Lecture #2

“To understand a science it is necessary to know its history” -Auguste Comte (1798-1857)

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History Remarks

• 1813 Telegraph Gauss&Weber

• 1861 Telephone Reis

• 1876 Telephone Bell (Bell System)

• 1877 First Finnish telephone connection in Helsinki

• 1878 Microphone Edison (General Electric)

• First Telephone Exchange in New Haven, USA

• 1865/6 Transatlantic Cables

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History Remarks

• 1884 Radio Telegraph Popov

• 1892 First Automatic Telephone Exchange in La Porte USA by Strowger

• 1896 Radio Telegraph Marconi

• 1898 First Automatic Telephone Exchange in Germany

• 1918 Radio Carrier System /USA 1920 Radio Broadcasting

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History Remarks

• 1927 TV Bell Labs

• 1929 TV BBC

• 1930 Coaxial cables

• 1931 Radiolinks

• 1937 PCM (64kbps) Reeves (Bell Labs)

• 1946 Cellular Radio (Bell Labs)

• 1947 Transistor (Bell Labs)

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History Remarks • 1957 Satellite data transmission

• 1961 Electronic Telephone Exchange

(Bell Labs)

• 1968 Digital Telephone Exchange GB

• 1969 ISDN (Integrated Services Digital

Network 2x64k+16k)

• 1970 Aloha-network (Hawaii)

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History Remarks

• 1974 Packet and Circuit Switched data

networks (CCITT X.25 and X.21)

• 1974 Arpanet/ Internet DoD/USA

• 1976 Optical Fiber in data transmission

• 1977 Ethernet 10Mbps Xerox

• 1978 ISO/OSI + CCITT x.200

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History Remarks

• 1972 Mobile Networks ARP

• 1981 Mobile Networks NMT Nordisk MobilTelefon

• 1984 MHS (Message Handling System) CCITT/ISO

– ODA (Open Document Architecture) CCITT/ISO

• 1984 Intelligent Networks (AIN Series) Bellcore

• 1987 GSM (Groupe Special Mobile, CEPT) -

Global System for Mobile Communications

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History Remarks

1989 HTTP/HTML in Cern by Tim Barners-Lee

1991 ATM (Asynchronous Transfer Mode 155 Mb/s)

1991 IN CS.1 (Intelligent Networks) by ITU and

ETSI

1994 WWW (World Wide Web)

1998 GPRS (General Packet Radio System)

2001 UMTS (Universal Mobile Telecommunication

System)

Multiplexing 1900: 25% of telephony revenues went to copper mines • standard was 18 gauge, long distance even heavier • two wires per loop to combat cross-talk • needed method to place multiple conversations on a single trunk

1918: “Carrier system” (FDM) • 5 conversations on single trunk • later extended to 12 (group) • still later supergroups, master groups, supermaster groups

1963: T-carrier system (TDM) • T1 = 24 conversations per trunk • later T3 = 28 T1s • still later SDH rates with 1000s of conversations per trunk

PSTN

Review

f

channels

t

timeslots

History • 1876 A. G. Bell telephone patent • 1878 The first exchange constructed in La Porte, the US

– could connect any two of the 21 subscribers – manual switching (!)

• 1892 first automatic exchange: Strowger Switch by Almon B. Strowger: an undertaker in Kansas City

• A 100 line Strowger switch: – each user has its

own selector – no concentrators – expensive

Strowger switch

via

se

lecto

rs

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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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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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Bandwidth on a Voice Circuit

• Human hearing ranges from about 20 Hz to about 14,000 Hz (some up to 20,000 Hz). Human voice ranges from 20 Hz to about 14,000 Hz.

• The bandwidth of a voice grade telephone circuit is 0 to 4000 Hz or 4000 Hz (4 KHz).

• Guardbands prevent data transmissions from interfering with other transmission when these circuits are multiplexed using FDM.

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Bandwidth on a Voice Circuit

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Bandwidth on a Voice Circuit

• It is important to note that the limit on bandwidth is imposed by the equipment used in the telephone network.

• The actual capacity of bandwidth of the wires in the local loop depends on what exact type of wires were installed, and the number of miles in the local loop.

• Actual bandwidth in North America varies from 300 KHz to 1 MHz depending on distance.

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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).

• Digital Signal 0 (DS0) is a basic digital signaling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel. The DS0 rate, and its equivalents E0 and J0, form the basis for the digital multiplex transmission hierarchy in telecommunications systems used in North America, Europe, Japan, and the rest of the world, for both the early plesiochronous systems such as T-carrier and for modern synchronous systems such as SDH/SONET.

• The DS0 rate was introduced to carry a single digitized voice call. For a typical phone call, the audio sound is digitized at an 8 kHz sample rate, or 8000 samples per second, using 8-bit pulse-code modulation for each of the samples. This results in a data rate of 64 kbit/s.

• Because of its fundamental role in carrying a single phone call, the DS0 rate forms the basis for the digital multiplex transmission hierarchy in telecommunications systems used in North America.

• To limit the number of wires required between two involved in exchanging voice calls, a system was built in which multiple DS0s are multiplexed together on higher capacity circuits. In this system, twenty-four (24) DS0s are multiplexed into a DS1 signal. Twenty-eight (28) DS1s are multiplexed into a DS3. When carried over copper wire, this is the well-known T-carrier system, with T1 and T3 corresponding to DS1 and DS3, respectively.

• Besides its use for voice communications, the DS0 rate may support twenty 2.4 kbit/s channels, ten 4.8 kbit/s channels, five 9.67 kbit/s channels, one 56 kbit/s channel, or one 64 kbit/s clear channel.

• E0 (standardized as ITU G.703) is the European equivalent of the North American DS0 for carrying . However, there are some subtle differences in implementation. Voice signals are encoded for carriage over E0 according to ITU G.711. Note that when a T-carrier system is used as in North America, robbed bit signaling can mean that a DS0 channel carried over that system is not an error-free bit-stream. The out-of-band signaling used in the European E-carrier system avoids this.

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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).

T1 technology was developed by AT&T in 1957 and is used in America and Asia

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DS-X Standards Bit Rate Multiplier

(CH)

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 Multiplier

( CH)

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 T4 —

— 565.148 Mbps 8192 — E5

Time Division Multiplexing

Multiplexing T1 streams into higher carriers.

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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.

http://www2.rad.com/networks/2003/e1_t1/e1_t1/frame.htm

Each frame has only one sample from each channel.

T1 Carrier

The T1 carrier (1.544 Mbps).

193 X 8000 = 1.544 Mbps

T1 Carrier

• 193rd bit is used for frame synchronization : a pattern of 010101… is looked for --- analog nodes cannot generate this pattern, digital users can but the chances are less.

•

Signaling(control) information in T1

• Notice : 8000 bps signaling information : too much : two possible approaches to reduce this :

– Common channel signaling : use of 193rd bit for signaling in alternate frames say odd frames and for data in even frames.

– Channel-associated signaling : each channel has its own private signaling subchannel – one of the eight user bits in every sixth frame is used for signaling

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T1 for Voice

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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.

E1 Carrier

• 32 channels : 30 for data + 2 for signaling

• Each group of four frames provides 64 bits of signaling : half for channel specific + half for frame sync

• Capacity : 32 X 8 X 8000 = 2.04 Mbps

E1

• A 2.048 Mbps point-to-point dedicated, digital circuit provided by the telephone companies in Europe. E1 is the European counterpart of the North American T1 line, which transmits at 1.544 Mbps, and E1 and T1 lines can be interconnected for international use. E2 through E5 lines provide multiple E1 channels.

• An E1 line uses two wire pairs (one for transmit, one for receive) and time division multiplexing (TDM) to interleave 32 64-Kbps voice or data channels.

Signal Rate

E0 64 kbit/s

E1 2.048 Mbit/s

E2 8.448 Mbit/s

E3 34.368 Mbit/s

E4 139.264 Mbit/s

E5 564.992 Mbit/s

T-carrier and E-Carrier Systems

North American Japanese European (CEPT)

Level zero (Channel data rate)

64 kbit/s (DS0) 64 kbit/s 64 kbit/s

First level 1.544 Mbit/s (DS1) (24 user channels) (T1)

1.544 Mbit/s (24 user channels)

2.048 Mbit/s (32 user channels) (E1)

(Intermediate level, US. hierarchy only)

3.152 Mbit/s (DS1C) (48 Ch.)

– –

Second level 6.312 Mbit/s (DS2) (96 Ch.) (T2)

6.312 Mbit/s (96 Ch.), or 7.786 Mbit/s (120 Ch.)

8.448 Mbit/s (128 Ch.) (E2)

Third level 44.736 Mbit/s (DS3) (672 Ch.) (T3)

32.064 Mbit/s (480 Ch.)

34.368 Mbit/s (512 Ch.) (E3)

Fourth level 274.176 Mbit/s (DS4) (4032 Ch.)

97.728 Mbit/s (1440 Ch.)

139.264 Mbit/s (2048 Ch.) (E4)

Fifth level 400.352 Mbit/s (DS5) (5760 Ch.)

565.148 Mbit/s (8192 Ch.)

565.148 Mbit/s (8192 Ch.) (E5)