14 Common FAQs About Telephone Optical Terminals
Aug 11, 2026
How to evaluate the performance of a telephone optical terminal How many optical ports and optical directions it has. How many E1 channels it provides. Whether it has an Ethernet interface. W
14 Common FAQs About Telephone Optical Terminals
- How to evaluate the performance of a telephone optical terminal
- How many optical ports and optical directions it has.
- How many E1 channels it provides.
- Whether it has an Ethernet interface.
- Whether it has a V.35 interface.
- Whether it has a V.11/V.24 interface.
- Whether it has an order wire telephone.
- Whether it has network management.
- Networking mode: whether it can form a self-healing ring.
- Number of optical ports 1: Point-to-point mode
The cheapest telephone optical terminal has only 1 optical port per pair of devices, forming a point-to-point working mode, also called terminal mode: TM. - Number of optical ports 2: Dual optical ports, single direction
Better telephone optical terminals have dual optical ports. Dual-port optical terminals are further divided into: 1 or 2 optical directions. With 1 optical direction and dual optical ports, it operates in 1+1 backup mode. Once the primary fiber is broken, the secondary optical port ensures continued operation. - Number of optical ports 3: Dual optical ports, dual directions
Part of the signal from the left optical port drops locally, while the other part continues to be transmitted from the right optical port. This working mode is called add-drop multiplexing mode: ADM. Optical terminals with ADM capability can form a dual-fiber self-healing ring. - Benefits of a dual-fiber self-healing ring
The benefit of a self-healing ring is that once the fiber cable is cut, the entire communication will not be interrupted. When the fiber cable is normal, information between stations travels via the outer ring. After a cable cut, information sent from Station 1 toward Station 2 is rerouted via the inner ring. When the fiber cable is repaired, the network automatically returns to normal. Only optical terminals with ADM capability can implement a self-healing ring. - Number of optical ports 4: Multiple optical ports, multiple optical directions
The best telephone optical terminals are SDH-based, with multiple optical directions, but the price is much higher. A detailed discussion of SDH equipment is given later, along with a network diagram that is a collection of various modes. - Number of E1 channels 1: PDH system
The simplest optical terminal has 4 E1 ports, commonly called small 8M.
30 analog telephones can be multiplexed into one 2M, called a primary group E1.
4 E1s can be multiplexed into one 8M, called a second-order group, also E2.
4 E2s can be multiplexed into one 34M, called a third-order group, also E3.
4 E3s can be multiplexed into one 140M, called a fourth-order group, also E4.
This system is called PDH, also known as the plesiochronous system. - Number of E1 channels 2: SDH system
The SDH system is a new system, born just 10 years ago. The smallest capacity SDH is 155 Mb/s, called STM-1, with 63 E1s.
4 STM-1s are multiplexed into one STM-4, also called 622.
4 STM-4s are multiplexed into one STM-16, called 2.5G.
4 STM-16s are multiplexed into one STM-64, called 10G.
It can be seen that SDH optical terminals are high-capacity optical terminals.
155 and 622 are the most widely used optical terminals in the SDH system. - Ethernet interface
Ordinary telephone optical terminals do not have an Ethernet interface. Optical terminals with an Ethernet interface are a new market demand of the past 2 years.
1 E1 channel can carry a 10M Ethernet signal.
4 E1 channels can carry a 100M Ethernet signal. - V.35 interface
Ordinary optical terminals do not have a V.35 interface.
The V.35 interface is also a market demand of the past 2 years.
The V.35 interface is a synchronous data interface with a rate of N×64 kb/s.
Usually 1 E1 is used to transmit one V.35 interface. - Auxiliary channels and order wire
Ordinary optical terminals generally have auxiliary channels. Auxiliary channels are low-speed asynchronous data interfaces, such as RS232. The order wire telephone is used for inter-station liaison, but is actually rarely used. Very cheap optical terminals do not have an order wire telephone, and some even do not have asynchronous data. - Network management system
Network management means computer network management, also called centralized monitoring. Connecting the operating status information of all station optical terminals to the network management computer in the central network management room is network management.
Generally, PDH optical terminals do not have a network management system.
Generally, SDH optical terminals all have a network management system. - Networking mode
Generally, PDH optical terminals can only do TM, i.e., point-to-point networking.
Generally, PDH optical terminals have only 1 optical port.
SDH optical terminals can work in both TM mode and ADM mode, and therefore can form a self-healing ring network.
A working mode with 3 or more optical directions is called DXC, i.e., cross-connect mode. - Specifications of telephone optical terminals
Telephone optical terminals have many specifications, but none of them are critical. The only somewhat useful one is transmission distance. Generally, optical terminals can transmit 20-60 km. When a longer distance is required, simply replacing the optical port module is sufficient. Another item is the optical port connector type. The more common types are FC (round) and SC (square). When ordering, it involves whether to include pigtails.