1CMPE 80N - Introduction to Networks and the Internet

CMPE 80N

Winter 2004

Lecture 5

Introduction to Networks and the Internet

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Announcements• First quiz on Friday, 01.16.– Covers material up to and including 01.14.– Closed books, notes, etc.

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Data Transmission • Analog and digital transmission.• Historically, communication infrastructure for

analog transmission.– Digital data needed to be converted: modems

(modulator-demodulator).

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Modems• MODEM = Modulator/Demodulator

– Converts digital to analog before transmitting over analog channel (e.g., telephone networks).

• To transmit data: DAC (digital-to-analog converter)

• To receive data: ADC (analog-to-digital converter)

• 2-way communication: needs two modems.– Each modem contains circuitry to encode

outgoing data and decode incoming data.

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Types of Communication• Half-duplex communication:– Only one party can talk at a time.• E.g., walkie-talkie.

• Full-duplex communication:– Both parties can talk at the same time.• E.g., telephone

• Modems use full-duplex communication.

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Modems (cont’d)• Modems contain complex circuitry to:– Modulate/demodulate the analog signal.• Allows for the transmission of moderately high

bit-rate over the telephone line.– Compress the data. • Reduces the amount of bits to be transmitted

– Detect bit errors due to transmission.• Achievable bit rates.– E.g., 14.4 - 56 Kb/s

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Input/Output Connections

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I/O Connections• How to connect peripheral devices to a

computer.• Serial and parallel connections.– Serial: bits are sent one at a time.– Sequence of bits sent in parallel using parallel

wires.

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I/O Connections: Standards• Standard: defines the details of a particular

technology.• RS-232 is a standard for serial

communication between digital devices.– It’s full duplex.– 20-30 Kb/s.– Can only connect one device at a time.• So if you want to connect a PC to many

devices, you need as many cables coming out of your PC.

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USB and FireWire• USB (Universal Serial Bus).

– Can connect many devices through a USB hub.– Bitrates: 12 Mb/s (USB 1.1) to 480 Mb/s (USB 2.0).– Provides power to small devices (e.g., mouse).

• Firewire (IEEE 1394).– Can connect many devices through a FireWire hub.– Bitrates: up to 400 Mb/s.– Very popular for video cameras and storage systems,

also to connect two devices (without a PC).– Can provide power to small devices (e.g., video

cameras).

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Some Considerations• RS-232, USB, Firewire, all have constraints

on the maximum length of the wire.• We already know a solution: modem.– Uses the telephone network.

• However, modems provide insufficient bitrate.– Also, when using the modem, you cannot use

the telephone for voice communication!

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Solution: Broadband Coonections

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Broadband Connection Types– Integrated Services Digital Network (ISDN).– Asymmetric Digital Subscriber Line (ADSL).– Cable Modem.– Wireless.– Satellite Links.

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ISDN• ISDN provides for communication of digitized

voice and data to subscribers over the conventional “local loop” (i.e., using the same wiring as for analog telephone).

• In the Basic Rate Interface (BRI), ISDN offers three separate digital channels (2B+D).• All on the same wire! (Multiplexing)

• Primary Rate Interface (23B+D).– Requires higher capacity lines than local loop!

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ISDN (cont’d)• The two B channels are intended to carry digital

voice, data, or digital streams– Bitrate of each B-channel: 64 kb/s (overall, 128

kb/s)• The D channel is used as a control channel– E.g., to request services which are then supplied

over the B channels, to carry caller ID information, etc.

– Bitrate of D channel: 16 kb/s.

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ISDN (cont’d)• To connect computer to ISDN, user needs a

special network termination device (NT1).– NT1 device a.k.a. ISDN modem.• A modem converts a digital signal to an analog

signal; ISDN is inherently digital, so no such conversion is necessary.

• Need to dial a number to start a connection

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ISDN (cont’d)• ISDN was initiated in 1984, and was available

in the USA in the early 90s.• It was an attempt to replace the analog phone

system with a digital voice+data system.– It never really succeeded…

• Currently, ISDN is obsolete, because it offers limited bitrate at a fairly high price.

• Still a possibility for Internet connection where other forms of broadband are not available.

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ADSL• ADSL allows transmission of high bit-rates over

local loop.– It does not require any changes in the wiring.

• In addition, it does not preempt the local loop.– A user can use the telephone for analog voice

communication and at the same time transmit data or stream video.

• It requires a splitter and a ADSL modem– The splitter separates voice/fax signals from

data stream.– PC to ADSL modem: typically USB.

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ADSL scheme

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ADSL (cont’d)• To achieve high bitrate transmission, ADSL

must use sophisticated technology– It is “adaptive”: ADSL modems at the two ends

probe the line between them to find its characteristics, and then agree to communicate using techniques that are optimal for that line.

• Depending on the characteristics of the wiring, different bit-rates can be achieved– If a house is too far form the “End office”

(switching center), ADSL is not available.

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ADSL (cont’d)• ADSL is asymmetric: it provides a higher bit-

rate downstream than upstream.– Downstream: 32 kb/s to 6.4 Mb/s (more typically, 1.5 Mb/s)– Upstream: 32 to 640 kb/s (more typically, 256 kb/s)

• Asymmetry is OK when high bitrate data is transmitted to the user.– E.g.: Video-On-Demand, Internet radio…

• In some cases, symmetric communication is preferable.– E.g.: Videoconferences

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Cable Modems• CATV (Community Antenna TV, or cable TV)

uses coax cable (less susceptible to interference)– 1-Km coax cable can accommodate bitrates of 1-2 Gb/s!

• Only one cable is used for a neighborhood– Different TV channels are multiplexed on it.

• Cable systems are designed to carry many more television signals than currently available.– There is unused capacity that can be used for

data communication!– >80% of US homes are already reached by

CATV

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Cable Modems (cont’d)• User can connect using a cable modem– A splitter separates the TV and the data signals.

• Problem: all users in the neighborhood share the same available capacity in the same cable!– If all users in the neighborhood transmit data at

the same time, the available bitrate is reduced.• E.g., if there are 50 Mb/s available, and 100 users in the

neighborhood use it simultaneously, each user has only 0.5 Mb/s

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Cable Modems (cont’d)• Coax cables from several neighborhoods

connect to a concentrator– The concentrator uses high capacity fiber optics

cables to connect to the head end, which is connected to the Internet.

• Communication is asymmetric– Originally, CATV was designed only for

downstream communication!• Available bitrates:– Downstream: 1.5 to 2 Mb/s– Upstream: 128 kb/s

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Satellite Systems• Digital communication satellites were

deployed by telecommunication companies as an alternative to terrestrial lines.– They can now be used as “local loop”

technology (e.g., DirectPC).• Advantages:– Can reach arbitrary geographic locations.– Does not require wiring.– Has high bandwidth.– Perfect for broadcasting (can reach many

users at once).

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Satellite Systems (cont’d)• Disadvantages:– It’s a shared medium (the bitrate depends on

the number of simultaneous users).– Delay (latency) can be relatively high (<1s)• Not ideal for playing interactive app’s (e.g.,

games).– You have to put a dish on your roof!

• Initially, uplink was not provided. – Needed to use a separate phone line to uplink

information.• Nowadays it is a two-way system.

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Physical Layer: Summary• Different types of signal:

– Analog and digital.• Analog communication infrastructure:

– Need to convert digital to analog before transmitting: ADC.

– DAC before entering computer.• Digitization:

– Sampling.– Sampling period and frequency (samples/sec or Hertz).– Sample representation (quantization).– Bit rate.

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Physical Layer: Summary (Cont’d)

• Modems.• Input/output connections.– RS 232.– USB.– Firewire.

• Broadband.– ISDN.– ADSL.– Cable modem.– Satellite.

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Other Wireless Networks

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Cellular Networks• Cellular phones:voice.• Cellular networks: shift from voice to data.• New wireless devices: pagers, PDAs.• New services: Web access, e-mail, instant

messaging, etc.

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Cellular Concept: Motivation• Early mobile radio systems: – Large coverage with single, high-powered

transmitter.– But, no frequency re-use due to interference.

• Since finite spectrum allocation, need: high capacity (number of users) with limited spectrum and wide coverage.

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Some Cellular Terminology• Mobile.• Base station.• Mobile Switching Center (MSC).• Handoff.• Cell.

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Cellular Architecture

cell cell

BSBSmobile

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Cellular Fundamentals• System-level idea, no major technological

changes.– Many low-power transmitters instead of single,

high power on (large cell).– Service area divided into small cells covered

by each low power transmitter.– Each transmitter (or base station) allocated a

portion of the spectrum.– Nearby BSs assigned different channel group

to minimize interference.– Scalability: as more users subscribe, more

BSs can be added using lower transmission power).

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Frequency Reuse

A

B

C

D

E

F

G

G

E

F

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Handoff/Handover

• Mobile hosts can change cells while communicating.

• Hand-off occurs when a mobile host starts communicating via a new base station.

• Handoff decision made based on signal strength.

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Cellular Networks: Evolution• Evidence of the wireless success!– Since 1996, number of new mobile phone

subscribers exceeded number of new fixed phone subscribers!

• 1st. Generation (1G): analog technology.– FDMA.– Analog FM.

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Second Generation (2G) • Most of today’s cellular networks use 2G

standards.• Early 90s.• Digital technology.– Lighter, smaller devices with longer battery

life.– Better reception and channel utilization.

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3G Wireless Networks• Multi-megabit Internet access, VoIP,

ubiquitous “always-on” access.• Single mobile device for everything

(integrated service approach).• New, world-wide standard.– International Mobile Telephone 2000 (IMT

2000)

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Wireless Local Area Networks• Local area network connectivity using

wireless communication.• IEEE 802.11 WLAN standard.• Example: WaveLan, Aironet• Wireless LAN may be used for– Last hop to a wireless host.– Wireless connectivity between hosts on the

LAN.

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Other WLAN Standards• HomeRF– Proponents of 802.11 frequency hoping-

spread spectrum (FH-SS).– HomeRF 2.0– 10 Mbps FH-SS.

• HIPERLAN– Europe, mid 1990s.– Similar capability to IEEE 802.11b.

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MANETs• Mobile, (wireless), multi-hop ad-hoc networks.• Formed by wireless hosts which may be

mobile.• Without (necessarily) using a pre-existing

infrastructure.• Routes between nodes may potentially

contain multiple hops.• Mobilitty cause routes to change.

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Multi-hop• May need to traverse multiple hops to reach

destination.

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Why MANETs ?• Ease of deployment.• Speed of deployment.• Decreased dependence on infrastructure.

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Many Applications• Personal area networking.– Cell phone, laptop, ear phone, wrist watch.

• Military environments.– Soldiers, tanks, planes.

• Civilian environments.– “Smart” environments.

• Emergency operations– Search-and-rescue– Policing and fire fighting– Monitoring and surveillance.