CSIT 220 (Blum) 1

Long Distance Digital Connection

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Telephony

• The prefix “tele” refers to doing or moving something over a distance

• The root “phone” refers to sound, especially that connected with speech

• Thus the telephone is a device that transmits speech over long distance

• Telephony (te lef’ e nē) is the science/technology associated with the transmission of speech over long distances

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Telephony (Cont.)

• Because an extensive phone system was already in place, long distance computer networks could use the phone system to transmit data and avoid the expense of lying down new transmission cables.

• The divisions between telephony, tele-communications and computer networking have become blurred.

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Analog or Digital

• Because speech is itself an analog signal (pressure waves in the atmosphere), traditional telephony has been based on transmission of analog electric and/or electromagnetic signals.

• However, the idea of digitizing audio signals predates computer networks by decades.

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PCM• Shannon and co-workers (Oliver and Pierce)

proposed Pulse Code Modulation in 1948.• Recall that when a digital signal reaches a repeater,

small amounts of noise can be eliminated from the signal before it is amplified and forwarded; whereas when an analog signal reaches a repeater the noise is amplified right along with the information.

• In an analog signal, all values are valid and so one cannot distinguish between the noise and the signal.

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Digitizing

• Consider for example an analog voltage signal. It can be continuous in two senses: 1. The voltage varies continuously in time

2. At a given instance, the voltage can take on any value from a continuum.

• To digitize the signal, the time continuum and the voltage continuum have to be converted into discrete sets of values.

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Analogy: Digitizing an image

Discretize spaceDiscretize color

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Sampling

• Breaking up the time continuum is known as “sampling.”

• Motion pictures are an example of sampling: a rapid succession of snapshots (still pictures) are taken, if the sampling frequency (the number of pictures/frames per second) is sufficiently high, the brain perceives the playback as continuous motion.

• Muybridge demo

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(pseudo)-Analog wave

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Continuous in time

Con

tin

uou

s va

lues

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Sampled Wave

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Another Nyquist Theorem

• Recall that signals can be thought of as being comprised of sine waves of various frequencies (Fourier).

• Nyquist says that to accurately represent a signal, one’s sampling frequency must be at least double its highest constituent frequency. – One wants to sample its ups and its downs.

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Nyquist Sampling Example• In the following sequence of graphs, a sine wave

is sampled. • The frequency of the sine wave is doubled each

time, while the sampling frequency is kept fixed.– Case E does not resemble a sine wave but alternates up

and down with the correct frequency – i.e. the sampling has captured the most essential feature

– Case F oscillates very quickly (alternating up and down), but its amplitude seems to vary at a much lower frequency. This was not a feature of the actual wave being sampled.

– Case G only has the slowly varying feature whereas the actual wave sampled varying quite rapidly.

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A: sf=10, f=0.159sf: sampling freq. F: freq.

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The sampling frequency is much higher the “actual” frequency and the behavior is captured well.

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B : sf=10, f=0.318

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C : sf=10, f=0.637

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D : sf=10, f=1.273

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E : sf=10, f=2.546

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The sampling frequency is about 4 times higher than the actual frequency – the actual oscillation is captured but the shape is not.

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F : sf=10, f=5.093

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The sampling frequency is about 2 times higher than the actual frequency – the actual oscillation is captured, but the shape is not and spurious low frequency effects have been introduced.

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G : sf=10, f=10.186

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The sampling frequency is about equal to the actual frequency – the actual oscillation is missed entirely, only the spurious low frequency effects remain. It’s like we sampled Mariah Carey and we got Barry White.

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The 8000 Hz decision• Most humans can hear sounds ranging in

frequency from 20 Hz to 20,000 Hz. • But for purposes of digitizing and transmitting

speech, the telephone industry decided that they wanted to represent accurately signals in a range up to 4000 Hz.

• Thus they opted for a sampling frequency of 8000 Hz (a la Nyquist). – A bandwidth of 3000 Hz was considered sufficient, but

it was upped to 4000Hz.

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Period

• Whereas the frequency (f) is the number of cycles that go by in a set amount of time (usually a second), the period (T) is the amount of time required for a single cycle.

• The frequency and period are reciprocals f = (1/T) or T=(1/f)

• If the sampling frequency is 8000 Hz, the sampling period is 0.000125 s = 125 s (microseconds) = (1/8000 Hz).

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The other half of the problem

• At the instant one is sampling, the signal can still take on an infinite number of values.

• Digitizing requires one to choose a discrete set of allowed values. – For example, to digitize an image one can choose two

values (black and white) or allow for shades of gray or allow for combinations of red, blue and green, etc.

• For the phone system, it was decided that 256 values would be allowed. – 256 values can be represented by 8 bits.

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Sine: 5 values

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Sine: 9 values

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Sine: 17 values

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Sine: 33 values

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A digital audio channel

• So the telephone company’s standard for digital audio required

64,000 bits/second

= 8,000 samples/second 8 bits/sample

• This 64 Kbps rate can be seen throughout telephony and is known as DS0 (digital signal).

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T-carrier system

• Bell introduced the first successful system for digitized voice transmission using the 64 Kbps rate (DS0) in the 1960s.

• The rate has been kept as a standard and the basis for subsequent standards.

• The US and Europe developed separate standards.

• The US standards were set up by ANSI (American National Standards Institute).

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DS-X Standards

Bit Rate Mult. T (US) E (Eur.)

DS0 64 Kbps 1 —— ——

DS1 1.544 Mbps

24 T1

—— 2.048 Mbps 32 —— E1

DS1C 3.152 Mbps 48 —— ——

DS2 6.312 Mbps 96 T2 ——

—— 8.448 Mbps 128 —— E2

C: concatenated

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DS-X Standards (Cont.)

Bit Rate Mult. T (US) E (Eur.)

— 34.368 Mbps 512 — E3

DS3 44.736 Mbps 672 T3 —

— 139.264 Mbps 2048 — E4

DS4/NA 139.264 Mbps 2176 — —

DS4 274.176 Mbps 4032 — —

— 565.148 Mbps 8192 — E5

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T-carrier system

• A T-1 line (1.544 Mbps), commonly used by businesses to connect to their Internet service provider (ISP), corresponds to 24 DS0 channels.

• A T-3 line (44.736 Mbps) corresponds to 28 T-1 lines or 672 (2428) DS0 and is commonly used by ISPs.

• One can also lease “a fractional T-1,” in which one rents some portion of the 24 channels in a T-1 line, with the other channels going unused.

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A leased line

• Unlike dial-up connections, a leased line is always active. It is a permanent telephone connection between two nodes.

• Usually used by businesses to connect distant offices.

• Typically one pays a fixed monthly rate based on the distance between the nodes and the speed of the circuit.

• The line is used exclusively by the lessee, so the carrier can assure a given level of quality.

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T1 and T3 • T1 and T3 lines are entirely digital. • They use pulse code modulation (PCM) and Time-

Division Multiplexing (TDM). • They provide full duplex capability by using four

wires – two wires (one for signal and one for return) for receiving – two for sending

• Originally the four wires were two twisted pair wires, but now they can be coaxial cable, optical fiber, or even wireless media like digital microwave.

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T1 for Voice (Shay Fig 3.20)

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T1 Frame• T1 can be used for voice or for data transmission. • Voice signals are sampled at 8000 Hz and each

sample is encoded using 8 bits. • With 24 such channels being multiplexed (TDM),

a 192-bit frame (24 channels 8 bits/channel) is sent every 125s.

• One bit separates consecutive frames, so each frame is actually a 193-bit block.

• The 193 bits/frame multiplied by 8,000 frames/sec yield 1.544 Mbps data rate.

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T1 (Cont.)

• T1 lines typically use copper wire and within or even between cities (metropolitan areas).

• A T1 Outstate System has been developed for longer distances between cities.

• It's probable that your Internet access provider is connected to the Internet as a point-of-presence (POP) on a T1 line owned by a major telephone network. – POP: Locations where an Internet Service Provider

offers access to its network.

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CSU/DSU• Channel Service Unit/Data Service Unit. • Required for both ends of a T1 or T3 line.• The CSU and DSU perform different tasks.

They are combined into one unit. • The CSU is a device that performs

protective and diagnostic functions, like: – Lightning protection– “Ones density:” if more than 15 consecutive 0’s

are sent, raises alarm or “stuffs”– Loopback (used for testing the lines)

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CSU/DSU (Cont.)

• The DSU is a device that connects a terminal to a digital line.

• Converts data, which is digital, to a form appropriate for the T1 line, which is also digital but uses different encoding, voltage values, etc. – A kind of modem.

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DSU/CSU (Fig 12.2)

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DSU/CSU

• The DSU/CSU combination may also take on some other duties – Multiplexing (putting many signals on one line) – Data Compression

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Comparison shopping

• There are various competing technologies for establishing long distance connections

• There are two basic criteria for the comparison – Transmission rate (a.k.a. capacity)– Cost

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Comparing Rates

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Comparing Rates (Cont.)

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Local loop

• Connection between the phone company’s Central Office and an individual home or business.

• To connect computers from an individual’s home or small office, one uses – Modem

– ISDN

– DSL

– Cable modem

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ISDN

• Integrated Services Digital Network• CCITT/ITU standards for digital transmission over

ordinary telephone copper wire– ITU International Telecommunications Union– CCITT, (now ITU-T) is an body for generating

telecommunications standards. • A service offered by telephone companies to connect

computers over long distances (MAN or WAN) – Since it’s telephone companies, that 64 Kbps rate will be

seen here too.

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ISDN (Cont.) • One can send voice, video or data at speeds

better than modems (modem’s best: 56 Kbps).– Hence I for “integrated.”

• Uses ISDN adapter in place of modem.

• Uses a mixture of packet switching and circuit switching.

dx

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ISDN Services

• There are two standard levels of ISDN service: Basic rate (interface) (for home use) and Primary rate (interface) (for business use).

• Basic rate: – Consists of three channels, two B (bearer)

channels and one D (delta) channel. – The B channels operate at 64 Kbps, are full

duplex and carry voice or data.

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ISDN Services (Cont.) – The D channel operates at 16 Kbps, is full

duplex and carries “signaling” (control) information used to request services from the B channels.

– One can use one B channel for voice and the second for data or they can be combined (“bonded”) to give the effect of one 128Kbps channel.

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ISDN Services• Primary rate:

– 23 64-Kbps B channels • Primary rate in Europe has 30 B channels

– 1 64-Kbps D channel, although sometimes addition D channels are installed.

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B-ISDN• The original ISDN is a baseband

technology (one signal). • Broadband ISDN, B-ISDN transmits many

signals simultaneously across the same medium. It has a transmission rates of 1.5 Mbps. – uses fiber optic cable – not widely available

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ISDN (dying technology)• When this standard was developed, good

modems were operating at 10Kbps (now they’re up to 56 Kbps) and alternatives like DSL and cable modems did not exist.

• Today it is not a much used technology.

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DSL

• Digital Subscriber Line• DSL uses fancy modulation schemes to improve

the transmission rate possible on basic copper wires (POTS, plain old telephone service).

• Sometimes called “last-mile technologies” since they are only used to connect a telephone (switching) station to a home and are not used between stations. – The distance to the office can play an important role

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DSL

• Recall that a voice signal uses approximately the lowest 4 KHz of bandwidth on what is typically twisted pair wire.

• It is possible to send higher frequency signals simultaneously, that is to use FDM (frequency division multiplexing).

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Discrete Multi-tone (DMT) modulation

• While the spectrum over 4 KHz is available for additional signals, the system (POTS) was not explicitly designed for its use.

• Consequently there may be variations in the quality of these higher frequency signals.

• Thus the modulation scheme, Discrete Multi-tone modulation (DMT), is “adaptive.” It searches for usable parts of the spectrum and varies the rates within channels depending on their signal-to-noise ratios.

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Adaptive

• For high S/N (good channels), DSL chooses a modulation scheme that encodes many bits per baud.– Recall how Shannon limits Nyquist.

• For low S/N (poor channels), DSL chooses a modulation scheme that encodes fewer bits per baud.

• DSL does not guarantee a specific rate, but does as well as conditions allow.

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Upstream/Downstream

USER

Upstream

Data from user

Downstream

Data to user

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Asymmetric DSL (ADSL)

• Typical Internet user’s transmission traffic is asymmetric, not the same in the upstream and downstream directions.

• The upstream traffic tends to consist of “requests” which are small packets.

• The downstream traffic tends to consist of large files (often multimedia files).

• ADSL builds in this asymmetry.

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Asymmetry

• The FDM (frequency division multiplexing) makes a number of channels available in a single wire.

• More of those channels are used for sending signals downstream than for sending signals upstream.

• Not good for businesses offering web services which require a higher upstream capacity. – They need Symmetric DSL.

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Upstream/Downstream Asymmetry

USER

Upstream

Up to 640 Kbps

Downstream

Up to 6.144 Mbps

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ADSL• ADSL divides the frequency range into 286 separate

channels. • 255 transmit downstream, 31 transmit upstream and 2

for control information. • The carriers for these channels are spaced at 4.1325

KHz intervals to keep signals from interfering. • The signal may then be “inverse multiplexed,” that is

one signal sent over many wires. – Recall multiplexing is several signals on one line, so inverse

multiplexing is one signal on many lines (channels).

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Other DSL Technologies• Symmetric DSL (SDSL)

– Symmetric transmission rates. – Encodes differently so it sometimes works where ADSL

does not.

• High-Rate DSL (HDSL) – 1.544 Mbps in both directions (same as T1).– Short distances only. – Requires 2 twisted pair wires, has some fault tolerance

since it can operate at reduced speeds with one wire.

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Other DSL Technologies

• Very-high bit rate DSL (VDSL) – Up to 52 Mbps. – Very restrictive on distances, so one comes into

neighborhoods (to an Optical Network Unit, ONU) with fiber and then on to the nearby houses with copper wiring.

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Other References

• http://www.webopedia.com

• http://www.whatis.com

• Understanding Data Communications & Networks (William Shay)

• Signals, The Science of Telecommunications (Pierce and Noll)

• Computer Dictionary, Mitchell Shnier