Computer network Learning Notes--Physical layer (2) __ Network and communication

Channel Multiplexing Technology

Multiplexing is the basic concept of communication technology, as shown in the following illustration, if a multiplexer is used at the transmitter, it can be combined to communicate using a shared channel. At the receiving end of the use of the distributor, the combined transmission of information sent to the corresponding end point.

When communicating, the multiplexer is always used in pairs with the splitter. The user-shared high speed channel between the multiplexer and the Distributor, the role of the splitter and the courage to the contrary, it uses the data transmitted by the high speed Channel to divide and then deliver to the corresponding user.

1, Frequency Division multiplexing FDM (frenquency Division Multiplexing)

All users of frequency division multiplexing consume different bandwidth resources at the same time (bandwidth refers to frequency bandwidth rather than data transmission rate).

When using frequency division multiplexing, if each user occupies the same bandwidth, then when the reused user increases, the total bandwidth of the multiplexed channel is widened.

2, Time Division Multiplexing TDM (Time Division Multiplexing)

Time division multiplexing is divided into a section of the length of the interval multiplexing frame (TDM frame), each of the users in the TDM frame in each of the fixed number of time slots. The time slots that each user occupies are periodically present (the length of the TDM frame is the period). Time-division multiplexing users occupy the same band width at different times. Time Division Multiplexing is more advantageous to the transmission of digital signals.

When a user temporarily has no data to send, the time slot allocated to the user in the TDMA frame can only be idle, and other users can not use these idle time slots even if they have data to send. This leads to low utilization of the channel after multiplexing.

3. Statistical Time Division Multiplexing STDM (statistic TDM)

It is an improved time division multiplexing, it can obviously improve the utilization of the channel. The concentrator often uses this statistical time division multiplexing. The following figure is a statistical time division multiplexing schematic diagram, a use of statistical time Division multiplexing concentrator to connect 4 low-speed users, and then set their data into a high speed circuit to send to a remote computer.

Each user has the data to send to the concentrator's input cache at any time, then the concentrator sequentially scans the input cache sequentially, putting the input data in the cache into the STDM frame. When a frame of data is full, send out. STDM frames are allocated gaps that are dynamically on demand. Therefore, statistical time division Multiplexing is also called asynchronous TDMA, because a user's slot is not cyclical. Note: The premise that a concentrator works correctly is to assume that each user is working intermittently.

Time division multiplexing may lead to waste of line resources

The intelligent multiplexer can provide the ability to store and forward the whole message, and make the users more reasonable to share the channel by queueing way. Both TDM and STDM frames are the frames that are divided in the bit stream transmitted by the physical layer, which is not the same as the "frame" of the data link layer that will be discussed at a later time, and cannot be confused.

3, Wavelength Division multiplexing WDM (Wavelength division multiplexing)

Wavelength division multiplexing is the frequency division multiplexing of light. Because of the high frequency of the optical carrier, it is customary to use wavelength rather than frequency to express the use of more than the optical carrier, so that the use of Wave Division multiplexing this term.

It is now possible to duplicate the optical carrier signal of dozens of or more paths on a single optical fiber, and the term "dense wavelength division multiplexing DWDM" is used.

The light signal decays after a distance, so the attenuation of the optical signal must be amplified to continue the transmission. Now there is a good erbium-doped fiber amplifier EDFA, which is a kind of optical amplifier, can be directly to the optical signal amplification, not to convert the light signal to the electrical signal amplification and then converted to light signals. The image in the transmission line is placed in EDFA.

Laying optical cables Underground is a costly project. So people always put as much fiber in an optical cable as possible, and then use dense wavelength division multiplexing for each fiber.

4, Code Division multiplexing CDM (Code Division multiplexing)

Code Division multiplexing is another way of sharing channels, in fact, more commonly used nouns are code division multiple access CDMA. Each user can use the same frequency band to communicate at the same time, because each user uses specially chooses different code type, therefore each user does not cause the interference (each user assigns the code slice sequence not only must each be different, but also must mutually orthogonal, namely the inner product is 0, one code piece vector and oneself normalized inner product is 1, The normalized inner product of the vector with its own code-chip is-1).

If there is an X station to accept the data sent by the S station, the X station must know the sequence of codes peculiar to S station. The X station uses the chip vector of s and the unknown signal received to compute the inner product. The X station receives the signal is the each station sends the code piece sequence The sum, will they and S's code slice vector to seek the inner product, can only then leave the S station to send the signal. When the S station sends 1 o'clock, the calculation of the inner product result is 1, when the S station sends the 0 o'clock, the inner product result is 1 (if the bit 0 is sent, then the binary inverse code of the sequence of the code is sent).

PS: The picture is taken from the book itself, relatively rough, just for the convenience of understanding.