IEEE 1588 Precision Time Protocol-Frequency synchronization on packet networks
Telecommunications networks are rapidly shifting from circuit-switching technology to packet-switching technology to meet the rapid expansion of bandwidth requirements for core and access networks. The traditional circuit-switched TDM network itself supports precise frequency synchronization across the entire network. To ensure high levels of QoS for end user equipment, access platforms such as wireless base stations and multi-service access points (MSAN) still rely on the synchronization functions provided on the network backhaul connection. In a telecommunications network, the ability to provide carrier-class synchronization quality to remote wireless base stations and access platforms via Ethernet is the key to the evolution of Ethernet backhaul networks.
Time transfer protocol
The telecommunications equipment that originally used the time transfer protocol used servo control loops to drive reference oscillators in remote network elements (such as street chassis access platforms and wireless base stations). The reference oscillators in these remote network elements used to resume synchronization from the T1 / E1 TDM backhaul connection. As long as the TDM transmission network can track the reference reference clock (PRC), the remote network elements can use relatively simple servo control to lock their oscillators to the return feedback clock that can track the PRC. The problem comes when the backhaul connection becomes Ethernet—the remote network element is isolated from the synchronization source. This article will discuss how to use IEEE 1588 Precision Time Protocol (PTP) on Ethernet to provide synchronization to remote network elements. Although Ethernet has gained widespread popularity and is an ideal medium for low-cost connections, it is not very suitable for applications requiring precise synchronization. Ethernet is inherently a non-deterministic network, and it is difficult to provide real-time or time-sensitive applications that require synchronization. PTP overcomes the Ethernet delay and jitter problem well through the hardware time stamping technology of the network physical layer. Therefore, using the Ethernet network to carry clock data packets can achieve unprecedented accuracy in the range of 100ns, thereby significantly saving costs.
Synchronization function of next generation network
GPS-based satellite receivers can provide accuracy of less than 100ns, and are often used in areas where precise time and frequency synchronization are critical, such as telecommunications, military, and aerospace applications. But the cost of improving accuracy is huge. GPS-based systems need to install outdoor antennas to ensure that they see the sky directly in order to receive low-power satellite transmission signals. This not only increases the cost, but also places an additional burden on the physical structure of the facility. For this reason, GPS is best used as a reference clock for telecommunications networks in central offices, and then uses other techniques to distribute synchronization and timing to remote devices. Telecom operators and equipment manufacturers are studying multiple new ways to provide synchronization via Ethernet.
* Adaptive Clock Recovery (ACR): Many non-standard solutions based on Circuit Emulation Services (CES) use ACR technology to regenerate network clocks in remote downstream units. However, operators have encountered some performance issues when using this technology, not to mention that some major operators are usually reluctant to deliver large-scale new business deployments to non-standard solutions for implementation.
* Synchronous Ethernet: ITU has recently completed the definition of synchronous Ethernet (G.8261, G.8262, G.8263) designed to meet the frequency synchronization needs provided by Ethernet transmission networks. The basic difference between existing Ethernet and synchronous Ethernet (Sync-E) is to send the PHY clock. The current IEEE 802.3 requires the transmission clock to achieve a 100 ppb (one billionth) free-running clock accuracy. In synchronous Ethernet, the accuracy of the sending clock must reach 4.6ppb, and it can be tracked to the first-level clock through the external SSU / BITS reference or receiving clock. By simply linking the send and receive clocks of Ethernet, synchronous Ethernet can be used to exchange data with SONET / SDH. The challenge for synchronous Ethernet is that all Ethernet switches must be upgraded to have synchronous Ethernet functionality along the entire path between PRC and terminal equipment.
* Network Time Protocol (NTP): NTP as the most popular protocol is widely used for time synchronization on LAN and WAN. The implementation cost of NTP is relatively low, and almost no hardware modification is required. However, the current version of NTP and implementation solutions cannot yet meet the higher precision requirements required for telecommunications network synchronization.
On the other hand, PTP can provide cost-effectiveness close to NTP by using the existing Ethernet distribution network, and achieve accuracy exceeding NTP by using hardware-based time stamping technology. PTP can coexist with normal network services on a standard Ethernet network using high-speed switches, while providing millisecond synchronization accuracy. The key to achieving this outstanding performance index is the hardware-assisted time stamping technology.
PTP working principle: hardware-assisted time stamping technology
The two main problems that must be overcome in network time keeping applications are oscillator drift and time propagation delay. Regardless of the protocol used, the oscillator drift problem can be alleviated by using a higher quality oscillator and obtaining time from a higher precision clock source (such as GPS). The problem of time transmission delay is more difficult to solve, and it has a dual nature: there are delays related to the processing of time packets by the operating system, and there are networks caused by routers, switches, cables, and other hardware between the source clock and the destination clock. Delay. PTP is the most successful in reducing operating system latency and jitter.
PTP combines an innovative method of time stamp exchange between a time stamp unit (TSU) and a master-slave clock. The TSU, located between the Ethernet media access control (MAC) and the Ethernet PHY receiver, simultaneously sniffs the input and output data streams, and issues a timestamp when the leading bit of the IEEE 1588 PTP packet is identified, used to accurately mark the PTP time The arrival or departure of data packets (see Figure 1).
F1: TSU is located between Ethernet MAC and Ethernet PHY receiver.
In order to estimate and reduce operating system delay, the master clock periodically sends a Sync message to the slave clock on the network according to the local clock. TSU marks the exact time of the sent Sync message. The slave clock also time stamps the arriving Sync message, then compares the arrival time with the departure time provided in the Sync message, so you can determine the amount of delay in the operating system, and finally adjust the clock accordingly.
By measuring the round-trip delay between the master clock and the slave clock, the network-related delay can be reduced. The slave clock periodically sends a delay request message (Delay_Req) ​​to the master clock, and then the master clock initiates a delay response message (Delay_Resp). Since both messages have precise time stamps, the slave clock can combine this information with the details from the Sync message to measure and adjust the delay introduced by the network. The detailed time stamp exchange protocol is shown in Figure 2.
Figure 2: The sequence of data packets used to transfer time from the PTP master clock to the PTP slave clock.
Sync packets are time stamped when they leave the master clock (T1) and arrive at the slave clock (T2). The follow-up packet transmits the Sync packet departure time to the slave clock. Delayed response packets are also time stamped when they leave the slave clock (T3) and arrive at the master clock (T4). Sync packets and Follow-Up packets are sent out periodically by the master clock as delay request and delay response packets. The formula used for slave clock correction is: 0.5 (T1 – T2 – T3 – T4).
Determine target accuracy
The PTP protocol uses hardware time stamping technology to provide sub-microsecond accuracy. The performance on the telecommunications WAN depends on the following three main factors:
* Resolution and accuracy of the timestamp engine in the master-slave clock (starting accuracy)
* Delay across the WAN / Packet Delay Variation (PDV), including hop count, load, and switch / router configuration
* Servo processing gain and oscillator implementation on the slave clock side (how efficient the PDV uncertainty is filtered out)
In the case of high initial accuracy, the packet delay variation (PDV) on the telecommunications network will soon become the dominant error factor in packet-based timing solutions. Two-layer switching networks that focus on QoS configuration and load changes can provide the best PDV performance. This situation is very suitable for IEEE 1588 PTP, because PTP is optimized for the two-layer switching environment. However, PDV is the dominant factor in the three-layer software routing network. The stability of the oscillator and the servo design on the slave clock side will become key performance factors to ensure that the synchronization requirements of the telecommunications network are met.
Select broadcast interval and oscillator type
In PTP, the target timing accuracy determines the frequency of synchronization message broadcast and what type of oscillator is used. More frequent broadcasts can get more precise synchronization, but they also generate more network traffic, although the bandwidth used is very small. Higher quality oscillators can also get more accurate synchronization. It seems tempting to use a lower quality oscillator while increasing the broadcast frequency to achieve the target accuracy more economically, but this approach is not recommended. Low-quality oscillators lack the stability required to provide high-precision PTP for telecommunications applications, so shortening the broadcast interval is usually not worth the gain.
Precision is also a function of the IEEE 1588 master clock. The IEEE 1588 master clock is also known as the highest-level clock (grandmaster) and is the final time source on the network. The most advanced clock is usually based on GPS, so it is very stable and very accurate. UTC (Coordinated Universal Time) accuracy is usually above 30ns RMS. By using such a high-precision clock and absolute time reference, the time on the PTP network can be well synchronized. The high-quality top-level clock also has other measurement characteristics that can be used to characterize the delay and jitter characteristics of the network element and measure the accuracy of the slave clock relative to the top-level clock.
Choose other hardware
PTP can play a good role on Ethernet networks where router buffer delay and switch delay affect the accuracy of time transmission. Figure 3 compares the delay and PDV measurements made on typical Ethernet switches, wire-speed routers, and software-based routers. Advanced wire-speed routers can provide fast switching comparable to traditional two-layer switching in terms of latency and PDV, making them ideal for PTP synchronous distribution applications. On the other hand, the high latency and PDV associated with software-based routers may become a limiting factor as described above.
Figure 3: Histogram showing the delays of the Ethernet switch (top), wire-speed router (middle) and software router (bottom). The performance of a wire-speed router is equivalent to a two-layer switch, while the PDV of a software router is two orders of magnitude higher.
The PTP protocol also introduces some special components, such as a boundary clock and a transparent clock, that is, a switch that has only one port to provide the PTP slave clock to the master clock, and other ports maintain accuracy by adding functions. Boundary clock refers to a multi-port switch that has one port from the PTP slave clock to the master clock, and the other ports from the master clock to the downstream slave clock. The boundary clock provides a good way to adjust synchronization to many subnets. However, the use of cascading boundary clocks can accumulate non-linear time offsets in the servo loop, which ultimately leads to unacceptable accuracy degradation.
Transparent clocks are another potential hardware option in PTP networks. This is a switch with PTP function, which can eliminate the internal reception and transmission delay of the switch by modifying the precision time stamp in Delay_Resp and Follow-Up messages, thereby improving the synchronization accuracy between the slave clock and the master clock. However, when the original data packet password checksum does not match the final data packet arriving at the slave clock, the transparent clock may also create security issues.
Application of PTP in telecommunications
Many telecommunications network equipment providers regard IEEE 1588 PTP as the most cost-effective method to meet the synchronization requirements of next-generation wireless and access platforms. For example, all GSM and UMTS base station frequencies must be synchronized to ± 50ppb (one billionth of a billion) to support network handover when mobile phones move from one base station to another. Failure to meet the 50ppb synchronization requirement will cause the call to be interrupted. To meet this requirement, the traditional approach of the base station is to lock the internal oscillator to the clock recovered from the T1 / E1 TDM backhaul connection. When the return channel becomes Ethernet, the base station is disconnected from the traditional network synchronous feedback connection. Figure 4 shows a typical deployment scenario where a wireless network using PTP provides synchronization to remote base stations. The base station will use PTP to recover the timing data packets from the device, and then used to control the base station's internal oscillator to meet the 50ppb requirement. The PTP slave device in the base station needs to access the carrier-level PTP highest-level clock deployed in the mobile switching center (MSC). Key considerations for deploying the most advanced PTP clock in MSC include:
Figure 4: To provide synchronization to the next-generation UMTS base station, the highest-level PTP clock deployed in the MSC / RNC is required. Synchronous data packets flow from the highest level clock to the slave clock in the base station.
* Integrate the most advanced clock function of PTP into the existing MSC synchronization platform (that is, BITS—building integrated clock supply system, and SSU—synchronization providing unit).
* Ethernet transmission unit configuration-fast exchange
* Oscillator selection and PTP slave / servo control
MSAN and IP-DSLAM are also required to support traditional TDM applications, such as T1 / E1 landing services. Device manufacturers use PTP as a method to distribute synchronization to remote terminal-based access platforms. ITU recently released the G.8261 standard with a view to establishing synchronization requirements for packet networks. The considerations in the specific implementation are the same as the above-mentioned wireless platform, the key is still to integrate the PTP's most advanced clock function into the BITS and SSU platforms of the telecom central office (Figure 5).
Figure 5: MSAN supporting traditional TDM services requires synchronous distribution from the central office via Ethernet backhaul. The PTP slave clock in MSAN can be synchronized with the PTP highest-level clock in the central office BITS / SSU.
PTP development prospects
Since its launch in 2002, PTP has received a high degree of attention, and its impact is increasing day by day. The hardware produced by many network equipment suppliers now supports PTP in the network system. The IEEE 1588 PTP protocol is continuing to improve in order to further improve accuracy, improve fault tolerance, and enhance management capabilities in telecommunications applications. Nanosecond precision, easy deployment, and cost-effective PTP are quietly changing the prospects of synchronization applications in many fields.
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