1. Zilong Ye, Ph.D. zye5@calstatela.edu
Optical Networking
Zilong Ye, Ph.D.

2. Wavelength-division multiplexing (WDM)
A single fiber link consists of a number of wavelength channels
Signal transmits in the optical domain
Each wavelength channel has a fixed wide of 50GHz in the spectrum domain, and can provide a very high bandwidth, e.g., 40Gbps
Each wavelength channel use the same modulation format
Wavelength-Division
Multiplexing (WDM)
Fiber Links
40G w1 w2 w3 w4 w5
40G
40G
40G
40G
WDM fiber links BPSK QPSK

3. The Routing and wavelength assignment (RWA) problem
Establishing a lightpath between a given source and destination requires the routing of the traffic, as well as the wavelength assignment on each physical link along the route
For example, in the figure below, the route between s and d is 6->1->2->3, and the signal is transmitted over the red channel on each physical link along the path.
Two lightpaths that share the same physical link are assigned with different wavelength channels, e.g., lightpaths s->d uses red channel on 1->2, while lightpath s’->d uses green channel on 1->2 1 2 s 6 3 d 5 4 S’

4. Wavelength continuity constraint
All-optical network requires signal to be transmitted only in the optical domain
The wavelength continuity constraint: the lightpath uses the same wavelength channel on each physical link along the route
The wavelength continuity constraint can be waived if wavelength converter is used; however, the wavelength converter is very expensive 1 2 1 2 s 6 3 d s 6 3 d 5 4 5 4
Without wavelength converter With wavelength converter

5. Routing solutions Fixed routing Fixed-alternate routing
Adaptive routing

6. Fixed routing Off-line calculation
Shortest-path algorithm: Dijkstra’s or Bellman-Ford algorithm
Advantage: simple
Disadvantage: high blocking probability and unable to handle fault situation

7. Fixed-alternative routing
Routing table contains an ordered list of fixed routes
e.g. shortest-path, followed by second-shortest path route, followed by third-shortest path route and so on
Alternate route doesn’t share any link (link-disjoint)
Advantage over fixed routing:
better fault tolerant
significantly lower blocking probability

8. Adaptive routing
Route chosen dynamically, depending on the network state
Adaptive shortest-cost-path
Each unused link has the cost of 1 unit; used link ∞; wavelength converter link c units.
Disadvantage: extensive updating routing tables
Advantage: lower blocking probability than fixed and fixed-alternate routing
Another form: least-congested-path(LCP)
Recommended form: shortest paths first, and use LCP for breaking ties

9. Wavelength assignment solutions
Random Assignment
First-Fit
Least-Used/SPREAD
Most-Used/PACK
Min-Product
Least Loaded
MAX-SUM
Relative Capacity Loss(RCL)
Wavelength Reservation
Protecting Threshold
Distributed Relative Capacity Loss(DRCL)

10. Random wavelength assignment
Randomly chosen available wavelength
Uniform probability
No global information needed

11. First-fit First available wavelength is chosen
No global information needed
preferred in practice because of its small overhead and low complexity
Perform well in terms of blocking probability and fairness
The idea behind is to pack all of the in-use wavelengths towards lower end and contineously longer paths towards higher end

12. Least used first Least used in the network chosen first
Balance load through all the wavelength
Break the long wavelength path quickly
Worse than Random:
global information needed
additional storage and computation cost
not preferred in practice

13. Most used first Select the most-used wavelength in the network
Advantages:
outperforms FF, doing better job of packing connection into fewer wavelength
Conserving the spare capacity of less-used wavelength
Disadvantages:
overhead, storage, computation cost are similar to those in LU

14. Traffic grooming No Traffic Grooming (left): w1 = 40G 10G w1 = 40G 10G
Con: consumes the entire wavelength, and may be wasteful
Pro: No intermediate Optical-Electrical-Optical (OEO) conversion
w1 = 40G
w1 = 40G
10G
w2 = 40G f1
10G f1 f2 f3
10G
10G
w3 = 40G f2
20G
20G f3
Wavelength-Division
Multiplexing (WDM)
Fiber Links
Wavelength-Division
Multiplexing (WDM)
Fiber Links
40G w1 w2 w3 w4 w5
40G
40G w1 w2 w3 w4 w5
40G
40G
40G
40G
40G
40G
40G
Traffic Grooming
No Traffic Grooming

15. Elastic optical network
The spectrum can be divided in a flexible way
Achieving an efficient spectrum utilization

16. Elastic spectrum allocation
250 km
400 Gb/s
200 Gb/s
100 Gb/s
1,000 km
Fixed format, grid
Adaptive
modulation
QPSK
16QAM
Path length
Bit rate
Conventional
design
Elastic optical path network
Elastic channel
spacing

17. Elastic transceiver
Elastic transceiver can be tuned to generate lightpaths with variable bit rate using different modulation

18. Flexible switching EONs WDM Networks
The optical nodes (cross-connect) need to support a wide range of switching (i.e., varying from sub-wavelength to super-wavelength)
WDM Networks
EONs

19. The routing and spectrum allocation (RSA) problem
Given a source and a destination, we need to determine the route and spectrum assignment.

20. Spectrum efficiency
Given a network traffic request, how to determine the spectrum width needed?
Spectrum width (GHz) = bandwidth requirement (Gbps) divided by the spectrum efficiency (bps/Hz)
E.g., a network traffic with a bandwidth requirement of 10Gbps, transmitted by a modulation format of spectrum efficiency of 2.5bps/Hz, needs a spectrum channel width of 4GHz.

21. Modulation format determines the spectrum efficiency and transmission reach
BPSK: 1.6 bps/Hz, 8000km
QPSK: 3.2 bps/Hz, 3000km
16QAM: 6.4 bps/Hz, 1000km

22. the RMSA problem Routing, modulation selection and spectrum assignment
Constraints:
The spectrum continuity constraint
The spectrum consecutive constraint
The transmission reach constraint

23. Network fragmentation
Due to the spectrum continuity constraint, there exists the network fragmentation problem in elastic optical network

24. Potential topics in addressing network fragmentation
Assessment matrix
Utilization entropy
Spectrum compactness
RMS based
Fragmentation-aware +
Proactive
Defragmentation
Passive
When to defrag, how to defrag, objective: (1) min # of spectrum slots (2) min service interruptions

25. Multipath routing Traffic splitting
Pros: potentially improve the network efficiency, reducing the network fragmentation
Cons: introducing more spectrum overhead (e.g., between each channel, there exists guard band), and introducing jitters in the receiving side and may need to be reassembled at the receiver

26. Optical Circuit Switching
Lightpaths are set up between source and destination nodes
No optical buffer needed at the intermediate nodes
Bit rate and protocol transparency
Setting up a connection takes a few hundreds of ms  Not suitable for short lived connections
Bandwidth allocated by one wavelength at a time, however, most applications only need sub- bandwidth
No statistical multiplexing  Inefficient bandwidth utilization when carrying bursty traffic

27. Optical Packet Switching
High bandwidth utilization due to statistical multiplexing
Need to buffer packets at intermediate nodes
Not feasible in the near future
Current optical switches (OXCs) too slow for packet switching
No practical optical buffer
Immaturity of high-speed optical logic
no practical optical control unit

28. The Challenge
How to efficiently support bursty traffic with high resource utilization as in packet switching while requiring no buffer at the WDM layer as in circuit switching?
Answer: Optical Burst Switching (OBS)

29. OBS Burst assembly/disassembly at the edge of an OBS network
Multiple IP packets aggregated into a burst at the ingress node
Data bursts disassembled at the egress node
Packets/bursts buffered at the edge during burst assembly/disassembly

30. OBS Separation of data and control signals in the core
For each data burst, a control packet containing the header information (including burst length) is transmitted on a dedicated control channel
A control packet is processed electronically at each intermediate OBS node to configure the OXCs
An offset time between a control packet and the corresponding data burst
The offset time is large enough so that the data burst can be switched all-optically without being delayed at the intermediate nodes
switching fabric

31. Advantages of OBS
No optical buffer or fiber delay lines (FDLs) is necessary at the intermediate nodes
Burst-level granularity leads to a statistical multiplexing gain absent in optical circuit switching
A lower control overhead per bit than in optical packet switching

32. OBS Building Blocks
Burst assembly: assembly of client layer data into bursts
Burst reservation protocols: end-to-end burst transmission scheme
Burst scheduling: assignment of resources (wavelengths) at individual nodes
Contention resolution: reaction in case of burst scheduling conflict

33. Burst Assembly
Aggregating packets from various sources into bursts at the edge of an OBS network
Packets to the same OBS egress node are processed in one burst assembly unit
Usually, one designated assembly queue for each traffic class
Create control packet and adjust the offset time for each burst

34. Burst Assembly Algorithms
Timer-based scheme:
A timer starts at the beginning of each assembly cycle
After a fixed time T, all the packets that arrived in this period are assembled into a burst.
Effect of time out value T
T too large: the packet delay at the edge will be too long.
T too small: too many small bursts will be generated resulting in a higher control overhead.
Disadvantage: might result in undesirable burst lengths.

35. Burst Assembly Algorithms
Burstlength-based scheme:
Set a threshold on the minimum burst length.
A burst is assembled when a new packet arrives making the total length of current buffered packets exceed the threshold.
Disadvantage: no guarantee on the assembly delay

36. Burst Assembly Algorithms
Mixed timer/threshold-based assembly algorithm:
A burst is assembled when either the burst length exceeds the desirable threshold or the timer expires
Address the deficiency of both timer-based and burstlength-based schemes

37. A Burst Reservation Protocol: Just-Enough-Time (JET)
Basic ideas
Each control packet carries the offset time and burst length
The offset time is chosen so that no optical buffering or delay is required at the intermediate nodes
Delayed reservation: the reservation starts at the expected arrival time of the burst

38. JET Control packet is followed by a burst after a base offset time
(h): time to process the control packet at hop h, 1  h  H
No fiber delay lines (FDLs) necessary at the intermediate nodes to delay the burst
At each intermediate node, T is reduced by (h)

39. JET
Use Delayed Reservation (DR) to Achieve efficient bandwidth utilization
Bandwidth on the output link at node i is reserved from the burst arrival time ts to the burst departure time ts + l (l = burst length)
ts = ta + T(i), where is the offset time remaining after i hops and ta is the time at which the processing of the control packet finishes
The burst is dropped if the requested bandwidth is not available
Can use FDLs at an intermediate node to resolve contention

40. JET