Download Optical Networking

OPTICAL NETWORKING
ZILONG YE, PH.D.
[email protected]
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
w1
w2
w3
w4
w5
40G
40G
40G
40G
40G
Wavelength-Division
Multiplexing (WDM)
Fiber Links
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
s
2
6
3
S’
5
4
d
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
s
2
6
1
3
5
4
Without wavelength converter
d
s
2
6
3
5
4
With wavelength converter
d
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):


f1
Con: consumes the entire wavelength, and may be wasteful
Pro: No intermediate Optical-Electrical-Optical (OEO) conversion
10G
f2
10G
f3
20G
w1
w2
w3
w4
w5
40G
40G
40G
40G
40G
w1 = 40G
w2 = 40G
f1
f2
f3
10G
10G
20G
w1 = 40G
w3 = 40G
Wavelength-Division
Multiplexing (WDM)
Fiber Links
No Traffic Grooming
w1
w2
w3
w4
w5
40G
40G
40G
40G
40G
Wavelength-Division
Multiplexing (WDM)
Fiber Links
Traffic Grooming
ELASTIC OPTICAL NETWORK
 The spectrum can be divided in a flexible way
 Achieving an efficient spectrum utilization
ELASTIC SPECTRUM ALLOCATION
Path length
Bit rate
Conventional
design
1,000 km
400 Gb/s
Fixed
format, grid
QPSK
Elastic
optical path
network
Adaptive
modulation
Elastic channel
spacing
1,000 km
200 Gb/s
QPSK
200 Gb/s
1,000 km
100 Gb/s
QPSK
250 km
250 km
400 Gb/s
100 Gb/s
16QAM
16QAM
ELASTIC TRANSCEIVER
 Elastic transceiver can be tuned to generate lightpaths with variable bit rate
using different modulation
FLEXIBLE SWITCHING
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
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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
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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)
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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
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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
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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
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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
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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
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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.
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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
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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
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A BURST RESERVATION PROTOCOL: JUSTENOUGH-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
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JET
H
T    ( h)
h 1
 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)
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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)
i
 (h) is the offset time remaining
 ts = ta + T(i), where T (i)  T  
1
after i hops and ta is the timeh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
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JET
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