1. Ch. 8 Multiplexing

2. Ch. 8 Multiplexing 8.1 Frequency-Division Multiplexing
8.2 Synchronous Time-Division Multiplexing
8.3 Cable Modems
8.4 Asymmetric Digital Subscriber Line
8.5 xDSL
8.6 Multiple Channel Access

3. 8.1 Frequency-Division Multiplexing
FDM--Definition
The division of a transmission facility into two or more channels by splitting the frequency band transmitted by the facility into narrower bands, each of which is used to constitute a distinct channel.

4. 8.1 Frequency Division Multiplexing (p.2)
FDM-- Figure 8.3
Incoming signals are each modulated using a different carrier frequency (N sources.)
The channels are separated by guard bands, which are unused portions of the spectrum.
The spectrum of the composite signal is shown in Figure 8.3b.
The receiver consists of bandpass filters and demodulators, centered around each carrier frequency.

5. 8.1Frequency Division Multiplexing (p.3)
Examples of FDM
Example 8.1Voiceband Signals
4 k Hz bandwidth (effective bandwidth 300 to 3400 Hz).
SSBSC--single sideband, suppressed carrier.
Use 64k Hz, 68k Hz, and 72k Hz carriers (Fig. 8-5).

6. 8.1 Frequency Division Multiplexing (p.4)
Analog Carrier Systems (Table 8.1)
FDM --earliest carrier system and still is common.
AT&T (North American Standard)
Group--12 voice channels
Supergroup--5 groups (60 voice channels)
Mastergroup-10 supergroups (600 voice channels)

7. 8.1 Frequency Division Multiplexing (p.5)
Wavelength Division Multiplexing
Multiple beams of light are transmitted at different frequencies on the same fiber.
1997--Bell Labs demonstrated 100 beams each operating at 10 G bps, for a total data rate of 1 trillion bits per second (1 terabit per sec).
Commercial systems with 160 and 256 channels are currently available.
Figure 8.5

8. 8.1 Frequency Division Multiplexing (p.6)
Problems with FDM carrier systems:
Crosstalk and intermodulation noise.
Must demodulate all signals for switching.
Inflexible.

9. 8.2 Synchronous Time-Division Multiplexing
STDM--Definition
A method of TDM in which time slots on a shared transmission line are assigned to I/O channels on a fixed, predetermined basis.
Each channel could carry a bit, byte, or block, depending on implementation.
In general, start and stop bits are stripped off, if asynchronous terminals are being multiplexed.
See Fig. 8.6.

10. 8.2 Synchronous Time-Division Multiplexing
STDM Link Control
Blocks of bits are the input sources (eg. HDLC).
Flow control, error control, etc. will be handled before and after the multiplexers.
Framing
There is some framing required.
Added-digit framing--a single bit is added to each frame; the bits will form a repetitive pattern.

11. 8.2 Synchronous Time-Division Multiplexing
Pulse Stuffing
Suppose that the outgoing data rate of the multiplexer, excluding framing bits, is higher than the sum of the maximum instantaneous incoming rates.

12. 8.2 Synchronous Time-Division Multiplexing
Excess capacity is used by stuffing extra dummy bits or “pulses” into each incoming signal until its rate is raised to that of a locally-generated clock signal.
Solves problems of synchronization among data sources.

13. 8.2 Synchronous Time-Division Multiplexing
Example STDM-- (Fig.8.8)
Digital and Analog Sources
Source 1 Analog
2 kHz bandwidth (16 kbps).
Source 2 Analog
4 kHz bandwidth (32 kbps).
Source 3 Analog
Sources 4-11: Digital
Each of the eight sources is a 7200 bps synchronous data stream.

14. 8.2 Synchronous Time-Division Multiplexing
Example STDM-- (Fig.8.8) (cont.)
Analog sources
Sampled and encoded using 4 bits.
Gathered into one 16-bit buffer .
Result is a 64 k bps multiplexed information stream.
Resulting analog source frame is Source 1 (4 bits), Source 2 (4 bits), Source 3 (4 bits), Source 2 (4 bits).
Digital sources
Each is increased to 8 k bps using pulse stuffing.
TDM signal: 64 k bps + 8 x 8 k bps =128 k bps.

15. 8.2 Synchronous Time-Division Multiplexing
Digital Carrier Systems
Standards
North American and ITU-T are different.
Table 8.3 (DS-1 through DS-4; Levels 1-5)

16. 8.2 Synchronous Time-Division Multiplexing
DS-1 Transmission Format (Fig. 8-9)
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.

17. 8.2 Time Division Multiplexing (p.9)
DS-1 Transmission Format (Fig. 8-9)(cont.)
Voice
Uses bit robbing.
Every sixth frame has one bit "robbed" for control signaling from each channel.
Data
Bit 8 is used for control signaling (8,000 bps.)
Bit 1-7 used for 56 kbps service.
Bit 2-7 used for 9.6, 4.8, and 2.4 kbps service.

18. 8.2 Synchronous Time-Division Multiplexing
SONET/SDH
An optical transmission interface.
Signal Hierarchy--Table 8.4.
Frame Formats--Fig.8.10 and 8.11.

19. 8.3 Cable Modem
A device that allows the user to access the Internet and other online services through a cable TV network.
Spectrum Division in North America
User-to-network data (upstream): 5-40MHz
Television Delivery (downstream): Mz
Network to user data (downstream: Mz

20. 8.4 Asymmetric Digital Subscriber Line
ADSL Design (Fig. 8.14)
ADSL provides more capacity down-stream than upstream.
Although originally conceived for video-on-demand, it is being used for Internet access.
Lowest 25kHz are reserved for voice (POTS)
Separate Upstream and Downstream (FDM).
Overlapping Upstream and Downstream (FDM with echo cancellation.)
Discrete Multitone Transmission (DMT) is used.

21. 8.5 xDSL
ADSL is one of several schemes for high-speed transmission on a subscriber line.
Other schemes are summarized in Table 8.8
High Data Rate Digital Subscriber Line
Single Line Digital Subscriber Line
Very High Data Rate Digital Subscriber Line

22. 8.6 Multiple Channel Access
Frequency Division Duplex
Two stations have a full duplex connection; each station transmits in a different band.
Time Division Duplex
Time compression multiplexing; data is transmitted in one direction at a time.

23. 8.6 Multiple Channel Access
Frequency Division Multiple Access—spectrum is shared among multiple stations.
A base station can communicate with a group of stations.
Satellite networks, cellular networks, WiFi, and WiMAX.
Figure 8.19(a)

24. 8.6 Multiple Channel Access
TDMA—time division multiple access
Usually used with a base station, also.
Each sub-channel is dedicated and not shared.
See Fig. 8.19(b)