Under the simplest conditions, a medium can carry only one signal at any moment in time. For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth.
The current techniques that can accomplish this include
- frequency division multiplexing (FDM)
- time division multiplexing (TDM)
- Synchronous vs statistical
- wavelength division multiplexing (WDM)
- code division multiplexing (CDM)
Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to each “user” or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies. A multiplexor accepts inputs and assigns frequencies to each device. The multiplexor is attached to a high-speed communications line. A corresponding multiplexor, or demultiplexor, is on the end of the high-speed line and separates the multiplexed signals
Analog signaling is used to transmits the signals. Broadcast radio and television, cable television, and the AMPS cellular phone systems use frequency division multiplexing. This technique is the oldest multiplexing technique. Since it involves analog signaling, it is more susceptible to noise.
Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users. Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
- Synchronous time division multiplexing, and
- Statistical, or asynchronous time division multiplexing.
Synchronous Time Division Multiplexing
The original time division multiplexing. The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern. T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing.
If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream. If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream.
Three types popular today:
- T-1 multiplexing (the classic)
- ISDN multiplexing
- SONET (Synchronous Optical NETwork)
Statistical Time Division Multiplexing
A statistical multiplexor does not require a line over as high a speed line as synchronous time division multiplexing since STDM does not assume all sources will transmit all of the time! Good for low bandwidth lines (used for LANs)
Wavelength Division Multiplexing (WDM)
Give each message a different wavelength (frequency). Easy to do with fiber optics and optical sources.
Dense Wavelength Division Multiplexing (DWDM)
Dense wavelength division multiplexing is often called just wavelength division multiplexing. Dense wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line. Different wavelength lasers (called lambdas) transmit the multiple signals. Each signal carried on the fiber can be transmitted at a different rate from the other signals. Dense wavelength division multiplexing combines many (30, 40, 50, 60, more?) onto one fiber.
Code Division Multiplexing (CDM)
Old but now new method. Also known as code division multiple access (CDMA). An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes. An advanced technique that allows multiple devices to transmit on the same frequencies at the same time. Each mobile device is assigned a unique 64-bit code (chip spreading code) To send a binary 1, mobile device transmits the unique code To send a binary 0, mobile device transmits the inverse of code. Receiver gets summed signal, multiplies it by receiver code, adds up the resulting values Interprets as a binary 1 if sum is near +64. Interprets as a binary 0 if sum is near –64.