Development of Transport Systems for Dedicated Service Provision Using Packet Transport Technologies

2.1 Accommodation on the packet transport network

(1) Handling accommodation efficiency and latency

To accommodate dedicated services on the packet transport network, processing is required in order to packetize fixed-rate user line signals in each type of dedicated service at a set period and to restore packet signals as user line signals (depacketization). Furthermore, there are standard values for latency time in dedicated service networks that must be satisfied.

One cause of increased latency that occurs in the packetizing process is buffering latency due to waiting for user line signals to reach packet lengths of a certain size. For this reason, it is necessary to shorten the period of packetization and reduce the amount of buffering to achieve lower latency. However, a smaller packet size means there will be proportionally more packet headers, which lowers the accommodation efficiency. In contrast, lengthening the period of packetization lowers the proportion of headers and hence raises the accommodation efficiency, but it also increases the amount of buffering, which increases latency. Thus, to optimize accommodation efficiency and latency, it was necessary to design an optimal packetization period.

In depacketizing, a cause of increased latency lies in the process of absorbing fluctuations in packet delay that occur on packet transport networks. Because user line signals are at a fixed rate, it is not possible to normally restore user line signals if there are fluctuations in packet latency. Hence, it is necessary to absorb packet latency fluctuations by buffering packets. If the size of the buffer for absorbing packet latency fluctuations is small, it is possible to reduce latency, but it is not possible to normally restore packets with large latencies as user line signals. In contrast, if the size of the buffer for absorbing packet latency fluctuations is large, it is possible to normally restore packets as user line signals, although the latency becomes large. For this reason, it is necessary to gain a sufficient understanding of latency fluctuation on packet transport networks so that an optimum buffer size can be set to absorb these fluctuations.

(2) Handling clock frequency transmissions

To handle fixed-rate user signals synchronized with clock frequencies in dedicated systems, it is necessary to ensure that the clock frequencies of each device match. However, because Ethernet is an asynchronous system, clock frequencies at the sending and receiving ends are not synchronized, and frequency deviations up to ±100 ppm occur at the sending and receiving ends. These frequency deviations cause misalignment in packetization and depacketization periods which accumulate and make it impossible to reproduce user line signals, and thus make it impossible to ensure quality of dedicated services. For this reason, clock frequencies must be transmitted via Ethernet.

2.2 High efficiency, high accommodation, energy saving

As new service items were added to dedicated service nodes, new equipment had to be developed, which created issues with installation space and accommodation efficiency. Hence, there are demands to accommodate multiple service items in single pieces of equipment to achieve greater efficiency and reduce space requirements. Lower power consumption has also become a requirement with the increased demand for greater energy saving in recent years.

2.3 Improved maintenance operability and reliability

When new dedicated service nodes are migrated to a system, the existing dedicated service nodes adopting the conventional SDH technology must coexist with the PTM adapters until all the circuits are migrated to the newly installed nodes. Hence, from the viewpoint of maintenance operations, it is necessary to seamlessly carry out circuit testing and alarm monitoring from the same control terminal without operators having to identify which equipment is SDH-based and which is PTM-based.

Furthermore, service continuity is also required for handling dedicated services. Hence, hitless switching functions are required so that services are not affected during maintenance work and failures.

3.1 Synchronous circuit emulation technologies

Synchronous circuit emulation technologies are used to achieve a level of communications quality similar to conventional dedicated services (latency, bit error rate). They consist of circuit emulation over packet (CEP) and synchronous Ethernet technologies. An overview of synchronous circuit emulation technologies is shown in Fig. 3.

Fig. 3. Synchronous circuit emulation technology.

CEP technologies are specified by the Internet Engineering Task Force (IETF) RFC5086 [1]. They convert fixed-rate line signals (including signaling signals) into packet signals. With transmission, fixed-rate line signals are packetized at 1–2 ms intervals and can be accommodated on the packet transport network by conversely depacketizing them upon reception.

Moreover, the PTM adapter is equipped with the following functions for packetizing and depacketizing. First, the PTM adapter can satisfy the latency requirement specified by dedicated services by setting optimal packetization periods for each service item. This also reduces the facility costs by improving the accommodation efficiency. Second, high-quality circuit emulation has been achieved by absorbing latency fluctuations that occur in the depacketization process. We have designed the optimal buffer size by simulating and measuring the expected maximum value on packet latency fluctuations with high accuracy, which is specified by dedicated services. We have also reduced the buffer size for latency fluctuations by depacketizing at an optimized timing with statistical processing. As a result, we have reduced the network latency.

Synchronous Ethernet technology is technology specified by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) in Recommendation G.8261 [2] for transmitting clock frequencies between devices via the physical layer of the Ethernet. After a reference clock frequency is generated in an oscillator in the transmitting device by using the clock frequency signal that was input, frequencies are converted to gigabit Ethernet (GbE) signaling speeds to send Ethernet signals. The transmitting equipment clock signals are reproduced by extracting the clock timing from the received Ethernet signal (the rise timing of the bit string) in the receiving equipment. This technology enables each piece of equipment to operate with the transmitted clock frequency, even via Ethernet.

3.2 High efficiency, high accommodation, energy saving

We have made it possible to reduce the maximum power consumption to about one-third that of conventional systems for the maximum number of accommodations by integrating existing dedicated service nodes required for each service item with the PTM adapters. We have also made more effective use of existing facilities by making subscriber interface packages compatible with existing equipment. Furthermore, we have drastically reduced the amount of space required by making it possible to house circuits in one rack, whereas five racks were previously required.

3.3 Improving maintenance operability

With the PTM adapters, we have made it possible to achieve seamless alert monitoring and line testing functions for SDH and packet transport segments from the same terminal by sorting the details required for testing and by coordinating each layer for SDH segments and packet transport segments. In terms of alert monitoring in particular, we have optimized determination criteria so that the transport error occurrence thresholds would be the same level for SDH and packets because the packet transport system detects errors packet-by-packet and discards errored packets, whereas the conventional SDH system detects errors bit-by-bit. This gives maintenance teams improved maintenance capabilities by enabling them to confirm normal operations in batches without having to consider the individual technologies used in each segment.

Additionally, we applied hitless switching technology to the PTM adapters to improve service quality. The hitless switching technology is illustrated in Fig. 4. This technology adjusts latency for short and long paths and uses systems to select first-come packets.

Fig. 4. Overview of hitless packet switching.

Moreover, since latency fluctuation is an issue with packet transport networks, we have enabled hitless switching with lower latency than the conventional system by using technology that efficiently absorbs packet latency differences between both paths through the use of statistical processing for the latency of each packet.