INTERNET AND WAN TELECOMMUNICATION

Transcription

1 INTERNET AND WAN TELECOMMUNICATION MULTIPLEXING LECTURER: MERCY CHITAURO

2 AGENDA Multiplexing Synchronous Time Division Multiplexing Statistical Time Division Multiplexing Frequency Division Multiplexing Code Division Multiplexing

3 MULTIPLEXING Multiplexing is used in high speed telecommunication lines. Multiple signals share one medium Multiplexing allows several txn sources to share a larger txn capacity Sharing the capacity of a data link is termed multiplexing data rate capacity of the txn medium > data rate required by the sending and receiving devices

4 TYPES Syncronous TDM Statistical TDM FDM WDM CDM

5 SYNCHRONOUS TDM TDM is a physical layer concept, TDM is independent of the Layer 2 protocol that has been used by the input channels. TDM slices time into smaller intervals so that the link carries the bits from multiple input sources Transmitter and the receiver both know exactly which signal is being sent.

6 TDM TDM is a digital multiplexing technique to combine data.

7 TIME-DIVISION MULTIPLEXING (TDM) Syncrhonous = Multiplexer allocates exactly the same time slot to each device at all times Regardless if they don t want to transmit Frames Time slots are grouped into frames. A frame consists of a one complete cycle of time slots, including one or more slots dedicated to each sending device.

8 TDM frames

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10 EXAMPLE A 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 timeslot. 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

11 TIME DIVISION MULTIPLEXING DS-1 Transmission Format Frame Structure (193 bits) 8 bits/channel 24 channels 1 framing bit. Data Rate 193 bits/frame x 8 k frames/sec =1.544 Mbps.

12 THE PROBLEM WITH SYNCHRONOUS TDM: UNFILLED SLOTS Douglas E. Comer 12

13 STDM

14 STATISTICAL TDM How can a multiplexing system make better use of a shared medium? One technique to increase the overall data rate is known as statistical TDM or statistical multiplexing some literature uses the term asynchronous TDM The technique is straightforward: select items for transmission in a round-robin fashion but instead of leaving a slot unfilled, skip any source that does not have data ready By eliminating unused slots statistical TDM takes less time to send the same amount of data Douglas E. Comer 14

15 STATISTICAL TDM Douglas E. Comer 15

16 STATISTICAL TDM Statistical multiplexing incurs extra overhead shown below: Consider demultiplexing: In a synchronous TDM system a demultiplexor knows that every N slot corresponds to a given receiver In a statistical multiplexing system, the data in a given slot can correspond to any receiver Each slot must contain the identification of the receiver to which the data is being sent Later chapters discuss identification mechanisms that are used with statistical multiplexing in packet switching networks and the Internet Douglas E. Comer 16

17 FREQUENCY DIVISION MULTIPLEXING Each channel occupies a part/ portion of the bandwidth of the link A channel is defined by its center frequency, and its bandwidth. To prevent interference, the channels are separated by guard bands, which are unused portions of the spectrum chapter 7. W. Stallings, Data and Computer communications

18 FDM N inputs multiplexer N channels, 1 link demultiplexer

19 FDM N sources are fed into a multiplexer the multiplexer modulates each signal onto a different frequency each modulated signal requires a certain bandwidth centered around its carrier frequency, referred to as a channel The modulated signals are summed to produce a composite signal chapter 7. W. Stallings, Data and Computer communications

20 Code Division Multiplexing (CDM) Old but now new method Also known as code division multiple access (CDMA) An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes Used for mobile communications 20

21 CODE DIVISION MULTIPLEXING allow multiple independent streams of digital data to exist in a given piece of bandwidth code" each zero and one as a much longer string of bits -- up to 64 bits in the case of CDMA technology We choose these codes (also called "chipping codes" or "Walsh" codes) so that they cause only minimal interference to one another Codes are orthogonal therefore send some number of these over the air, at the same frequency, at the same time, with very little degradation or interference A given receiver just looks for the codes of its corresponding transmitter, converts the codes back to ones and zeros

22 CODE DIVISION MULTIPLEXING (CDM) I've found the best way to explain CDMA technology is via the following analogy. Suppose you're at a cocktail party at the United Nations. Dozens of languages are being spoken simultaneously, but someone who speaks only English, for example, will have little trouble hearing and understanding someone speaking English even among all the noise that the other traffic represents. As long as you're listening for what you're interested in, this usually works. Of course, the surrounding noise might eventually drown out whomever you're listening to -- but, you get the point. wer/cdma-technology-explained

23 END Any questions?