Application of Synchronization Status Message Based on the Self-healing Ring Network with Synchronous Digital Hierarchy

Abstract

This paper studies how to avoid timing loop in the self-healing ring (SHR) network with synchronous digital hierarchy (SDH) and analyzes the generation and mechanism of timing loop in SDH local ring network as well as the harmfulness of timing loop to the network. Based on these results, a new method is proposed to avoid the generation of timing loop in the multiplex section protection ring by combining the K1 and K2 bytes in APS protocol of the multiplex section with S1 byte in the synchronization status message (SSM). This method improves the synchronization performance of SDH SHR network. Introduction In the SDH network, since the payload is asynchronously transferred, the different rates in the sender and receiver cause pointer justification, which is supposed to result in the jitter and wander of output signals. The greater jitter is bound to result in the loss of frame (LOF), while the excessive wander is supposed to lead to the sliding of terminal equipments. Therefore, attention has been paid to satisfy the requirements of ITU-T standard for the jitter and wander of the output signals in the SDH network synchronization. In the SDH network, when all the nodes are synchronous, pointer justification events rarely happen; while when the synchronization quality is degraded, these events frequently occur. In turn, multiple pointer justification events bring about stricter requirements for the synchronizer which has a phase smoothing effect on the boundary of SDH/plesiochronous digital hierarchy (PDH). Hence, the synchronous performance of the network is crucial. By reducing the slip rates and the times of pointer justification, the sliding damage and the influence of jitter and wander on network services are reduced. Besides, Through effectively controlling the sliding, this paper attempts to have all equipments in the network using the reference clock with a same frequency [1.2.3]. Generation of timing loop Timing loop refers to the phenomenon that clocks track their output signals directly or indirectly through the network. The SDH equipment clocks (SEC) are able to receive multiple external synchronous signals, which are automatically switched based on the priority. When there is no system safety monitor (SSM), timing loops are easy to be formed. When the timing loop passes through a ring network, synchronization supply units (SSU) are equipped in the places where a timing signal enters into and leaves the SDH ring network. SSU1 is configured in the entering place of timing into the SDH ring to form a gateway equipment, namely, SDH equipment timing source (SETS1). SSUk is provided in the leaving place of timing to form SETSn. As presented in Figure 1, when the main timing path breaks down, timing loops are formed between two network units. At this moment, the clock is gradually skewed, resulting in large frequency offset. As a result, network services are interrupted. The clock of node locks its signal so that a timing loop is generated. If the reference timings of all node clocks in the ring network are independent, timing loops are impossible to occur. In the usage of local ring network with multi-clock, the timings of all the nodes are from the external network. This scheme has high requirements for the performance of clocks and high economic cost. In addition, 4th International Conference on Mechatronics, Materials, Chemistry and Computer Engineering (ICMMCCE 2015) © 2015. The authors Published by Atlantis Press 640 there occur periodic slides, so this kind of synchronization mode is not applicable in network-connection. Synchronization of SHR In users’ local network, the section discusses whether the network supports SSM or not and the method how to realize the network synchronization by avoiding timing loops in the path protection ring and the multiplex section protection ring. SSM of SDH equipments. According to ITU-T, it is stipulated that the S1 byte (b5~b8) in the multiplex section overhead (MSOH) of SDH and Sa4 in E1 signals are considered as SSM to indicate the quality grade of synchronous clocks for current signal flows, as shown in Table 1. Based on this standard, top-grade clocks are automatically selected as the timing sources, so that the synchronization quality of clocks is furthest improved. Table 1 SSM coding S1(b5~b8) Grade of SDH synchronization quality 0000 Unknown synchronization quality 0010 G.811 Reference clock signal 0100 G.812 Clock signal of transit exchange 1000 G.812 Clock signal of local exchange 1011 SDH Equipment Timing source signal 1111 Not applied in the synchronization . Synchronization of SDH path protection ring. When the path protection ring is applied to the local network, SHR generally has a central node to control the whole timing and external timing is Figure 2 Timed transmissions in different modes without failure A