Modems and Beyond Chapter 5 Copyright 2001 Prentice Hall

2 Modems n Problem – Computer is digital – Telephone line is analog – Need translation device called a modem Digital Signal Modem Analog Signal

3 Modems n Note: – Device is digital;Transmission line is analog – Cannot just say “modem translates between analog and digital” (For analog device and digital transmission line, need something else) Digital Device Analog Transmission Line

4 Modulation n A Modem is a Modulator and Demodulator – Modulation is converting outgoing digital device signals into analog transmission line signals – Demodulation is converting incoming analog transmission line signals into digital device signals Digital Signal Modem Analog Signal

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

6 Modem Forms n Internal Modem – On printed circuit board inside PC – Does not take up desktop space – Can be complex to install--must open up computer

7 Modem Forms n External Modem – Easy to install (just plug into serial port) – Takes up desk space – Needs wire to serial port (adds to wiring clutter) – Needs electrical power

8 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

9 Modem Connection for Internal Modem Phone Line to Telephone Phone Line to Wall Jack

10 Modem Connection for External Modem Phone Line to Telephone Wall Power: usually uses “brick” transformer Serial Cable to Serial Port Phone Line to Wall Jack

11 Modem Standards n Two Modems Must Follow Same Standards – Speed standards (modulation method) – Error correction and compression standards – Facsimile standards – Modern modem standards are created by the ITU-T

12 Modem Standards n Training Period – When two modems first start talking, they negotiate standards to use; settle on highest common standards

13 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)

14 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 n MNP Level 5 – Proprietary standard of Microcom – Once widely used – Provides both error correction and compression – Now largely superceded by V.42 and V.42 bis New

15 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

16 V.90 Modems n Telephone Bandwidth is Limited (Chapter 4) – Bandwidth limits speed – Limits speed to about 35 kbps – When you transmit, limited to 35 kbps 35 kbps

17 V.90 Modems n Telephone Bandwidth is Limited – Limits occur when you transmit in analog to the phone company n Analog-to-digital converter (ADC) at the telephone company limits you to about 35 kbps Telephone Network Telephone Network ADC PC V.34 modem 33.6 kbps

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

19 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 56 kbps modem ISP Digital Switches and Trunk Lines 56 kbps

20 V.90 Modems n Receive at 56 kbps – Telephone company transmits in analog to subscriber at 56 kbps – Digital to audio converter (DAC) does not limit speed to less than 56 kbps Telephone Network Telephone Network DAC PC 56 kbps modem ISP Digital Link 56 kbps

21 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 56 kbps modem ISP Digital Link DAC

22 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 56 kbps modem ISP Digital Link No V.90 Modem!

23 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

24 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

25 Transmission Line Versus ISP n Transmission line may be provided by a transmission carrier, not the ISP n Users have to pay separate fees to carrier and ISP n Some ISPs also provide transmission line – But not all do – Transmission line costs must always be considered PC ISP Transmission Line

26 Translator Devices n Device versus Line Analog LineDigital Line Analog Device Codec Digital Device ModemDSU Translator Transmission Line Device

ISDN Integrated Services Digital Network Codecs DSUs

28 ISDN n Get ISDN line from telephone company – 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

29 ISDN n Two B Channels are 64 kbps – Original idea: one for voice, one for data 1101 B Channel B Channel

30 ISDN n 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

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

32 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 n Need “ISDN modem” (expensive)

33 ISDN Modem n ISDN “Modem” is a Misnomer – Modem is for for digital device, analog line – ISDN line is digital n ISDN modem really contains – 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.)

34 ISDN Modem n Codec – Translates analog device signal into digital signal for propagation – Constantly samples the intensity of the voice analog signal Sample 1/8,000 sec

35 ISDN Modem n Codec – Divides each second into 8,000 sampling periods – Only measures intensity of voice signal in each Sample 1/8,000 sec Sampling Period Intensity Value

36 ISDN Modem n Codec – Measures voice intensity as an 8-bit intensity value (0-255) – Overall, sends 8 bits 8,000 times per second (64 kbps) Sample 1/8,000 sec Sampling Period Intensity Value

37 ISDN Modem n Codec – This is why telephone channels are 64 kbps n Designed for digitized voice n Often steal 8 kbps for supervisory signaling, so 56 kbps Sample 1/8,000 sec Sampling Period Intensity Value

38 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

Cable Modem Service Broadband Internet Access

40 Cable Modem Service n Service of Cable Television Companies – Deliver 10 Mbps downstream to the home – Capacity is shared by multiple subscribers, so real speed is more limited – Limited to about 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

41 Cable Modem Sharing in Perspective n Sharing is Not as Extreme as it May First Appear – Usually, shared within Blocks of 500 Houses – Only Some Households in Block will Subscribe – Only Some Subscribers will be Online at Any Moment – Only Some of Online Subscribers will Send and Receive at Any Moment – Only these will Share the Capacity n So Sharing Does Not Reduce Speed Unless the Adoption Rate is Large

42 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 – Best alternative today beyond V.90 modem – Cost-competitive with adding a second phone line to handle your modem communication

DSLs Digital Subscriber Lines

44 Digital Subscriber Lines (DSLs) n Offered by Telephone Companies – Lines to customer premises are subscriber lines, which connect subscribers to the telephone system – These are digital – Hence “digital subscriber line” Telephone Network Telephone Network ISP DSL Modem DSL Modem DSL

45 Digital Subscriber Lines (DSLs) n Offered by Telephone Companies – Several types of digital lines for subscribers n Some are for homes, others for businesses – For residential customers, usually multiplexes regular phone, high-speed data n Can use existing phone line coming into house n Can use Internet without tying up phone Telephone Network Telephone Network ISP DSL Modem DSL Modem Existing Phone Line

46 Digital Subscriber Lines n Most common for home is Asymmetric DSL (ADSL) – Upstream and downstream speeds are different – Upstream at 64 kbps or more – Downstream at 256 kbps to a few Mbps – Asymmetric speed is good for WWW – About $50 per month incl. ISP; more for faster service Telephone Network Telephone Network ISP DSL Modem DSL Modem ADSL 64 or more kbps 256 kbps-a few Mbps

47 Digital Subscriber Lines n G.Lite (G.992.2) Standard – New ADSL standard from ITU-T – Up to 1.5 Mbps downstream speed – No carrier installation is necessary – Not being widely adopted by ADSL vendors yet Telephone Network Telephone Network ISP DSL Modem DSL Modem ADSL 64 or more kbps Up to 1.5 Mbps New

48 Digital Subscriber Lines n Digital Subscriber Line Access Multiplexer – Telco must install a DSLAM at end switching office Telephone Network Telephone Network ISP DSLAM DSL

49 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 DSL Splitter DSL DSLAM Voice Network Computer Phone New

50 DSLs Versus Cable Modem Service n Both Expected to Sell Well n Forecast for Worldwide Installations in 2004 – Cable modem lines: 9 million – DSLs: 10 million – Source: Insight Research n Cable is Available Mostly in Residential Areas n Cable is only widespread in some countries, especially the United States

51 Broadband Access Lines in Perspective n Limited to Transmission Between Customer Premises and the ISP – ISP Speed can Limit Service Speed – Internet Backbone Speed can Limit Service Speed – Server Host Access Line Speed can Limit Service Speed – Server Host Processing Speed can Limit Service Speed n Broadband Access Lines are Not Panaceas

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

53 GEO Satellites n In geosynchronous orbit – Appear to be stationary in the sky n Far from the ground – 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

54 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 – Do not focus as precisely as large dishes, so satellites cannot be spaced closely together – However, inexpensive – Used when there must be many ground stations

55 LEO Satellites n Low Earth Orbit (LEO) Satellites – Below First Van Allen Radiation band, which peaks at 3,000 km (1,800 miles) – A typical orbit is 1,000 km or 600 miles Omnidirectional Antenna New: Not in Book

56 LEO Satellites n Low Earth Orbit (LEO) Satellites – Need far less power to reach than 36,000 km (22,300 mile) geosynchronous satellites – So can get by with omnidirectional antenna – Can use phone of reasonable size, cost – Access anywhere is the world, although local laws may limit legality Omnidirectional Antenna

57 MEO Satellites n Medium Earth Orbit (MEO) Satellites – Between First and Second Van Allen Radiation Band, which 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 New

58 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 – Fast enough for DSLs, cable modems – Faster version coming (USB-2, ~480 Mbps)

59 Ports n Firewire (IEEE 1394) – 400 Mbps and faster – Not available on most new PCs – Fast enough for DSLs, cable modems n Ethernet NIC (10 Mbps) – Network interface card used in PC networks – Printed circuit board – Must be installed inside PC systems unit – Fast enough for DSLs, cable modems

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

61 Small Office Service n Serial 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

62 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)

63 Connecting Two PCs at Home n Physically, One PC (the Gateway PC) Connects to the Hub or Switch and to the Broadband Line – Gateway (router) PC needs two Network Interface Cards, one for hub, one for broadband modem Gateway PC Broadband Line to ISP ADSL/Cable Modem Hub or Switch NIC

64 Connecting Two PCs at Home n Gateway PC Needs Gateway Software – On NIC to broadband line, acts like a single station – Gets temporary IP address from ISP DHCP – Sends and receives to ISP using this IP address Gateway PC Send, Receive with Temporary IP Address

65 Connecting Two PCs at Home n Gateway PC Needs Gateway Software – Each PC is set up with a permanent IP address – So gateway PC has two addresses, one bound to each NIC Gateway Program Temporary IP Address Permanent IP Address Permanent IP Address

66 Connecting Two PCs at Home n Gateway Software Does Network Address Translation (NAT) – When a PC sends using permanent IP address – Gateway software replaces source IP permanent address with temporary IP address ISP expects Gateway PC Temporary IP Address Permanent IP Address Permanent IP Address

67 Connecting Two PCs at Home n Gateway Software Does Network Address Translation (NAT) – When a PC receives – Gateway software replaces ISP temporary IP destination address with receiver’s permanent IP address, passes on to PC Gateway PC Temporary IP Address Permanent IP Address Permanent IP Address