Multiplexing Lab

Data Communication, Lecture 9. Share one link among many signals with FDM and TDM, compare synchronous and statistical TDM, and spread a signal with FHSS and DSSS. Works offline.

Channels side by side in frequency · grey = guard bands

How wide must the link be?

Minimum link bandwidthFormulan·B + (n − 1)·G

Example 6.2: five 100 kHz channels with 10 kHz guards need 540 kHz. Example 6.1: three 4 kHz voices, no guards, starting at 20 kHz.

Inputs (left) are interleaved into frames on the link · S = synchronising bit

Interleave the inputs

Input bit durationSlot durationFrame durationFrame rateLink bit rate

Example 6.7: four 1 kbps inputs, 1 bit per slot → link 4 kbps, slot 0.25 ms, frame 1 ms. Example 6.10: 2 kbps inputs, 8-bit slots, 1 sync bit → 8,250 bps.

Synchronous TDM: every input gets a slot, even when idle (empty = grey)
Statistical TDM: only waiting data is sent; each slot carries an address

Stop wasting empty slots

Synchronous: slots wastedStatistical: slots usedAddress bits per slot

Statistical TDM fills the link only with real data, so the link can be slower than the sum of the inputs, but every slot must carry an address.

FHSS: the carrier hops between M frequencies in a pseudo-random order
DSSS: each data bit is XORed with an 11-chip Barker code

Spread the signal on purpose

Barker code (11 chips)10110111000Chip rate for 1 Mbps11 Mchips/sSpread bandwidth vs original≈ 11 ×