Is a 155m optical module a hundred-megabit or a Gigabit?

The rate of a 155M optical module is neither strictly "hundreds of megabits" (100Mbps) nor "gigabits" (1000Mbps), but a specific rate level—155.52Mbps, mainly derived from the STM-1 (Synchronous Transport Module level 1) rate in the SDH (Synchronous Digital Hierarchy)/SONET (Synchronous Optical Network) communication standard.
 

Specifically:

The standard rate of a hundred-megabit optical module is 100Mbps (such as the Fast Ethernet FE standard);

The standard rate of a gigabit optical module is 1000Mbps (such as the Gigabit Ethernet GE standard);

The rate of a 155M optical module is 155.52Mbps, the most basic rate level in the early SDH/SONET system, often used in traditional telecommunication transmission networks. It belongs to a different technical system than the Ethernet "hundreds of megabits" and "gigabits," and the rates do not directly correspond.

The 155M optical module (rate 155.52Mbps) originates from the SDH/SONET system. Its application scenarios mainly revolve around traditional telecommunications transmission, early data communication, and specific leased line services, as follows:

1.  Basic SDH/SONET transmission network support

As the core optical interface of STM-1 (Synchronous Transport Module level 1) in the SDH system, the 155M optical module is a basic component of traditional telecommunications backbone networks and metropolitan area networks.

Used for fiber optic connections between SDH equipment (such as multiplexers, add-drop multiplexers ADM, cross-connect devices DXC) to achieve the convergence and transmission of multiple services.

As the "smallest granule" of the SDH network, it can carry multiple low-rate signals (such as 2Mbit/s, 34Mbit/s, 140Mbit/s) for multiplexing, building a hierarchical transmission network.

2.  Traditional voice and telecommunications service support

Voice transmission Early fixed telephone network (PSTN) voice signals are transmitted after PCM encoding via the SDH network. The 155M optical module, as a basic link, carries a large number of voice channels (155M can carry approximately 1920 voice signals).

ISDN and low-speed data services Supports Integrated Services Digital Network (ISDN), X.25, and other early data services, providing narrowband data communication services for enterprises.

3.  Transitional medium-speed data communication link

Medium-speed interconnection of enterprise networks and campus networks Before the popularization of Gigabit Ethernet (GE) (early 2000s), 155M optical modules were often used for cross-building/cross-regional connections between switches and routers in enterprise or campus networks, serving as a medium-speed data link (rate higher than hundreds of megabits, lower than gigabits) to meet moderate bandwidth needs.

Router and server interconnection Some high-end servers or storage devices (such as early minicomputers) connect to core routers via 155M optical modules to achieve centralized data transmission.

4.  Leased line and point-to-point data transmission services

Enterprise leased line services In the "leased line" services provided by operators (such as DDN leased lines, frame relay, ATM leased lines), the 155M optical module serves as the underlying transmission carrier, providing stable fiber optic links for point-to-point data communication between enterprises (such as interconnection between headquarters and branches), supporting file transfer, database synchronization, etc.

Financial and government dedicated networks In areas with high stability requirements (such as interconnection of bank branches, government data transmission), SDH networks were widely used in the early days. The 155M optical module, as a transmission unit between nodes, ensures low latency and high-reliability data interaction.

5.  Broadcast television and monitoring transmission

Digital television signal transmission Early digital television signals (such as from satellite receiving stations to television stations, and from television stations to cable network front ends) were transmitted via SDH networks. The 155M optical module was used for signal relay and distribution in fiber optic links.

Long-distance video surveillance Before the popularization of IP surveillance, some large-scale security systems (such as highways, oil fields, and mining areas) transmitted high-definition video signals via SDH networks. The 155M optical module provided stable fiber optic transmission capabilities, supporting the convergence transmission of multiple monitoring signals.

Summary

The application scenarios of the 155M optical module mainly revolve around the traditional SDH/SONET transmission system primarily carrying voice, data, and video services. It is widely used in telecommunications operator networks, enterprise leased lines, and traditional broadcasting and monitoring systems. With the popularization of Gigabit and 10-Gigabit optical modules, it is gradually relegated to "legacy system maintenance" scenarios, but it still has some applications in networks that need to be compatible with old equipment.