Why Rings? The Foundation of Network Resiliency

While a simple point-to-point link is the most direct way to connect two locations, it has a critical weakness: a single failure (like a backhoe cutting the fiber cable) completely severs the connection. To build highly reliable networks, especially in metropolitan and core areas, a more robust physical layout is needed. The solution is the ring topology.

Imagine a single road connecting City A and City B. If there's an accident, traffic stops. Now imagine a ring road that connects A and B, but also passes through cities C and D. If the road between A and B is blocked, traffic can simply be rerouted the "long way around" through D and C to reach its destination. A ring inherently provides two physically diverse paths between any two nodes, making it the cornerstone of SDH/SONET's celebrated resiliency. The workhorses that form the "interchanges" on these digital ring roads are the Add-Drop Multiplexers (ADMs).

The Two Philosophies of Ring Protection

Within the ring architecture, SONET and SDH implement two primary philosophies for protecting traffic against failures. They differ in what they protect and how efficiently they use bandwidth.

Deep Dive: Path-Switched Rings (UPSR / SNCP)

In North American SONET terminology, this is known as a Unidirectional Path-Switched Ring (UPSR). In international SDH, it's called Sub-Network Connection Protection (SNCP). Both describe the same mechanism.

Architecture and Operation

A UPSR is typically a 2-fiber ring where traffic flows in opposite directions on each fiber. The protection mechanism follows the 1+1 "bridge and select" principle:

1. Permanent Bridge: When a service (e.g., a VC-12 carrying a T1 line) needs to be sent from Node A to Node C, the ADM at Node A creates a permanent bridge. It transmits identical copies of the VC-12 onto both rings simultaneously (one traveling clockwise, e.g., via B, and the other counter-clockwise, e.g., via D).
2. Continuous Monitoring and Selection: The destination ADM at Node C receives both copies of the VC-12, one from each direction. Its internal switching fabric continuously monitors the health of both signals based on information in the Path Overhead. It selects the signal from the pre-designated "working" path to send to its local tributary port.
3. Instantaneous Switching: If a fiber cut occurs between nodes B and C, the signal on the clockwise path is lost. The selector at Node C detects this failure (e.g., a Loss of Signal or high bit error rate) and instantly switches its output to use the signal it was already receiving from the counter-clockwise path.

Characteristics of UPSR/SNCP

• Very Fast: The switchover time is extremely fast (typically well under 50ms) because no complex signaling is required. The backup signal is already at the destination.
• Simple: The logic is straightforward and robust.
• Inefficient Bandwidth Usage: Its major drawback is inefficiency. Since every service is duplicated, 100% of the ring's bandwidth is consumed to deliver 50% of the working capacity. This makes it costly for high-capacity core transport but ideal for access networks where traffic is often broadcast-like (e.g., distributing video signals).

Deep Dive: Line-Switched Rings (BLSR / MS-SPRing)

For core and long-haul networks where bandwidth efficiency is paramount, the preferred solution is the Bidirectional Line-Switched Ring (BLSR) (SONET name) or Multiplex Section-Shared Protection Ring (MS-SPRing) (SDH name). This is a more complex but far more efficient shared protection scheme. We will focus on the common 2-fiber implementation.

Architecture and Bandwidth Allocation

A 2-fiber BLSR uses a pair of fibers between each node, one for clockwise transmission and one for counter-clockwise. The capacity on each fiber is logically divided in half:

• Working Capacity (50%): Used to carry high-priority service traffic under normal conditions.
• Protection Capacity (50%): Reserved and kept empty. It is only used to carry rerouted traffic in the event of a failure.

The "Loopback" Switch: Surviving a Cable Cut

BLSR's powerful resiliency comes from its primary protection mechanism against catastrophic failures like a full cable cut: the loopback switch.

1. Normal Operation: Traffic between, for example, Node A and Node C flows on the working channels over the shortest path (e.g., through Node B). The protection channels on all links are idle.
2. Failure: A construction crew accidentally severs the entire cable bundle between Node B and Node C. Both fibers are cut.
3. Detection and Signaling: The ADMs at Node B and Node C immediately detect a Loss of Signal. They use the APS bytes (K1, K2) in the Multiplex Section Overheadto broadcast a failure alert to all other nodes on the ring.
4. Coordinated Switch: The nodes adjacent to the break (B and C) execute a loopback switch:
   • At Node B: Working traffic arriving from Node A that was destined for Node C is now internally bridged from its working input to the protection channels of the link going back towards Node A.
   • At Node C: Similarly, working traffic arriving from Node D destined for Node B is looped back onto the protection channels of the link going back towards Node D.
5. Result: The network is reconfigured in milliseconds. The traffic that previously used the short path is now rerouted the "long way around" the ring, utilizing the previously empty protection capacity. The entire ring temporarily acts as one long, linear chain, transparently bypassing the physical break.

4-Fiber Rings and a Comparison

For even greater capacity and resiliency, operators can deploy 4-Fiber BLSR/MS-SPRing architectures.

• Architecture: It uses four fibers between each node: two working fibers (one clockwise, one counter-clockwise) and two dedicated protection fibers (one for each direction).
• Capacity: It provides double the working capacity of a 2-fiber ring.
• Protection: It offers more advanced protection, capable of performing a faster "span switch" (simply moving traffic from the working fiber to its dedicated parallel protection fiber) for single-fiber failures, and reverting to a "ring switch" (loopback) for full cable cuts.