches generally allow spare SCTs to switch to the line without losing frame synchronization, many chips can provide this function 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 meet the bandwidth of 1.544 Mbps or 2.048 Mbps. In this case, T1 and E1 are usually bound together or logically combined into a unit. According to the user's demand, the newly launched SCT usually provides multiple port densities in a single-chip package. 2.8 Low voltage power supply Traditional framers and LIU products generally require + 5V power supply. However, in order to ensure low power consumption requirements, most of the newly launched products use 3.3V single power supply, but also allow 5V input / output interface.
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2 Design points
figure 1
2.3 Jitter attenuator