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
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;
The following are the main specifications of ISDN:
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 For home computer/Internet users, it has largely been replaced by Cable Modem and Asymmetric Digital Subscriber Line (ADSL) discussed below.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
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:
Table 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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