Multiplexing

Multiplexing Techniques and PSTN Hierarchy 1

Multiplexing Transmitting several calls on the same physical connection (such as a wire pair) – Space Division multiplexing (SDM) – Frequency division multiplexing (FDM) – Time division multiplexing (TDM) – Wavelength division multiplexing (WDM) 2

Space Division Multiplexing (SDM) • Channels are represented by ki • Space is shown as Si • First three channels are mapped to the shown spaces.

k1

k2

k3

k4

k5

k6

channels ki

c t

c t

s1

• Problem: – One channel one space – (Used in old telephone systems) – a pair of copper wires to each subscriber – Another example : One car - one lane!!!

f s2

f

c t

s3

f

S - space k - channel 3

Time Division Multiplexing (TDM) • A channel gets the whole spectrum for a certain amount of time • Advantages – only one carrier in the medium at any time k1 k2 k3 k4 k5 k6 – throughput high even c for many users f • Disadvantages – precise synchronization necessary t • Applications – Telephony 4

Frequency Division Multiplexing • Separation of the whole spectrum into smaller frequency bands • A channel gets a certain band of the spectrum for the whole time k1 k2 k3 k4 k5 k6 • Advantages c – no dynamic coordination necessary f – works also for analog signals • Disadvantages – waste of bandwidth if the traffic is distributed unevenly t – guard spaces/bw • Applications 5 – Radio , TV

Time and frequency multiplex • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Advantages k1 k2 k3 k4 k5 – better protection against c tapping – higher data rates • But – precise coordination required • Application t 2.18.1 – GSM,GPRS

k6

f

6

Code multiplex • Each channel has a unique code • All channels use the same k1 spectrum at the same time • Implemented using spread spectrum technology • Advantages – bandwidth efficient – no coordination and synchronization necessary – good protection against interference and tapping • Disadvantages – lower user data rates – more complex signal regeneration • Applications – CDMA Mobile

k2

k3

k4

k5

k6

c

f

t

2.19.1

7

Wavelength Division Multiplexing

Prisms in WDM multiplexing and demultiplexing

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Plesiochronous digital hierarchy • Time division multiplexing in PSTN

9

30-Channel PCM frame format

10

24-Channel multiplexing process

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Transmission Techniques - T1 T2 and T3

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Difference between a T1 and DS1 • T1 defines a physical attribute of a transmission system operating at 1,544 Mb/s Cabling Regenerator Spacing Connectors • DS1 defines a 1.544Mb/s digital signal format Pulse shapes Frame Format • A T1 carrier typically carries a DS1 Signal • A DS1 may also be carried with a higher rate multiplex system such as DS3 or SONET 13

Digital Trunks - E1 E2 E3 and E4

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Plesiochronous digital hierarchies

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