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Optical Time Division Multiplexing And Demultiplexing

Optical Time Division Multiplexing And Demultiplexing. Packet Switched Service. Second generation optical network provides packet switched services Packet-switched services are provided by using electronic networks Rate of transmission is slow Requires to set up a lightpath for each packet

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Optical Time Division Multiplexing And Demultiplexing

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  1. Optical Time Division Multiplexing And Demultiplexing

  2. Packet Switched Service • Second generation optical network provides packet switched services • Packet-switched services are provided by using electronic networks • Rate of transmission is slow • Requires to set up a lightpath for each packet • Packet switching is inefficient in terms of lightpath since transmission time is much shorter than the time to setup light path

  3. Need for phonic packetswitching • Requirement to go for higher standard to increase the rate of transmission • Switching functions are performed optically which increases the speed • Overcomes the drawbacks of packet switched services in terms of lightpath

  4. OTDM • Time Division Multiplexing is done by passing multiple optical signals into a single data stream • Done by using either bit-by-bit basis or packet-by-packet basis where framing pulses are used • In bit-interleaved case,if n input data streams are multiplexed,framing pulse are used every n bits • In packet-interleaved case,framing pulse helps in marking the boundary between packets

  5. OTDM Communication System • 10 Gbps is commercially available • There are two types: -Bit Interleaved TDM -Slotted (packet) TDM

  6. Bit-interleaved optical multiplexer

  7. Packet-interleaved optical multiplexer

  8. Bit Interleaving multiplexer • Multiplexing is done by generating periodic pulse train using a mode-locked laser • The pulse train is spitted into n no of data streams,i=1,2,…n • Here some data streams are delayed and others remain undelayed • Delaying is done by passing some data streams through optical fibers • Undelayed pulse stream is used as framing pulse • Each data stream externally modulates the appropriately delayed periodic pulse stream • Combination of delayed pulse and framing pulse are given to bit-interleaved TDM

  9. Optical multiplexer to create bit-interleaved TDM stream

  10. Bit Interleaving Demultiplexer • Multiplexed input is split into two streams is using 3-db coupler • For extracting jth multiplexed stream,the stream is delayed by j • To extract the framing pulse a thresholding operation is performed on the delayed stream • A logical AND operation is performed between framing pulse stream and multiplexed pulse stream to extract the jth stream • The output of logical AND gate is a pulse if both inputs have pulses

  11. Optical demultiplexer of a bit-interleaved TDM

  12. Packet Interleaving multiplexer • Periodic stream of narrow pulses is externally modulated by the data stream • If bit interval is T, the separation between successive pulse is also T. • Should reduce the interval between successive pulse to t which is done by passing modulator through compression stages • If size of each packet is l bits, output goes through k=[log2l] compression stages

  13. Optical multiplexer to create packet-interleaved TDM

  14. Cont.. • Compression stage consists of a pair of 3-db coupler,two SOA used as on-off switches and a delay line • In the 1st compression stage,bits 1,3,5,7… are delayed by T-t • In the 2nd compression stage,the pair of bits (1,2),(5,6),(9,10).… are delayed by 2(T-t) • In the 3rd compression stage,bits (1,2,3,4),(9,10,11,12)... are delayed by 4(T-t) • The jth compression stage,has a delay line of value 2^j-1(T-t) • The delay encountered by the pulse i, passing through kth compression stage is ((2^k) –i)(T-t)

  15. Compression stage

  16. Packet Interleaving Demultiplexer • Decompressing is done by passing the compressed packet through a set of decompression stages • No of stages required is k=log[l],l denotes the packet length • We use five AND gates to convert the incoming high-speed stream into five parallel streams • And gate used has two inputs ,one from data stream,the other from control pulse stream which are offset from each other and hence selects different pulses • In this way the first parallel stream contains 1,6,11…of packet,the second contains 2,7,12..and so on.

  17. Packet interleaved Demultiplexer

  18. conclusion • Optical multiplexing and demultiplexing provides efficient way of transmission compared to other media • Provides higher bit rate of the order of 100Gb/s • Operations are performed optically at very high speeds

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