The Evolution of PCM Technology in Optical Terminals within Optical Fiber Communication Systems

    PCM encoding principles and rules: The PCM digital interface complies with the G.703 standard, supporting asymmetric or symmetric transmission via 75Ω coaxial cable or 120Ω twisted pair. The transmission line code is HDB3, which contains timing information. The receiving end can recover timing through decoding to achieve clock synchronization. Fb is the frame synchronization signal, C2 is the clock signal at 2.048Mbps, and data is valid on the falling edge of the clock. The E1 interface has a PCM frame structure: one multiframe consists of 16 frames, each frame is 125μs and divided into 32 timeslots. The zero timeslot of even frames carries synchronization information code 0011011, the zero timeslot of odd frames carries alarm indication signals, timeslot 16 carries signaling information, and the remaining timeslots carry data, with each timeslot transmitting 8 bits of data.

    In fact, E1 is just one standard (manifestation) of PCM. When we refer to PCM equipment, it generally means 30 telephone channels transmitted over an E1 line, also known as an E1 line multiplexer. In addition to transmitting telephone FXO/FXS signals, such equipment also supports other service interfaces including 2/4W audio, magneto, RS232, same-direction 64K, V.35, V.24, and Ethernet (in private networks such as military applications, video conferencing and telephony are transmitted simultaneously over one E1 line, requiring telephony to intelligently and dynamically occupy bandwidth—i.e., no bandwidth is occupied when the phone is on-hook, and video conferencing has priority functionality). This technology has been applied for many years and is widely used in public and private network communications.

    E1 is generally transmitted via optical terminals (SDH/PDH) or HDSL. In recent years, with the proliferation of optical fiber, Fiber-to-the-Home (FTTH) has been put on the agenda, and a fiber-optic PCM device has emerged in the industry. That is, signals such as telephone FXO/FXS, 2/4W audio, magneto, RS232, same-direction 64K, V.35, V.24, and Ethernet are transmitted directly over optical fiber without the need for SDH/PDH relay. Alternatively, traditional PDH equipment can transmit the aforementioned 2/4W audio, magneto, and other signals in addition to E1 signals. Since most applications involve ordinary telephone signals, this type of equipment is also called a "Telephone Optical Terminal."

    This is not yet the latest PCM technology. Recently, Tianwei Telecom launched IPPCM, also known as network PCM equipment, where telephone signals are transmitted directly over IP/Ethernet. This technology immediately attracted significant attention from private network customers in customs, public security, and military sectors upon its release.

    TOIP is a telephone multiplexer designed for IP network environments. This device achieves transparent conversion of PHONE or E1 lines to IP packets, enabling information to be transmitted over IP-based Gigabit Ethernet backbone networks. In this way, voice and data no longer require high-level processing such as VoIP gateways, and can be transmitted directly over IP networks. TOIP captures information streams from up to TDM ports and converts them into information packets for transmission over IP networks. Against the backdrop of rapidly growing demand for TDM access over high-speed packet-switched networks, the overall utility of TOIP is to enable new and existing operators and enterprises to achieve transparent telephone or E1 connectivity in new-generation packet-switched network environments, extending the service life of existing equipment. This product has already achieved considerable reliability.

    1) TOIP applications not only provide excellent voice quality but also preserve existing PBX system features. It is easy to install and maintain, cost-effective, and supports Modem, fax, and video at various rates.

    2) In TDM over IP, synchronous bit streams are packetized into packets, then IP headers are added, and the packets are sent to the destination over the IP network. At the destination, the original bit stream is recovered by removing the IP header, concatenating the packets, and restoring the clock. In this way, traditional telephone services and E1 leased line services can be delivered over high-speed data backbone networks (such as Gigabit Ethernet, wireless networks, optical fiber, etc.).

    3) The most notable feature of TOIP equipment is that it performs no compression on voice, providing voice quality that VoIP cannot match. It requires no complex signaling or protocol processing, makes no software or hardware changes to the existing telephone network, and leverages low-cost IP networks for voice transmission.