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Broadcast&Select WDM networks - 1
Optical Networks: from fiber transmissionto photonic switching
Broadcast-and-Select Networks
Fabio Neri and Marco MelliaTLC Networks Group Electronics Department
e-mail: [email protected]://www.tlc-networks.polito.it/
[email protected] tel. 011 564 [email protected] tel. 011 564 4173
Broadcast&Select WDM networks - 4
Broadcast-and-select networks
No routing, but full connectivity: information from a source is broadcasted to all receivers; receivers select the information directed to them, and discard the rest
Similar to traditional LAN operation
All nodes have visibility on all network traffic, but do not need to process (switch) all of it
The aim is to limit the bandwidth processed by network nodes, finding the proper balance between photonic and electronic technology
Broadcast&Select WDM networks - 5
Broadcast-and-select networks Typical topologies: star, bus and ring
It is possible to have star of stars, ring of rings, etc.
starcoupler
1
7 3
8 2
56 4
Broadcast&Select WDM networks - 6
Passive couplers Passive component, to couple or split signals in different
fibers
Can be fiber based, or realized in waveguides Can be wavelength selective When
= , the input power on one fiber is (as a first
approximation) halved on each output fiber (3 dB coupler) When 1 (0.9 - 0.95), we have a tap
output 1
output 2input 2
input 1
O1 =
I1 + (1-) I2
O2 = (1-) I1 + I2
Broadcast&Select WDM networks - 7
Broadcast-and-select networks
12345678
12345678
3-dB coupler
Using a passive star coupler it is possible to build the broadcast star topology
overall n/2 log2 n 22 (3 dB) couplers log2 n 22 devices crossed on all paths: same power loss
for every node pair
Broadcast&Select WDM networks - 8
A bus topology requires 2n 22 couplers Losses along the path are typically larger
(linear with n) Losses are different for each node pair
1 2 3 4 5 6 7 8
3-dB coupler
Broadcast-and-select networks
Broadcast&Select WDM networks - 9
Broadcast-and-select networks
Each node is usually attached to two fibers: one to transmit, one to receive
W WDM channels are available Tx and Rx operate on a single WDM channel at
a time (to reduce electronic bandwidth) It is possible to observe collisions and contention
Collision: two or more transmitters transmit on the same channel at the same time
Contention: a single receiver must tune to two or more channels at the same time
We need a Medium Access Control (MAC) protocol
Broadcast&Select WDM networks - 10
Broadcast-and-select networks Nodes can be equipped with one or more tx and rx
devices, which may be tunable or fixed Tunable txs and rxs are more expensive (and tunable rxs
usually cost more than tunable txs) Connectivity may be limited due to components or
complexity constraints
For example, if node i has a fixed tx on i and a fixed rx on |i-1|N , a ring logical topology results
Traffic will be then routed using multi-hop paths, going through a number of intermediate nodes, where OEO conversion is performed
Broadcast&Select WDM networks - 11
Broadcast-and-select networks With limited connectivity, a logical topology is built
over the (broadcast star) physical topology For example: 2 fixed tx/rx per node allow to build a
shuffle topology
1
2
3
4
5
6
7
8
1
2
3
4
1234
5
786
91011
12
13
1516
14
1234
5
786
Broadcast&Select WDM networks - 12
Broadcast-and-select networks Different resource allocation strategies can be adopted
when the traffic pattern is relatively stable (flow duration much larger than propagation delays), or when a dynamic, packet by packet, network control is necessary
Often time is slotted, and statistical time multiplexing is adopted
Tuning time of tx/rx may be a non-negligible and must be taken into proper account
One (or more) channel can be devoted to signaling (almost necessary in broadcast stars )
Slot synchronization does not come for free, since the slot time is small (guard times must be small compared to the slot duration)
Bit synchronization must be faced as always
Broadcast&Select WDM networks - 13
Slot synchronization
Broadcast&Select WDM networks - 14
Synchronization problems Varying propagation delays must be equalized (ranging) Slot phase (when the slot starts) and frequency (how long does
it last) information must be distributed; chromatic dispersion and tuning latencies must be taken in proper account using guard times at slot boundaries
The bit frequency (not the bit phase) can be broadcasted (to simplify the receiver design), or acquired at each receiver
The bit phase must be acquired at receivers Burst mode receivers are necessary: each rx can receive from
a different source in different time slots (we no longer have point-to-point channels); therefore bit synchronization and decision threshold must be acquired for each new reception
Broadcast&Select WDM networks - 15
Slotted Aloha / Slotted Aloha Broadcast-and-select star network; N nodes;
W
Broadcast&Select WDM networks - 16
Slotted Aloha / Slotted Aloha When node x has to transmit a packet:
It selects according to some criteria (e.g. at random) a transmission channel T
It transmits a control frame (containing destination node and chosen data channel) using c
The data frame is transmitted on T in the next minislot Every node
Keeps listening to the control channel When a transmission to its address is detected, it tunes the rx on
channel T There can be collisions (on c and on T ) and contention (on T ) This is called tell-and-go approach (data is assumed to be
received after large propagation delays)
Broadcast&Select WDM networks - 17
Slotted Aloha / Slotted Aloha
Broadcast&Select WDM networks - 18
Slotted Aloha / Slotted Aloha Two tx/rx pairs per node are necessary:
One fixed pair locked to the control channel One tunable pair for data channels
To avoid useless transmissions on the data channels after collisions on control and data channels, it is possible to use a wait-and-see approach: do not transmit data until the control frame is received back (after a propagation delay); we get:
higher throughput higher access delays
To reduce hardware costs, it is possible to use only one tunable tx/rx pair (with TDM implementation of the control channel)
Variable-size packets can be accommodated by specifying the packet length (in minislots) in control packets
Broadcast&Select WDM networks - 19
DT-WDMA Broadcast-and-select star network; N nodes;
W=N channels plus a control channel on c Time is divided into minislots (signaling) and
slots (data), with N minislots per data slot
tdata N=6
Nodes are equipped with a fixed transmitter and a tunable receiver for data, plus a fixed tx/rx pair on c
t
control
data t
Broadcast&Select WDM networks - 20
DT-WDMA Due to transmitter-dedicated channels, no
collision is possible on data channels No collision on the control channel, due to TDMA
access There could be contention on the data channel;
we assume that all nodes use the same algorithm to solve these contentions, so that no explicit ack is necessary
The destination id is the only information carried by the control frame (source and data channel are known due to the TDMA control access)
Broadcast&Select WDM networks - 21
Scheduling protocols It is possible to increase performance keeping the same DT-
WDMA scheme, but delaying transmission of data frames If data frame allocation is decided by all nodes with the
same rules after having received all reservations, contentions can be avoided
data
control
end-to-end delay
reservationsfor slot X
slot X
Access to the control channel can become random (instead of deterministic TDM) to improve scalability
Broadcast&Select WDM networks - 22
Larger delays at low loads Larger maximum throughput
Performance of scheduling
Broadcast&Select WDM networks - 23
Scheduling protocols The previous schemes are distributed: no central control
node, no delay in collecting reservations They can be viewed as extensions of traditional and
successful LAN MAC protocols to a multi-channel setup If access delays are increased, and network resources
are allocated to signaling, it is possible to collect all requests in a central node (or in all network nodes), and compute a schedule for transmissions in the next slot, or next set of slots (frame)
In case of a centralized scheduler, the outcome of the scheduling algorithm must be returned to network nodes
The scheduling (or resource allocation) algorithm can operate on the knowledge of the traffic request matrix
R[i,j]
Broadcast&Select WDM networks - 24
Switches and B&S networks Broadcast-and-select networks are
equivalent to packet switches Nodes in the network correspond to input/output
ports (linecards) of the switch The central star corresponds to the switching
fabric The same equivalence holds between MAC
algorithms and scheduling algorithms in switches similarities for input buffering, output buffering,
and speed-up can be easily recognized
Broadcast&Select WDM networks - 25
Output buffer switch
switching fabricoutput 1
output P
input 1a1
input PaN
..
..
..
..
..
..
xP
x1
Broadcast&Select WDM networks - 26
Output buffering In the packet switch: the switching fabric
and the output card buffers must operate at the aggregate speed Limited scalability
In the B&S network: output buffering is equivalent to have W=N channels, and N receivers in each node
In both systems, it is possible to show that this complexity is not needed
Broadcast&Select WDM networks - 27
Input buffer switch
input 1a1
input PaN
..
..
..
output 1
output P
..
..
..
scheduler
xP
x1
D
switching fabric
Broadcast&Select WDM networks - 28
Input buffering Considering W=N WDM channels, the B&S
network behaves as a rearrangeably non- blocking crossbar (if W
Broadcast&Select WDM networks - 29
Input buffer switch with VOQ
aP
input P
a1
input 1
scheduler
output 1
output P
..
..
..
..
..
..
x1,1x1,P
a1,1
a1,P
1
P
xP,1xP,P
aP,1
aP,P
1
P
D
switching architecture
Broadcast&Select WDM networks - 30
Matching probleminput output input output
Broadcast&Select WDM networks - 31
Scheduling problem Matching algorithms on bipartite graphs
are used
requests maximummatching
maximalmatching
Maximum matching has a complexity O(N2.5) Requests can be weighted (priority, queue
length, age, ) leading to weighted matching
Broadcast&Select WDM networks - 32
Scheduling By increasing the access delay, it is possible to solve a
time/frequency scheduling problem considering several slots in a time frame
Input is the traffic request matrix (which can be static in case of persistent requests, or dynamic)
Constraints: no more than one transmission per TX and no more than one transmission per RX in each time slot; tuning latencies may have to be taken into account
Utility: find the minimum number of slots (minimum frame size) to satisfy all requests, or minimize losses for a fixed frame
Off-line or on-line traffic scenarios; single hop or multihop strategies
Broadcast&Select WDM networks - 33
Scheduling example (I) A traffic rate matrix (node to node rates are normalized
to channel capacity) is given (N=4 nodes):0.0 0.7 0.1 0.10.1 0.0 0.5 0.2 0.5 0.1 0.0 0.40.4 0.2 0.4 0.0
Each row sum is the total tx rate of one node: must be less than or equal to the number of transmitters available at that node (we assume one)
Each column sum is the total rx rate of one node: must be less than or equal to the number of receivers available at that node (we assume one)
Broadcast&Select WDM networks - 34
Scheduling example (II) A frame size F must be decided; longer frames give
finer granularity but more complexity; we take F=10 Rates must be translated in slots per frame; in our
case:0 7 1 11 0 5 2 5 1 0 44 2 4 0
A time-wavelength plan must be built, subject to constraints (no more than one packet can be received and transmitted by a node in a slot)
We assume that i delivers information to receiver i
Broadcast&Select WDM networks - 35
Scheduling example (III) Largest demands are served first (scanning the traffic matrix
by rows) The 4 slots from 4 to 3 cannot be accommodated (the red
slots cannot be used due to constraints); we move forward in time four transmissions from 2 to 3
3 3 3 3 3 4 4 4 41 1 1 1 1 1 12 2 2 2 2
3 3 3 31 2 3 4 5 6 7 8 9 10 1 2 3 4 5
1234
Broadcast&Select WDM networks - 36
Scheduling example (IV) The 2 slots from 4 to 2 cannot be accommodated
(the red slots cannot be used due to constraints); we move forward in time two transmissions from 1 to 2
3 3 3 3 3 4 4 4 41 1 1 1 1 1 14 4 4 4 2 2 2 2 22 2 3 3 3 31 2 3 4 5 6 7 8 9 10 1 2 3 4 5
1234
Broadcast&Select WDM networks - 37
Scheduling example (V) The slots from 3 to 2 cannot be accommodated; we
swap in time the last two transmissions to 1, 2 and 3
3 3 3 3 3 4 4 4 4 21 1 1 1 4 1 1 1 44 4 4 4 2 2 2 2 2 12 2 1 3 3 3 31 2 3 4 5 6 7 8 9 10 1 2 3 4 5
1234
Broadcast&Select WDM networks - 38
Scheduling example (VI) The final scheduling (just one of the several
possible) is a sequence of switching configurations or of input/output permutations
3 3 3 3 3 4 4 4 2 4 3 3 3 3 1 1 1 1 4 1 1 1 4 3 1 1 1 1 4 4 4 4 2 2 2 2 1 2 4 4 4 4 2 2 1 3 3 3 3 2 2 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5
1234
Broadcast&Select WDM networks - 39
Broadcast-and-select testbeds Several B&S testbeds were described since the early 90s:
Lambdanet [Bellcore 1990]: 18
1.5 Gb/s with 2 nm channel spacing Testbed NTT [NTT 1993]: 100
622 Mb/s with 10 GHz channels spacing
Rainbow I [IBM 1990]: 32
300 Mb/s with 1 nm channel spacing Rainbow II [IBM 1996]: 32
1 Gb/s with 1 nm channel spacing SONATA [E.C. ACTS 1999]: 800
622 Mb/s with 6.25 GHz (0.05 nm) channel spacing
Today the interest in B&S networks for metro networks has decreased
Passive Optical Networks (PONs) take a similar approach for high-speed access and traffic concentration
Broadcast&Select WDM networks - 40
Considering ring topologies With an approach similar to B&S star networks,
consider a ring topology for applications in metropolitan area networks
Rings help to distribute synchronization information, permit spatial reuse of resources, ease the design of distributed access schemes, ease fault recovery, but introduces larger losses
Usually N users, WN WDM channels, tunable transmitters and fixed receivers
Examples: CORD, Daisy+SR3, Hornet, Wonder Recently interconnected WDM rings have been
proposed (e.g. IST DAVID project)
Broadcast&Select WDM networks - 41
Arrayed Waveguide Grating (AWG) Generalization of Mach-Zehnder interferometers Wavelength-routing capabilities
Largely used as WDM mux/demux
AWG
11, 21, 31, 4112, 22, 32, 4213, 23, 33, 4314, 24, 34, 44
11, 22, 33, 4414, 21, 32, 4313, 24, 31, 4212, 23, 34, 41
Broadcast&Select WDM networks - 42
AWG-based networks AWGs have very interesting features to
implement single-hop interconnections using tunable TXs and (tunable) RXs
TT1TT2
TTN
RX1RX2
RXN
AWG No broadcasting Max total
bandwidth = N RXs can be fixed
Broadcast&Select WDM networks - 43
AWG-based network with couplers
Max total bandwidth = N2 Packet scheduling needed Tunable RXs (and TXs) needed
AWG+
+
TTTTTTTT+
RXRXRXRX+
TTTTTTTT
TTTTTTTT
RXRXRXRX+
RXRXRXRX+
couplers couplers
Broadcast&Select WDM networks - 44
AWG-based networks with couplers
The space equivalent is a 3-stage setup, with a full mesh in the central stage
Boxes are non-blocking switches (e.g. crossbars)
TXTXTXTX
RXRXRXRX
TTTxTXTX
TTTTTTTX
RXRXRXRX
RXRXRXRX
Broadcast&Select WDM networks - 45
AWG-based networks
One channel available between any pair of groups of terminals
AWG+
+
TTTTTTTT+
RXRXRXRX+
TTTTTTTT
TTTTTTTT
RXRXRXRX+
RXRXRXRX+
Broadcast&Select WDM networks - 46
AWG-based networks
AWG+
+
RXRXRXRX+
TTTTTTTT
TTTTTTTT
RXRXRXRX+
-converter
WDM mux WDM demux
One extra channel available between any pair of groups of terminalsScheduling is more complex
Diapositiva numero 1Broadcast-and-select networksBroadcast-and-select networksPassive couplersBroadcast-and-select networksBroadcast-and-select networksBroadcast-and-select networksBroadcast-and-select networksBroadcast-and-select networksBroadcast-and-select networksDiapositiva numero 13Synchronization problemsDiapositiva numero 15Diapositiva numero 16Diapositiva numero 17Slotted Aloha / Slotted AlohaDiapositiva numero 19Diapositiva numero 20Scheduling protocolsPerformance of schedulingDiapositiva numero 23Switches and B&S networksOutput buffer switchOutput bufferingDiapositiva numero 27Input bufferingInput buffer switch with VOQMatching problemDiapositiva numero 31Diapositiva numero 32Scheduling example (I)Scheduling example (II)Scheduling example (III)Scheduling example (IV)Scheduling example (V)Scheduling example (VI)Diapositiva numero 39Considering ring topologiesArrayed Waveguide Grating (AWG)AWG-based networksAWG-based network with couplersAWG-based networks with couplersAWG-based networksAWG-based networks