CSC778 - Optical Networking Rudra Dutta, Fall 2007 General Overview: Static Traffic Network Design

Copyright Rudra Dutta, NCSU, Fall, Overview Static Traffic – Variation of traffic over time - no variation – Where and why? – A view of the network – Traffic networks and transport networks Network Design – Capacity, flow, provisioning – Design goals

Copyright Rudra Dutta, NCSU, Fall, Network Traffic What is traffic? – That which occupies / is carried by links Traffic is offered to the network by/at network nodes – Network is made of end nodes, intermediate nodes, and links – All traffic ultimately originated by end-nodes – However, for hierarchical networks, aggregation may occur In some network paradigms, E2E traffic is recognizable at all “places” in network In others, components within aggregated traffic not recognizable inside network “Forwarding” at L1 versus L3 – Are telephone calls just (very) long packets? – Consider the flexibility / choice of actions afforded to intermediate stations

Copyright Rudra Dutta, NCSU, Fall, Network View Because of scalability, hierarchy seems inevitable Nature of end-nodes and intermediate nodes vary All links are TDM (FDM modeled as separate links)

Copyright Rudra Dutta, NCSU, Fall, Traffic Aggregation - Static Traffic Consider lowest level networks Assume each station injects traffic steadily – Number of bits injected per time unit is constant for each source Due to aggregation, magnitude increases as traffic climbs hierarchy – But constant nature of traffic remains Aggregation/dis-aggregation process is straightforward for intermediate nodes – Effectively same as slotted TDM Therefore static traffic is stable - remains static at higher levels of hierarchy Utilization of links / provisioning of capacity is not a problem / not interesting – Capacity, of course, must increase at higher levels

Copyright Rudra Dutta, NCSU, Fall, Bursty Traffic Traffic is generated intermittently at each end node – Assume (peak) rates are known Question of capacity and aggregation become intertwined – One approach: pretend each end node is a steady source at its peak rate, then provision as before Aggregation will be easy – Another approach: provision for average Do bursts arrive deterministically? Sometimes link will be busy when traffic arrives to use it Must store-and-forward, or discard Question of slotting TDM comes in - work conservation

Copyright Rudra Dutta, NCSU, Fall, A View of Aggregation bandwidth burstiness

Copyright Rudra Dutta, NCSU, Fall, “Elastic” Traffic Read: – “Elastic Traffic Effects on WDM Dynamic Grooming Algorithms”, R. Lo Cigno, E. Salvadori, Z. Zsoka, Globecom 2004 – Also reference [10] of the above – Need to primarily read the definition and discussion on “elasticity” of Internet traffic Briefly: – Source-to-destination traffic flows in the Internet are not static as generated – Congestion in network will slow down bursts – In response, flow duration will increase

Copyright Rudra Dutta, NCSU, Fall, Empirical Observations Many sources around the net – Not all are equally comprehensive or thorough, use judgment CAIDA is a good source – Source for this set of data

Copyright Rudra Dutta, NCSU, Fall, About Loss Loss may occur on the link – Usually very little in guided medium - ignore – Usually handled by L2 transmissions or ignored Loss may occur at intermediate nodes – Store-and-forward buffers are finite - may overflow – Other mechanism at intermediate node may discard Does retransmission occur? – May not be required / desired – If desired, May be at L2, on link May be at L4, E2E

Copyright Rudra Dutta, NCSU, Fall, About Delay Controversial proposition: – “If delay is not important, capacity is not important” – “If delay is important, capacity must be large OR aggregation must be slotted OR both” Delay for static traffic / slotted TDM aggregation – Small, constant Delay for bursty traffic / statistical TDM – Large range Delay for circuits - telephony – Very small and constant, operative quantity is blocking ratio

Copyright Rudra Dutta, NCSU, Fall, Delay for Single Link Bursty traffic, statistical TDM Usual M/M/1 assumptions – In reality, traffic process is heavier-tailed D(,  ) = 1 / (  - ) “Statistical Multiplexing Gain”

Copyright Rudra Dutta, NCSU, Fall, Blocking in Telephony Average call rate Average holding time  Offered traffic load or intensity a =  a c / c! B(a,c) =  a k / k!

Copyright Rudra Dutta, NCSU, Fall, Static Traffic Performance Give “matrix” of traffic demand components – Static, “always-on” – Usually aggregate – Measured or estimated Delay - fairly constant for each demand Blocking - none; loss - none – Except in unusual circumstances Performance is measured globally – Various objectives – Delay or throughput (global, across all components) – Revenue, fairness, protection, …

Copyright Rudra Dutta, NCSU, Fall, Transport, Demand, Capacity Traffic Networks and Transport Networks Traffic networks: where stochastic demand picture is operative – Short term switching/routing Transport networks: where traffic demands of static magnitude are seen to be operative – (Semi-) Permanent – QoS considerations paramount – Demands seen to be injected at transport network nodes, lower level networks not visible Links must have capacity to carry traffic – But routing can be designed on basis of traffic

Copyright Rudra Dutta, NCSU, Fall, Flows Multiply defined term In this context, the traffic associated with a path or route Sometimes (esp. in Internet context) defined as source-to-destination traffic Routing defines flows, but routing can be in terms of flows – If only s-d is considered, routing can be by flow (s-d) Instead of by destination only – But same s-d traffic can be split up and routed variously Requires slotted TDM approach, to “mark” flows

Copyright Rudra Dutta, NCSU, Fall, Summation L3-switched/routed traffic can be thought of as static at a high level of network At this level, a transport view of network is appropriate, using slotted TDM This approach is indispensable when strong guarantees must be made w.r.t. delay, variability of delay, and bandwidth Capacity of links becomes important in meeting such guarantees Capacity, routing, and other variables can be thought of as “control knobs” in the ensuing design problem

Copyright Rudra Dutta, NCSU, Fall, Multi-layer Networks Generalized protocol layering can create complicated networks – Better thought of as multiple layers – Each layer satisfies flow constraints Generally, demand is specified in one layer and capacity in another – Must assume some mapping method - possibly constrained Digital Transmission Optics Media Cross- Connect IP Networks Ckt-switched voice Private Line

Copyright Rudra Dutta, NCSU, Fall, Management Cycle Near Real-Time Capacity Mgmt, Netw Engg. Network Planning

Copyright Rudra Dutta, NCSU, Fall, Design Problems Capacity Design – Given topology, traffic matrix, (constrained routing), find minimum links capacities that will work – Utilization maximization Fair Networking – Max-min fairness - user fairness – Proportional fairness - mixture of user and network Topology Design – Link installation cost included – Possibly virtual links in multilayer networks Restoration Design – Backup paths as well as working paths – Governed by failure criteria