Network topology I

Apr. 26, 2018


TDM and network topology handout
More on TDMA handout
More on WDMA handout

• Multiplexing vs multiple access: multiplexer between nodes and shared medium.
all nodes directly connected to shared medium.
• Time domain (TDM or TDMA): Signal consists of time slots, each of which carries bits of 1 input. Time slots are grouped into frames with additional bits for frame and slot synchronization.
TDM requires input signals with the same bit rate, i.e. freq justification input clocks into a common clock.
TDMA Stores low bit rate signal in a buffer and use guard time to sync.
• Freq Domain (FDM or FDMA): corresponds to WDM in optics.
• Code-domain medium access (CDMA): In direct seq., system assigns a unique code seq. for each user and signal carries the code seq. Receiver will use the code seq. to decode signal.
In Freq. hopping, signal carried by a sinusoidal carrier with a freq. that is hopping in a pseudo-random manner. Each user will be assigned a unique freq.-hop codes.
Near-far problem - signal near to substation may interfere those from remote locations.
• Routing of signal: Req. switching array including I/O, switching fabric and control.
Try to achieve - max. throughput, max. switching speed, min. number of crosspoints, min. blocking probability, min. delay & loss, scalability and broadcasting & multicasting.
• Switch architectures: multiple-stage (layer) switches reduce cross points, e.g. a 3-stage switches instead of (nr)x(nr) cross points, has2mnr+mr2 Potential problem - internal blocking, reqm2n1
Use of two dimensional switches, e.g. time & space switching. System structure - time-space-time (TST); time slot interchange (TSI) spatial switch TSI. E.g. L T1 line with 24 DS0 channels(24L)2 cross points but TST only requiresL2 cross points.
Scalable switches,e.g. star coupler with MZ filters.
• Circuit switching vs packet switching: circuit switching - seq or route is pre-defined.
Packet switching - route defined dynamically by packet, e.g. Batcher-Banyan.
Batcher network sorts input packets in ascending order according their destination.
Banyan network routes packet to output port, e.g. 1st stage routes by most significant bit of output port addr and 2nd stage least significant bit. Guarantee non internal blocking but blocking can occur when two packets sent to the same output port.
Circuit switching is preferable for voice and video in high speed optical communication.
Packet switching requires high speed switching as transmission speed increases.
Optical communication systems have problem in adjusting clock since photons are difficult to store.
• Topology: blink>Ring, star and bus.
For optical network in a bus topology, we have hybrid (2x2 directional coupler) instead of T connector. Power coupling coef from bus to downstream is x while coupling coefficient from upstream bus to down stream bus is 1-x.
For bidirectional comm., use double bus or re-entrant bus.
Use ring for reliability. Passive star coupler for combining and broadcasting signals.
Star with switch is used for routing from an input to a particular outputs.
Logical topology, e.g. logical ring formed by a physical star.
• Considerations: Power loss and dynamics range.
Power loss increases with number of nodes N.
In general, re-entrant bus loss > bus loss > star coupler loss. Bus lossηbus=(1x)N2x210αN/10 withxbus,opt=2/N
Re-entrant bus lossηr,bus=x(1x)2N3x10α(2N1)/10 withxr,bus,opt=2/(2N1)
Star coupler lossηStar=N110αlog2N/10
Note:α is coupling loss in dB, total coupling power loss from bus to node is10α/10x and coupling power loss from upstream to down stream10α/10(1x)
Dynamic range - power ratio of max. rec power to min. power. Dynamic range for star coupler is 0dB.
Dynamic range for busDRbus=10(N2)α/10/(1x)N2
Dynamic range for re-entrant busDRr,bus=10(2N4)α/10/(1x)2N4
• Bottlenecks: Direct modulation 1-10GHz
TDM require freq. justification
Optical systems cannot perform complicated operations like those in electronics

trade speed for simplicity.


Last Modified: April 11, 2018
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