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# STDM, T-1 lines, TDM and FDM

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How does STDM yield more efficient use of the bandwidth?

How many T-1 lines are needed to make up one OC12 line ? Need to see the calculated out to understand this

Explain the differences between TDM and FDM?

See the attached file.

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1. How does STDM yield more efficient use of the bandwidth?
STDM: statistical time division multiplexing:
TDM becomes inefficient when traffic is intermittent because the time slot is still allocated even when the channel has no data to transmit. Statistical time division multiplexing was developed to overcome this problem
For the following reasons, STDM yield more efficient use of the bandwidth.
âž¢ A system developed to overcome some inefficiencies of standard time division multiplexing, where time slices* are still allocated to channels, even if they have no information to transmit.
âž¢ STDM uses a variable time slot length and by allowing channels to compete for any free slot space.
âž¢ It employs a buffer memory which temporarily stores the data during periods of peak traffic. This scheme allows STDM to waste no high-speed line time with inactive channels.
âž¢ STDM requires each transmission to carry identification information (i.e. a channel identifier).
âž¢ To reduce the cost of this overhead, a number of characters for each channel are grouped together for transmission.
âž¢ The scheduler is run once every time slice to choose the next process to run. If the time slice is too short then the scheduler will consume too much processing time but if it is too long then processes may not be able to respond to external events quickly enough.
Note: time slices (Or "time quantum", "quantum") The period of time for which a process is allowed to run uninterrupted in a pre-emptive multitasking operating system

2. How many T-1 lines are needed to make up one OC12 line? Need to see the calculated out to understand this

For this purpose, we need to use this standard list which gives the speed of respective lines.
âž¢ DS0 - 64 kilobits per second
âž¢ ISDN - Two DS0 lines plus signaling (16 kilobits per second), or 128 kilobits per second
âž¢ T1 - 1.544 megabits per second (24 DS0 lines)
âž¢ T3 - 43.232 megabits per second (28 T1s)
âž¢ OC3 - 155 megabits per second (84 T1s)
âž¢ OC12 - 622 megabits per second (4 OC3s)
âž¢ OC48 - 2.5 gigabits per seconds (4 OC12s)
âž¢ OC192 - 9.6 gigabits per second (4 OC48s)
OC12 ----------------â†’ 622 megabits per second
T1 -------------------â†’ 1.544 megabits per second
Hence we need 622/1,544 = 403 T1 lines to make up the speed of 1 OC12 line

A T1 line can carry 24 digitized voice channels, or it can carry data at a rate of 1.544 megabits per second. If the T1 line is being used for telephone conversations, it plugs into the office's phone system. If it is carrying data it plugs into the network's router.
A T1 line can carry about 192,000 bytes per second -- roughly 60 times more data than a normal residential modem. It is also extremely reliable -- much more reliable than an analog modem. Depending on what they are doing, a T1 line can generally handle quite a few people. For general browsing, hundreds of users are easily able to share a T1 line comfortably. If they are all downloading MP3 files or video files simultaneously it would be a problem, but that still isn't extremely common.
The other end of the T1 line needs to be connected to an ISP.
3. Explain the differences between TDM and FDM?
TDM: Time division multiplexing.
A type of multiplexing where two or more channels of information are transmitted over the same link by allocating a different time interval ("slot" or "slice") for the transmission of each channel. i.e. the channels take turns to use the link. Some kind of periodic synchronizing signal or distinguishing identifier is usually required so that the receiver can tell which channel is which.
TDM becomes inefficient when traffic is intermittent because the time slot is still allocated even when the channel has no data to transmit. Statistical time division multiplexing was developed to overcome this problem.

FDM: Frequency division multiplexing.
The simultaneous transmission of multiple separate signals through a shared medium (such as a wire, optical fibre, or light beam) by modulating, at the transmitter, the separate signals into separable frequency bands, and adding those results linearly either before transmission or within the medium. While thus combined, all the signals may be amplified, conducted, translated in frequency and routed toward a destination as a single signal, resulting in economies which are the motivation for multiplexing. Apparatus at the receiver separates the multiplexed signals by means of frequency passing or rejecting filters, and demodulates the results individually, each in the manner appropriate for the modulation scheme used for that band or group.
Bands are joined to form groups, and groups may then be joined into larger groups; this process may be considered recursively, but such technique is common only in large and sophisticated systems and is not a necessary part of FDM.
Neither the transmitters nor the receivers need be close to each other; ordinary radio, television, and cable service are examples of FDM. It was once the mainstay of the long distance telephone system. The more recently developed time division multiplexing in its several forms lends itself to the handling of digital data, but the low cost and high quality of available FDM equipment, especially that intended for television signals, make it a reasonable choice for many purposes.

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