Optical Communications and Networking Nov. 27, 2017 1
What is a Core Network? A core network is the central part of a telecommunication network that provides services to customers who are connected by the access networks. A core network usually has a mesh topology that provides any-to-any connections among devices on the network. Network design: pushing the intelligence and decision making into the access networks and keeping the core network dumb and fast. Primary functions of core networks Traffic aggregation and data switching Core 2
Optical Core Networks SONET/SDH aggregates low-speed data streams and works as the transport layer. SONET/SDH are standardized protocols that transfer multiple digital bit streams over optical fibers. WDM (the optical layer) is the physical layer for wavelength routing. IP SONET/SDH WDM 3
Cross-Layer Networking Traffic is carried by all-optical connections (lightpaths) that each occupies a wavelength channel. Optical circuit switching Packet forwarding from lightpath to lightpath is performed in the IP routers Electrical packet switching 4
SONET/SDH SONET: Synchronous Optical Networks North America SDH: Synchronous Digital Hierarchy Europe, Japan SONET/SDH Standards convergence Time division multiplexing Synchronous network 5
Synchronous Multiplexing Asynchronous multiplexing Have to stuff extra bits for accommodating the clock difference For a 155 Mb/s signal, a 20 ppm (parts per million) variation in clock rate means? Difficult to de-multiplex a low-speed stream without completely demultiplexing the whole stream. Synchronous multiplexing All clocks are perfectly synchronized to a single master clock. 6
Synchronous Multiplexing The basic signal rate in SONET is 51.84 Mb/s The basic signal rate in SDH is 155.52 Mb/s The optical interface corresponding to 155.52 Mb/s is called OC-3 (Optical Carrier-3). OC-3: 155.52 Mb/s, OC-12: 622.08 Mb/s OC-24: 1.244 Gb/s, OC-48: 2.488 Gb/s OC-192: 9.953 Gb/s, OC-768: 39.814 Gb/s 7
WDM Wavelength Routing 8
Optical Add/Drop Multiplexer (OADM) 9
OADM Architectures 10
Parallel Architectures All incoming channels are demultiplexed. The loss through the OADM is fixed, independent of how many channels are dropped/added. Not very cost-effective for handling a small number of channels as all channels need to be multiplexed and demultiplexed. Loss could be high. Modular version - two stage multiplexing: wavelength band => wavelength channels. 11
Serial Architectures Wavelength are dropped and added individually as single channel (SC-) OADM In order to drop and add multiple channels, several SC-OADMs are cascaded. Loss is proportional to the number of channels dropped, and it can be a modular architecture. Band drop: try to make a compromise between the parallel and serial architectures. 12
Reconfigurable Optical Add/Drop Multiplexer (ROADM) 13
ROADM Architectures 14
Optical Cross-connect (OXC) 15
Optical Cross-Connect OXC is used to switch optical signals in a wavelength routing network. OXC can be functionally divided into a switch core and a port complex. The switch core performs the switching functionality. The port complex houses linecards that are used for communications. 16
Problems in Optical Core Networks Network Design Given a set of connection requests (traffic matrix), network design inserts network resource to support them with a cost-effective way. Static problem as the requests are known. Network Provisioning Given a network with finite resource, network provisioning allocates network resource to support dynamic connection requests. Dynamic problem as the requests are unknown. 17
Problem Statement of Network Design Physical topology: G(V,E), G is the topology, V is the set of nodes, and E is the set of links. Wavelength channels per link: W Traffic Matrix: a NxN matrix, N is the number of nodes in the network, and (i,j)-th element is the average traffic load flowing from node i to node j. Design objectives: minimize average delay, minimize equipment cost, minimize energy consumption, multiobjective optimization. 18
Design Constraints Wavelength continuity Along the routing path, the wavelength of a lightpath cannot be changed when there is no wavelength conversion. Capacity The used wavelength channels on each link should be less than or equal to W. Flow All required traffic should be supported. Single-route 19
Design Tasks Lightpath routing or virtual topology design The nodes of the virtual topology correspond to the nodes in the physical topology. Each link between a pair of nodes in the virtual topology corresponds to an optical lightpath between the them. Wavelength assignment Assign wavelength channel to each lightpath. Routing and wavelength assignment (RWA). 20
Example of RWA 21
Routing ILP Formulation 22
Routing Algorithms Solve ILP problem Heuristics Shortest path routing (Dijkstra) K-Shortest path routing Weighted path routing (modified Dijkstra) Least congested path routing (load balancing) Adaptive shortest path routing 23
Wavelength Assignments Graph Coloring problem Construct an auxiliary graph Lightpaths => nodes in the auxiliary graph If two lightpaths share a common link => a link between the corresponding nodes in the auxiliary graph Color the nodes in the auxiliary with minimal number of colors such that no adjacent nodes have the same color 24
Wavelength Assignments Heuristics First-fit: number the wavelength on the links, and use the first available wavelength with the lowest index Random-fit Least-used: assign the wavelength that is least used in the network. Most-used: assign the wavelength that is most used in the network. 25
Traffic Model for Network Provisioning Erlang: dimensionless unit for the statistical measure of traffic load in network. Carried traffic: Average number of concurrent requests carried by the network over a period of time. One Erlang of carried traffic: a single resource being in continuous use for a period of time. 26
Traffic Model of Network Provisioning Offered traffic: Average number of concurrent requests arrived in the network over a period of time. Request arrival rate: λ, Poisson process Average request-holding time: h, Exponentially distributed Erlang: E = λh Blocking: there is no enough network resource to serve the offered traffic. 27
Blocking Probability Assumption: An unsuccessful request, because of insufficient resource, is not queued or retried, but lost forever. The blocking probability P b of a path: m: # of wavelength channels on the path E = λh: offered traffic to the path 28
Blocking Probability In WDM mesh network, the model becomes more complicated and it is hard to provide an analytical expression. Discrete time simulations: 1. Generate Network Topology 2. Generate requests for a service period 3. Try to serve the requests and record blockings 4. Repeat 2-3 until statistical accuracy has been achieved. 29
Blocking Probability 30
Wavelength Conversion Wavelength Conversion (WC) eliminates the wavelength continuity constraint in WDM networks WC makes wavelength assignment trivial, as wavelengths used on successive links along a path can be independent of each other. WC can effectively improve the blocking probability, especially for requests routed through long paths. 31
Gain of Wavelength Conversion H: # of links along a path W: # of wavelengths per link p: the probability that a wavelength is used on any fiber link Blocking Probability when WC exists: Achievable utilization for a given blocking probability: 32
Gain of Wavelength Conversion Blocking Probability without WC: Achievable utilization for a given blocking probability: Gain of WC: 33
Network Survivability Customers lease lightpaths from the service provider. A Service-Level Agreement is signed to make sure the provider commits to providing a certain availability for the lightpath. A common requirement: the lightpath will be available 99.999% of the time. Down-time per year: 5 minutes 34
Network Survivability (0.9999999) 100 < 0.99999!! 35
Network Survivability There are many elements along a lightpath. The only practical way of obtaining 99.999% availability is to provide service continuously when failures present. Protection: providing redundant capacity 10 ~ 100 milliseconds recovery time. Restoration: recovering service during failure. 1 ~ 10 seconds recovery time. 36
When Protection and Restoration are needed? Link Failure Fiber cut Node Failure Power outage Nature Disasters Component Failure Regular Maintenance 37
How to Protect? 1:1 Protection Dedicated Protection High availability 1:N Protection Shared Protection High cost-efficiency 38
Protection for Link Failures Path Protection: The request is rerouted end-to-end on an alternate path. The protection path can be either dedicated or shared. Link Protection: The failed link is protected by a spare one. Example: pre-configured-cycle (p-cycle) in mesh networks. 39
Path Protection Path Protection Shared Path Protection 40
Link Protection: p-cycle Link failure recovery p-cycle configuration 41