Networking and Telecommunications

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Networking and Telecommunications

• 2. Telecommunications

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Transmission Characteristics

• Medium
• Air Waves (satellite, microwaves, wireless lans
  (infrared, radio waves)
• Electrical /Copper (Twisted Pair, Coax)
• Light (fiber optic via glass/plastic cable)

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• Transmission Signals
• Parallel - all 8 bits at same time
• serial 1 bit at a time (rs232), sequential
• Transmission Types
• Asynchronous start/stop bits, one character at
  a time
• Synchronous fast, multiple characters in a
  block of data. No start/stop bits.

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• Analog (continuous sine wave)
• Watch (with hands),speedometer
• digital transmissions (discrete pulses of 0 or 1)
• Computers/binary
• TV-discrete channels

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Analog vs. Digital

• Internal to Computer (digital, binary)
• Transmissions originally analog
• Phone systems analog totally at first
• Computers used modems to talk over analog phone
  lines (and connections between CO were analog)
• Today Connections between CO are digital.
  Connections between home and CO (last mile) are
  still mostly analog
• Cable Modems and DSL connections from home
  becoming Digital

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Why Digital better then Analog?

• Faster (higher capacity)
• 56k vs. Mbps
• Better data integrity (less errors), distortion
  of pulses easier to correct
• Better Security
• Lower Costs Today
• No analog to digital conversion needed (no modem)
• Easier to integrate voice, video, data

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• Transmission Codes Encoding
• (Morse Code)
• EBCDIC (8 bit)
• ASCII (8bit vs 7bit)
• Parity Even vs. Odd
• Half Duplex vs. Full Duplex

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EBCDIC vs. ASCII (link)
EBCDIC
Character
ASCII
A 1100 0001 0100 0001
B 1100 0010 0100 0010
Z 1110 1001 0101 1010
5 1111 0101 0011 0101
! 0101 1010 0010 0001
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Morse Code
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Waves

• Sound, Electrical, Radio, Electromagnetic
• Sine Wave, measured in Hertz (Hz)
• Characteristics Amplitude and Frequency
• Bandwidth difference from high to low frequency
  range

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• Speed (modems)
• Baud vs. BPS
• 300, 1200, 2400, 9600, 14.4, 28.8, 56k

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Modems
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Circuit Switching Networks

• Temporary", "dedicated", communications channel
• Voice (phone)

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Packet Switching Networks

• Store and forward, non-dedicated
• Shared communications channel (packets
  interleaved on same line)
• Packets may take different paths, arrive in
  different order (reassemble at end)
• X.25 old WAN standard for PSN
• Useful for data

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Telecom Hardware

• Hosts (mainframe, mini)
• Micros, workstations (desktop systems)
• Terminals (vt100, 3270, X-terminals)
• Automatic Teller Machines (ATM - Banks)
• POS (Point of Sales - cash registers)
• Fax
• Satellite Dishes, Satellites, radar towers
• Terminal Server (Access Server)
• MUX (multiplexer) - TDM and FDM
• Modems

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Satellite Dish
ATM Machine
Fax Machine
Terminal
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Terminal Servers/Access Servers

• Connect modem pool to internal network
• Allows outside users to connect in
• And internal users to dial-out
• Authentication is normally used

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Multiplexers

• Allows multiple systems to share single common
  communications channel
• Often used with mainframes/terminals, as well as
  phone systems
• 2 Main Methods used
• TDM Time Division Multiplexing
• FDM Frequency Division Multiplexing

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Transmission (telecom.) Media

• electrical/copper
• Phone wire
• TP - 2 pair (4 wire, RJ-11), 4 pair (8 wire, RJ-
  45)
• Coax
• rs232
• light fiber optic (glass/plastic)
• air satellite/microwave

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Fiber Optics Cable
Phone Cable (RJ-11)
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• Dial-up lines - analog, non-dedicated, (POTS
  -Plain Old Telephone Service)
• Leased lines - digital, dedicated, often
  fiber-based
• 56k
• T1 - 1.544 Mbps
• T3 - 44 Mbps
• OC3/OC12

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Modems

• POTS (Plain Old Telephone Service)
• Modulator/demodulator, digital--gtanalog--gtdigital
  
• Carrier wave (base, neutral wave)
• Amplitude modulation (am), frequency modulation
  (fm)
• Phase modulation (like radio, am/fm)

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Frequency Modulation of a Carrier Wave
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• Baud vs. bps - detectable events (signaling
  events)
• Detect changes in carrier wave
• Types of modems - internal/external

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Modem Terms

• Standards - V.32 (9600), V.32bis (14,400 or 14.4)
• V.34 (28.8), V.90 (56k) ---gt 56k max speed
• Data compression
• Error detection (parity, checksum)
• Encryption

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Internal Modem
External Modem
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Cable and DSL Modems

• Also called broadband
• Sometimes 56k and below is referred to as
  narrowband
• Higher speeds then 56k and always online (may be
  security risk)

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DSL Modem
Cable Modem
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Cable Modems

• Cable- digital modem, higher speeds (500k to 2
  meg)
• Different upload/download speeds
• Shared technology
• Uses Cable TV coax cable
• Security Concerns

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DSL

• Digital Subscriber Line - digital modem
• Fast speeds but typically less then cable modems
• Different upload/download speeds
• switched/dedicated technology
• Uses new installed line or existing phone line
  with filters
• Speeds of 144k up to 6 meg
• Typically 144k to 600k for home users

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Bandwidth

• Speed
• Info carrying capacity of a transmission facility
• Range of frequencies, given in hertz (hz),
  Cycles/sec, that can be accommodated without
  signal degredation
• voice lines 3000 hz (300 to 3300 hz)

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Note DSL and Cable Modems range from 500,000
bps to 2,000,000 bps and thus would be in the
around the T1 range (2-4 lane highway).
(wireless is similar)
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Transmission Problems

• Attenuation - loss of signal strength over a
  distance
• Crosstalk - interference from neighboring wires
• EMI/RFI - electro magnetic interference- radio
  frequency interference
• Line noise
• Solutions - amplifiers, repeaters, shielding,
  twisted pair, fiber

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RS-232

• Asynch. serial communications standard (1969)
• 25 pins (DB-25), copper based (electrical signal)
• Handshaking (modems)
• DCE/DTE
• Null modem connection
• 50 feet max
• Used with modems, printers, terminals
• Typical 2,3,7 (send, receive, ground)

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RS-232
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RS-232
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RS-232
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RS-232
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• Summary

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Telecommunications

• Low level
• Bits
• Encoding
• Parity
• Duplex
• Serial
• Asynch
• etc..

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Hardware

• Modems
• Mux
• T1
• T3
• terminal server
• (access server)
• rs232
• etc..

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Software

• Connect to host
• telnet/tn3270/ssh
• terminal emulation
• log into host
• windows terminal
• hyperterminal
• File transfer
• ftp/sftp

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Fun Facts

• 1876 Bell electrical speech machine
• 1878 1st phone book
• New Haven Conn., 1 page (50 names)
• 1883 1st yellow pages. Why yellow?
• 1929 1st President with phone in White House ??
• (Herbert Hoover)

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Herbert
Hoover
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More Fun Facts

• Alexander Graham Bell 29 telephone
• Photophone (speech via light waves)
• Bell and Elisha Gray (2 hours apart)
• First Rotary Phone (non crank) 1929
• First mobile phone 1924 NYC police

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• THE END