Sharing the medium Many Users/Data Streams One Device

Multiplexing Space - division –physically separate channels (wires) Time - division –sharing a CPU in multiprogramming OSs –telephone connections to a switching station Frequency - division –tv channels on a cable line –telephone conversations on a TRUNK line –radio stations sharing the airwave

Space division User 5 User 6 User 7 User 8 User 1 User 2 User 3 User 4

Time and Frequency division User 5 User 6 User 7 User 8 User 1 User 2 User 3 User 4 Medium

Time Division time frequency User 1 and User 5 User 2 and User 6 User 3 and User 7

Frequency Division time frequency User 1 and User 5 User 2 and User 6 User 3 and User 7

Other Examples ASK –creates bands of freqs for modem transmission –in a different band for each direction allows for Frequency-Division Multiplexing TDM –PCM encoded audio for telephones TDM –Aloha protocols (satellites) Ethernet protocols (LANs)

Telephone lines (TDM) 4000Hz -> 8000 samples/sec -> 8 bits/sample-> bps Channel 24 Channel 3 Channel 2 Channel1 One every 125 microsecs T1 Line One call 8 bits 24 calls -> 24*8 bits 8000 times per second -> bps plus timing

North American Standards T T T T Mbps Voice ChannelsCarrier These carriers can be used at a business to provide aggregate data rates rather than divide them into voice channels.

Statistical Multiplexing Making the use of the medium more efficient Examples –cars on the highway –seats reserved on an airline flight –lines for making phone calls All overbook. Do not provide sufficient capacity to meet maximum demand. Provide less capacity. Save money. Usually good enough!

Data Transmission Example TDM SITE1SITE1 SITE2SITE2 A4A3A2A1A4A3A2A1 B4B3B2B1B4B3B2B1 C4C3C2C1C4C3C2C1 D4D3D2D1D4D3D2D1..A 2 A 1..B 2 B 1..C 2 C 1..D 2 D 1 D3C3B3A3D3C3B3A3 D4C4B4A4D4C4B4A4... Fully Utilized!

Data Transmission Example NOT Fully Utilized (9/16) SITE1SITE1 SITE2SITE2 A 4 A 3 …A 1 B 4 ….B 2 B 1 C 4 …….C 1 ……….D 1 …..A 1..B 2 B 1 …..C 1 …..D 1 ……….A 3...C 4 B 4 A 4... How Do We Make This More Efficient?

Under-Allocate SITE1SITE1 SITE2SITE2 A 4 A 3 …A 1 B 4 ….B 2 B 1 C 4 …….C 1 ……….D 1 …..A 1..B 2 B 1 …..C 1 …..D 1 A C 4 B 4 A bits overhead per frame saves wasted slots. Less capacity required. Unable to meet Maximum Demand. Overhead

Queueing Statistical multiplexing generates a whole new science Underallocating generates potential waiting lines –gas station –bank tellers –on-ramps at interstate –your personal “to-do” list ……………. Computer simulation –when to change resource amount (more tellers)

Multiplexing a subtle distinction Users trying to make calls –Statistical –Some users have to wait to gain access Calls actually on the line –Not Statistical –Once on, you consume the line as long as you are connected Any of the 3 basic multiplexing strategies can be utilized statistically or not (statically)

A better example.. See other notes for cable television architecture