2A Flashcards
The process of combining two or more users into one composite signal for simultaneous transmission to the distant end de-multiplexer.
Multiplexing
It is an efficient method of sending signals from multiple users together across a single transmission link.
Multiplexing
The key to multiplexing is to have more transmitting bandwidth than the total amount needed by all of the users combined. If the total
bandwidth of all of the users coming in exceeds that of the system output bandwidth, then one or more users may experience…?
a delay or loss in communications
Most defense communications systems today use a ___ ___ ___.
time-based multiplexer
There are two basic types of multiplexing:
Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM).
FDM multiplexes users based on ___, while TDM multiplexes users by assigning a ___ ___ to each user.
frequency
time slot
A multiplexing technique that uses modulation to create analog signals for each user.
Frequency Division Multiplexing (FDM)
It is the older of the two multiplexing techniques, but it is still
used in some applications today.
FDM
As depicted in the given picture, each user will be modulated
into an analog signal representing only a portion of the overall frequency spectrum. After all users are modulated, the modulated signals are combined together prior to transmission. This combined signal will make up the entire transmitting bandwidth across a single channel. User A will occupy the ___ frequency all the way up to User C occupying the ___ frequency. At the receive end, the entire bandwidth is applied to every bandpass filter. Each distant end receiver has a bandpass filter that will only pass the frequency to which it is tuned. This means that receiver A will only get the ___ frequency that was transmitted by user A all the way up to receiver C getting the ___ frequency as transmitted by user C.
lowest
highest
FDM was extensively used in…?
telephone systems, Line of Sight (LOS), tropospheric scatter (troposcatter) links, and relay networks,
Most of the technology in civilian and military communications today utilizes which multiplexing method?
Time Division Multiplexing (TDM)
TDM is considered the ___ ___ use of a frequency. Not only does it subdivide the intelligence across a bandwidth, but TDM also interleaves the smaller portions together when it is all transmitted to the distant end.
round robin
TDM combines two or more digital signals onto the same transmission path by assigning each channel to a different time slot. This means that a sample of each channel is applied in sequence to the transmission line at an extremely fast rate. TDM samples a channel for a short period of time, then samples the next channel. A ___ ___ is used to control the sampling. After each channel is sampled, the process is repeated. Each bit of sampled intelligence will be separated by a short period of time and this time-divided signal is then transmitted to the distant end. At the distant end, the receive de-multiplexer is a switching device synchronized to the local transmit multiplexer. As the intelligence is received, it is successively routed to the correct channel by the switching device. As long as the multiplexer and de-multiplexer are synchronized, and the individual samples are recombined quickly enough, the original signal from the transmit end can
be received and understood at the receiving end.
switching device
The distant end multiplexer receives an aggregate signal and de-multiplexes (breaks apart or separates) all individual signals into their own respective outputs/channels.
Demultiplexing
