CSC778 - Optical Networking
Rudra Dutta, Fall 2007
General Overview: Static Traffic Network Design
Overview
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Static Traffic
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Variation of traffic over time - no variation
Where and why?
A view of the network
Traffic networks and transport networks
Network Design
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Capacity, flow, provisioning
Design goals
Copyright Rudra Dutta, NCSU, Fall, 2007
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Network Traffic
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What is traffic?
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That which occupies / is carried by links
Traffic is offered to the network by/at network nodes
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Network is made of end nodes, intermediate nodes, and links
All traffic ultimately originated by end-nodes
However, for hierarchical networks, aggregation may occur
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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
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Are telephone calls just (very) long packets?
Consider the flexibility / choice of actions afforded to
intermediate stations
Copyright Rudra Dutta, NCSU, Fall, 2007
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Network View
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Because of scalability, hierarchy seems inevitable
 Nature of end-nodes and intermediate nodes vary
 All links are TDM (FDM modeled as separate links)
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Copyright Rudra Dutta, NCSU, Fall, 2007
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Traffic Aggregation - Static Traffic
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Consider lowest level networks
Assume each station injects traffic steadily
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Due to aggregation, magnitude increases as traffic
climbs hierarchy
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But constant nature of traffic remains
Aggregation/dis-aggregation process is straightforward
for intermediate nodes
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Number of bits injected per time unit is constant for each source
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
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Capacity, of course, must increase at higher levels
Copyright Rudra Dutta, NCSU, Fall, 2007
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Bursty Traffic
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Traffic is generated intermittently at each end
node
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Assume (peak) rates are known
Question of capacity and aggregation become
intertwined
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One approach: pretend each end node is a steady
source at its peak rate, then provision as before
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Aggregation will be easy
Another approach: provision for average
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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, 2007
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A View of Aggregation
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burstiness
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bandwidth
Copyright Rudra Dutta, NCSU, Fall, 2007
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“Elastic” Traffic
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Read:
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“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:
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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, 2007
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Empirical Observations
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Many sources around the net
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Not all are equally comprehensive or thorough, use
judgment
CAIDA is a good source
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Source for this set of data
http://www.caida.org/outreach/isma/9901/slides/gmiller_ISMA99/sld001.htm
Copyright Rudra Dutta, NCSU, Fall, 2007
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About Loss
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Loss may occur on the link
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Loss may occur at intermediate nodes
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Usually very little in guided medium - ignore
Usually handled by L2 transmissions or ignored
Store-and-forward buffers are finite - may overflow
Other mechanism at intermediate node may discard
Does retransmission occur?
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May not be required / desired
If desired,
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May be at L2, on link
May be at L4, E2E
Copyright Rudra Dutta, NCSU, Fall, 2007
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About Delay
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Controversial proposition:
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Delay for static traffic / slotted TDM aggregation
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Small, constant
Delay for bursty traffic / statistical TDM
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“If delay is not important, capacity is not important”
“If delay is important, capacity must be large OR
aggregation must be slotted OR both”
Large range
Delay for circuits - telephony
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Very small and constant, operative quantity is
blocking ratio
Copyright Rudra Dutta, NCSU, Fall, 2007
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Delay for Single Link
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Bursty traffic, statistical TDM
Usual M/M/1 assumptions
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In reality, traffic process is heavier-tailed
D(l, m) = 1 / (m - l)
“Statistical Multiplexing Gain”
Copyright Rudra Dutta, NCSU, Fall, 2007
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Blocking in Telephony
Average call rate l
 Average holding time t
 Offered traffic load or intensity a = lt
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ac / c!
 B(a,c) = -------------------
S ak / k!
Copyright Rudra Dutta, NCSU, Fall, 2007
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Static Traffic Performance
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Give “matrix” of traffic demand components
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Static, “always-on”
Usually aggregate
Measured or estimated
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Delay - fairly constant for each demand
 Blocking - none; loss - none
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Except in unusual circumstances
Performance is measured globally
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Various objectives
Delay or throughput (global, across all components)
Revenue, fairness, protection, …
Copyright Rudra Dutta, NCSU, Fall, 2007
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Transport, Demand, Capacity
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Traffic Networks and Transport Networks
 Traffic networks: where stochastic demand
picture is operative
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Transport networks: where traffic demands of
static magnitude are seen to be operative
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Short term switching/routing
(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
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But routing can be designed on basis of traffic
Copyright Rudra Dutta, NCSU, Fall, 2007
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Flows
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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
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If only s-d is considered, routing can be by flow (s-d)
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Instead of by destination only
But same s-d traffic can be split up and routed
variously
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Requires slotted TDM approach, to “mark” flows
Copyright Rudra Dutta, NCSU, Fall, 2007
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Summation
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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, 2007
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Multi-layer Networks
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Generalized protocol
layering can create
complicated networks
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Better thought of as
multiple layers
Each layer satisfies flow
constraints
Generally, demand is
specified in one layer
and capacity in another
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Must assume some
mapping method possibly constrained
Copyright Rudra Dutta, NCSU, Fall, 2007
Ckt-switched
voice
Private
Line
IP
Networks
CrossConnect
Digital
Transmission
Optics
Media
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Management Cycle
Near Real-Time
Capacity Mgmt, Netw Engg.
Network Planning
Copyright Rudra Dutta, NCSU, Fall, 2007
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Design Problems
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Capacity Design
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Fair Networking
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Max-min fairness - user fairness
Proportional fairness - mixture of user and network
Topology Design
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Given topology, traffic matrix, (constrained routing), find
minimum links capacities that will work
Utilization maximization
Link installation cost included
Possibly virtual links in multilayer networks
Restoration Design
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Backup paths as well as working paths
Governed by failure criteria
Copyright Rudra Dutta, NCSU, Fall, 2007
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