Broadcast-and-select networks
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-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 usually
cost more than tunable txs)
• For example: 2 fixed tx/rx per node allow to build a shuffle
topology
Level of performance
It is measured by
Packet delay: The delay is specified from the
start of the packet being transmitted at the
source to the end of the packet being received
at the destination and
through put of network : data that are
successfully received by their destination
Slot synchronization
Synchronization algorithm
• Predicts the time of arrival of the next sync
pulse at its receiver
• Estimates the round trip delay
• A node transmits the information one round
trip delay prior to the estimated time of
arrival.
Slotted Aloha/Slotted Aloha protocol
Some basic assumption
• No. of wavelength or channel, W<N
• Each node must have a fixed –tuned
transmitter and receiver operating at control
wavelength
Slotted Aloha/Slotted Aloha
• Aloha used in single channel broadcast
network where any node that has a packet to
send transmits it immediately.
• Whereas In slotted aloha time is divided into
slots whose length is equal to the maximum
sized packet
In multichannel network
• The time is divided into slots for data as well
as the control channel
Basic Slotted Aloha/Slotted Aloha
protocol
Operation:
When a data to be send :
Control packet is sent in control slot
Data packet is sent in data slot
• Every node
– Keeps listening to the control channel
– When a transmission to its address is detected, it tunes
the rx on channel λT
This is called “tell-and-go” approach (data is assumed to
be received after large propagation delays)
There can be collisions and contention.
Basic Slotted Aloha/Slotted Aloha
protocol
Control wavelength protocol : Slotted Aloha
Data wavelength protocol: Slotted Aloha
Thus the name Slotted Aloha / Slotted Aloha
Modified SA/SA
• To avoid useless transmissions on the data
channels after collisions on control and data
channels, it is possible to use a “wait-andsee” approach: do not transmit data until the
control frame is received back (after a
propagation delay); we get:
• higher throughput
• higher access delays
Access delays
Defined as the delay between the time at
which a packet is available for transmission
at a node and the time at which it is first
transmitted
Through put analysis
Some assumptions are required
1. Independent transmission
2. Large “n”
3. Uniform traffic
Independent transmission
In any slot and for any node the probability that
the node has a packet for transmission is p,
independent of all other slots and all other
nodes.
Large “n”
• No. of nodes in the network n is very large
compared to W
Assume that
• W is fixed but n  ∞ and p 0 then
• The expected no. packets in the network is
Gnp
Uniform traffic
• Data packets is equally likely to be transmitted
on any one of the W data channels
Throughput analysis
Expected no. of data packets that are successful
in any data slot for basic SA/SA is
LGe-G/W.(e-G/W)2(L-1)
And for modified SA/SA
LGe-G/W.(1- Ge-G/W)2(L-1)
DT-WDMA
• Dynamic Time-Wavelength Division Multiaccess
• This protocol assume that the
– Number of nodes n is equal to the number of wavelengths W.
– Each node has a 1 Fixed Tr, 1 Tunable Rx and 1 Fixed Rx for the control
channel
– The size of the data slots is n times the control slots.
– The data slots do not overlap in time.
• When ever a node, say xi, has a data packet to send, it sends a
control packet in a control slot and the data packet in the data slot
immediately following it.
• Each node continuously monitors the control channel. Thus when
node xj , receives a control packet send by node xi it knows that
the next data slot has a packet intended for it being transmitted on
wavelength I so it tunes its TR to that wavelength.
• Features : Data packets never collide also control packet never
collide.
Scheduling protocol is a self study. ..
Test beds
Test beds
Topology
No.of
wavelength
Wavelength
Spacing
Bit Rate per
wavelength
Lambdanet
Star
18(18)
2nm
1.5Gb/s
NTT
Star
100(100)
10GHz
622 Mb/s
Rainbow I
Star
12(32)
1nm
300 Mb/s
Rainbow II
Star
4(32)
1nm
1 Gb/s
Starnet I
Star
2(80)
10GHz
1.25/2.5Gb/s
BBC
Interconnected
Star
-(16)
4nm
2.5 Gb/s