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

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

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’

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

Routing solutions Fixed routing Fixed-alternate routing Adaptive routing

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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)

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

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

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

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

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

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.

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

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

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

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)

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

JET