Day 8

Multiplexing

More than 1 signal per cable• Typically a single cable can carry a

single connection– Not good if you want a cable to be

able to send lots of separate• Telephone calls• TV channels• Data connections

Multiple ways to slice things• By Frequency

– Frequency Division Multiplexing

• By time– Synchronous Time Division

Multiplexing – Statistical Time Division Multiplexing

• Other special application– Wavelength Division Multiplexing– Discrete Multitone– Code Division Multiplexing

Frequency Division Multiplexing• Examples

– FM Radio– TV/Cable TV– D-AMPS (old cellular) – 802.11b/g (3 channels)

Frequency Division Multiplexing

Guard Bands• Used to prevent “cross talk”

between the channels.• Empty frequencies between

channels.

Synchronous time division• Each station gets a chance to talk.

– If a station has anything to say, their data is inserted into their block.

– If nothing to say, the block is left empty

– Everyone gets equal time.

Synchronous Time Division Multiplexing

T1/DS1 – 1.544Mb/s• The T1 is a high speed data line

designed to interconnect phone switches.– Also available to end users to provide high

speed data or lots of phone lines

• It uses synchronous time division multiplexing

• 24 Channels – Each frame gets 1 byte from each of 24

devices– 8000 frames per second are transmitted– 56k/channel for voice, 64k for data.

• Extra control bit is used giving 7 bits which is all that is necessary for voice.

• ~$400/month + install

ISDN• 2 B channels (64k)• 1 D channel (16k)• Signal

– Control bits– 8 bits from B1– Control bits– D bit– Control bits– 8 bits from B2

SONET• Synchronous Optical Network

– Single clock• Atomic clock

– Synchronous Transport Signals• OC 1 – 51.84Mbps

– 8000 frames/second 6480 bits/frame• OC 3 = 3*OC1 = 155.52Mbps

– ~ $40k/month• OC48 = 2.5Gbps

– ~$80k/month• OC192 = 192 OC1 =~ 10Gbps

– ~$1/4million per month• 1, 3, 9, 12, 18, 24, 36, 48, 96, 192

• Gige ~$15k/month

Statistical Time Division• Not everyone has something to

say at all times.– Why not give the traffic to those who

do

• If so, we need to add addresses so we know who said what.

• Multiplexer must be smart

Size of frames• Either they must be set in stone• Or you must transmit the size so

the other side can tell where one frame ends and the next starts

Wavelength Division Multiplexing• Using laser color to differentiate

streams• You can now multiplex multiple

OC192’s together on a single cable.– Has been used to create up to

1.6Tbps over a single fiber pair

Discrete Multitone Multiplexing• Examples

– DSL– Digital FM Radio– 802.11a/g

• A specific example of frequency division multiplexing– Data in a particular channel is

transmitted on multiple different frequencies at once.

– Frequencies are chosen to avoid interference with other data

Code Division Multiplexing• Each user has a code:

– A: 10111001– B: 01101110– C: 11001101

• If you want to send a 1 you send your code, if you want to send a 0 you send the inverse of your code (flip all 0’s and 1’s)

• Receiver gets the sum of all this, adds each code (1= 1 or 0= -1).– +8 means they sent a 1, -8 means

they sent 0