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Intro to Telecom Fig 6.2 Analog and Digital Signals Digital signal Analog Fig. 6.4 signal Analog Continuous fluctuations over time between high and low voltage Digital A discrete voltage state Fig 6.3 Source/Signal Combinations Analog Signal Analog Voice, Telephone, Source Television.... Digital Signal Voice over digital media, Audio files, CODEC Digital Fax, Any Computer Computer Source over POTS, Digital over digital T-V lines (T-1, ATM, Frame relay...) Basic Modulation Techniques Amplitude modulation (AM) Converts digital data to analog signals using a single frequency carrier signal High-amplitude wave denotes a binary 1 Low-amplitude wave denotes a binary 0 Frequency modulation (FM) Uses a constant amplitude carrier signal and two frequencies to distinguish between 1 and 0 Phase modulation Uses a phase shift at transition points in the carrier frequency to represent 1 or 0 Examples: Analog shifts Data Transmission Speeds Measured in bits per second (bps) Kilobits per second (kbps) Megabits per second (Mbps) Gigabits per second (Gbps) Types of Communications Media Guided Media Twisted wire cable Coaxial cable Fiber-optic cable Unguided Media Microwave transmission - satellite Microwave transmission - terrestrial Cellular transmission Infrared transmission Cable/Wire Types Twisted Pair Wire A cable consisting of pairs of twisted wires The twist helps the signal from “bleeding” into the next pair Cheapest Limited bandwidth Coaxial Cable Inner conductor wire surrounded by insulation, called the dielectric Dielectric is surrounded by a conductive shield, which is in turn covered by a layer of nonconductive insulation, called the jacket More expensive than twisted pair, but higher bandwidth Twisted Pair Fig 6.4 Coaxial Cable Fig 6.5 Cable/Wire Types, Continued Fiber Optic Cable Consists of many extremely thin strands of solid glass or plastic bound together in a sheathing Transmits signals with light beams No risk of sparks, safe for explosive environments More expensive than coaxial, but more bandwidth Different colors of light are used to simultaneously send Multiple signals Fiber Optic Cable Fig 6.6 Microwave Transmission Fig 6.7 Satellite Fig 6.8 Cellular Fig 6.9 Table 6.1 Communications Efficiency A large part of telecommunication expense is cost of the medium Several approaches are used to efficiently use the medium Multiplexing Switching Compressing Multiplexing: Time Division and Frequency Division Time division multiplexing (TDM) is where multiple incoming signals[Figure are sliced6.14] into small time intervals Frequency division multiplexing (FDM) is where incoming signals are placed on different frequency ranges Multiplexing Freeway Analogy • Frequency division multiplexing is analogous to having a 3-lane freeway. Each car has its own lane, three cars drive simultaneously in the same direction. • Time division multiplexing is analogous to a freeway-onramp: cars enter the on-ramp one at a time, and drive in single file. Frequency Division of Cable Base video width is 4.2 MHz with guard bands 6 MHz Ch 1Ch 2Ch 3Ch 4Ch 5Ch 6 ….…………….... Ch n The default is 6 Mega Hertz slices of bandwidth per channel Cable modem --Cable Bandwidth-Q: What limits the bandwidth on coaxial cable? A: The bandwidth of the amplifier. Cable phone gets 4 KHz slices Switching Switching further advances the objective of efficiently utilizing the circuit Two types: Circuit switching (e.g., public telephone network) requires end-to-end physical connection Packet switching (e.g. Internet) breaks up messages into small “packets” and routes them individually. No end-to-end physical connection required. Can be virtual circuit (all packets travel through same route) or datagram (packets may travel through any route) Circuit Switching You Switch To communicate a physical connection must be made and maintained Your Mom Packet Switching Packets thrown into the internet ‘cloud’ either independently find the path from point to point (datagram) of follow the same path (virtual circuit) The message Header Message contents Direction of transmission Start of Header SOH Start of Text (STX) If variable length header used Trailer Block Check End Character of Text (ETX) BCC If variable length message used A Simple Protocol Stack Application Protocol Application Transport Protocol Transport Network Access Network Protocol Application Transport Network Access A Simple Protocol Stack, Continued • The application uses the protocol for its layer/level to determine how it should format its message for an application at a different computer • However, it does not worry about getting the message to the application • The transport layer is responsible for making sure that the message arrives at the correct application at the correct computer • However, it does not concern itself with how it gets there. That is the responsibility of the network layer. The transport layer is only concerned with reliability of the communication • The network layer determines how the message should be presented to the network Formatting and Decoding a Message Protocols strip header information from the message Protocols add header information to the message Application Application Data Transport Transport Transport Header Network Access Network Access Network Header Communications Protocols Fig 6.22 Relationship of TCP/IP to OSI OSI TCP/IP 7 Application 6 Presentation 5 Session control 4 Transport control Host to Host 3 Network control Internet 2 Data link control 1 Physical link control Process / Application Network Access Controls the user’s interface and applications between two hosts, e.g.: • File transfer protocol (ftp) • HTTP (Hypertext trans. protocol) • Telnet • SMTP (Simple mail transfer protocol) • SNMP (Simple Network Mgt protoc’l) • NNTP (Net news transport protocol) TCP: Virtual circuit maintained, ack UPD: No acknowledgment IP: routing, fragmentation, assembly ICMP: Above IP, error handling ARP: Address resolution sw to hw addr RARP: hardware to sw address convert Physical layer, such as Ethernet or Token Ring Fig 6.23 Ethernet Evolution New, taking over Predominant Legacy 10 Mbps Ethernet Old installations 100 Mbps Ethernet Most new installations 1000 Mbps Gigabit Ethernet Battling ATM Ethernet Pros and Cons •Operates by contention – packets collide •Inefficient – many aborted transmissions •Rates of only 37% of raw wire speed •10 Gbit Ethernet on the way •Inexpensive •Simple circuitry •Cheapest bandwidth ratios Token Ring T data T data 40008065402 Token Ring Pros and Cons Very efficient – 75% of raw bandwidth A better technology Expensive Used for mission critical applications like banking Lost battle to fast-Ethernet (like beta vs. VHS) ATM •Sends 53-byte cells – not variable length packets like Token Ring and Ethernet •Hardware knows where header ends and data begins •Speeds up to 622 Mbps •Predictable throughput rates = very reliable, guaranteed service •Military, Safety valve in nuclear power reactor…. No Delay or Jitter!!! Body Header ATM Pros and Cons •Very fast •Reliable – mission critical applications •Efficient bandwidth >75% of raw capacity •No delays or sequence re-configuring •Very expensive – and complex •Not compatible with 10/100 Mbps Ethernet installations •Most applications do need this efficient management of data cells – only messages used in Body Header real time need ATM Connectivity Type Bandwidth # Users Rel.Cost Modem 28.kbps 1-5 1 DSL 256+ Kbps 1-50 2 ISDN 128 Kbps 5-50 3 T1 (DS1) 1.54 Mbps 50-500 10 T3 (DS3) 45 Mbps 4000+ 100 ATM 155-622 Mbps 10,000 200+ Synchronous Optical Network (SONET) Define Optical Carrier Levels (OC) Basic transmission rate STS-1 51.84 Mbps OC-3 = 3*51.84 Mbps = 155.52 Mbps OC-12 = 12* 51.84 Mbps = 622.08 Mbps OC-48 = 2.488 Gbps OC-768 = ????? Bringing in the fiber 48 strands - OC 48 96 strands - OC 96 Dense Wave Division Multiplexing 48 strands can yield OC – 192 Optical Switches –do not convert from light to electricity and back to light. 100% light. Current Status: Fiber Massive investments by telecoms in 1990s. Current fiber utilization at 2.5%!!!! Mostly between major corporate infrastructures in major cities. CO to CO Limitations on last mile to smaller infrastructures Abundance trickled to equipment manufacturers as well; predicted to last through 2002 Brief History of Telecom 1837 - Invention of the telegraph 1876 - Alexander Graham Bell invents the telephone 1876 - Edison invents the electric bulb and the phonograph 1880 - American Bell founded 1892 - Telephone system regulation begins in Canada 1893 - Broadcasting was started in Budapest. 1906 - Lee de Forest invents the vacuum tube. 1910 - Interstate Commerce Commission starts to regulate telcos 1914 - Underground cables link Boston, NYC and Washington 1925 - Bell Telephone Laboratories founded 1930 - AT&T introduces much higher quality insulated wire 1934 - Federal Communications Commission (FCC) founded 1945 - AT&T lays 2000 miles of coax cable 1952 - The first database was implemented on RCA's Bizmac computer 1954 - Gene Amdahl developed the first computer operating system for the IBM 704. 1968 - Carterfone court decision permits non-Bell telephone equipment to be used 1970 - Court permits MCI to provide long-distance services 1984 - Breakup of AT&T 1984 - Cellular phones enter service 1996 - Telecommunications Act of 1996 deregulates U.S. telephone system