The photonic layer is the lowest SONET layer and it is responsible for transmitting the bits to the physical medium. The section layer is responsible for generating the proper STS-N frames which are to be transmitted across the physical medium. It deals with issues such as proper framing, error monitoring, section maintenance, and orderwire. The line layer ensures reliable transport of the payload and overhead generated by the path layer.
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The photonic layer is the lowest SONET layer and it is responsible for transmitting the bits to the physical medium.
The section layer is responsible for generating the proper STS-N frames which are to be transmitted across the physical medium. It deals with issues such as proper framing, error monitoring, section maintenance, and orderwire. The line layer ensures reliable transport of the payload and overhead generated by the path layer. It provides synchronization and multiplexing for multiple paths.
It modifies overhead bits relating to quality control. It takes data to be transmitted and transforms them into signals required by the line layer, and adds or modifies the path overhead bits for performance monitoring and protection switching.
Please help improve it to make it understandable to non-experts , without removing the technical details. The main functions of network management thereby include: Network and network-element provisioning In order to allocate bandwidth throughout a network, each network element must be configured. Performance management Network elements have a very large set of standards for performance management. The performance-management criteria allow not only monitoring the health of individual network elements, but isolating and identifying most network defects or outages.
Higher-layer network monitoring and management software allows the proper filtering and troubleshooting of network-wide performance management, so that defects and outages can be quickly identified and resolved.
This interface can also be attached to a console server , allowing for remote out-of-band management and logging. This is for local management of that network element and, possibly, remote management of other SONET network elements.
Generally, section overhead regenerator section in SDH is used. To handle all of the possible management channels and signals, most modern network elements contain a router for the network commands and underlying data protocols. Nevertheless, as network architectures have remained relatively constant, even newer equipment including multi-service provisioning platforms can be examined in light of the architectures they will support. Thus, there is value in viewing new, as well as traditional, equipment in terms of the older categories.
Regenerator[ edit ] Traditional regenerators terminate the section overhead, but not the line or path. Regenerators extend long-haul routes in a way similar to most regenerators, by converting an optical signal that has already traveled a long distance into electrical format and then retransmitting a regenerated high-power signal. Since the late s, regenerators have been largely replaced by optical amplifiers.
Also, some of the functionality of regenerators has been absorbed by the transponders of wavelength-division multiplexing systems. STS multiplexer and demultiplexer[ edit ] STS multiplexer and demultiplexer provide the interface between an electrical tributary network and the optical network.
Add-drop multiplexer[ edit ] Add-drop multiplexers ADMs are the most common type of network elements. Traditional ADMs were designed to support one of the network architectures, though new generation systems can often support several architectures, sometimes simultaneously.
ADMs traditionally have a high-speed side where the full line rate signal is supported , and a low-speed side, which can consist of electrical as well as optical interfaces. The low-speed side takes in low-speed signals, which are multiplexed by the network element and sent out from the high-speed side, or vice versa.
Advanced DCSs can support numerous subtending rings simultaneously. These architectures allow for efficient bandwidth usage as well as protection i. Switching is based on the line state, and may be unidirectional with each direction switching independently , or bidirectional where the network elements at each end negotiate so that both directions are generally carried on the same pair of fibers.
Unidirectional path-switched ring[ edit ] In unidirectional path-switched rings UPSRs , two redundant path-level copies of protected traffic are sent in either direction around a ring.
A selector at the egress node determines which copy has the highest quality, and uses that copy, thus coping if one copy deteriorates due to a broken fiber or other failure. UPSRs tend to sit nearer to the edge of a network, and as such are sometimes called collector rings. Any other nodes on the ring could only act as pass-through nodes. BLSRs switch at the line layer. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring on the protection fibers.
BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability to support "extra traffic" that can be pre-empted when a protection switching event occurs. In four-fiber ring, either single node failures, or multiple line failures can be supported, since a failure or maintenance action on one line causes the protection fiber connecting two nodes to be used rather than looping it around the ring. BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities.
Because a BLSR does not send redundant copies from ingress to egress, the total bandwidth that a BLSR can support is not limited to the line rate N of the OC-N ring, and can actually be larger than N depending upon the traffic pattern on the ring.
The worst case is when all traffic on the ring egresses from a single node, i. In this case, the bandwidth that the ring can support is equal to the line rate N of the OC-N ring. This is why BLSRs are seldom, if ever, deployed in collector rings, but often deployed in inter-office rings. Synchronization[ edit ] Clock sources used for synchronization in telecommunications networks are rated by quality, commonly called a stratum.
Synchronization sources available to a network element are:[ citation needed ] Local external timing This is generated by an atomic cesium clock or a satellite-derived clock by a device in the same central office as the network element.
The interface is often a DS1, with sync-status messages supplied by the clock and placed into the DS1 overhead. Line-derived timing A network element can choose or be configured to derive its timing from the line-level, by monitoring the S1 sync-status bytes to ensure quality.
Holdover As a last resort, in the absence of higher quality timing, a network element can go into a holdover mode until higher-quality external timing becomes available again. In this mode, the network element uses its own timing circuits as a reference.
Timing loops[ edit ] A timing loop occurs when network elements in a network are each deriving their timing from other network elements, without any of them being a "master" timing source. This network loop will eventually see its own timing "float away" from any external networks, causing mysterious bit errors—and ultimately, in the worst cases, massive loss of traffic.
The source of these kinds of errors can be hard to diagnose.
For this reason, before WDM protection schemes were defined, SONET protection mechanisms were mainly adopted to guarantee optical network survivability. In the case of a link or network failure, the simplest mechanism for network survivability is automatic protection switching APS. APS techniques involve reserving a protection channel dedicated or shared with the same capacity of the channel or element being protected. In a BLSR, every link can carry both the working and backup traffic at the same time and hence does not require backup links. There are two architectures for BLSRs. In a four-fiber BLSR, two fibers are used as working fibers and the other two are used as protection fibers, to be utilized in the case of a failure. Four-fiber BLSRs use two types of protection mechanisms during failure recovery, namely ring and span switching.
UNDERSTANDING SONET BLSRs
Synchronous optical networking