1. Multiplexing
Multiplexing: Combining multiple data (voice) channels for transmission on a common medium. Multiple devices sharing one physical link.
Demultiplexing: Recovering the original separate channels from a multiplexed signal.
Multiplexing and demultiplexing are performed by a multiplexer.
The two common forms of multiplexing are frequency-division multiplexing (FDM) and time-division multiplexing (TDM).

2. Frequency-Division Multiplexing (FDM)
Useful bandwidth of the medium exceeds the required bandwidth of the channel.
Each signal is modulated to a different carrier frequency.
Carrier frequencies are separated so signals do not overlap (guard bands).
E.g. broadcast radio, television, cable television, etc.
Channel allocated even if there is no data to be sent.

3. Frequency-Division Multiplexing (FDM)

4. FDM System- Transmitter
Analog or digital inputs: mi (t), i = 1,n.
Each is modulated onto a subcarrier: fi.
Signals summed to produce a composite baseband: mb(t).
fi chosen such that there is no overlap.

5. FDM System- Band

6. FDM System- Receiver
mb(t) is passed through n bandpass filters with response centered on fi.
Each si(t) component is demodulated to
recover the original analog/digital data.

7. FDM of Three Voiceband Signals
Effective spectrum of voice: Hz.
Amplitude modulation at 64-kHz carrier: produces an 8 kHz bandwidth (60-68kHz).
For efficiency use of bandwidth, only transmit lower sideband.
Three carriers at 64, 68 and 72 kHz.
Crosstalk (overlap): use guard band to avoid.
Intermodulation noise: nonlinear effects of amplifiers in one channel can produce frequency components in other channels.

8. FDM of Three Voiceband Signals

9. Analog Carrier Systems
Long distance voiceband signals over high-capacity links (coaxial cable, microwave).
AT&T (USA) designated a hierarchy of FDM schemes.
Group:
– 12 voice channels (4kHz each) = 48kHz.
– Range from 60kHz to 108kHz.
Supergroup:
– 60 channels.
– FDM of 5 group signals on carriers between 420 kHz and 612 kHz.
– Each group is treated as a separate signal with 48 kHz bandwidth.
Mastergroup:
– 10 supergroups.
– 600 voice channels with a bandwidth of 2.52 MHz
so original signal can be modulated many times

10. Analog Hierarchy

11. North American and International FDM Carrier Standards

12. Example
Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?
For five channels, we need at least four guard bands. This means that the required bandwidth is at least
5 x x 10 = 540 KHz,
as shown in the Figure.

13. Example
Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM
The satellite channel is analog. We divide it into four channels, each channel having a 250-KHz bandwidth. Each digital channel of 1 Mbps is modulated such that each 4 bits are modulated to 1 Hz. One solution is 16-QAM modulation. The figure shows one possible configuration.

14. Example
The Advanced Mobile Phone System (AMPS) uses two bands. The first band, 824 to 849 MHz, is used for sending; and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 KHz in each direction. The 3-KHz voice is modulated using FM, creating 30 KHz of modulated signal. How many people can use their cellular phones simultaneously?
Each band is 25 MHz. If we divide 25 MHz into 30 KHz, we get In reality, the band is divided into 832 channels.

15. Wavelength Division Multiplexing
Multiple beams of light at different frequency
Carried by optical fiber
A form of FDM
Each color of light (wavelength) carries separate data channel
1997 Bell Labs
100 beams
Each at 10 Gbps
Giving a total 1 terabit per second (Tbps)
Commercial systems of 160 channels of 10 Gbps now available
Lab systems (Alcatel) 256 channels at 39.8 Gbps each
Total 10.1 Tbps
Over 100km

16. WDM Operation Same general architecture as other FDM
Number of sources generating laser beams at different frequencies
Multiplexer consolidates sources for transmission over single fiber
Optical amplifiers amplify all wavelengths
Typically tens of km apart
Demux separates channels at the destination
Mostly 1550nm wavelength range
Was 200MHz per channel
Now 50GHz

17. WDM

18. Dense Wavelength Division Multiplexing
DWDM
No official or standard definition
Implies more channels more closely spaced that WDM
200GHz or less

19. Synchronous Time-Division Multiplexing
Data rate of the medium exceeds the data rate of the digital signal to be transmitted.
Multiple digital signals interleaved in time: May be at the bit level or blocks of bytes.
Time slots preassigned to the data sources and fixed.
Time slots allocated even if no data is sent (like FDM).
Time slots do not have to be evenly distributed amongst sources.

20. Synchronous Time-Division Multiplexing

21. Synchronous TDM System
Interleaving:
– Can be compared to a very fast rotating switch which selects each device at a constant rate and a fixed order.
– Each device sends a fixed number of bits in its timeslot.
Weakness of Synchronous TDM: fixed time slot allocations can lead to empty slots when a device has nothing to send.

22. TDM System Transmitter: Receiver:
Digital inputs, mi(t), i = 1,n, are briefly
buffered.
Buffers are scanned sequentially to form
a composite signal: mc(t).
Scanning is rapid enough so buffers are
emptied before more data arrives.
Data organized into frames of one cycle.
Receiver:
Interleaved data is demultiplexed and
routed to destination buffers.

23. TDM Link Control no headers and trailers
data link control protocols not needed
flow control
data rate of multiplexed line is fixed
if one channel receiver can not receive data, the others must carry on
corresponding source must be quenched
leaving empty slots
error control
errors detected & handled on individual channel
Flow control and error control can be provided on a per-channel basis by using a data link control
Note that the transmitted data stream depicted in Stallings DCC8e Figure 8.6b does not contain the headers and trailers that we have come to associate with synchronous transmission, as the control mechanisms provided by a data link protocol are not needed. It should be clear that, as far as the multiplexer and demultiplexer (Figure 8.1) are concerned, flow control is not needed. The data rate on the multiplexed line is fixed, and the multiplexer and demultiplexer are designed to operate at that rate. If one of the individual output lines attaches to a device that is temporarily unable to accept data, transmission must continue, since the remaining output lines are expecting to receive data at predetermined times. Thus, the channel in question will carry empty slots, but the frames as a whole will maintain the same transmission rate.
The reasoning for error control is the same. It would not do to request retransmission of an entire TDM frame because an error occurs on one channel. The devices using the other channels do not want a retransmission nor would they know that a retransmission has been requested by some other device on another channel. Again, the solution is to apply error control on a per-channel basis.

24. Data Link Control on TDM

25. Framing Bits
There is no flag or SYNC characters bracketing TDM frames.
Must provide a synchronising mechanism.
Most common mechanism: Added digit framing.
– One control bit is added to each TDM frame:
Looks like another channel - “control channel”
– Identifiable bit pattern used on the control channel.
– E.g. alternating : : : which is unlikely to be sustained on a data channel.
– Receiver can compare incoming bit patterns on each channel with the sync pattern.
– Once synchronized, the receiver continues to monitor the control channel.

26. Synchronous TDM System Example
Each device sends 250 characters/second = 250*8 = 2000 bps.
Transmission is character interleaved, and each frame has one framing bit.
Therefore, the devices create 2000*4=8000 bits of data per second, and the multiplexer adds 250 bits of overhead per second.

27. Pulse/Bit Stuffing
It is possible to connect devices of different rates using different time slot allocations.
Since a fixed number of bits are transmitted in each time slot, each device must have a data rate which is an integer multiple of the channel rates.
For example: a device three times faster than the other devices uses three time slots.
Problem - What do we do for a device 3.75 times faster.
Solution - Pulse/Bit Stuffing:
– The multiplexer adds extra dummy bits or pulses into the devices data stream to force the integer speed relationship.
– Therefore the 3.75 times faster device will be raised to 4 times faster.
– Stuffed bits are inserted at fixed locations in the frame and removed at the demultiplexer.

28. Example TDM of Analog and Digital Sources

29. North American and International TDM Carrier Standards

30. Digital Carrier Systems
Hierarchy of TDM.
USA/Canada/Japan use one system and ITU-T use a similar (but different) system.
Digital Signal (DS) Service DS-1 format: Multiplexes 24 channels.
Each frame has 8 bits per channel plus one framing bit: 8*24+1 = 193 bits per frame.
The DS service is implemented using T lines.

31. Digital Carrier Systems
The first bit is a framing bit, used for synchronization.
Voice channels:
8-bit PCM used on five of six frames.
7-bit PCM used on every sixth frame; bit 8 of each channel is a signaling bit.
Data channels:
Channel 24 is used for signaling only in some schemes.
Bits 1–7 used for 56-kbps service (8-bit data or control)
Bits 2–7 used for 9.6-, 4.8-, and 2.4-kbps service.
subrate multiplexing
additional bit is robbed from each channel to indicate which subrate multiplexing rate is being provided

32. Digital Carrier Systems Format

33. Digital Carrier Systems Hierarchy

34. SONET/SDH
SONET: Synchronous Optical Network is an optical transmission interface (proposed by BellCore and standardized by ANSI).
SDH: Synchronous Digital Hierarchy (defined by ITU-T).
Both standards are compatible and designed for the high-speed transmission capability of optical fiber.
Signal Hierarchy:
– Lowest level: Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level
1 (OC-1).
– Mbps.
– STS-1 carries a single DS-3 or a group of lower rate signals (DS1, DS1C, DS2) plus
ITU-T rates (e.g Mbps).
– Multiple STS-1 signals combined into an STS-N signal.
– ITU-T lowest rate is Mbps (STM-1) and corresponds to SONET STS-3.

35. SONET/SDH Signal Hierarchy

36. SONET Frame Format The basic SONET building block is the STS-1 frame
STS-1 frame contains 9 rows of 90 octets = 810 octets every 125µs 51.84 Mbps.

37. SONET STS-1 Overhead Octets

38. Statistical TDM
In synchronous TDM many slots are wasted (empty slots).
Statistical or Asynchronous TDM allocates time slots dynamically based on demand.
The multiplexer scans the input lines and collects data until the frame is full.
The data rate on the multiplexed line is lower than the aggregate rates of the input lines.
may have problems during peak periods (must buffer inputs)

39. Statistical TDM

40. Statistical TDM Frame Formats
Address information is required for proper delivery of data in the frames: greater overhead.
Generally use a synchronous protocol such as HDLC.
Include one (b) or more (c) sources of data in each HDLC frame (a).

41. Performance Output data rate is less than the aggregate input rates.
Anticipate the average rate of input is less than the multiplexed capacity.
May cause problems during peak periods:
– Buffer inputs.
- smallest possible buffer and smallest possible data rate ?
– Keep buffer size to minimum to reduce delay.

42. Performance

43. Performance I = the number of input resources
R = data rate of each source
M= effective capacity of multiplexed line (after accounting for overhead).
α = mean fraction of time each source is transmitting 0 < α <1
K = M/IR fraction of multiplexed line capacity (compression factor, i.e K = 1 synchronous TDM)
The value of K can be bounded α< K <1

44. Performance MUX as a single server queue
Total delay = time spent waiting + service time
Delay depends on the pattern of arriving traffic and the characteristics of the server.
Assume poisson arrivals and const service time
λ = α I R (λ average arrival rate)
Ts = 1/M ( Ts time it takes to transmit one bit)
The utilization of total link capacity
ρ = λ Ts = α I R/M = α /K = λ/M
Buffer size depend on ρ , and not directly on M (look at the two cases in the book)
As utilization rises, so do the buffer requirements and delay.

45. Performance

46. Performance Data are transmitted in 1000-bit frame
Utilization is expressed as a percentage of the total line capacity.
Utilization above 80% is not desirable

47. Cable Modems dedicate two cable TV channels to data transfer
each channel shared by number of subscribers, using statistical TDM
Downstream
cable scheduler delivers data in small packets
active subscribers share downstream capacity
also allocates upstream time slots to subscribers
Upstream
user requests timeslots on shared upstream channel
Headend scheduler notifies subscriber of slots to use
To support data transfer to and from a cable modem, a cable TV provider dedicates two channels, one for transmission in each direction. Each channel is shared by a number of subscribers, and so some scheme is needed for allocating capacity on each channel for transmission. Typically, a form of statistical TDM is used. In the downstream direction, cable headend to subscriber, a cable scheduler delivers data in the form of small packets. Because the channel is shared by a number of subscribers, if more than one subscriber is active, each subscriber gets only a fraction of the downstream capacity. An individual cable modem subscriber may experience access speeds from 500 kbps to 1.5  Mbps or more, depending on the network architecture and traffic load. The downstream direction is also used to grant time slots to subscribers. When a subscriber has data to transmit, it must first request time slots on the shared upstream channel. Each subscriber is given dedicated time slots for this request purpose. The headend scheduler responds to a request packet by sending back an assignment of future time slots to be used by this subscriber. Thus, a number of subscribers can share the same upstream channel without conflict

48. Cable Modem Scheme
A form of statistical TDM is typically used with Cable Modems, as illustrated in Stallings DCC8e Figure This also shows the request and allocation of upstream time slots.

49. Asymmetrical Digital Subscriber Line (ADSL)
Modem technology designed to provide high-speed digital data transmission over ordinary telephone wire.
Link between the subscriber and the network (local exchange): Local loop.
Uses currently existing twisted pair cables:
– Can carry broader spectrum than just voice bandwidth (1 MHz or more).
ADSL Design:
– Asymmetric: Greater capacity downstream than upstream.
– Frequency-division multiplexing:
Lowest 25kHz for voice: Plain old telephone service (POTS).
Use echo cancellation or FDM to give two bands for downstream and upstream.
Use FDM within bands each band.
– Range of up to 5.5 km.

50. ADSL Channel Configuration
ADSL uses frequency division multiplexing (FDM). There are three elements of the ADSL strategy, shown above in Stallings DCC8e Figure 8.17:
• Reserve lowest 25 kHz for voice (POTS - plain old telephone service). The voice is carried only in the 0 to 4 kHz band; the additional bandwidth is to prevent crosstalk between the voice and data channels.
• Use either echo cancellation or FDM to allocate two bands, a smaller upstream band and a larger downstream band. Echo cancellation is a signal processing technique that allows transmission of digital signals in both directions on a single transmission line simultaneously. In essence, a transmitter must subtract the echo of its own transmission from the incoming signal to recover the signal sent by the other side.
• Use FDM within the upstream and downstream bands. In this case, a single bit stream is split into multiple parallel bit streams and each portion is carried in a separate frequency band.
When echo cancellation is used, the entire frequency band for the upstream channel overlaps the lower portion of the downstream channel. This has two advantages compared to the use of distinct frequency bands for upstream and downstream: the higher the frequency, the greater the attenuation., and echo cancellation design is more flexible for changing upstream capacity. The disadvantage of the use of echo cancellation is the need for echo cancellation logic on both ends of the line.

51. ADSL Channel Configuration
Advantages of echo cancellation compared to the use of distinct frequency bands
The higher the frequency, the greater the attenuation. With the use of echo cancellation, more of the downstream bandwidth is in the “good” part of the spectrum.
The echo cancellation design is more flexible for changing upstream capacity. The upstream channel can be extended upward without running into the downstream (overlap is extended).
disadvantage of the use of echo cancellation is the need for echo cancellation logic on both ends of the line

52. Discrete Multitone (DMT)
Available transmission bandwidth is divided into 4kHz subchannels, each having its own carrier frequency.
DMT modem sends out test signals on each subchannel to determine the signal-to-noise ratio.
assign more bits to subchannels with better signal-to-noise ratio.
Use quadrature amplitude modulation (QAM): easy to assign variable number of bits.
Presently 256 downstream subchannels with each 4kHz subchannel carrying 60 kbps:
– Mbps possible, impairments bring this down to 1.5 Mbps to 9 Mbps.

53. DMT Transmitter
Stallings DCC8e Figure 8.19 provides a general block diagram for DMT transmission. After initialization, the bit stream to be transmitted is divided into a number of substreams, one for each subchannel that will carry data. The sum of the data rates of the substreams is equal to the total data rate. Each substream is then converted to an analog signal using quadrature amplitude modulation (QAM), described in Chapter 5. This scheme works easily because of QAM’s ability to assign different numbers of bits per transmitted signal. Each QAM signal occupies a distinct frequency band, so these signals can be combined by simple addition to produce the composite signal for transmission.

54. DMT Transmitter

55. xDSL
ADSL is one of a number of recent schemes for providing high-speed digital transmission of the subscriber line. Many scheme are used which are collectively referred to as xDSL.
High-speed digital transmission of the subscriber line.
High data rate Digital Subscriber Line (HDSL):
– More cost-effective means of delivering a T1 data rate (1.544 Mbps).
– Up to 2 Mbps over two twisted pair lines within a bandwidth of 196 kHz.
– Range of about 3.7 km.
Single line Digital Subscriber Line (SDSL):
– HDSL is not suitable for residential subscribers because of two twisted pair.
– SDSL provides same service as HDSL but over a single twisted-pair line.
Very high data rate Digital Subscriber Line (VDSL):
– Similar to ADSL but a much higher data rate and shorter distance (up to 1.4 km).

56. Comparison of xDSL Alternatives

57. Problem Assignments Solve all the review questions
Try the following problems:
8.1, 8.8, 8.9, 8.10, 8.11, 8.14, 8.17