CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Modem Modulation + Demodulation.
Modulation is the process of encoding a set of binary digits into an analog electrical signal.
Demodulation is the process of decoding an analog electrical signal into a set of binary digits.
- During the establishment of a modem dial-up circuit, a modem signals the tones associated with
the telephone keypad. These tones, which are generated by phones as well, are interpreted as
dual-tone multi-frequency (DTMF) tones by a CO switch. Once the circuit is established by the
Telco, the modems at both ends must agree the modem standard to use for communication.
There are many modem standards and most modems support several standards. Modems are
able to probe, negotiate, and use the best standard that is supported by the modems at both ends.
Volume
High High
Low Low
Low High Low
Amplitude Modulation Frequency Modulation Phase Modulation AM + FM Combined
Figure 21-4: Modulation Schemes
- Modulation scheme with combination of AM and FM:
Amplitude Frequency Binary Code
Low Low 00
Low High 01
High Low 10
High High 11
- Amplitude modulation (AM) represents info by varying the amplitude of the carrier signal.
Frequency modulation (FM) represents info by varying the frequency of the carrier signal.
- Baud is a measure of the number of symbol changes or signaling events per second. A baud is a
single encoded analog energy that can represent 1 or more bits. To archive higher bit rates,
modems use certain modulation techniques to encode more than 1 bit in the analog signal.
With the AM + FM combined modulation scheme in Figure 21-4, a modem which is running at
14000 baud per second can transmit data at 28000bps.
- Most computers use PPP as the data link (L2) protocol over the L1 service provided by modems.
PPP is the good choice as modems traffic is transmitted asynchronously, while PPP support
both synchronous and asynchronous communications.
- Other reasons why PPP is being used in modem communications are:
i) PPP provides the capability to dynamically assign an IP address for a device connected
to the other end of a PPP link.
ii) PPP supports authentication (via PAP and CHAP), which is commonly being used to
authenticate dial-in users for the access to the ISP network.
- V.92 is the current modem specification defined by International Telecommunications Union
(ITU). It transmits at 56kbps (downstream) and 33kbps (upstream), which is same as V.90 but
offers a reduced handshake time (faster connection setup), increased compression throughput,
and the on-hold feature.
147
Phase Shifts
0 1 0 1 0 1 1 0 0 1 0 1
00 01 10 11
Time
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