1. Introduction to Survivable Rings
ECE Module 7
Introduction to Survivable Rings
W. D. Grover
TRLabs & University of Alberta
© Wayne D. Grover 2002, 2003,2004

2. Two main types of “survivable ring”....(1) UPSR
Unidirectional Path-switched Ring...Principle of operation

3. Two main types of “survivable ring”....(1) UPSR
Unidirectional Path-switched Ring ...
Unidirectional - because in normal operation all working demand flows in one direction only. i.e., A sends to B clockwise, B also sends to A clockwise
Path-switched - because in restoration each receiver selects an alternate end-to-end path through ring, regardless of where actual break occurred.

4. UPSR Animation... Working fibre Tail-end Switch Protection fibre l1 15 2
Protection fibre 3 4 l1

5. UPSR ...line capacity requirement
Consider a bi-directional demand quantity between nodes A, B: dA,B. - A to B may go on the short route - then B to A must go around the longer route
Thus, every (bi-directional) demand pair circumnavigates the entire ring.
Hence in any cross section of the ring, we would find one unidirectional instance of every demand flow between nodes of the ring.
Therefore, the line capacity of the UPSR must be:
A -> B A E B
B -> A C D
“ The UPSR must have a line rate (capacity) greater (or equal to) the sum of all the (bi-directional) demand quantities between nodes of the ring. “

6. The “access” demand pattern
Notes on the UPSR
Can be thought of as a number of virtual 1+1 APS set-ups sharing a single set of high-speed transmission systems to obtain “economy of scale”.
Economy of scale arises since one OC-96 (say) optical Tx / Rx pair, is a lot less expensive than 96 OC-1 Tx / Rx !
UPSRs are inherently 2-fibre structures.
Primary use is in “access” applications.
- distances are not great - under pure “hubbed” demand pattern UPSR is as efficient as BLSR.
UPSR need not “revert” after protection switching.
UPSR switching decisions are independent on a tributary-by-tributary basis:
- switching on one channel has no effect on other channels.
The “access” demand pattern

7. Two main types of “survivable ring”....(2) BLSR
Bi-directional Line-switched Ring...Principle of operation (“4-fibre” BLSR illustrated)

8. Two main types of “survivable ring”....(2) BLSR
Bi-directional Line-switched Ring...Principle of operation (“4-fibre” BLSR illustrated)
Bi-directional - because in normal operation working demand flows travel in opposite directions over the same route through the ring
Line-switched - because in restoration the composite optical line transmission signal is switched to the other direction around the ring (on the other fibre pair) specifically around the failed section. Note implication: Protection fibre capacity must equal the largest-working capacity cross-section of any span on the ring.
“ The BLSR must have a line rate (capacity) greater (or equal to) the largest sum of demands routed over any one span of the ring. “

9. (4 fibre) BLSR Animation...
Working fibres 1
Loop-back 5 2
Protection fibres 3 4 l1
Loop-back

10. BLSR can also be in a “2-fibre” variant
Each fibre has its tributary channels arranged in two groups - Working - Protection
The set of 4 channel groups on two fibres then acts logically just like a 4-fibre BLSR
For the same demand pattern the required line rate is doubled
fibre 1
fibre 2
Ex: OC-48 2 BLSR: fibre 1 (cw) fibre 2 (ccw)
- Channels 1-24 Working Channels 1-24 Working
- Channels Protection Channels Protection
“loopback”

11. BLSR ...line capacity requirement to serve its demands
Start by considering how BLSR demand routing differs from UPSR....
Protection fibres
Working fibres
Protection fibre
Working fibre 1 2 3 4 5 l1
UPSR: every demand pair circumnavigates ring
BLSR: demand pair can be routed over shortest path. Not all spans “see” any given demand pair
 opportunity for “bandwidth reuse”

12. BLSR ...Bandwidth re-use improves BLSR efficiency
Concept of “bandwidth re-use” in a BLSR....
The example shows one timeslot (or “channel”) being reused on 4 spans to serve three different demand pairs.
Q. what demand pattern lends itself to perfect bandwidth re-use ?
Now note: the blue demand (1-3) could equally well have gone on route as (since same distance used).
If so, what would effect be on required line-rate capacity ?
Implication: BLSR line-rate requirement depends on how the set of demands it is to serve are loaded into it !
Demand 1-4
Demand 1-3 1
Demand 1-4 4
Time Slot #1 2
Demand 3-4 3
Demand 3-4
Demand 1-3