Encoding Modulation and the Physical Layer

Posted On 2005-11-1 by FortyPoundHead
Keywords: Encoding Modulation and the Physical Layer
Tags: Networking Tutorial 
Views: 1490

The Physical layer doesn't get any respect. This negligence results partly from the fact that there are few end-user interventions once a network's cabling or wireless infrastructure is successfully put into place. The actual behavior of electromagnetic signals and raw digital sequences is generally built into hardware interfaces and is beyond any ordinary fiddling or tuning. Nevertheless, a bit of education about the Physical layer can help prepare you for a deeper understanding of new wireless, optical, and local-loop technologies that will have important future roles in every network.

The Physical layer is responsible for turning some medium into a bit pipe. Copper cabling is the most common data-networking medium of LANs and in the local loop, while fiber optic cabling is prevalent in most wide area networks-and is perhaps the local-loop and the LAN medium of the future. Radio frequency bands also play a role in some data networks, including signals that are relayed via satellites. Some local networks and point-to-point links employ infrared signals propagated through space, and visible light lasers are sometimes used to transmit data without the aid of a fiber optic cable.

Each medium has its strengths, vulnerabilities, and quirks. Cost is obviously an important factor, which explains the prevalence of copper cable in today's infrastructures. Performance, in the form of reliable throughput, is closely related to cost. The rapidly increasing price/performance of fiber optic cabling accounts for the growing hegemony of that medium as it migrates from interstate backbones to metropolitan areas, neighborhoods, and campuses. Although wireless systems have the powerful cost advantages of no cables, trenches, or poles, their relatively low reliability, low privacy, and low throughput offset these advantages.

The main task of the Physical layer is to make the best use of the medium at hand. The laws of physics constrain a medium's potential for reliable bit throughput, but so do regulations devised by the people who need to share limited resources. For example, T1 lines interfere with analog voice, ISDN, and Asymmetric Digital Subscriber Lines (ADSLs) so much that aggregated cables or binder groups can't include T1s.

Analog and Digital Data

Data that needs to be communicated may be in analog or digital form. Analog data is continuous, taking on innumerable values within a range. Voices, images, and temperature readings from a sensor are all examples of analog data. Digital data takes on a limited number of discrete values. In the limiting, and most common case, digital data takes one of two values: zero or one. Logical values such as true or false, integers, and text are commonly encountered examples of digital data.

In order to manipulate or communicate data, it must be encoded as some kind of signal, usually an electrical or electromagnetic signal. Analog data can be encoded as an analog signal. Perhaps the most common example is a plain old telephone in the local loop, though a cassette tape player, the video and audio components of a TV program, and many other household media use analog signals to represent analog data.

Analog data is also commonly encoded with digital signals. If a phone call travels beyond the local exchange carrier's central office into the long distance network, it will be digitized. If you scan an image or capture a sound on the computer, you're converting analog data to digital signals. This analog-to-digital conversion is usually accomplished with a special device or process referred to as a codec, which is short for coder-decoder.

Digital data is routinely converted to analog signals. The most common example is when you make use of the omnipresent voice infrastructure for computer connectivity and employ a modem to represent your bits in the form of audible tones. (Modem is short for modulator-demodulator, which performs the inverse of what a codec does-though in most cases, of course, both a codec and a modem perform both analog-to-digital and digital-to-analog conversions.) Modulation can be considered to be a special case of encoding, though the terms tend to overlap in ordinary usage. Technically speaking, modulation involves combining two signals, either of which can be analog or digital, to produce a resultant signal, which can be analog or digital. Encoding, then, is the representation of data by a signal using any method.

Finally, digital data is also regularly represented by digital signals. Any time you send e-mail, load a file, or download Web pages, you're encoding digital data with digital signals.

Signal Obstacles

The enemies of both analog and digital signals include attenuation, noise, and crosstalk. Attenuation is the tendency of a signal to get weaker with distance. Analog signals must be amplified before they become too diminished to be detectable. Unfortunately, analog signals accumulate noise with repeated amplification. Digital signals, while they are degraded by attenuation, can be detected and repeated indefinitely with no loss of data. This property is one of the principal reasons digital communication became increasingly important in the last years of the 20th century.

Noise is the backdrop of the universe. Atoms and molecules in motion create random electromagnetic signals that prevent any communication channel from being perfectly clear. Of course, all sorts of events, from elevator motors and electric mixers to lightning and solar flares, also contribute noise to our communications environment. Some encoding techniques are less susceptible to particular kinds of noise than others. Crosstalk is a special form of noise that is induced by other signals on a common medium.

Digital signal transmission is used in LANs, where cable lengths are relatively short and thus not subject to severe attenuation. (Attenuation increases with increasing frequency, and digital transmissions have high frequency components, which means that channels with constrained bandwidth aren't suitable for high throughput digital transmissions.) The best known examples of digital transmission on telephone facilities are T1 lines and ISDN.

The simplest representation of digital signals is a line code known as Non Return to Zero Level, or NRZ-L. This code is the archetype of what a digital signal looks like, although there are innumerable variations in how best to transmit a digital signal across various media. NRZ-L has severe limitations in practice, but it has real-life applications in RS-232 links and in data storage on hard disk drives.

Two of the biggest shortcomings of NRZ-L are its DC component and its inability to carry synchronization information along with the data. If an NRZ-L signal has a sequence of ones, the signal can't pass through such electrical components as transformers and capacitors, which only conduct when the signal is changing. As for synchronization, correct timing is essential for a receiver to identify the discrete states of the digital signal. If a series of ones appears in an NRZ-L transmission, the receiver will require an additional synchronization signal to be aware of how many there are.

T1 lines often use a line code called Bipolar with 8 Zero Substitution. B8ZS is a variant of Bipolar Alternate Mark Inversions. (Marks and spaces are just a terminological variation on zeros and ones.) Bipolar AMI solves the DC component problem by alternating the polarity of ones-zeros are represented by no signal, the first one is a positive signal, the second one is a negative signal, and the signal values of subsequent ones alternate. However, with a long string of zeros, Bipolar AMI signals can lose self-synchronization. The 8 Zero Substitution trick takes care of the problem by breaking the alternation rule when it comes across a sequence of eight consecutive zeros. By sticking ones in the places of the fourth and fifth zeros, and in the places of the seventh and eighth zeros, with the first substitute one incorrectly having the polarity of the previous one, and the third substitute one incorrectly having the polarity of the second substitute one, the receiver recognizes an intentional violation and concludes that there is in fact a sequence of eight zeros. This coded violation ensures that there will never be a sequence of more than seven successive no-signal bit times. The rules of mark inversion also add a degree of Physical-layer error detection to this encoding method; noncoded violations will indicate spoiled bits.

Ethernet uses a type of digital signal known as Manchester encoding. A one is indicated by a high/low transition in the middle of a bit, while a zero is indicated by a low/high transition in the middle of a bit. Based on the previous discussion, you can see that Manchester encoding has no DC component and is fully self-synchronizing. If there is no transition in a bit time, you have a Physical-layer error indication. The drawback to applying this line code more widely is that its bandwidth requirement is twice the baud rate; in other words, there are significant spectral components as high as 20MHz, which are no problem on coaxial cable or on short distances of twisted-pair cabling, but not suitable for long distances.

ISDN lines make use of a line code known as 2 Binary 1 Quaternary (2B1Q). Symmetric Digital Subscriber Line (SDSL) and High-Bit Rate Digital Subscriber Line (HDSL) also employ this encoding method. A line with 2B1Q encoding uses four distinct signaling levels, with data represented in 2-bit units (see Figure 1e). By encoding two bits with each signal transition, 2B1Q represents the distinction between bits per second and baud rate. The baud rate of a signal is the number of signal transitions per second, and it can't be higher than the bandwidth of the channel. The number of bits per signal element, represented by L, is given by log2L. Thus a code with eight signal elements could encode 3 bits per baud, a code with 16 signal elements could encode 4 bits per baud, and a code with 256 signal elements could encode 8 bits per baud. In the case of ISDN, the bandwidth the signal occupies is 80KHz, the baud rate is 80Kbaud, and the raw data rate is 160Kbits/sec.

Digital Signals, Analog Transmissions

A vast infrastructure exists for analog signaling, and much of it can readily transport digital signals as well. The telephony local loop, the cable TV infrastructure, and practically every form of wireless communication are inherently analog transmission media that have been adapted for digital signals.

The earliest modems used a technique known as Frequency-Shift Keying (FSK, see Figure 2a) to represent digital data. FSK devices, such as the Bell 103 modem, used one tone (1070Hz) for zeros and another tone (1270Hz) for 1s. Amplitude-Shift Keying is one way of describing the modulation of digital data over fiber optic cable. In this case, no light represents a zero while the presence of light above a threshold level represents a one. The third attribute of a sinusoidal signal is its phase, and Phase-Shift Keying (PSK, see Figure 2c) is widely used in modems. Nowadays, modems commonly use a combination of phase and amplitude modulation to encode multiple bits in a single signaling event or symbol.

Cable modems and ADSL make use of a signaling technique called Quadrature Amplitude Modulation (QAM). With QAM, the carrier signal is split into two signals, shifted in phase by 90 degrees. Each component is modulated with ASK, using as many as 16 amplitude levels to represent as many as 256 different states. One ADSL modulation technique, Discrete Multitone (DMT), divides the available twisted-pair spectrum above 25KHz into 256 downstream subchannels of 4KHz each. QAM is then applied to each subchannel according to its individual performance. At best, a single subchannel may be able to carry as many as 60Kbits/sec. Theoretically then, ADSL could provide throughput rates as high as 15.36 Mbits/sec.

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