Modems and Beyond Chapter 5 Copyright 2001 Prentice Hall Revision 2: July 2001

2 Modems n Problem – Computer (device)is digital – Telephone line is analog – Need translation device called a modem n Modulation: converting outgoing digital device signals to analog transmission line signals n Demodulation: converting incoming analog transmission line signals into digital device signals Digital Signal Modem Analog Signal

3 Modulation n Example: Frequency Modulation – 1 is a high-frequency vibration – 0 is a low-frequency vibration

4 Modem Forms n Internal Modem – On printed circuit board inside PC – Does not take up desktop space – Complex access--must open computer Phone Line to Telephone Phone Line to Wall Jack

5 Modem Forms n External Modem – Easy to install (just plug into serial port) – Takes up desk space – Needs wire to serial port (wiring clutter) – Needs electrical power Phone Line to Telephone Wall Power: usually uses “brick” transformer Serial Cable to Serial Port Phone Line to Wall Jack

6 Modem Forms n PC Card Modems – Fit in PC Card slots on notebooks – Easy to install – Do not take up desktop space – Expensive – Few desktop PCs have PC Card slots

7 Modem Standards n Two Modems Must Follow Same Standards – Modern modem standards are created by the ITU-T – Speed standards (modulation method) – Error correction and compression standards – Facsimile standards n Training Period – When two modems first start talking, they negotiate standards to use; settle on highest common standards

8 Modem Speed Standards (ITU-T) n V.90 – Receive at 56 kbps but send at only 33.6 kbps – Not all phone lines, ISPs support 56 kbps – Fall back to 33.6 kbps if cannot support n V.34 – 33.6 kbps send and receive n V.32 bis – 14.4 kbps send and receive – bis means second (version of the standard)

9 Modem Standards: Error Correction and Compression n ITU-T – V.42: Error detection and correction – V.42 bis: Compression (up to 4:1) – Most newer modems have both

10 Modem Standards n Facsimile Modem Standards – Most modems also act as fax modems – Facsimile requires different modulation than data transmission – V.14: 14.4 kbps – V.29: 9.6 kbps

11 New Modem Standards n V.92 – Can transmit faster than 33.6 kbps but only if the line has unusually high top frequency cutoff – If transmits faster, reception speed falls below 56 kbps – Modem on hold: can talk a short time without breaking the connection – Cuts call setup time in half n V.44 – Can compress webpages better, cutting download time in half.

12 V.90 Modems n Telephone Bandwidth is Limited – Limits speed to about 35 kbps n Upload (Sending) Speed is Limited – Analog-to-digital converter (ADC) at the telephone company filters & converts your incoming signal – Limits you to about 35 kbps 35 kbps Telephone Network Telephone Network ADC PC V.90 modem 33.6 kbps

13 V.90 Modems n However, Receive at 56 kbps – ISP connects with digital line to the Telco – Sends at 56 kbps – No analog-to-digital converter to filter and so limit speed Telephone Network Telephone Network DAC PC V.90 modem ISP Digital Link No ADC! 56 kbps

14 V.90 Modems n Receive at 56 kbps – 56 kbps digital channels inside phone network – Only local loop is analog today – Trunk lines and switches support 56 kbps transmission Telephone Network Telephone Network DAC PC V.90 modem ISP Digital Switches and Trunk Lines 56 kbps

15 V.90 Modems n Receive at 56 kbps – Telephone company transmits in analog to subscriber at 56 kbps – Digital to analog converter (DAC) does not limit speed to less than 56 kbps. No filtering. Telephone Network Telephone Network DAC PC V.90 modem ISP Digital Link 56 kbps

16 V.90 Modem n Telephone Company Does Not Have to Do Anything Differently – ADCs and DACs are already in place for ordinary voice service – Lack of change in phone system allowed fast implementation Telephone Network Telephone Network ADC PC V.90 modem ISP Digital Link DAC

17 V.90 Modem n ISP Does Have to Do Things Differently – Digital line to telephone network – Special equipment at ISP – Does not use a V.90 modem – If used V.90 modem, could only send at 33.6 kbps Telephone Network Telephone Network DAC PC V.90 modem ISP Digital Link No V.90 Modem!

18 V.90 Modem n User Needs a V.90 Modem – Does not need a new phone line – Not all phone lines will work, but it is never worth the cost to install a new line for V.90 modems

19 Alternatives to Ordinary Phone Lines & Modems n Faster Speeds; Called Broadband Services n Also Higher Cost – Translation devices more expensive than modems – Faster (more expensive) transmission line to ISP – ISP may charge more because of heavier transmission load – Faster interface than 232 serial port may be needed

20 Transmission Line Versus ISP n Transmission line may be provided by a transmission carrier, not the ISP – Users have to pay separate fees to carrier and ISP – Some ISPs also provide transmission line – Transmission line costs must always be considered in addition to hardware n External Modem n High-Speed ports

21 Ports n Need port fast enough for line n 232 Serial port to kbps: only V.34, V.90 or ISDN n USB (universal serial bus): 12 Mbps – Available on all new PCs – Faster version coming (USB-2, ~480 Mbps) n Firewire (IEEE 1394) – 400 Mbps and faster – Not available on most new PCs n Ethernet NIC (10 Mbps) – Network interface card used in PC networks – Printed circuit board – Must be installed inside PC systems unit

22 ISDN n ISDN line from telephone company (dial-up digital service) – Multiplexes (mixes) three channels on one UTP wire pair to the desktop 1101 B Channel B Channel 2B+D Multiplexed Onto One Set of Wires ISDN Modem D Channel: control signals

23 ISDN n Two B Channels are 64 kbps – Original idea: one for voice, one for data – Can now “bond” the two B channels for 128 kbps data if ISP supports it n D Channel is 16 kbps (for supervisory signaling) Telephone Network Telephone Network ISP ISDN Modem ISDN Modem 128 kbps B B Bonding

24 ISDN Costs n Must install an ISDN line – Will cost more than telephone line (3X) n Actually, could use your existing phone line – But then would need one B channel for telephony

25 ISDN Costs n Line is expensive – $60-$80/mo plus installation fee – ISP charge is separate; may charge more for ISDN access n Dial Up: Not always connected – Do not have to pay for full-time use – Good if usage is small per day, say to upload sales data from retail once per night – Good for backup to always-on services n Need “ISDN modem” (expensive)

26 ISDN Modem n ISDN “Modem” – Modem is for for digital device, analog line – ISDN line is digital – Codec to link analog telephone to digital ISDN line – Data service unit (DSU) to translate between PC digital format and ISDN digital format (voltage levels, timing, etc.) Station Analog Digital Codec Modem DSU

27 ISDN Modem n Codec – Translates analog device signal into digital signal for propagation – Divides each second into 8,000 sampling periods – Constantly samples the intensity of the voice analog signal in each sample – Measures voice intensity as 8-bit value (0-255) – Sends 8 bits 8,000 times per second (64 kbps) Sample 1/8,000 sec Sampling Period Intensity Value

28 DSU (Data Service Unit) n DSU translates between different digital formats – Device and line are both digital, but still must have translator n Different bit rates n Different number of possible states n Different voltage levels for the states n Different ways to represent ones and zeros DSU Digital Line

29 Cable Modem Service n Service of Cable Television Companies – 10 Mbps downstream n Capacity is shared by multiple subscribers (real throughput 300 – 500 kbps) n Shared within Blocks of 500 Houses – 64 kbps to 256 kbps upstream – Does not tie up telephone line – Always available Cable TV Network Cable TV Network Also ISP Functions Also ISP Functions Cable Modem Cable Modem

30 Cable Modem Service n Cost is about $50 per Month – Includes ISP service! – Installation usually costs $100 to $150 and includes a cable modem and a network interface card – Cost-competitive with adding a second phone line to handle your modem communication

31 Digital Subscriber Lines (DSLs) n Offered by Telephone Companies – subscriber lines: connect customer premises to telephone system – Entirely digital – Different subscriber lines for home & business Telephone Network Telephone Network ISP DSL Modem DSL Modem DSL

32 Digital Subscriber Lines (DSLs) n Residential customers: multiplexes regular phone n Can use existing phone line coming into house n Can use Internet without tying up phone – Most common Asymmetric DSL (ADSL) Telephone Network Telephone Network ISP DSL Modem DSL Modem Existing Phone Line

33 Digital Subscriber Lines n Lower-speed upstream and higher-speed downstream n About $50 per month includes ISP – Businesses pay more for higher speeds (up to 1.5 Mbps) n DSL service requires DSLAM (Digital Subscriber Line Access Multiplexer) n G.Lite (G.992.2) Standard – Up to 1.5 Mbps downstream speed Telephone Network Telephone Network ISP DSL Modem DSL Modem ADSL 64 or more kbps 384 kbps-640 kbps

34 Digital Subscriber Lines n Digital Subscriber Line Access Multiplexer – Telephone company must install at end switching office to serve DSL users Telephone Network Telephone Network ISP DSLAM DSL

35 Digital Subscriber Lines n Splitting Voice and Data – Voice and data are split at home by the DSL modem – Voice and data are also split at the telephone company’s first switching office DSL Modem DSLAM DSL Data Network Voice Network Computer Phone

36 Wireless Internet Access n Reach the Internet by Radio – Terrestrial (uses earth stations) – Satellite-based ISP

37 GEO Satellites n In geosynchronous orbit – Appear to be stationary in the sky – 36,000 km (22,300 miles) – Need much power to send/receive – Need dish antennas to concentrate signals – Must point dish at the satellite – Impractical for portable computers

38 VSATs n Very Small Aperture Terminals (VSATs) – Small dishes (1 meter in diameter or less) – Do not concentrate signal as well as large dishes, so inefficient – Less precise focus then large dishes, satellites cannot be spaced closely together – However, inexpensive – Used when there must be many ground stations

39 LEO Satellites n Low Earth Orbit (LEO) Satellites – Below First Van Allen Radiation band, peaks at 3,000 km (1,800 miles) – A typical orbit is 1,000 km or 600 miles n Medium Earth Orbit (MEO) Satellites – Between 1st and 2 nd Van Allen Radiation Band, peaks at 15,000 km (about 9,000 miles) – Typical distance is 6,000 km or 4,000 miles – Farther away than LEOs, so need more power – But travel more slowly across sky, so fewer hand- offs, fewer satellites needed Omnidirectional Antenna

40 Small Office Service n Serial Router Connects Internal Stations to Shared Broadband Access Service Small Office Serial Router Shared Broadband Line ISP

41 Small Office Service n Serial Routers (Also Called Access Routers) are Very Simple – One or more ports to connect the serial port to the LAN – One port to connect to the high-speed access line, which is serial – Much less expensive than general-purpose routers because no complex router forwarding decision caused by multiple possible output ports Serial Router

42 Small Office Service n Serial Router Connects Internal Stations to Shared Broadband Access Service – Companies with several stations pay much more for line than when connecting a single PC – Each machine gets its own IP address via DHCP – Serial router may be a user PC with added software – Often, router is called a gateway (the old name for router)