What is frequency division multiplexing, time division multiplexing?

Frequency division multiplexing (FDM), is the division of the channel bandwidth into a number of non-overlapping frequency bands, one for each signal station. The commonly heard FM radio is a typical example of FDM.

Time-Division Multiplexing (TDM), is the use of each signal sampling value in time does not overlap each other, so as to achieve multiple signals of the same channel transmission at the same time. This is still well understood.

Code division multiplexing (CDM) is a technique in which each channel is used as a coding channel to achieve bit transmission (a specific sequence of pulses). This type of coded transmission is accomplished by transmitting a unique time series of short pulses, but time slices are used instead for longer bit durations. Each channel, has its own code and can be transmitted as well as asynchronously demultiplexed over the same fiber.

Packet-switched networks

Packet-switched networks are a new type of switched network after circuit-switched and message-switched networks, which are mainly used for data communications, such as X.25, Frame Relay, DPT, SDH, GE, and ATM are examples of packet switching. Packet switching is a store-and-forward switching method, which divides the user's message into packets of a certain length (which can be of fixed and indefinite length), and forwards them in packets as store-and-forward. Therefore, it has a higher utilization rate than circuit switching, a smaller delay than message switching, and the ability to communicate in real time. Packet switching utilizes the principle of statistical time division multiplexing to multiplex a data link into multiple logical channels, which finally constitutes an information transmission path between the calling and called users, called virtual circuit (i.e., VC, where two user terminal devices need to establish a logical connection through the network before they start to send and receive data from each other), to realize packet transmission of data. Some packet switching networks support statistical multiplexing, some do not support statistical multiplexing, for example, SDH does not support statistical multiplexing, its bandwidth is fixed, support for statistical multiplexing technology is mainly frame relay, ATM and IP, the following are introduced separately.

(1) Frame Relay

Frame Relay is a fast packet-switched transmission technology developed on the basis of X.25 packet-switching technology, user information is transmitted in frames (variable length), and statistical multiplexing of user information flow.

(2) ATM

ATM supports connection-oriented (non-physical logical connection) services, with great flexibility, according to the actual needs of multimedia services can be dynamically allocated communication resources, for a particular service, the transmission rate varies with the rate of arrival of the information, so ATM has the ability to adapt to any type of service with statistical multiplexing.

(3) DPT

DPT (Dynamic Packet Transport) is a new generation of optimized dynamic packet transport technology created by Sisco, absorbing the advantages and overcoming the shortcomings of SDH, and combining the efficient use of IP routing technology for broadband and the rich business integration capability with the high-bandwidth and reliable self-healing function of fiber optic loops. The IP routing technology for efficient use of broadband and rich service convergence capabilities and the high bandwidth of fiber optic loops and reliable self-healing function is closely integrated, as all nodes have a fairness mechanism and support bandwidth statistical multiplexing, which can exponentially increase the available bandwidth network.

(4) Gigabit Ethernet

GE (Gigabit Ethernet) is an extension of Ethernet technology, is the third generation of Ethernet, which mainly deals with data services, broadband broadband metropolitan backbone network using the mainstream technology. Ethernet switch port (RJ45) with the user channel utilization is usually not the same, there will often be a channel is very busy, some channels in the idle state, even if all the ports of the Ethernet switch are in the communication state, but also involves the different demand for bandwidth, and the characteristics of the data exchange lies in the suddenness of the only through the statistical multiplexing, that is, the dynamic allocation of bandwidth in order to reduce the busy and idle phenomenon, thus maximizing the busy and idle phenomenon, so as to minimize the time required for data exchange. The only way to reduce the busy-idle phenomenon is through statistical multiplexing, i.e. dynamic allocation of bandwidth, thus maximizing the use of network bandwidth.