SDH Networks - Protection Mechanism

SDH Networks - Protection Mechanisms 

Network failures, whether due to human error or faulty technology,  can be very expensive for users and network providers alike. As a result, the subject of so called fall-back  mechanisms is very important. A wide range of standardized mechanisms are incorporated into synchronous networks in order to compensate for failures in network elements.

Protection Mechanism  evolution:

  v  80' s –

-          DCS-based Mesh Restoration of DS3 Facilities

-          Centralized (EMS/NMS)

-          Path-based, failure-dependent, after fault detection and isolation

-          Capacity-efficient but slow (~ minutes)

  v  90' s –

-          ADM-based Ring Protect ion of SONET/SDH Facilities

-          Distributed

-          Path-based (UPSR)  or span-based (BLSR) , pre-determined

-          Fast (“ 50 msec” )  but capacity-inefficient

  v  2000' s  –

-          OXC-based Mesh Protect ion/Restorat ion

-          Distributed

-          Path-based, failure-independent, pre-determined and pre-provisioned

-          Capacity-efficient AND fast  (10’s – 100's msec)

Automatic protection switching (APS):

Two basic types of protection architecture are distinguished in APS.

One is the linear protection mechanism used for point-to-point connections. 

The other basic form is the so called ring protection mechanism which can take on many different forms. 

Both mechanisms use spare circuits or components to provide the back-up path. Switching is controlled by the overhead bytes K1 and K2.

Linear protection : The simplest form of back-up is the so-called 1+1 APS. Here, each working line is protected by one protection line. If a defect occurs, the protection agent in the network elements at both ends switch the circuit over to the protection line. The switch over is triggered by a defect such as LOS. Switching at the far end is initiated by the return of an acknowledgment in the backward channel.

1+1 architecture includes 100%redundancy, as there is a spare line for each working line. Economic considerations have led to the preferential use of 1:N architecture, particularly for long-distance paths. In this case, several working lines  are protected by a single back-up line. If switching is necessary, the two ends of the affected path are switched over to the back-up line.

The 1+1 and 1:N protection mechanisms are standardized in ITU-T Recommendation G.783.

The reserve circuits can be used for lower-priority traffic, which is simply interrupted if the circuit is  needed to replace a failed working line.

Ring protection : A ring is the simplest and most cost-effective way of linking a number of network elements. Various protection mechanisms are available for this type of network architecture, only some of which have been standardized in ITU-T Recommendation G.841.

A basic distinction must be made between ring structures with unidirectional and bi-directional connections. Figure below shows the basic principle of APS for unidirectional rings. Let us assume that there is an interruption in the circuit between the network elements A and B. Direction y is unaffected by this fault. An alternative path must, however, be found for direction x. 

The connection is therefore switched to the alternative path in network elements A and B. The other network elements (C and D) switch through the back-up path. This switching process is referred to as line switched. 

A simpler method is to use the so-called path switched ring (see figure). Traffic is transmitted simultaneously over both the working line and the protection line. If there is  an interruption, the receiver (in this case A) switches to the protection line and immediately takes up the connection

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.

Because the same data is sent around the ring in both directions, the total capacity of a UPSR is equal to the line rate N of the OC-N ring. For example, in an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, 100 percent of the ring bandwidth (N=3) would be consumed by nodes A and D. Any other nodes on the ring could only act as pass-through nodes.

The SDH equivalent of UPSR is SubNetwork Connection Protection (SNCP); SNCP does not impose a ring topology, but may also be used in mesh topologies.

Bi-directional rings : In this network structure, connections between network elements are bi-directional. This is indicated in figure by the absence of arrows when compared with first figure.The overall capacity of the network can be split up for several paths each with one bi-directional working line, while for unidirectional rings, an entire virtual ring is required for each path. If a fault occurs between neighboring elements A and B, network element B triggers protection switching and controls network element A by means of the K1 and K2 bytes in the SOH.

Bidirectional line-switched ring

Bidirectional line-switched ring (BLSR) comes in two varieties: two-fiber BLSR and four-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies from ingress to egress. 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. In the best case, all traffic is between adjacent nodes. The worst case is when all traffic on the ring egresses from a single node, i.e., the BLSR is serving as a collector ring. 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.

The SDH equivalent of BLSR is called Multiplex Section-Shared Protection Ring (MS-SPRING).

Despite predictions of SDH reduction in favour of lower cost transmission, such as Gigabit  Ethernet, still the vast majority of traffic in the operators transport networks runs of SDH.

Apart from the need for well-defined protection switching strategies, SDH on reconfigurable optical networks is quite straightforward. When the cost for an OXC port is lower than for a DXC port an optical bypass layer can be formed reducing the total network cost, especially if protection is done in the optical layer.

ASON

Automatically Switched Optical Network (ASON) is a network management feature which enables dynamic control of transmission networks through an automated management of network resources. ASON functionality is typically made available in higher capacity SDH/SONET and DWDM networks. It enables fast end-to-end service provisioning, re-routing and restoration. As per ITU-T G.8080, the key goals for ASON are:

-          Facilitate fast and efficient configuration within transport layer network

-          Reconfigure or modify connections

-          Perform a protection/restoration function