Intro to Telecom

Fig 6.2

Analog and Digital Signals

Fig. 6.4Analog signal

Digital signal

Analog Continuous fluctuations over time between high and low voltage

Digital A discrete voltage state

Fig 6.3

Source/Signal Combinations

AnalogSignal

DigitalSignal

AnalogSource

Voice, Telephone,Television....

Voice overdigitalmedia,Audio files,CODEC

DigitalSource

Fax, Any Computerover POTS, DigitalT-V

Computerover digitallines (T-1,ATM,Framerelay...)

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

[Figure 6.14]Time division multiplexing (TDM) is where multiple incoming signals are sliced 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

--Cable Bandwidth--

Ch 1Ch 2Ch 3Ch 4Ch 5Ch 6 Ch n….……………....

Base video width is 4.2 MHz with guard bands 6 MHz

The default is 6 Mega Hertz slices of bandwidth per channelCable modem

Cable phone gets 4 KHz slicesQ: What limits the bandwidth on coaxial cable?

A: The bandwidth of the amplifier.

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

Your Mom

To communicate a physical connection must be made and maintained

Switch

medium

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 Trailer

Startof Header Start

of Text

Endof

Text

BlockCheck

Character

SOH (STX) (ETX) BCC

If variable length header used If variable length message used

Direction of transmission

A Simple Protocol Stack

Application

Transport

NetworkAccess

Application

Transport

NetworkAccess

Application Protocol

Transport Protocol

Network Protocol

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

Application

Transport

NetworkAccess

Application

Transport

NetworkAccess

Transport Header

Data

Network Header

Protocols add header information to the message

Protocols strip header

information from the message

Communications Protocols

Fig 6.22

Relationship of TCP/IP to OSI

6

7

4

5

2

3

1

Presentation

Application

Transport control

Session control

Data link control

Network control

Physical link control

OSI

Process /Application

Internet

Host to Host

NetworkAccess

TCP/IP 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)IP: routing, fragmentation, assemblyICMP: Above IP, error handlingARP: Address resolution sw to hw addrRARP: hardware to sw address convert

TCP: Virtual circuit maintained, ackUPD: No acknowledgment

Physical layer, such as Ethernet or Token Ring

Fig 6.23

Ethernet Evolution

10 MbpsEthernet

100 MbpsEthernet

1000 MbpsGigabitEthernet

Legacy

Predominant

New, taking over

Old installations

Most new installations

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

data

T

data

T40008065402

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!!!

HeaderBody

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 real time need ATM

HeaderBody

Connectivity

Type Bandwidth # Users Rel.CostModem 28.kbps 1-5 1DSL 256+ Kbps 1-50 2ISDN 128 Kbps 5-50 3T1 (DS1) 1.54 Mbps 50-500 10T3 (DS3) 45 Mbps 4000+ 100ATM 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