Link protection

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Link protection is designed to safeguard networks from failure. Failures in high-speed networks have always been a concern of utmost importance. A single fiber cut can lead to heavy losses of traffic and protection-switching techniques have been used as the key source to ensure survivability in networks. Survivability can be addressed in many layers in a network and protection can be performed at the physical layer (SONET/SDH, Optical Transport Network), Layer 2 (Ethernet, MPLS) and Layer 3 (IP).

Contents

Protection architectures like Path protection and Link protection safeguard the above-mentioned networks from different kinds of failures. In path protection, a backup path is used from the source to its destination to bypass the failure. In Link protection, the end nodes of the failed link initiate the protection. These nodes detect the fault responsible to initiate the protection mechanisms in order to detour the affected traffic from the failed link onto predetermined reserved paths. Other types of protection are channel-, segment- and p-cycle protection.

In older high-speed transport networks, the SONET layer (also SDH) was the main client wavelength-division multiplexing (WDM) layer. For this reason, before WDM protection schemes were defined, SONET protection mechanisms were mainly adopted to guarantee optical network survivability. When the WDM layer was created, the optical networks survivability techniques in consideration were mainly based on many elements of SONET protection in order to ensure maximum compatibility with the legacy systems (SONET systems). Hence some of the WDM-layer protection techniques are very similar to SONET/SDH protection techniques in the case of ring networks. [1]

Ring-Based protection

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. A four-fiber BLSR.jpg
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.

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. [2] When a shared protection technique is used, an APS protocol is needed to coordinate access to the shared protection bandwidth. [3] An example of a link-based protection architecture at the Optical Transport Network layer is a Bidirectional Line Switched Ring (BLSR). In a BLSR, every link can carry both the working and backup traffic at the same time and hence does not require backup links. Unlike a UPSR (see SONET), in a BLSR, under normal circumstances, the protection fiber is unused and this is beneficial to ISP's since they can use the protection fiber to send lower priority traffic (using protection bandwidth) like data traffic and voice traffic.

There are two architectures for BLSRs. The four-fiber BLSR and the two-fiber BLSR. 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. In span switching, when the source or destination on a link fails, traffic gets routed onto the protection fiber between the two nodes on the same link and when a fiber or cable cut occurs, service is restored using the ring switching mechanism.

In a two-fiber BLSR, the protection fibers are contained within the working fibers (like a four-fiber BLSR) and both the fibers are used to carry working traffic whilst keeping only half the capacity on each fiber for protection purposes. Two-fiber BLSRs also benefit from the ring switching but cannot perform span switching like a four-fiber BLSR.

Due to its efficiency in protection, BLSRs are widely deployed in long haul and interoffice networks, where the traffic pattern is more distributed than in access networks. Most metro carriers have deployed two-fiber BLSRs, while many long-haul carriers have deployed four-fiber BLSRs since they can handle more load than two-fiber BLSRs. [1]

Mesh-based protection

The techniques mentioned above for SONET and WDM networks can also be applied to mesh network architectures provided there are ring decompositions for the mesh architectures; and use well defined protection-switching schemes to restore service when a failure occurs. The three most notable ring-based protection techniques for mesh networks are ring covers, cycle double covers and p-cycles (pre-configured protection cycles).

The main goal of the ring cover technique is to find a set of rings that covers all the network links and then use these rings to protect the network against failures. Some network links in the ring cover might get used in more than one ring which can cause additional redundancy in the network and because of this reason, scaling down redundancy is the primary focus of this technique.

The cycle double covers technique provides one protection fiber for each working fiber (like in SONET rings) keeping 100% redundancy. This technique was initially proposed to remove the additional redundancy issue caused by the ring cover scheme. [4]

The p-cycle technique is based on the property of a ring to protect not only its own links, but also any possible links connecting two non-adjacent ring nodes called chordal links. By doing this, p-cycles reduce the redundancy required to protect a mesh network against link failure. There are two types of p-cycles namely link p-cycles and node p-cycles. Link p-cycles protect all channels on a link whereas a node p-cycle protects all the connections traversing a node.

One of the best features of p-cycles is its ability to allow savings in spare resources and they are also recognized to be the most efficient protection structures as for capacity minimization. However, p- cycle planning is an NP-hard problem and is not scalable. [1]

Another technique called the generalized loopback technique can be included under ring-based approaches. Although it is not strictly considered as one of the mesh-based ring protection techniques, its usage of a loopback operation is similar to the APS operation in rings to switch the signal from working to the redundant capacity. [4]

Protection in Ethernet

Ethernet links use link aggregation as a mechanism to recover from failures. Even when a link fails, its link capacity gets reduced but the communication system keeps working without interruptions in data flow. [5]

Other terms used to describe link aggregation include IEEE 802.1ax (formerly knows as 802.3ad), link bundling or NIC teaming.

Protection in IP

In recent years, packet based networks made a big leap and almost every single service provided (voice, IP-TV, etc.) is IP based. This is due to the reason that the IP layer has long provided best-effort services. [3]

IP uses dynamic, hop-by-hop routing of packets and if there is a link failure, the routing protocols (OSPF or IS-IS) operates in a distributed manner and updates the routing table at each router in the domain. This process can get slow and cause heavy delays in the network. In order to avoid slow recovery, every IP link can be protected using protocols at the lower levels which will help the IP links to recover by itself instead of waiting for the IP routing table to change. For example, IP links can be realized by protected MPLS using Label Switched Paths – LSPs (IP over MPLS).

Protection in MPLS Networks

Between LSRs A and B a tunnel (orange) is set up. When the link between LSR A and B fails, the initial LSP (black) is redirected down the orange tunnel so that there is no disruption of data flow between A and B. MPLS fast-reroute technique.jpg
Between LSRs A and B a tunnel (orange) is set up. When the link between LSR A and B fails, the initial LSP (black) is redirected down the orange tunnel so that there is no disruption of data flow between A and B.

MPLS based networks use fast re-route as its network resiliency mechanism. In MPLS fast re-route, MPLS data can be directed around a link failure without the need to perform any signaling when a failure is detected.

One form of fast re-route is called Link Protection. [6] In this protection, an LSP tunnel is set up through the network to provide a backup for a vulnerable physical link. The LSP provides a parallel virtual link. When the physical link fails, the upstream node switches traffic to the virtual link so that data continues to flow with a minimal disruption.

The capacity of the backup LSP should be sufficient to carry the protected LSPs. Depending on the LSPs, the capacity needs to be configured. For instance, if all the LSPs are to be protected, then the net capacity should equal the bandwidth of the protected link. By doing this, the backup bandwidth would increase if multiple links were to be protected. On the other hand, by leaving some LSPs over the link unprotected, the backup bandwidth can be reduced. [6]

See also

Related Research Articles

Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses. Whereas network addresses identify endpoints, the labels identify established paths between endpoints. MPLS can encapsulate packets of various network protocols, hence the multiprotocol component of the name. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

<span class="mw-page-title-main">Synchronous optical networking</span> Standardized protocol

Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization.

<span class="mw-page-title-main">Wavelength-division multiplexing</span> Fiber-optic communications technology

In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. This technique enables bidirectional communications over a single strand of fiber as well as multiplication of capacity.

<span class="mw-page-title-main">Self-healing ring</span> Telecommunications loop network topology

A self-healing ring, or SHR, is a telecommunications term for loop network topology, a common configuration in telecommunications transmission systems. Like roadway and water distribution systems, a loop or ring is used to provide redundancy. SDH, SONET and WDM systems are often configured in self-healing rings.

<span class="mw-page-title-main">Ring network</span> Network topology in which nodes form a ring

A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node – a ring. Data travels from node to node, with each node along the way handling every packet.

Resilient Packet Ring (RPR), as defined by IEEE standard 802.17, is a protocol designed for the transport of data traffic over optical fiber ring networks. The standard began development in November 2000 and has undergone several amendments since its initial standard was completed in June 2004. The amended standards are 802.17a through 802.17d, the last of which was adopted in May 2011. It is designed to provide the resilience found in SONET and Synchronous Digital Hierarchy networks but, instead of setting up circuit oriented connections, provides a packet based transmission, in order to increase the efficiency of Ethernet and IP services.

Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol (PPP) over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes (LEDs) over optical fibre at high line rates. POS is defined by RFC 2615 as PPP over SONET/SDH. PPP is the Point to Point Protocol that was designed as a standard method of communicating over point-to-point links. Since SONET/SDH uses point-to-point circuits, PPP is well suited for use over these links. Scrambling is performed during insertion of the PPP packets into the SONET/SDH frame to solve various security attacks including denial-of-service attacks and the imitation of SONET/SDH alarms. This modification was justified as cost-effective because the scrambling algorithm was already used by the standard used to transport ATM cells over SONET/SDH. However, scrambling can optionally be disabled to allow a node to be compatible with another node that uses the now obsoleted RFC 1619 version of Packet over SONET/SDH which lacks the scrambler.

<span class="mw-page-title-main">Metro Ethernet</span> Metropolitan area network based on Ethernet standards

A metropolitan-area Ethernet, Ethernet MAN, carrier Ethernet or metro Ethernet network is a metropolitan area network (MAN) that is based on Ethernet standards. It is commonly used to connect subscribers to a larger service network or for internet access. Businesses can also use metropolitan-area Ethernet to connect their own offices to each other.

In telecommunications and computer networking, connection-oriented communication is a communication protocol where a communication session or a semi-permanent connection is established before any useful data can be transferred. The established connection ensures that data is delivered in the correct order to the upper communication layer. The alternative is called connectionless communication, such as the datagram mode communication used by Internet Protocol (IP) and User Datagram Protocol, where data may be delivered out of order, since different network packets are routed independently and may be delivered over different paths.

Dynamic Packet Transport (DPT) is a Cisco transport protocol designed for use in optical fiber ring networks. In overview, it is quite similar to POS and DTM. It was one of the major influences on the Resilient Packet Ring/802.17 standard.

MPLS Fast Reroute is a local restoration network resiliency mechanism. It is actually a feature of resource reservation protocol (RSVP) traffic engineering (RSVP-TE). In MPLS local protection each label-switched path (LSP) passing through a facility is protected by a backup path which originates at the node immediately upstream to that facility.

In telecommunications, subnetwork connection protection (SNCP), is a type of protection mechanism associated with synchronous optical networks such as synchronous digital hierarchy (SDH).

<span class="mw-page-title-main">Optical mesh network</span> Optical network using a mesh topology

An optical mesh network is a type of optical telecommunications network employing wired fiber-optic communication or wireless free-space optical communication in a mesh network architecture.

Shared risk resource group is a concept in optical mesh network routing that different networks may suffer from a common failure if they share a common risk or a common SRG. SRG is not limited to Optical mesh networks: SRGs are also used in MPLS, IP networks, and synchronous optical networks.

<span class="mw-page-title-main">Multicast lightpaths</span> Type of computer communication

A multicast session requires a "point-to-multipoint" connection from a source node to multiple destination nodes. The source node is known as the root. The destination nodes are known as leaves. In the modern era, it is important to protect multicast connections in an optical mesh network. Recently, multicast applications have gained popularity as they are important to protecting critical sessions against failures such as fiber cuts, hardware faults, and natural disasters.

Path protection in telecommunications is an end-to-end protection scheme used in connection oriented circuits in different network architectures to protect against inevitable failures on service providers’ network that might affect the services offered to end customers. Any failure occurred at any point along the path of a circuit will cause the end nodes to move/pick the traffic to/from a new route. Finding paths with protection, especially in elastic optical networks, was considered a difficult problem, but an efficient and optimal algorithm was proposed.

Segment protection is a type of backup technique that can be used in most networks. It can be implemented as a dedicated backup or as a shared backup protection. Overlapping segments and non-overlapping segments are allowed; each providing different advantages.

The p-Cycle protection scheme is a technique to protect a mesh network from a failure of a link, with the benefits of ring like recovery speed and mesh-like capacity efficiency, similar to that of a shared backup path protection (SBPP). p-Cycle protection was invented in late 1990s, with research and development done mostly by Wayne D. Grover, and D. Stamatelakis.

Fast automatic restoration (FASTAR) is an automated fast response system developed and deployed by American Telephone & Telegraph (AT&T) in 1992 for the centralized restoration of its digital transport network. FASTAR automatically reroutes circuits over a spare protection capacity when a fiber-optic cable failure is detected, hence increasing service availability and reducing the impact of the outages in the network. Similar in operation is real-time restoration (RTR), developed and deployed by MCI and used in the MCI network to minimize the effects of a fiber cut.

Deterministic Networking (DetNet) is an effort by the IETF DetNet Working Group to study implementation of deterministic data paths for real-time applications with extremely low data loss rates, packet delay variation (jitter), and bounded latency, such as audio and video streaming, industrial automation, and vehicle control.

References

  1. 1 2 3 Guido Maier; Achille Pattavina; Simone De Patre; Mario Martinelli (2002). "Optical Network Survivability: Protection Techniques in the WDM Layer". Photonic Network Communications.
  2. ↑ "SONET/SDH Automatic Protection Switching". 2005-02-02. Retrieved 2012-12-13.
  3. 1 2 Optical Networks, A practical perspective. Morgan Kaufmann. 2010. pp. 511–569. ISBN   978-0-12-374092-2.
  4. 1 2 Path Routing in Mesh Optical Networks . John Wiley and Sons, Ltd. 2007. pp.  32–57. ISBN   978-0-470-01565-0.
  5. ↑ "Link Aggregation - LAG" . Retrieved 2012-12-12.
  6. 1 2 Protection and Restoration in MPLS Netowrks. Metaswitch Networks. 2001. pp. 29–36.
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