Multiplexing Technologies - MULTIMEDIA

Modem communication links usually have high capacity. This became even more true after the introduction of fiber - optic networks. When the link capacity far exceeds any individual user's data rate, multiplexing must be introduced for users to share the capacity.

In this section, we examine the basic multiplexing technologies, followed by a survey on several modern networks, such as ISDN, SONET, and ADSL.

Basics of Multiplexing

  1. Frequency Division Multiplexing (FDM). In FDM, multiple channels are arranged according to their frequency. Analogously, radios and televisions are good examples of FDM — they share the limited bandwidth of broadcast bands in the air by dividing them into many channels. Nowadays, cable TV resembles an FDM data network even more closely, since it has similar transmission media. Ordinary voice channels and TV channels have conventional band widths of 4 kHz for voice, 6 MHz for NTSC TV, and 8 MHz for PAL or SECAM TV.

    For FDM to work properly, analog signals must be modulated first, with a unique carrier frequency fc for each channel. As a result, the signal occupies a bandwidth Bs centered at fc. The receiver uses a band - pass filter tuned for the channel - of - interest to capture the signal, then uses a demodulator to decode it.

    Basic modulation techniques include Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM). A combination of Amplitude Modulation and Phase Modulation yields the Quadrature Amplitude Modulation (QAM) method used in many modern applications.

    Digital data is often transmitted using analog signals. The classic example is a modem (modulator - demodulator) transmitting digital data on telephone networks. A carrier signal is modulated by the digital data before transmission, then demodulated upon its reception to recover the digital data. Basic modulation techniques ate. Amplitude - Shift Keying (ASK), Frequency - Shift Keying (FSK), and Phase - Shift Keying (PSK). QPSK (Quadrature Phase - Shift Keying) is an advanced version of PSK that uses a phase shift of 90 degrees instead of 180 degrees. As QAM, it can also combine phase with amplitude, so as to carry multiple bits on each subcarrier.

  2. Wavelength Division Multiplexing (WDM). WDM is a variation of FDM that is especially useful for data transmission in optical fibers. In essence, light beams representing channels of different wavelengths are combined at the source and transmitted within the same fiber; they are split again at the receiver end. The combining and splitting of light beams is carried out by optical devices [e.g., Add - Drop Multiplexer (ADM)], which are highly reliable and more efficient than electronic circuits. Since the bandwidth of each fiber is very high (> 25 terahertz for each band), the capacity of WDM is tremendous — a huge number of channels can be multiplexed. As a result, the aggregate bitrate of fiber trunks can potentially reach dozens of terabits per second.

    Two variations of WDM are
    • Dense WDM (DWDM), which employs densely spaced wavelengths to allow a larger number of channels than WDM (e.g., more than 32).
    • Wideband WDM (WWDM),which allows the transmission of color lights with a wider range of wavelengths (e.g., 1310 to 1557 nm for long reach and 850 nm for short reach) to achieve a larger capacity than WDM.
  3. Time Division Multiplexing (TDM).As described above, FDM is more suitable for analog data and is less common in digital computer networks. TDM is a technology for directly multiplexing digital data. If the source data is analog, it must first be digitized and converted into Pulse Code Modulation (PCM) samples.

    In TDM, multiplexing is performed along the time (t) dimension. Multiple buffers are used for m (m > 1) channels. A bit (or byte) will be taken from each buffer at one of the m cycled time slots until a frame is formed. The TDM frame will be transmitted and then demultiplexed after its reception.

    The scheme described above is known as Synchronous TDM, in which each of the m buffers is scanned in turn and treated equally. If, at a given time slot, some sources (accordingly buffers) do not have data to transmit, the slot is wasted.

    Asynchronous TDM (or Statistical TDM) gathers the statistics of the buffers in this regard. It will assign only k (k < m) time slots to scan the k buffers likely to have data to send. Asynchronous TDM has the potential for higher throughput, given the same carrier data rate. There is, however, an overhead, since now the source address must also be sent, along with the data, to have the frame demultiplexed correctly.

    Traditionally, voice data over a telephone channel has a bandwidth of 4 kHz. According to the Nyquist theorem, 8,000 samples per second are required for a good digitization. This yields a time interval of 125 μsec for each sample. Each channel can transmit 8 bits per sample, producing a gross data rate (including data and control) for each voice channel of 8 x 8,000 = 64 kbps. In North America and Japan, a T1 carrier1 is basically a synchronous TDM of 24 voice channels (i.e., 24 time slots), of which 23 are used for data and the last one for synchronization.

    Each T1 frame contains 8 x 24 = 192 bits, plus one bit for framing. This yields a gross data rate of 193 bits per 125 μsec — that is, 193 bits/sample x 8,000 samples / sec = 1.544 Mbps.

    Four T1 carriers can be further multiplexed to yield a T2. Note that T2 has a gross data rate of 6.312 Mbps, which is more than 4 x 1.544 = 6.176 Mbps, because more framing and control bits are needed. In a similar fashion, T3 and T4 are created. Similar carrier formats have been defined by the ITU - T, with level 1 (E1) starting at 2.048 Mbps, in which each frame consists of 32 time slots: 8 x 32 x 8,000 = 2.048 Mbps. Two slots are used for framing and synchronization; the other 30 are for data channels. The multiplexed number of channels quadruples at each of the next levels — E2, E3, and so on.

Integrated Services Digital Network (ISDN)

For over a century, Plain Old Telephone Service (POTS) was supported by the public circuit - switched telephone system for analog voice transmission. In 1980s, the ITU - T started to develop ISDN to meet the needs of various digital services (e.g., caller ID, instant call setup, teleconferencing) in which digital data, voice, and sometimes video (e.g., in videoconferencing) can be transmitted.

By default, ISDN refers to Narrowband ISDN. The ITU - T has subsequently developed Broadband ISDN (B - ISDN). Its default switching technique is Asynchronous Transfer Mode (ATM) which will be discussed later.

ISDN defines several types of full - duplex channels;

  • B (bearer) - channel.64 kbps each. B - channels are for data transmission. Mostly they are circuit - switched, but they can also support packet switching. If needed, one B - channel can be readily used to replace POTS.
  • D (delta) - channel.16 kbps or 64 kbps. D - channel takes care of call setup, call control (call forwarding, call waiting, etc.), and network maintenance. The advantage of having a separate D - channel is that control and maintenance can be done in realtime in D - channel while B - channels are transmitting data.

The following are the main specifications of ISDN:

  • It adopts Synchronous TDM, in which the above channels are multiplexed.
  • Two type of interfaces were available to users, depending on the data and subscription rates:
  • Basic Rate Interfaceprovides two B - channels and one D - channel (at 16 kbps). The total of 144 kbps (64 x 2 + 16) is multiplexed and transmitted over a 192 kbps link,
  • Primary Rate Interfaceprovides 23 B - channels and one D - channel (at 64 kbps) in North America and Japan; 30 B - channels and two D - channels (at 64 kbps) in Europe. The 23B and ID fit in TI nicely, because T1 has 24 time slots and a data rate of 24 slots x 64 kbps / slot m 1,544 kbps; whereas the 30B and 2D fit in El, which has 32 time slots (30 of them available for user channels) and a data rate of 32 x 64 = 2,048 kbps.

Because of its relatively slow data rate and high cost, narrowband ISDN has generally failed to meet the requirement of data and multimedia networks. For home computer/Internet users, it has largely been replaced by Cable Modem and Asymmetric Digital Subscriber Line (ADSL) discussed below.

Synchronous Optical NETwork (SONET)

SONET is a standard initially developed by Bellcore for optical fibers that support data rates much beyond T3. Subsequent SONET standards are coordinated and approved by ANSI in ANSI T1.105, T1.106 and T1.107. SONET uses circuit switching and synchronous TDM.

Table Equivalency of SONET and SDH

Equivalency of SONET and SDH

In optical networks, electrical signals must be converted to optical signals for transmission and converted back after their reception. Accordingly, SONET uses the terms Synchronous Transport Signal (STS) for the electrical signals and Optical Carrier (OC) for the optical signals.

An STS - 1 (OC - 1) frame consists of 810 TDM bytes. It is transmitted in 125 μsec, — 8,000 frames per second, so the data rate is 810 x 8 x 8,000 = 51.84 Mbps. All other STS - N (OC - N) signals are further multiplexing of STS - 1 (OC - 1) signals. For example, three STS - 1 (OC - 1) signals are multiplexed for each STS - 3 (OC - 3) at 155.52 Mbps.

Instead of SONET, ITU - T developed a similar standard, Synchronous Digital Hierarchy (SDH), using the technology of Synchronous Transport Module (STM). STM - 1 is the lowest in SDH — it corresponds to STS - 3 (OC - 3) in SONET.

The above table lists the SONET electrical and optical levels and their SDH equivalents and data rates. Among all, OC - 3 (STM - 1), OC - 12 (STM - 4), OC - 48 (STM - 16), and OC - 192 (STM - 64) are the ones mostly used.

Asymmetric Digital Subscriber Line (ADSL)

ADSL is the telephone industry's answer to the last mile challenge — delivering fast network service to every home. It adopts a higher data rate downstream (from network to subscriber) and lower data rate upstream (from subscriber to network); hence, it is asymmetric.

ADSL makes use of existing telephone twisted - pair lines to transmit Quadrature Amplitude Modulated (QAM) digital signals. Instead of the conventional 4 kHz for audio signals on telephone wires, the signal bandwidth on ADSL lines is pushed to 1 MHz or higher.

ADSL uses FDM (Frequency Division Multiplexing) to multiplex three channels:

  • The high speed (1.5 to 9 Mbps) downstream channel at the high end of the spectrum.
  • A medium speed (16 to 640 kbps) duplex channel
  • A POTS channel at the low end (next to DC, 0 - 4 kHz) of the spectrum.

Table Maximum Distances for ADSL Using Twisted - Pair Copper Wire

Maximum Distances for ADSL Using Twisted - Pair Copper Wire

The three channels can themselves be further divided into 4 kHz subchannels (e.g., 256 subchannels for the downstream channel, for a total of 1 MHz). The multiplexing scheme among these subchannels is also FDM.

Because signals (especially the higher - frequency signals near or at 1 MHz) attenuate quickly on twisted - pair lines, and noise increases with line length, the signal - to - noise ratio will drop to an unacceptable level after a certain distance. Not considering the effect of bridged taps, ADSL has the distance limitations shown in the above table when using only ordinary twisted - pair copper wires.

The key technology for ADSL is Discrete Multi - Tone (DMT). For better transmission in potentially noisy channels (either downstream or upstream), the DMT modem sends test signals to all subchannels first. It then calculates the signal - to - noise ratios, to dynamically determine the amount of data to be sent in each subchannel. The higher the SNR, the more data sent. Theoretically, 256 downstream subchannels, each capable of carrying over 60 kbps, will generate a data rate of more than 15 Mbps. In reality, DMT delivers 1.5 to 9 Mbps under current technology.

The following table offers a brief history of various digital subscriber lines (xDSL). DSL corresponds to the basic - rate ISDN service. HDSL was an effort to deliver the T1 (or E1) data rate within a low bandwidth (196 kHz). However, it requires two twisted pairs for 1.544 Mbps or three twisted pairs for 2.048 Mbps. SDSL provides the same service as HDSL on a single twisted - pair line. VDSL is a standard that is still actively evolving and forms the future of xDSL.


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