2. 9.2
9-2 DIAL-UP MODEMS
• Traditional telephone lines can carry frequencies
between 300 and 3300 Hz,
• BW= 3000 Hz.
• All this range is used for transmitting voice,
• Recently there have been used for Data
Taransmission
Modem Standards
Topics discussed in this section:
3. 9.3
Figure 9.6 Telephone line bandwidth
◼ Frequency range: 300 to 3300 Hz for a bandwidth of 3000 Hz
◼ Since voice transmission can tolerate a lot of noise and distortion, the
whole phone line bandwidth is used for voice transmission
◼ However, data transmission requires more accuracy and it is less tolerant
to noise and distortion that usually exists around the bandwidth edges
◼ →Therefore, the effective bandwidth for data transmission should be
smaller than the actual cable bandwidth
◼ Effective bandwidth is usually 2400 Hz (from 600 to 3000 Hz)
4. 9.4
Figure 9.7 Modulation/demodulation
◼ The modulator creates a band-pass analog signal from
the binary data
◼ The demodulator recovers the binary data from the
modulated signal
5. 9.5
Figure 9.8 The V.32 constellation and bandwidth
◼ Uses trellis-coded modulation and encoding technique
◼ Trellis-coded modulation:
◼ It is QAM with an extra bit for error correction
◼ Data stream is divided into n-bit blocks (e.g. 4 bits)
◼ One extra bit is added to each block (for error detection)
◼ Transmits (n+1)-bit patterns (e.g 4+1=5 bits)
◼ V.32 uses 32-QAM with baud rate of 2400 (Why 2400?)
◼ Therefore, 4-bit data bits in a 5-bit patterns
◼ Bit rate = 4 x 2400 = 9600 bps
6. 9.6
Figure 9.8 The V.32bis constellation and bandwidth
◼ V.32 bis: Uses 128-QAM
◼ Therefore, 6-bit data bits in a 7-bit patterns
◼ Bit rate = 6 x 2400 = 14400 bps
◼ It has an automatic fall-back and fall-forward feature that allows the
modem its speed up or down depending on the quality of the signal.
7. 9.7
V.90 Modem Standard
◼ Shannon’s capacity of the phone line is 33.6 kbps for digital line
◼ If quantization noise is removed, the capacity increases to 56 kbps
◼ Why 56 kbps?
◼ Sampling rate = 8 000 samples/s with 8 bits/sample (1 bit for control)
◼ Data rate = 8 000 samples/s * 7 bits/sample = 56000bits (56 kbps)
◼ V.90 Modems:
◼ Used only when the other side uses digital signaling (i.e.; no
sampling point)
◼ Asymmetric: download and upload speeds are diiferent
◼ →Upload speed with sampling = 33.6 kbps
◼ →Download speed from internet without sampling = 56 kbps
10. 9.10
9-3 DIGITAL SUBSCRIBER LINE (DSL)
• After traditional modems reached their peak data rate,
telephone companies developed another technology, DSL,
to provide higher-speed access to the
• Internet. Digital subscriber line (DSL) technology is one of
the most promising for supporting high-speed digital
communication over the existing local loops.
1. ADSL (Asymmetric DSL)
2. ADSL Lite
3. HDSL (High Data Rate DSL)
4. SDSL (Symmetric DSL)
5. VDSL ( Very High Data Rate DSL)
Topics discussed in this section:
11. 9.11
DSL Technology: Introduction
◼ DSL (Digital Subscriber Line) technology is an
alternative high-speed link to the dialup via
telephone lines
◼ DSL consists of a set of technologies, each is
suited for certain needs and applications:
◼ ADSL: for residential customer (homes)
◼ SDSL: for business customer
◼ HDSL: alternative to the T1 service using twisted-pair
cables
◼ VDSL: short-range on several media types
12. 9.12
ADSL Technology
◼ ADSL (Asymmetric DSL) provides higher downstream
data rates than upstream
◼ good for residential users who download more than
upload
◼ The bandwidth of the local loop was intentionally divided
unevenly for such purpose
◼ Why can DSL achieve higher data rate than modems?
◼ The actual BW of the twisted pair is 1.1MHz
◼ The filter at the telephone company limits the BW to 4
KHz to allow multiplexing of a large number of signals
16. 9.16
The ADSL Adaptive Technology
◼ The 1.1MHz BW of the twisted pair cable is
theoretical
◼ The actual BW is dependent on:
◼ The distance to the nearest local switching office
◼ The size of the cable
◼ The signaling used
◼ The noise sources affecting the cable
◼ Etc.
◼ The ADSL technology is adaptive. It changes the
data rate based on the condition and type of the
local loop cable
17. 9.17
ADSL is an adaptive technology.
The system uses a data rate
based on the condition of
the local loop line.
Note
18. 9.18
The ADSL Modulation Technique
◼ The ADSL uses a modulation technique called DMT
(Discrete Multitone Technique)
◼ DTM combines QAM and FDM
◼ Typically, the 1.104MHz BW is divided into 256 channels of
4.312 kHz each
◼ Ch 0: used for voice communications
◼ Ch 1-5: used as a guard band between voice and data
◼ Ch 6-30: used for upstream data (24 ch) and control (1 ch)
◼ Each channel uses 4KHz out of the 4.312KHz with 15 bit/baud QAM
◼ Upstream data BW = 24 x 4000 baud/sec x 15 bit/baud = 1.44 Mbps
◼ Ch 31-255: used of downstream data (244 ch) and control (1 ch)
◼ Downstream data BW = 244 x 4000 x 15 = 13.4 Mbps
◼ In fact, due to high-level noise at some carrier
frequencies, the actual data BW’s are as follows:
◼ Upstream: 64 Kbps to 1 Mbps
◼ Downstream: 500 Kbps to 8 Mbps
22. 9.22
Other DSL Technologies
◼ SDSL (Symmetric DSL)
◼ Divides the BW equally between upstream and downstream (2.3
Mb/s)
◼ Suitable for business customer
◼ HDSL (High Data Rate DSL)
◼ Rate = 1.544 Mb/s →An alternative to the T1 service
◼ The T1 uses AMI encoding, which is very susceptible to
attenuation at high frequencies. Repeater needed after 1 Km
◼ HDSL uses 2B1Q encoding for about 2 Mbps for 4 Km
◼ Uses 2 twisted pair cables for full-duplex transmission
◼ VDSL (Very High Data Rate DSL)
◼ Similar to ADSL using coaxial, fiber, or twisted-pair cables
◼ Short-range: 1 to 3 Km
◼ Data BW: 50-55 Mb/s downstream and 1.5-2.5 upstream
◼ ADSL Lite (or Universal ADSL or Splitterless ADSL)
◼ Splitting is done at the telephone company
◼ Uses 256 carriers with 8-bit modulation rather than 15-bit
