Multiplexing No. 1  Seattle Pacific University Multiplexing: Sharing a single medium between multiple users Kevin Bolding Electrical Engineering Seattle Pacific University Based on Chapter 8 of William Stallings, Data and Computer Communication, 7 th Ed.

Multiplexing No. 2  Seattle Pacific University Sharing Multiplexing is all about sharing Multiple users want to use the same medium Cost savings Fewer wires/fibers Use of large capacity links Statistical usage Necessity Airwaves are not private property! So, how can we share? Any way that we can filter out everybody else’s signal

Multiplexing No. 3  Seattle Pacific University Methods of Multiplexing Frequency (wavelength) division Each channel gets a portion of the total bandwidth Use band-pass filtering Time division Each channel gets the whole bandwidth for a portion of the time Use time-slot filtering – Synchronous Use demand-driven techniques - Asynchronous Code division Each channel has an individual digital code Transmits on many bands at once (spread-spectrum) Uses digital processing to filter out signals

Multiplexing No. 4  Seattle Pacific University Frequency Division Multiplexing FDM can be used any time a channel’s required bandwidth is less than the medium’s total bandwidth Simply assign each channel a portion of the bandwidth kHz Single speech signal AM Modulated to 64kHz note: dual sidebands kHz transmit only one sideband kHz 0 Multiplexed with other signals 72 Based on Stallings, Fig. 8.5 Also called Wavelength Division Multiplexing (WDM)

Multiplexing No. 5  Seattle Pacific University Time Division Multiplexing Use all of the bandwidth for each channel Divide the usage based on time slots Normally used only with digital data Mux Synchronous TDM Each channel has a fixed, regularly occurring slot It’s 4:03: , this must be channel 3…

Multiplexing No. 6  Seattle Pacific University North American TDM Standards NameVoiceMbps Channels DS DS-1(T1) DS-1c DS DS-3(T3) DS NameDataPayload Rate (Mbps)Rate (Mbps) OC OC OC OC OC OC AT&TSONET OC OC

Multiplexing No. 7  Seattle Pacific University Asynchronous TDM Synchronous TDM reserves space for the maximum channel rate Always allocated, even if input stream is idle Wiser allocation: Allocate a slot for a channel only when it is needed Issues How do we know what channel a slot is for? Put a header in each slot (packet) How do we manage all of the different needs of input streams? Asynchronous TDM – Use packets (datagrams) instead of time slots

Multiplexing No. 8  Seattle Pacific University Code Division Multiplexing Instead of allocating discrete time/frequency units, allow multiple users to use the whole bandwidth Use digital coding techniques to separate users Each sender has a unique digital code All data is encoded with this code; receiver separates signals by codes Spread-spectrum technique Signal 10x Spreading Code Encoded signal (10x BW) Shannon’s Law: C=B log 2 (SNR+1) SS: Large bandwidth, low power

Multiplexing No. 9  Seattle Pacific University CDMA – Walsh Codes Hadamard-Walsh codes are mutually orthogonal After being combined, they can all be separated back out Walsh functions of order 2 (can combine two sequences) The (0) code is used to transmit a binary 0, the (1) for a binary 1 W 2 0(0) = W 2 0(1) = W 2 1(0) = W 2 1(1) = To transmit: Sum codes from all channels Ch : Ch : Sum: 0 +2 Ch : Ch : Sum: 2 0 Ch : Ch : Sum: 0 -2 Ch : Ch : Sum: -2 0 All summed combinations are unique – can separate out the original code Note: 2-times spreading – Each bit becomes two chips

Multiplexing No. 10  Seattle Pacific University Larger Walsh Codes Walsh codes are (nearly) mutually orthogonal codes of any degree Some correlation in larger codes, but minimal W87W W86W W85W W84W W83W W82W W81W W80W80 An 8-way Walsh code (Note: Use negative of code to send 0) CDMA uses 64-bit Walsh codes 64x Spreading Can support 64 simultaneous transmissions on the same frequency band

Multiplexing No. 11  Seattle Pacific University Using Walsh Codes: 8-sender Example Walsh matrix: Multiply data to send by row. Spreads each bit 8x On the common channel, all signals are effectively summed when combined in airwaves Each row represents 8 chips sent by that sender Sending (Modulating) Process Time for 1 Bit 8 Chips This is sent on the channel over one bit time (8 chip times) Data to send by 8 senders 1C7 0C6 1C5 0C4 0C3 0C2 1C1 1C0 (Binary 1 represented by +1, Binary 0 represented by -1)

Multiplexing No. 12  Seattle Pacific University Using Walsh Codes: 8-sender Example Walsh matrix: Multiply received data by column. Receiving (Demodulating) Process Sum rows 8  Binary 1 -8  Binary 0 1C7 0C6 1C5 0C4 0C3 0C2 1C1 1C This is sent on the channel over one bit time (8 chip times) Each channel recovers the original bit sent to it

Multiplexing No. 13  Seattle Pacific University Multiplexing Summary Three basic methods of division: Frequency Time Code (digital) Can combine methods: Frequency-division into large bands, then time- division within each band SONET works this way Time-division over a single CDMA channel