1. Lecture 5
Multiplexing

2. Time Division Multiplexing

3. TDM: Time Slots and Frames
In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter

4. TDM: Example 1
Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find (a) the duration of 1 bit before multiplexing, (b) the transmission rate of the link, (c) the duration of a time slot, and (d) the duration of a frame?
a) The duration of 1 bit is 1/1 Kbps, or s (1 ms).
b) The rate of the link is 4 Kbps.
c) The duration of each time slot 1/4 ms or 250 μs.
d) The duration of a frame 1 ms.

5. Interleaving
Interleaving can be done by bit, by byte, or by any other data unit

6. Empty Slots Synchronous TDM is not efficient in many cases
Statistical TDM can improve the efficiency by removing the empty slot from the frame

7. Data Rate Management
Multiple-slot allocation / Pulse stuffing

8. Frame Synchronizing
Synchronization between the multiplexing and demultiplexing is a major issue in TDM

9. TDM: Example 2
We have four sources, each creating 250 characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (a) the data rate of each source, (b) the duration of each character in each source, (c) the frame rate, (d) the duration of each frame, (e) the number of bits in each frame, and (f) the data rate of the link.
1. The data rate of each source is 2000 bps = 2 Kbps.
2. The duration of a character is 1/250 s, or 4 ms.
3. The link needs to send 250 frames per second.
4. The duration of each frame is 1/250 s, or 4 ms.
5. Each frame is 4 x = 33 bits.
6. The data rate of the link is 250 x 33, or 8250 bps

10. Synchronous Time Division Multiplexing
the multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit.
Many types popular today:
T1 multiplexing
E1 multiplexing
ISDN multiplexing
SONET (Synchronous Optical NETwork)
SDH (Synchronous Digital Hierarchy)

11. Synchronous Time Division Multiplexing (T1)
The T1 (1.54 Mbps) multiplexor stream is a continuous series of frames of both digitized data and voice channels.
24 separate 64Kbps channels

12. T-1 Line for Multiplexing Telephone Lines
Synchronous Time Division Multiplexing (T1)
T-1 Line for Multiplexing Telephone Lines

13. Synchronous Time Division Multiplexing (T1)
T-1 Frame Structure

14. DS and T Line Rates

15. Synchronous Time Division Multiplexing (E1)
Synchronous Time Division Multiplexing (E1)
E1 Frame description
Frame time length 125 microsec
Frame Length 256 bits = 30*voice channel + 1 synch + 1 signaling channel
Rate 256 bits/frame x 8000 frames/second = Mbps

16. E Line Rates
European use a version of T lines called E lines

17. Synchronous Time Division Multiplexing (ISDN)
The ISDN multiplexer stream is also a continuous stream of frames. Each frame contains various control and sync info.

18. Synchronous Time Division Multiplexing (SONET)
Synchronous Optical Network (SONET) Framing
STS (Synchronous Transport Signal) frame consists of:
90 columns x 9 rows x 8 bits/byte x 8000 frames/sec = Mbps

19. Synchronous Time Division Multiplexing (SDH )
Synchronous Digital Hierarchy (SDH) Framing
The STM-1 frame consists of 9 rows of 270-column bytes, for a total of 2430 bytes.
270 columns x 9 rows x 8 bits/byte x 8000 frames/sec = Mbps
Synchronous Payload Envelope (SPE) for an actual data rate of Mbps.
258 columns x 9 rows x 8 bits/byte x 8000 frames/sec = Mbps

20. SDH / SONET – Higher level of multiplexing

21. Statistical TDM

22. Statistical TDM Addressing is required in Statistical TDM
Slot size: the ratio of the data size to address size must be reasonable to make transmission efficient
No synchronization bit: no need for frame-level sync.
Bandwidth: normally less than the sum of the capacities of each channel
This type of TDM is used in Asynchronous Transfer Mode (ATM) networks.

23. OFDM (Orthogonal Frequency Division Multiplexing)
Different symbols are transmitted over different subcarriers

24. OFDM (Orthogonal Frequency Division Multiplexing)
Transmission of QAM symbols on parallel subcarriers
Overlapping, yet orthogonal subcarriers
Orthogonal Frequency Division Multiplexing (OFDM) is special form of multi-carrier modulation, patented in It is particularly suited for transmission over a dispersive channel.
In a multipath channel, most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel.
OFDM is significantly less sensitive to intersymbol interference than conventional modulation such as BPSK or QAM, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequency-time window which ensures little or no distortion of the waveform. In practice, it means that bits are transmitted in parallel over a number of frequency-nonselective channels.

25. OFDM (Orthogonal Frequency Division Multiplexing)
Multi-carrier Modulation
OFDM (Orthogonal Frequency Division Multiplexing)
Basic FFT,OFDM transmitter and receiver

26. Asymmetric Digital Subscriber Line (ADSL)
IFFT

27. Bandwidth Division in ADSL
There is no set way that the bandwidth is divided
Channel 0 for voice
Upstream
24 channels x 4 kHz x 15 bits/Hz = 1.44 Mbps (ch 6:ch 30)
Downstream
224 channels x 4 kHz x 15 bits/Hz = 13.4 Mbps (ch 31:ch 255)

28. DSL: Actual Bit Rate Because of the high signal/noise ratio Upstream
Normally below 500 kbps
Downstream
Normally below 8 Mbps

29. Asymmetric Digital Subscriber Line (ADSL)

30. Asymmetric Digital Subscriber Line (ADSL)

31. Customer Site: ADSL Modem

32. Telco Site: DSLAM
Digital subscriber line access multiplexer