Multiplexing refers to a scheme that allows multiple logical signals to share a single physical channel. Two common types of multiplexing are time-division multiplexing (TDM) and statistical time-division multiplexing (STDM).
TDM was invented to maximize the amount of voice traffic carried over a medium. Before multiplexing, each telephone call required its own physical link. This was an expensive and unscalable solution. TDM divides the bandwidth of a single link into separate time slots. TDM transmits two or more channels (data stream) over the same link by allocating a different time slot for the transmission of each channel. In effect, the channels take turns using the link.
TDM is a physical layer concept. It has no regard for the nature of the information that is multiplexed on to the output channel. TDM is independent of the Layer 2 protocol that has been used by the input channels. TDM increases the capacity of the transmission link by dividing transmission time into smaller, equal intervals so that the link carries the bits from multiple input sources.
In the figure, a multiplexer (MUX) at the transmitter accepts three separate signals. The MUX breaks each signal into segments. The MUX puts each segment into a single channel by inserting each segment into a time slot.
A MUX at the receiving end reassembles the TDM stream into the three separate data streams based only on the timing of the arrival of each bit. A technique called bit interleaving keeps track of the number and sequence of the bits from each specific transmission so that they can be quickly and efficiently reassembled into their original form upon receipt.
Compare TDM to a train with 32 railroad cars. Each car is owned by a different freight company, and every day the train leaves with the 32 cars attached. If one of the companies has cargo to send, the car is loaded. If the company has nothing to send, the car remains empty, but stays on the train. Shipping empty containers is not very efficient. TDM shares this inefficiency when traffic is intermittent, because the time slot is still allocated even when the channel has no data to transmit.
STDM was developed to overcome this inefficiency. STDM uses a variable time slot length allowing channels to compete for any free slot space. It employs a buffer memory that temporarily stores the data during periods of peak traffic. STDM does not waste high-speed line time with inactive channels using this scheme. STDM requires each transmission to carry identification information or a channel identifier.
Sonet and SDH
On a larger scale, the telecommunications industry uses the Synchronous Optical Networking (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely-related standards that specify interface parameters, rates, framing formats, multiplexing methods, and management for synchronous TDM over fiber.
The figure displays SONET, which is an example of STDM. SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signals. It then sends the signals out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) x n. Thus, traffic arriving at the SONET multiplexer from four places at 2.5 Gb/s goes out as a single stream at 4 x 2.5 Gb/s, or 10 Gb/s. This principle is illustrated in the figure, which shows an increase in the bit rate by a factor of four in time slot T.
Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol (PPP) over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes (LEDs) over optical fibre at high line rates. POS is defined by RFC 2615 as PPP over SONET/SDH. PPP is the Point to Point Protocol that was designed as a standard method of communicating over point-to-point links. Since SONET/SDH utilises point-to-point circuits, PPP is well suited for use over these links. Scrambling is performed during insertion of the PPP packets into the SONET/SDH frame to solve various security attacks including denial-of-service attacks and the imitation of SONET/SDH alarms. The most important application of POS is to support sending of IP packets across Wide Area Networks. Large amounts of traffic on the Internet are carried over POS links.