What is OTN Network Delay

What is Delay?

In a communication system, delay is a critical technical index, which refers to the length of time it takes for data to be transmitted from one network device to another in the network.

The delay mainly includes three parts: transmission delay, propagation delay, and processing delay.

• Transmission delay: the time spent from the beginning of the original data entering the sending end of the network device to the time spent from the sending end until it is completely in the transmission medium. The size of this delay depends on the amount of data, channel bandwidth, and processing performance of the network device.
• Propagation delay: the time it takes for the data to be sent from the sending end to be received at the receiving end. The size of this delay depends on the transmission distance, transmission medium, the number of intermediate network devices/processing performance.
• Processing delay: the time it takes for a network device to start receiving information and reduce it to the original data. This delay depends on the processing performance of the network device.

How is the delay calculated for data transmission from network device 1 to network device 3?

As can be seen from the figure, it is still divided into three parts for delay calculation:

• Transmission delay of network device 1:

The time spent by A’s pending data to be transmitted starting from its entry into the sending end of network device 1 to the time it is sent out from the sending end of network device 1 until it fully enters the transmission link between network device 1 and network device 2.

This part of the delay is strongly influenced by the performance of network device 1. If the processing speed is fast, the transmission delay is low.

• The data passes through network device 2 and the transmission delay from network device 1 to network device 3:

This part of the delay is affected by the transmission distance the number of intermediate devices in the network and the processing performance. It can be clearly concluded that if the transmission distance is close, the intermediate network devices are less, and the processing performance of the intermediate network devices is fast, the transmission delay is low.

• Processing delay of data restored by network device 3:

This part of the delay is greatly affected by the processing performance of network device 3. If the processing speed is fast, the processing delay is low.

What is Delay in OTN Networks?

The definition of delay in a generalized communication system was explained earlier. Then what is the delay of OTN network?

An OTN network is a communication system that uses OTN (Optical Transport Network) devices as the network equipment and optical fiber as the transmission medium. The latency of an OTN network is the amount of time it takes for data to be transmitted from the OTN device at the sending end to the OTN device at the receiving end.

Like general-purpose communication systems, the latency of an OTN network consists of three components: transmission delay, propagation delay, and processing delay.

Below is a typical OTN network system composition and delay distribution diagram.

Transmission Delay

The transmission delay is the time it takes for the raw data to enter the transmitting end of the OTN device at the transmitting end, to be sent out from the transmitting end, and until it is completely in the line fiber.

The raw data undergoes the following two main processing flows at the sending end of OTN equipment before the data can be sent to the line fiber for transmission.

1. Through the truck – OTU (Optical Transponder Unit) converted to OTN equipment can be transmitted to the data. OTU to process the data spent on the instantaneous delay of the order of 10μs ~ 100μs.

2. Through the checkpoint – WDM achieves a fiber-combined transmission of multiple data optical signals. The instantaneous delay time spent by the combining/splitting unit to process the data is in the order of ns.

Propagation Delay

Propagation delay is the time it takes for data to be sent from the sending OTN device to be received by the receiving OTN device. The data sent from the sending OTN device is mainly transmitted via optical fiber. The delay of optical fiber is of the order of 5μs/km.

Since optical signals in optical fibers may cause dispersion effects, dispersion compensation modules may be needed to compensate for the problems caused by dispersion. Therefore, the propagation delay must also include the transmission delay caused by the dispersion compensation modules.

When the transmission distance is long, repeaters (i.e., optical amplification units) need to be added along the OTN network to regenerate and amplify the optical signals before continuing to transmit. From the above analysis, we can see that the propagation delay is affected by optical fibers, dispersion compensation modules, and optical amplification units.

Processing Delay

The size of the processing delay depends on the “receiving OTN device”. From the above “OTN network system composition and delay distribution diagram”, we can see that the data processing process of the receiving OTN device is exactly the reverse process of the data processing of the sending OTN device.

Through the checkpoint (multiplexing/demultiplexing unit), the multiplexed data optical signal in the optical fiber is restored to a single-channel data optical signal.

Through the truck (OTU unit), the single-channel data optical signal is converted into original data. Therefore, we can assume that the processing delay is the same as the transmission delay, mainly affected by the optical forwarding unit OTU and the multiplexing/demultiplexing unit.

Therefore, we can see that the OTN network delay is mainly affected by the transmission link (such as optical fiber) and physical devices (such as OTU unit, multiplexing/demultiplexing unit, and optical amplification unit).

How to Reduce the Delay of OTN Network?

As we know, 5G network requires ultra-low delay, and even the end-to-end delay of the whole network is required to be as low as 1ms. OTN network, as an important bearer network of 5G network, generally starts from the two factors of transmission link and physical device analyzed above to reduce the delay in order to meet the ultra-low delay requirement of 5G network.

Method 01 Optimize transmission link

Use a minimalist network architecture design, reduce intermediate forwarding nodes, reduce intermediate optical amplification units, construct a single-hop network, and reduce transmission delay. Use coherent optical communication technology, which does not need dispersion compensation modules, and can eliminate the additional transmission delay introduced by dispersion compensation modules.

Coherent optical communication does not mean that the transmission light is coherent light communication, and any optical communication system uses laser. It is because the sending end uses coherent modulation and the receiving end uses coherent technology for detection in the coherent optical communication system, so it is called coherent optical communication.

Method 02 Optimize physical devices

• Optimize the OTU unit and reduce the delay introduced by the OTU unit.
• Use RAMAN amplifier instead of EDFA to reduce the delay of the optical amplification unit.
• Use advanced optical layer technology to realize direct and switching of optical signals, and reduce the number of optical-electrical-optical regenerations.