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 slotData 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 λTThis 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 AlohaData 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-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

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 required1. Independent transmission2. 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

Gnp

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/SALGe-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

wavelengthWavelength 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