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Today’s access networks are largely based on fiber links. The natural choice for enterprises, campuses, residential neighborhoods and SOHOs, fiber is now beginning to reach as far as the home. Users are connected to the backbone through a mesh of carriers COs, regional offices and service providers’ points-of-presence (POP). Convergence of voice, data and video services is expected to greatly increase bandwidth requirements. These services will utilize FTTX technology, extending the reach of fiber to the customer premises and home.

Case A: Not Protected Case B: Protected against fiber or interface failure by Ethernet protocols (not against Metro switch failure) Case C: Dual homing architecture. Fully protected (disregarding CPE itself) Any protection is by layer 2-3 protocols

Figure 1

Figure 1 depicts a typical Metro Ethernet network providing the increasingly popular GbE services. As can be seen in the figure, Ethernet-based services can be protected through protocols such as IEEE 802.3ad (LACP), IEEE 802.1W (RSTP) or IEEE 802.17 (RPR). Yet protocol-layer solutions have many disadvantages:

Fiber breaks and other physical layer failures account for the vast majority of all network failures. Fiber breaks are most critical, since they may occur during off-hours and take extended periods to repair. Fiber protection is therefore essential in achieving high network availability.

Add-on all-optical protection systems provide the fastest and most cost-effective method for protecting fiber-based access networks. Since they are usually transparent to data rates and protocols, these systems are versatile and can be deployed and re-deployed in various environments (Sonet/SDH, Ethernet, ATM, IP, FC, etc.).

Dual homing is a special form of access network protection. In this topology, a single CPE is connected through two fibers to two different nodes within the metro core networks, protecting access to the CPE due to either fiber breaks or malfunction of the network elements. Lynx systems offer fiber protection at the physical layer (less than 50 milliseconds restoration time) and leave the protection of those rare and less critical equipment failures to the slower protocol-layer protection protocols (e.g., OSPF for IP traffic or RSTP for Ethernet traffic).

Figure 2 illustrates the same GbE access network shown in Figure 1, with the addition of Lynx protection systems.

Case A: Not Protected Case B: Protected against fiber or interface failure by Ethernet protocols(not against Metro switch failure) Case C: Dual homing architecture. Fully protected (disregarding CPE itself)Any fiber protection is in physical layer

Figure 2

Key challenges in access protection systems include controlling costs and ensuring that end-to-end optical power insertion losses due to the protection system are not too high. Protection systems usually employ a 1+1 scheme where the transmitted signal is split at the ‘bridge’ into two fibers (‘working’ and ‘protection’). During normal operation, a selector at the receiving end selects the traffic arriving from the working fiber; during failure, the selector chooses traffic arriving from the protection fiber. The insertion loss of these systems is the combined loss of the bridge and the selector along with the connectors, taps, etc. A major loss component is the bridge splitter: splitting the signal creates a practical loss of 3.6dB plus the added losses from the conectors and taps of the bridge and selector. Altogether, the insertion losses are significantly above the theoretical loss of 3.0dB.

To achieve lower insertion loss, the 1:1 protection scheme is typically used. This method uses a switch at both the transmitting and receiving ends, routing the signal to the active path. However, 1:1 protection systems require special care in synchronizing both ends of the link and monitoring the non-active link.

The figures below shows the 2 different methods discussed above: 1+1, 1:1.

Figure 3.1: 1+1 fiber protection method

Figure 3.1 depicts a 1+1 fiber pair protection scheme. A splitter bridge sends the Tx signal to both fiber pairs (the working link and the protection link) and a 2x1 selector switch selects the signal from one of the fiber pairs. In default mode, traffic from the working link is selected. Upon a failure in the working link, the selector near the Rx side of the failed traffic detects the failure, switches over, and selects the traffic from the protection link. This switching scheme is called “single-ended” or “non-synchronized”, since each selector operates independently of the other, and there is no coordination between the link ends. This type of protection is also called “unidirectional”, since if a link has a failure in only one direction (one fiber), the protection switching is done only for that direction (see figure 3.1 b).

Figure 3.2: 1:1 fiber protection method

Figure 3.2 depicts a 1:1 fiber pair protection scheme. Here both the bridge and the selector are switches. In default mode, the bridge routes the Tx signal towards the working link, and the selector chooses the Rx traffic from the working link. When the working link fails, both bridge and selector switchover to the protection link. This scheme is more complex than the 1+1 scheme presented in Figure 3.1, because both ends of the link need to be synchronized (hence the terms “dual-ended” or “synchronized”). The protection scheme is also called “i-directional”, because both directions are switched upon any failure, even if it is only a single direction. Figure 3.2 b demonstrates the synchronized and bidirectional nature of the protection scheme, by showing an example of a unidirectional failure that results in bi-directional switching.

Figure 3.3: 1+1 bidirectional (fiber-pair) (assuming only bidirectional failures)

Figure 3.3 depicts a bidirectional 1+1 scheme. In this scheme both the bridge and the selector at one side (left in this example) are switches, and both the bridge and the elector at the other side are splitters. This means that the signal transmitted from left to right is split between the two links, and selected from one of them (1+1). The signal from left to right is routed only to the working link, and selected from both. The left-to-right direction is not exactly 1+1 (since the signal is routed at the bridge rather than split), nor exactly 1:1 (since the signal is combined from both links and not selected from one of them). We chose the term “1+1 bidirectional” for such a protection scheme because of its dominant 1+1 architecture, and the fact that it always performs bidirectional switching. Since switchover can be performed only at one side (left in this case), this side performs monitoring and activates both switches simultaneously in case of failure. Note that this scheme is only effective when failures are bi-directional. That is, both fibers of a link are routed within the same cable, so any cut affects both fibers.

Figure 3.4: 1:1 WDM (Bi-directional)

Figure 3.4 depicts a 1:1 scheme for a single fiber carrying bi-directional traffic. This type of traffic is usually denoted WDM bi-directional because both directions are coupled into a single fiber using a WDM coupler and different Tx wavelengths at both ends. A common implementation is that one end transmits in 1550nm while the other in 1310nm. Any failure in such architecture is of bi-directional nature because the same fiber carries both directions of traffic. Any switchover at one end must be synchronized with a switchover at the other end.

In many cases the carrier provides the protection systems on both ends of the link, one at the CO or POP, the other at the customer’s site. This equipment as well as any carrier-owned CPE at the customer’s site needs to be managed by the carrier remotely. While in some cases a separate management link is available, in other cases management traffic is carried in-band. Typical in-band management mechanisms introduce additional losses and may affect the reliability of the management channel.

Figure 4 shows the position of the in-band management in the network.

Figure 4: Star topology with in-band management

A popular method for saving fibers in the access network is the WDM bi-directional fiber (See Figure 3 above). In this system the transceiver is connected to a WDM coupler that combines the transmit wavelength and the receive wavelength into a single fiber carrying both directions of traffic. Transceivers that carry both directions of a WDM link are called ‘diplexers’. Transceivers that also combine CATV analog video are named ‘triplexers’. This application requires a 1:1 protection scheme and must avoid monitoring issues due to crosstalk between the Rx signal and the reflected Tx signal.

To learn more about the Lynx products that address the access fiber networks protection, click:

Fiber Protection Products